Sarcolemmal distribution of I(Ca) and I(NCX) and Ca(2+) autoregulation in mouse ventricular myocytes

The balance of Ca(2+) influx and efflux regulates the Ca(2+) load of cardiac myocytes, a process known as autoregulation. Previous work has shown that Ca(2+) influx, via L-type Ca(2+) current (I(Ca)), and efflux, via the Na(+)/Ca(2+) exchanger (NCX), occur predominantly at t-tubules; however, the role of t-tubules in autoregulation is unknown. Therefore, we investigated the sarcolemmal distribution of I(Ca) and NCX current (I(NCX)), and autoregulation, in mouse ventricular myocytes using whole cell voltage-clamp and simultaneous Ca(2+) measurements in intact and detubulated (DT) cells. In contrast to the rat, I(NCX) was located predominantly at the surface membrane, and the hysteresis between I(NCX) and Ca(2+) observed in intact myocytes was preserved after detubulation. Immunostaining showed both NCX and ryanodine receptors (RyRs) at the t-tubules and surface membrane, consistent with colocalization of NCX and RyRs at both sites. Unlike I(NCX), I(Ca) was found predominantly in the t-tubules. Recovery of the Ca(2+) transient amplitude to steady state (autoregulation) after application of 200 µM or 10 mM caffeine was slower in DT cells than in intact cells. However, during application of 200 µM caffeine to increase sarcoplasmic reticulum (SR) Ca(2+) release, DT and intact cells recovered at the same rate. It appears likely that this asymmetric response to changes in SR Ca(2+) release is a consequence of the distribution of I(Ca), which is reduced in DT cells and is required to refill the SR after depletion, and NCX, which is little affected by detubulation, remaining available to remove Ca(2+) when SR Ca(2+) release is increased. NEW & NOTEWORTHY This study shows that in contrast to the rat, mouse ventricular Na(+)/Ca(2+) exchange current density is lower in the t-tubules than in the surface sarcolemma and Ca(2+) current is predominantly located in the t-tubules. As a consequence, the t-tubules play a role in recovery (autoregulation) from reduced, but not increased, sarcoplasmic reticulum Ca(2+) release.