(Circulation. 1996;94:1475-1482.)
© 1996 American Heart Association, Inc.
Articles |
Rapid Shortening During Relaxation Increases Activation and Improves Systolic Performance
the Cardiology Unit, The University of Vermont, Burlington.
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Abstract |
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Background Previous studies in cardiac muscle and isolated heart preparations generally have attributed positive effects of ejection to greater length-dependent activation. However, there have been some reports of an ejection-related increase in contractile function that is independent of end-diastolic volume (EDV) history. The present study was designed to more fully characterize the mechanoenergetic results of the latter effect in the intact ventricle.
Methods and Results A servomotor was used to initiate left ventricular volume reduction (VR) at end systole, with EDV kept constant. Seven isolated, red blood cell-perfused rabbit hearts were studied at constant EDV during isovolumic contraction, slow VR (5.0±0.9 EDV/s), and rapid VR (26.8±5.1 EDV/s). Compared with isovolumic beats, VR caused an enhancement in contractility. This effect was greater for rapid VR and required >50 beats to attain steady state. Rapid VR increased developed pressure by 15% (92.2±23.7 [mean±SD] versus 105.9±27.6 mm Hg), maximum dP/dt by 17% (1223±401 versus 1435±505 mm Hg·s-1), and Emax (slope of the end-systolic pressure-volume relation) by 13% (69.4±19.9 versus 78.6±23.0 mm Hg/mL) (all P<.01). Left ventricular oxygen consumption (
O2) was unchanged with slow VR and decreased by 8% with rapid VR (0.0744±0.0194 versus 0.0683±0.0141 mL O2·beat-1·100 g-1; P<.05). In separate hearts (n=8), costs (basal metabolism and excitation-contraction coupling) were estimated by use of 2,3-butanedione monoxime. Compared with control, rapid VR was associated with a 26% increase in nonmechanical O2 (0.0248±0.0021 versus 0.0312±0.0022 mL O2·beat-1·100 g-1; P<.01), consistent with an increase in calcium cycled per beat.
Conclusions Ejection after end systole has a positive effect on ventricular performance that cannot be ascribed to length-dependent activation and is likely related to an increase in calcium available for activation. Similarly, an increase in nonmechanical O2 associated with ejection suggests a positive interaction between myofilament shortening and activator calcium cycling.
Key Words: calcium • contractility • mechanics • sarcoplasmic reticulum
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Introduction |
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On the basis of early studies, cardiac muscle shortening (ejection) was considered to have a predominantly negative effect on contractile performance compared with isometric (ISOV) contraction.1 This was ascribed to deactivation of shortening and viscous resistance.2 Recently, a more complicated picture has emerged as positive effects of shortening and ejection have been characterized in isolated heart muscle and in the intact ventricle. In isolated muscle preparations, shortening has been associated with a prolonged time course of contraction3 4 and an increase in the amplitude of the intracellular calcium transient.5 The latter effect is consistent with a length-dependent decrease in myofilament calcium affinity and subsequent displacement of calcium from the contractile proteins.5 6 The energetic consequences of shortening have varied in muscle preparations, with reports of both a decrease7 and an increase8 in myocardial energy consumption (
O2) compared with isometric contraction. In addition, a precise understanding of the interaction between shortening and alterations in the partitioning of energy input between that used for cross-bridge formation and E-C coupling is lacking. It is conceivable that shortening results in differential energetic effects, ie, an increase in energy consumption due to increased cycling of activator calcium and a decrease in energy consumption due to deactivation of cross bridges. In the intact ventricle, several recent studies have demonstrated a positive effect of ejection on LV pressure generation. Over a range of contraction modes, including volume withdrawal by rapid flow pulses,1 9 volume ramp ejection,10 11 and ejection controlled by an impedance afterload,12 a consistent increase in ES pressure of "ejecting" beats compared with ISOV beats at matched ESV has been observed. This positive effect has been ascribed to a beat-to-beat "memory" of greater length-dependent activation in the ejecting beat due to the larger EDV and average volume compared with an ISOV beat at the same ESV.
