Abstract 3046: Critical Role of Endocardial Endothelium in Regulation of Force-Frequency Relationship in Rat Ventricular Myocardium
Force-frequency relationship (FFR) is an important property of cardiac muscle. A normal (positive) FFR has been thought to involve intrinsic mechanisms including sarcolemmal L-type channels and Na+/Ca2+ exchangers, Ca2+ release from the sarcoplasmic reticulum (SR), and regulation of SR Ca2+ release (i.e. phospholamban and calmodulin). Here, we report that endocardial endothelium (EE) also exerts significant impact on FFR. Rat trabeculae from the right ventricle were mounted between a force transducer and a motor arm, superfused with K-H solution (pH 7.4) at room temperature, and loaded with fura-2 salt via ionotophoresis. EE was damaged by brief (<1 sec) exposure to 1% Triton. Damage of EE resulted in ~15% decrease in force at baseline (i.e. [Ca2+]o=0.5 mmol/L, stimulation rate=0.5 Hz, and at 22 °C) without affecting [Ca2+]i transient (iCa2+). Before EE damage (control), both force and iCa2+ increased as the stimulation frequency increased from 0.5 Hz up to 3.0 Hz (force increased by: 243±35%; iCa2+ by: 165±25%). After EE damage, the increases in both force and iCa2+ were significantly blunted as stimulation frequency increased (force: 110±28%, p>0.05; iCa2+: 47±10%, p>0.05 vs. control, n=9). High dose of isopreterenol (ISO, 5 nmol/L) increased force and iCa2+ and low dose of ISO (0.1 nmol/L) had no effect at baseline, but neither of the doses improved the positivity of FFR. Endothelin-1 (ET-1, 10 –100 nmol/L) also increased baseline force but did not affect FFR. However, FFR was significantly improved in the presence of low dose of ISO (0.1 nmol/L) plus ET-1(20 nmol/L) (force: 203±63%; iCa2+: 94±15%, p<0.05 vs. after EE damage, n=5). These results indicate that damage of EE leads to significantly blunted FFR. Furthermore, the reversal of FFR towards normal (positive) by low dose ISO + ET-1 suggests cross-talks between the signaling pathways involving β- and ET-1 receptors, and that a balance must be maintained between the two pathways for a normal (positive) FFR. The results also implicate that dysfunction of EE may have contributed to the flat (or even negative) FFR in heart failure. Future studies are focused on the molecular balance between the two pathways in regulating FFR in cardiac muscle.