L-type Ca²⁺channel function and expression in neonatal rabbit ventricular myocytes

L-type Ca(2+) channel-mediated, Ca(2+)-induced Ca(2+) release (CICR) is the dominant mode of excitation-contraction (E-C) coupling in the mature mammalian myocardium but is thought to be absent in the fetal and newborn mammalian myocardium. Furthermore, the characteristics and contributors of E-C coupling at the earliest developmental stages are poorly understood. In this study, we measured [(3)H](+)PN200-110 dihydropyridine binding capacity, functionality and expression of the L-type Ca(2+) channel, and cytosolic [Ca(2+)] ([Ca(2+)](i)) at various developmental stages (3, 6, 10, 20, and 56 days old) to characterize ontogenetic changes in E-C coupling. We found that 1) the whole cell L-type Ca(2+) channel peak current (I(Ca)) density increased slightly in parallel with cell growth, but the current-voltage relationship, the steady-state activation, and the maximum DHP binding and binding affinity did not exhibit significant developmental changes; 2) sarcoplasmic reticulum Ca(2+) dependence of inactivation rates of L-type Ca(2+) channel and peak of I(Ca) density were only observed after 10 days of age, which temporally coincides with transverse (T)-tubule formation; 3) the relationship between [Ca(2+)](i) and voltage changed from a linear relationship at the earliest developmental stages to a "bell-shaped" relationship at the later developmental stages, presumably corresponding to a switch from reverse-mode Na/Ca exchange-dependent to I(Ca)-dependent E-C coupling; and 4) the expression of two different splice variants of Ca(V)1.2, IVS3A and IVS3B, switched from predominantly IVS3A at the earliest stages to IVS3B at the later developmental stages. Our data suggest that whereas the density of functional dihydropyridine receptors (DHPRs) increases only slightly during ontogeny, the enhancement of functional coupling between DHPR and ryanodine receptor is dramatic between the second and third weeks after birth. Furthermore, we found that the differential expression of splice variants during development temporally correlated with the appearance of I(Ca)-dependent E-C coupling and T-tubule formation.