In contrast, one previous report1 demonstrated an ejection-related increase in contractile performance that was independent of ED loading. In that study, Yasumura and coworkers used rapid volume withdrawal (8 to 19 EDV/s) beginning at ES in isolated canine hearts and found an increase in Emax of the volume withdrawal contractions compared with ISOV contractions at matched EDV. The time course of this effect was not described. These results cannot be explained by length-dependent activation and suggest some other effect related to shortening. ESV withdrawal contractions were also found to have a significant decrease in total O2 that was proportional to the speed of withdrawal. Because quick release of cardiac muscle during relaxation results in transient increases in intracellular calcium concentration, we hypothesized that the positive effect of rapid VR during relaxation in the intact ventricle is due to an increase in myocyte calcium available for activation. If this hypothesis is correct, such a shortening-related increase in contractility would be expected to be associated with an increase in nonmechanical O2 (O2 for basal metabolism and E-C coupling) because of the increased energetic requirements of calcium handling and a decrease in mechanical O2 (O2 for cross-bridge cycling) caused by cross-bridge deactivation. The effect on total O2 would depend on the balance between these two effects.
In the present study, we characterized the positive effect of rapid volume withdrawal at ES in the isolated, red blood cell-perfused rabbit heart and tested this hypothesis by partitioning total O2 into nonmechanical and mechanical components. To accomplish this, we used a recently described technique using the drug BDM to estimate nonmechanical O2.13 14 We found that rapid VR during relaxation resulted in a gradually appearing positive inotropic effect that was associated with substantial increases in nonmechanical O2 and decreases in mechanical O2.
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Methods |
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Heart Preparation
We used an isolated rabbit heart preparation with retrograde perfusion of bovine red blood cells suspended in a KH buffer. The details of this preparation have been reported elsewhere.14 15 Adult male, nonfasted New Zealand White rabbits (weight, 2.4 to 3.8 kg) were premedicated with fentanyl (0.044 mg/kg IM) and droperidol (2.2 mg/kg IM), anesthetized with ketamine (20 mg/kg IM) and xylazine (1 mg/kg IM), and ventilated through a tracheostomy with a respirator. After median thoracotomy, the rabbits were anticoagulated with heparin (1000 U/kg IV). The venae cavae and pulmonary veins were ligated, and a perfusion cannula was immediately connected to the aortic root 4 to 5 mm above the aortic valve. The heart was excised from the chest after initiation of retrograde perfusion. Coronary perfusion pressure was kept constant (80 to 85 mm Hg) by a pressurized flow chamber. The temperature of the isolated heart was maintained at 36°C to 37°C.
The LV was vented at the apex, the left atrium was opened, and the chordae tendineae were cut. A thin-walled latex balloon was placed in the LV and secured at the level of the mitral valve ring. LV pressure was measured inside the LV balloon by a high-fidelity micromanometer catheter (Millar Instruments), and LV volume was controlled by a computer-driven linear servomotor. The operating characteristics of the servomotor are described below. Coronary blood flow was collected by continuous drainage of the collapsed RV and was measured by timed collection in a graduated cylinder. Thebesian flow was ignored because it is negligible in rabbit heart.15 Coronary arteriovenous O2 content difference was measured continuously with an AVOX analyzer (AVOX Systems), which was calibrated at intervals throughout each experiment with a Lex-O2-Con O2 content analyzer (Lexington). Heart rate was controlled (160 to 210 bpm) by epicardial pacing from the LV.
