Mechanisms underlying variations in excitation–contraction coupling across the mouse left ventricular free wall

Ca²⁺ release during excitation–contraction (EC) coupling varies across the left ventricular free wall. Here, we investigated the mechanisms underlying EC coupling differences between mouse left ventricular epicardial (Epi) and endocardial (Endo) myocytes. We found that diastolic and systolic Ca²⁺]_i was higher in paced Endo than in Epi myocytes. Our data indicated that differences in action potential (AP) waveform between Epi and Endo cells only partially accounted for differences in Ca²⁺]_i. Rather, we found that the amplitude of the Ca²⁺]_i transient, but not its trigger – the Ca²⁺ current – was larger in Endo than in Epi cells. We also found that spontaneous Ca²⁺ spark activity was about 2.8-fold higher in Endo than in Epi cells. Interestingly, ryanodine receptor type 2 (RyR2) protein expression was nearly 2-fold higher in Endo than in Epi myocytes. Finally, we observed less Na⁺–Ca²⁺ exchanger function in Endo than in Epi cells, which was associated with decreased Ca²⁺ efflux during the AP; this contributed to higher diastolic Ca²⁺]_i and SR Ca²⁺ in Endo than in Epi cells during pacing. We propose that transmural differences in AP waveform, SR Ca²⁺ release, and Na⁺–Ca²⁺ exchanger function underlie differences in Ca²⁺]_i and EC coupling across the left ventricular free wall.