Abstract 5308: T and L-type Ca2+ Currents Differentially Regulate Myocardial Ischemia-Reperfusion Tolerance
Ca2+ plays a central role in excitation-contraction coupling (ECC) and stress related signaling in cardiomyocytes. Dysregulation of myocyte Ca2+ homeostasis is a central factor mediating ischemia reperfusion (IR) tolerance, myocardial hypertrophy, apoptosis, and heart failure progression. Until recently, the global Ca2+ transient was thought to be responsible for both ECC and stress signaling. This study explored the hypothesis that Ca2+ influx through T (TTCC) and L (LTCC) type Ca2+ channels differentially regulate the global Ca2+ transient and myocyte stress responses.
Methods: Mice with cardiac specific expression of the β2a subunit of the LTCC (β2a) or the α1G isoform of the TTCC (α1G), and wild type (WT) controls were studied. T and L type Ca2+ currents were measured in isolated myocytes. Developed LV pressure (DP) and IR tolerance was measured in Langendorff perfused isovolumic hearts (6Hz, 2mM Ca2+, EDP=10mmHg) during a 15 minute period of global ischemia followed by 30 minutes of reperfusion.
Results: TTCC current was present only in α1G myocytes (32±3 pA/pF, n=15). LTCC current was increased (32±1.6 pA/pF, n=10) in β2a mice versus WT (12±0.4 pA/pF, n=11). Total Ca2+ influx, measured using action potential voltage clamp, was equivalent in α1G (150±23 fC/pF, n=6) and β2a (129±12 fC/pF, n=9) but both were significantly (3 fold) greater than in WT (46±5 fC/pF n=6; p<0.01). Baseline LV developed (DP) pressure (WT: 105±11, α1G:130±9, β2a: 141±3 mmHg, P<0.05) was increased above WT in α1G but was significantly greater in β2a versus both WT and α1G. Only β2a mice had hypertrophy (HW/BW ratio; WT: 6.3±0.2, α1G: 5.9±0.2, β2a: 6.9±0.5 mg/g, P<0.05). Ischemia caused a significant EDP increase in β2a but not in either WT or α1G (WT: 8.6±2.6, α1G: 6.9±1.3, β2a: 30.1±2.2 mmHg, P<0.01). These differences were sustained through reperfusion. At 30 minutes reperfusion, both WT and α1G had significantly more functional recovery of DP vs. β2a (WT: 84±5, α1G: 79±0.9, β2a: 56±2 % baseline, P<0.05).
Conclusions: Equivalent quantities of Ca2+ influx through T- and LTCC have different effects on contractility, hypertrophy and IR responses, suggesting that Ca2+ influx through T- and LTCC enter different cellular microdomains to elicit unique functional and stress responses.
This research has received full or partial funding support from the American Heart Association, AHA Great Rivers Affiliate (Delaware, Kentucky, Ohio, Pennsylvania & West Virginia).