Perfusate Preparation
The nonrecirculated perfusate consisted of bovine red blood cells suspended in KH buffer. Fresh, whole cow blood was obtained from a local slaughterhouse and prepared as described by Marshall.16 The details of initial separation and daily washing of the red blood cells have been described elsewhere.14 The red blood cells were used within 4 days of collection. The KH buffer used for washing and final red blood cell suspension had the following composition (in mmol/L): 108.0 NaCl, 4.0 KCl, 2.5 CaCl2, 1.4 KH2PO4, 25.0 NaHCO3, 11.0 dextrose, and 10.0 sodium pyruvate (all from Sigma Chemical Co). Gentamicin (3 mg/L) was added to the buffer to suppress growth of bacteria. Before each experiment, the red blood cells were suspended to a final hematocrit of 35% in a mixture of KH buffer and insulin (10 IU/L). The perfusate was equilibrated in a silicone elastomer tubing oxygenator with 98% O2/2% CO2 to achieve a PO2 of >100 mm Hg and a PCO2 of 
40 mm Hg; pH was adjusted to 7.35 to 7.45 by addition of NaOH. The temperature of the perfusate was maintained between 35°C and 37°C with water jackets around the oxygenator and the pressurized chamber in the arterial line.
The average time between heart excision and the start of data collection was 20 minutes (range, 15 to 30 minutes). Data collection for either the VR or BDM protocol required an average of 60 minutes (range, 40 to 90 minutes). We have validated the mechanical and energetic stability of this preparation previously.13 To further ascertain stability, the LV developed pressure at maximum LV volume was measured again after data collection. On average, there was a decrement of <1% in developed pressure. Hearts with decreases >5% were excluded.
Servomotor Control System
Contraction mode was varied by use of a volume- and pressure-integrated servomotor directed by a computer control system.17 The servomotor system is composed of a linear motor, a piston-cylinder device (based on a ground-glass syringe), an LVDT, an analog position controller, and a high-current amplifier. The piston barrel is connected to one end of the metal motor coil shaft. The flared end of the cylinder is attached to the LV balloon and placed in the mitral annulus. The piston-cylinder device allows a maximal infusion or withdrawal of 1.5 mL from the initial EDV at a resolution of 0.01 mL. The LVDT (Trans-Tek, Inc) is mounted on the back of the linear motor. Its core is attached to the other end of the motor coil shaft. The LVDT has a frequency response of 1 kHz and resolution of ±0.015 mm. This allows precise measurement of piston position and, with proper calibration, instantaneous LV volume. Piston position is controlled by a classic analog PD controller. A volume command from the computer control system is used as reference input to the PD controller. Output voltage from the controller is sent to a high-current amplifier. An electric current is then delivered to the motor coil to change volume in the LV to match the volume command. Thus, the volume-servo system controls volume in the LV balloon according to the volume command signal from the computer control system. Similarly, a PD compensator controls pressure (by controlling volume) at any level commanded by the computer. With this system, LV pressure or volume can be altered over a wide range of rates at any point in the cardiac cycle.
The computer control system is based on an Intel 486 processor on an IBM-compatible computer and supervises the volume-servo system. An analog/digital convertor samples instantaneous LV pressure and volume (calculated from the LVDT position signal). In addition, a digital-to-analog convertor sends the volume command to the analog PD controller of the volume-servo system and another sends the pressure command to the analog PD controller. One digital input/output line is used to pace the heart under computer control and another is used to switch between analog pressure or volume control during the ejection cycle. Because the computer control system operates in real time with a 1-ms update cycle, the volume-servo system can be made to respond to any pressure or volume command signal almost instantaneously.
BDM Partitioning of O2
We used a recently described method to partition O2 in the intact heart using the negative inotropic drug BDM.13 14 In previous whole-heart experiments, mechanical unloading has been used to partition energy consumption (O2) into mechanical (cross-bridge cycling) and nonmechanical (basal metabolism plus E-C coupling) components (reviewed in Reference 18), ie, the mechanically unloaded O2 has been considered to represent nonmechanical O2. However, mechanical unloading cannot unequivocally distinguish mechanical from nonmechanical O2 because the mechanically unloaded LV undergoes shape changes and pressure fluctuations in the negative range with each contraction.15 Therefore, under mechanically unloaded conditions, an uncertain amount of energy continues to be used for cross-bridge cycling. We have demonstrated previously13 that such cross bridge-related O2 is a significant portion of mechanically unloaded O2. Furthermore, use of mechanical unloading to study energy partitioning during ejecting beats obviously alters the loading conditions of interest. To determine the energetic effects of ejection on partitioning of O2, it is mandatory to use a technique that does not require a change in contraction mode.
