Functional expression and inactivation of L-type Ca ²⁺ currents during murine heart development - Implications for cardiac Ca ²⁺ homeostasis

Background/Aims: The expression and regulation of Ca ²⁺ signaling in embryonic cardiomyocytes has been shown to be different from those in adult heart cells, particularly the L-type Ca ²⁺ channel current (I _CaL ) increases during development. However, little is known about the underlying reasons for this increase of I _CaL density and developmental changes in the process of I _CaL inactivation, a critical regulator of intracellular Ca ²⁺ homeostasis. In the present work, we therefore studied functional differences of I _CaL between embryonic and fetal cardiomyocytes and its interaction with intracellular Ca ²⁺ homeostasis and Ca ²⁺ -induced Ca ²⁺ release (CICR). Moreover, we examined the process of voltage- (VDI) and Ca ²⁺ - dependent inactivation (CDI) of I _CaL during murine embryonic heart development. Methods: The electrophysiological characteristics of I _CaL inactivation were analyzed in embryonic ventricular cardiomyocytes of early (E9.5-11.5) and late developmental, fetal (LDS, E16.5-18.5) stages and of adult mice using the whole-cell patch-clamp technique. Results: Fast, Ca ²⁺ -dependent inactivation kinetics (τ _f ) were significantly accelerated in LDS-derived cardiomyocytes (2.53±1.43 ms, n=9) as compared to EDS (5.09±2.19 ms, n=8, p>=0.009), whereas slow, voltage-dependent inactivation time constants (τ _s ) were unchanged. In cardiomyocytes derived from LDS we observed an increase in the maximal gating charge (Q _max ), suggesting an increase in the number of L-type Ca ²⁺ channels at the sarcolemma, whereas the channel open probability (P _o ) was unchanged. Accordingly a significantly higher I _CaL density was found in LDS (-14±2.26 pA/pF, n=14) versus> EDS-derived cardiomyocytes (-10.03±1.43 pA/pF, n=13, p>