The BDM method was introduced by Alpert et al19 as a new approach to partitioning energy utilization in isolated rabbit heart muscle. The technique is based on the myofilament selective negative inotropic effect of low-concentration BDM (<6 mmol/L) in rabbit heart muscle.20 21 In isometrically contracting rabbit heart muscle at constant length, perfusion with up to 6 mmol/L BDM results in a linear decline in the relation between initial heat and TTI. Extrapolation of this relation to the initial heat-axis intercept (ie, zero TTI) provides an estimate of tension-independent heat or the energy used for E-C coupling. The method has been validated in isolated muscle by two independent tests.19 We subsequently adapted this technique to partition O2 in the red blood cell-perfused, isolated rabbit heart.13 14 Similar to the initial heat-TTI relation in isolated rabbit heart muscle, we reported a linear relation between O2 and FTI up to about 5 mmol/L perfusate (BDM) under ISOV contraction conditions with constant volume. By analogy, we consider extrapolation of the O2-FTI relation to the O2 axis to be an estimate of nonmechanical O2, which includes O2 for both E-C coupling and basal metabolism but not cross-bridge cycling. Several additional validations of this approach have been obtained in the isolated rabbit heart. The myofilament selectivity of low-concentration BDM was verified by the demonstration that <10 mmol/L BDM had no effect on the intracellular calcium transient measured by indo 1 fluorescence.13 Nonmechanical O2 estimated by BDM changed appropriately with the administration of isoproterenol, an agent known to increase energy costs for E-C coupling (M.W.W., MD, et al, unpublished data, 1995, and Reference 22). Finally, the BDM estimate of nonmechanical O2, corrected for basal metabolic O2 and normalized for unit mass, was found to be very similar to estimates of E-C coupling energy consumption in isolated muscle. In contrast, a similar correction of the mechanically unloaded O2 yielded values of E-C coupling energy consumption that were approximately twice those obtained in isolated heart muscle. Thus, the BDM estimate of nonmechanical O2 seems much more reasonable than the mechanically unloaded O2.
Experimental Protocol
LV pressure, LV volume, coronary blood flow, and arteriovenous O2 content difference were measured in seven hearts while contraction mode was varied randomly between ISOV and VR beginning at ES and EDV was kept constant. VR was performed at two rates, slow (9.0±1.7 mL/s) and rapid (50.0±8.3 mL/s). VR was begun at ES and continued until LV pressure was reduced to the control value of LVEDP. This resulted in an average EF of 50.2±7.6% and minimum LV pressure of 4.0±4.3 mm Hg during rapid VR and an average EF of 43.9±5.2% and minimum LV pressure of 6.4±4.2 mm Hg during slow VR. After each VR, the withdrawn volume was returned to the LV by no later than a time period equal to 90% of the cardiac cycle length. EDV was set at a value that resulted in an LVEDP of 8 to 12 mm Hg (mean, 9.3±5.5 mm Hg) during initial conditions. EDV was kept constant during VR and ISOV conditions. Mechanical and energetic variables were recorded during steady state, with a delay of at least 2 minutes between a change in contraction mode and data acquisition.
In eight additional hearts, BDM was infused at five or six incremental perfusate concentrations 
5 mmol/L with an infusion pump (Baxter Health Care) while contraction mode was varied randomly between ISOV and rapid VR at the same EDV. Measurements were made during each contraction mode under control conditions and then after stable conditions were present at each BDM.
In three hearts, basal metabolism was measured after arrest with an infusion of 20% KCl solution as previously described.14 Coronary blood flow and arteriovenous O2 difference were measured >15 minutes after arrest as the LV volume mode was varied between ISOV and rapid VR, analogous to the above contraction modes.
At the end of each protocol, the remnant great vessels and atria were trimmed, the ventricles were separated, and the RV free wall and LV (including septum) were weighed. LV and RV weights were 4.75±0.74 g and 1.46±0.39 g, respectively.
Data Analysis
Mechanical Measurements
LV pressure, LV volume, and arteriovenous O2 difference were recorded on a strip chart and collected with a computer (Gateway 2000) at 5-ms sampling intervals and stored for off-line analysis. Each contraction mode was ISOV until the time of maximum LV pressure, which was considered to be ES. ED was considered to be LV pressure when dP/dt was 10% of maximum positive dP/dt. Maximum dP/dt was calculated from the digitized LV pressure. sFTI was calculated as the integrated total wall force between ED and ES with use of an assumed spherical geometry for the LV.14 Each mechanical data point was determined as the average of complete cardiac cycles over 4 seconds. The LV volume was calculated as the sum of the intraventricular balloon fluid volume plus the small volume of the balloon walls and connector extending into the LV. V0, the mechanically unloaded LV volume at which maximum systolic pressure is zero, was determined during ISOV contractions in each heart. PVA, a measure of the total mechanical energy output of the LV,2 18 was calculated as the sum of two areas: a triangle bounded by the ES, ED, and V0 points and a smaller area that reflected the curvilinear ED pressure-volume relation. For the purposes of the present study, we assumed the ESPVR was linear15 23 and represented by the line that connected the peak pressure-volume coordinate and V0 for each contraction mode. Thus, the first area was calculated as (Pes-Ped)x(V-V0)/2, where Pes and Ped are ES and ED pressures and V is ventricular volume. The second area was approximated by Pedx(V-V0)/4.23 Emax, the slope of the ESPVR, was defined as LV developed pressure/V-V0.
O2 Measurement
Total O2 consumption per minute was calculated as the product of coronary flow (mL/min) and arteriovenous O2 content difference (vol %) and was normalized for heart rate to yield total O2 per beat (mL O2/beat). O2 of the mechanically unloaded RV was considered to be constant and was calculated as the product of mechanically unloaded total O2 multiplied by the ratio of RV weight to biventricular weight.12 23 LV O2 was calculated as total O2 minus RV O2 and was normalized for 100 g LV wet weight to give LV O2 consumption (mL O2·beat-1·100 g-1). Mechanical efficiency (%) was calculated as the ratio of PVA to total LV O2 after both parameters were converted to standard energy units (joules).24
Statistics
Data are reported as mean±SD. For mechanical and energetic parameters, we tested for differences between contraction modes using repeated measures ANOVA followed by Student-Newman-Keuls test if the F test was positive. With the BDM method, nonmechanical O2 is the O2 intercept of the O2-sFTI relation obtained under control conditions and as perfusate (BDM) is varied. In individual hearts, the O2 intercept was estimated by linear regression analysis of this relation during both ISOV and VR contraction modes. We have described previously14 the use of a repeated measures ANOVA with multiple linear regression and dummy variables to detect differences in the slope and O2 intercept of regression lines of pooled O2-sFTI data obtained with the BDM method. This approach provides a rigorous test of differences because it accounts for between-subject variability. With this method, all O2-sFTI data were pooled and fitted to the following regression model:
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O2 intercept of the pooled O2-FTI relation for control; Bc, the slope of the pooled O2-FTI relation for control; A
, the intercept difference between control and VR; and B, the slope difference between control and VR. For both ANOVA and the pooled regression model, we required a value of P<.05 to indicate a statistically significant difference. |
Results |
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Cardiac Mechanics
Representative recordings from a single heart under steady-state conditions during each contraction mode are shown in Fig 1A
. Compared with ISOV contraction, VR at ES is associated with a steady-state increase in peak LV pressure and dP/dt. The positive effect on contractility is greater during rapid VR, with a 15% increase in developed pressure in this example. Note that the EDV is constant between contraction modes. Slow tracings obtained as the contraction mode was switched from ISOV to rapid VR are shown in Fig 1B. The positive effect due to VR had a gradual onset after the abrupt change in contraction mode, with >50 beats required to reach a new steady state.
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Table 1 summarizes the group cardiac mechanical variables in seven hearts during control and VR contraction modes. The rate of rapid VR at ES was about five times greater on average than the rate of slow VR (26.8±5.1 versus 5.0±0.9 EDV/s). There was a trend toward a lower LVEDP during all VR that was not statistically significant. Compared with ISOV contractions, rapid VR was associated with a 15% increase in LV developed pressure (92±24 versus 106±28 mm Hg; P<.01), a 17% increase in maximum LV dP/dt (1223±401 versus 1435±505 mm Hg·s-1; P<.01), and a 13% increase in Emax (69±20 versus 79±23 mm Hg/mL; P<.01). Slow VR was associated with a smaller effect on contractile function, a 7% increase in LV developed pressure (P<.01), a 5% increase in Emax (P<.05), and a trend toward an increase in maximum dP/dt.
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Cardiac Energetics
Group effects on cardiac energetic parameters due to alteration in contraction mode are summarized in Table 2. VR was associated with a modest increase (<12%) in coronary blood flow at constant coronary perfusion pressure that was not significant at the P<.05 level. Consistent with the increase in LV pressure at the same EDV, PVA increased 6% with slow VR and 14% with rapid VR compared with ISOV contraction (1548±406 versus 1640±461 versus 1765±492 mm Hg·mL-1·beat-1·100 g-1, respectively; each P<.01). Despite the increase in PVA, O2 was unchanged with slow VR and decreased by 8% with rapid VR (0.0744±0.0195 versus 0.0683±0.0141 mL O2·beat-1·100 g-1; P<.05). Thus, rapid VR increased mechanical efficiency compared with ISOV contraction (14.0±2.5 versus 17.2±3.9%; P<.01).
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Nonmechanical O2
At constant EDV, there was a linear O2-sFTI relation during infusion of incremental BDM 5 mmol/L during both contraction modes. The linear regression of O2 on sFTI in individual hearts is summarized in Table 3. O2-sFTI relations were highly linear for all hearts during each contraction mode (ISOV r=.91±.05, rapid VR r=.94±.04), and as depicted in Fig 2, nonmechanical O2 determined from linear regression in individual hearts was uniformly larger for rapid VR than ISOV contractions. With use of the multiple linear regression model for pooled data, the O2 axis intercept of the O2-sFTI relation obtained with the BDM method was 0.0248±0.0021 mL O2·beat-1·100 g-1 for ISOV contraction and 0.0312±0.0022 mL O2·beat-1·100 g-1 for rapid VR contractions (P<.01), which amounted to an increase of 26% in nonmechanical O2. The slope of the O2-sFTI relation was significantly lower for the rapid VR contraction compared with ISOV contraction. The latter must be the case given the decrease in total O2 for VR contraction compared with ISOV contraction during control conditions and the above increase in O2 intercept of the rapid VR contractions. The fit of the group BDM data to the multiple linear regression model is summarized in Table 4. Finally, there was no significant difference in the O2 for basal metabolism between ISOV and VR contraction mode. O2 after KCl arrest was 0.616±0.130 mL O2·min-1·100 g-1 for ISOV LV volume and 0.614±0.122 mL O2·min-1·100 g-1 (P=.98) during rapid VR volume mode at the same EDV.
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Discussion |
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The present study characterizes the mechanoenergetic effects of ejection (VR) at ES in the isolated rabbit ventricle. There were three major findings. First, VR results in a gradually appearing increase in systolic force generation. This effect cannot be ascribed to length-dependent activation, and its magnitude is proportional to the rate of VR. Second, nonmechanical
O2 increases during VR; this effect supports a significant alteration in activation due to ejection. Finally, total LV O2 decreases during VR.
Mechanical Effects of Ejection
Several recent reports10 12 25 have characterized positive aspects of ejection on the ES pressure of the intact ventricle. Positive effects have been demonstrated over a range of afterload histories with a variable relationship to EF12 25 and a time course that ranges from the first beat10 25 to a gradual appearance10 after the onset of ejection. In general, however, previous studies have focused on a comparison of ISOV and "ejecting" beats at matched ESV. Accordingly, the positive effects observed have been ascribed to greater length-dependent activation of ejecting beats compared with ISOV beats at the same ESV. Shortening deactivation has been invoked as an opposing effect, proportional to EF. Thus, the ES pressure of ejecting beats has been considered to reflect a balance between these two opposing influences during ejection.4 25 In contrast, the current study describes a positive effect of ejection on LV pressure that is independent of length-dependent activation. Such shortening activation may represent a third effect that mediates cardiac contractility during ejection.
Two previous studies described analogous effects. Yasumura and coworkers1 performed rapid LV volume withdrawal at ES and found an average 10% steady-state increase in Emax for ESV withdrawal contractions compared with ISOV beats at the same EDV. We used an identical contraction mode with entirely analogous mechanical results. At an average VR rate (13.5 EDV/s) that was intermediate between our slow and rapid VR rates (5 and 27 EDV/s), Yasumura et al1 reported an intermediate increase in Emax. The time course of the positive effect they observed was not reported. If one assumes that it was gradual, the similarity of these results between species would support a fundamental interaction between VR during relaxation and ventricular contractile function. Sugiura and coworkers10 used a volume ramp to simulate ejection in isolated dog hearts. They reported a first-beat increase in LV pressure for ejecting contractions with matched ESV and small stroke volume and a gradually appearing further increase that was independent of stroke volume. They speculated that the latter, gradually appearing effect was due to a long-term increase in calcium availability, which in turn was related to a length-dependent alteration in calcium handling. Our results show a very similar gradual increase in LV pressure at the same EDV. Thus, the present study suggests that an alternate mechanism specifically related to ejection, rather than a change in EDV, accounts for a gradually appearing increase in LV pressure after the initiation of ejection. The time course of the VR effect and the associated increase in nonmechanical O2, discussed below, support an ejection-related increase in activation.
An alternative mechanism for the contractile effect of VR is an increase in myocardial turgor (Gregg effect) induced by VR at ES. However, this is unlikely for several reasons. The LV was vented such that VR rapidly eliminated afterload but did not create a suction force on the endocardium. Furthermore, VR did not decrease LV chamber compliance, because LVEDP tended to be lower with VR. Although coronary blood flow showed a trend toward a modest (<10%) increase during VR, we have reported previously23 that an adenosine-induced doubling of coronary blood flow in our isolated rabbit heart preparation is required to produce an LV pressure increase comparable to that associated with VR.
Increase in Nonmechanical O2 With Ejection
In the present study, nonmechanical O2 increased by 26% in association with rapid VR. Basal metabolism has been shown to be independent of contraction mode in isolated muscle.19 Our results in the arrested heart confirm that altered basal metabolism cannot explain the observed increase in nonmechanical O2. Accordingly, the increase in nonmechanical O2 can be attributed specifically to an increase in energy utilization associated with E-C coupling. Our results reflect an approximate 50% increase in O2 for activation on the basis of prior reports14 that E-C coupling accounts for roughly 50% of total nonmechanical O2 at moderate loading conditions. Because calcium reuptake by the SR calcium ATPase is the predominant energetic cost associated with activation, these results strongly support a link between muscle shortening and calcium cycled per beat. As discussed below, studies that used calcium transients5 6 suggested a similar link, but their methodology does not provide information regarding the total amount of calcium cycled.
As mentioned previously, one possible mechanism underlying an increase in calcium cycled per beat is that VR displaces calcium from troponin C, resulting in an increase in free calcium, and that the latter effect in some way has a positive influence on calcium reuptake and subsequent activation.4 There is evidence from cardiac muscle preparations to support such an interaction. Rapid shortening of isometrically contracting cardiac muscle, particularly during relaxation,6 results in a transient increase (spike) in the intracellular calcium signal.26 Myofilament calcium displacement from troponin C is thought to be the basis of the calcium signal increase.27 A mechanism to link this calcium displacement to a subsequent increase in contractile force has not been established, but we and others4 have speculated that the effect may be mediated by the SR. For example, a net shift in the time course of SR calcium uptake due to abrupt calcium displacement after ES could influence the kinetics of transfer of calcium from uptake to release pools. A recent study by DeTombe and Little4 supports this possibility; their study demonstrated altered inotropic effects of ejection in rat cardiac muscle strips when Sr2+, which is not handled by the SR, was substituted for calcium. Their results were consistent with a shortening-related myofilament displacement of calcium followed by sequestration by the SR. Alternatively, the source of the additional calcium cycled could be external and could be mediated via sarcolemmal calcium transport. For example, mechanosensitive sarcolemmal ion channels could be the source as a result of greater cyclic stretch and deformation that occurs with ejecting beats. Although these channels transport mainly monovalent cations, increased intracellular Na+ could in turn result in increased Ca2+ via the Na-Ca exchanger.28 29 Furthermore, a mechanosensitive sarcolemmal calcium channel has been reported.30
The precise alteration in calcium cycling that underlies these results is beyond the scope of the current study. However, the present results demonstrate a link between ventricular ejection, increased activation, and a subsequent increase in contractile performance. Although the contraction mode used was chosen to isolate and maximize such an interaction, it is possible that this effect participates in the regulation of cardiac contractility during normal ejection.
Influence of Ejection on Total O2
The final major finding of the present study is the decrease in total O2 associated with VR. An important aspect of this result is the dependence of O2 reduction on the rate of VR. We found no change in O2 during VR at an average of 5 EDV/s and an 8% decrease in O2 when the VR rate was 27 EDV/s. A similar result was reported by Yasumura et al1 in the dog ventricle. Thus, this effect on energy utilization appears to be dependent on the rate of VR and presumably muscle shortening. Previous work7 in cardiac muscle demonstrated a large decrease in O2, with quick releases (2 to 4 muscle lengths/s) after peak isometric tension, which supports the concept that during isometric contraction, maintenance of tension after ES is an energy-consuming process, ie, cross-bridge formation and associated splitting of ATP continue during relaxation. Our results are consistent with an analogous time course of cross-bridge energy utilization during ISOV contraction in the beating heart. In addition, our results support the occurrence of a considerable magnitude of cross-bridge-related energy utilization after ES in ISOV contractions. When the increase in nonmechanical O2 during rapid VR is combined with the decrease in total O2, mechanical O2 decreased by 25% compared with ISOV contraction. The magnitude of this energy saving is comparable to that observed during the aforementioned quick-release experiments. Although the precise mechanism responsible for the shortening-related decrease in total O2 remains unclear, our results once again clearly indicate that it is a decrease in the mechanical component that is responsible.
In summary, our results support a novel effect of a positive interaction between myofilament shortening and activator calcium cycling. The physiological significance of interactions between shortening and calcium cycling remains to be determined. Additional experiments that use more normal ejection will be required to accomplish this. Furthermore, elucidation of mechanisms that underlie shortening-calcium cycling interactions may provide insight into elements of the pathophysiology of heart failure. Recently,31 isolated cardiac muscle obtained from myopathic human hearts was shown to display abnormalities in the calcium transient that were modest during isometric contractions but increased markedly when the specimens were allowed to shorten. Other reports32 showed that the protein level of SR calcium ATPase, which would be involved in the reuptake of a shortening-related increase in calcium in our hypothesis, is selectively reduced in human dilated cardiomyopathy. Thus, the current results may be relevant to a mechanism that is specifically impaired in heart failure. Additionally, there are clinical situations in which myocardial load is altered acutely either at ES or during relaxation itself. Examples include acute mitral regurgitation and ventricular septal defect, in which shortening continues past the time of aortic valve closure with continuing ejection into the left atrium or the right ventricle. Similarly, acute aortic regurgitation increases load after ES. It is possible that an acute alteration in the "baseline" level of shortening activation is involved in the response to these pathological situations.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This study was supported by American Heart Association Grant-in-Aid No. 93006340 and NHLBI grant No. HL-51201. We thank Stephen P. Bell and Judit Fabian for excellent technical assistance and Dr Bryan K. Slinker for help in statistical analysis.
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Footnotes |
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Reprint requests to Matthew W. Watkins, MD, Cardiology Unit, Medical Center Hospital of Vermont-McClure 1, Burlington, VT 05401.
Received December 12, 1995; revision received February 6, 1996; accepted March 26, 1996.
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