Postnatal maturation of excitation-contraction coupling in rat ventricle in relation to the subcellular localization and surface density of 1,4-dihydropyridine and ryanodine receptors.

Abstract

To better understand excitation-contraction coupling in cardiac muscle, we investigated the main Ca2+ channels involved in that process in adult and neonatal rat ventricle. Voltage-dependent (L-type) Ca2+ channels and sarcoplasmic reticulum Ca2+ release channels were labeled by means of [3H] (+)-PN200-110 and [3H]ryanodine, respectively. The number of [3H]ryanodine binding sites (per gram tissue) increased more than that of [3H] (+)-PN200-110 binding sites over the postnatal period (2.1-fold versus 1.35-fold, respectively). After equilibration of microsomal fractions in density gradient, ryanodine receptors were characterized by a heavy distribution pattern that did not change appreciably between days 1 and 30 after birth. In neonatal tissue, 1,4-dihydropyridine receptors were found mainly in low-density subfractions, together with other sarcolemmal constituents, whereas in adult tissue, they were recovered predominantly in high-density subfractions, together with ryanodine receptors. Thus, after birth, and in parallel with the development of T tubules, there was a progressive concentration of L-type Ca2+ channels in junctional structures of high equilibrium density, where they were situated close to the Ca2+ release channels of the sarcoplasmic reticulum. In adult ventricle, L-type channels were, on an average, threefold more abundant in T tubules than in external sarcolemma. In parallel mechanical studies, we found that the inhibitory action of ryanodine on systolic contraction was much more pronounced in adult than in neonatal right ventricle, and that, conversely, neonatal tissue was more sensitive that adult tissue to inhibitors of L-type channels. We conclude that, in view of the presumed mechanism of Ca2+ release from the sarcoplasmic reticulum, that is, Ca(2+)-induced Ca2+ release, the predominant localization in adult rat ventricle of the major Ca2+ entry pathway in the vicinity of the Ca2+ release pathway is of great functional significance. Furthermore, owing to the relative stoichiometry of Ca2+ entry and Ca2+ release channels in junctional structures (about 1:9), a physical link between these channels is not likely to be involved in the modulation of Ca2+ release from the sarcoplasmic reticulum in cardiac muscle.

Developmental Changes in Excitation–Contraction Mechanisms of the Mouse Ventricular Myocardium as Revealed by Functional and Confocal Imaging Analyses

“…These results indicate that the functional role of Ca 2+ release from the SR increases during development. Similar developmental increases in inotropic sensitivity to ryanodine have been observed in the postnatal rat (14,15), rabbit (16), late fetal guinea pig (17), and middle age chick embryo (18), and also in the postnatal mouse (3). In the mouse ventricle, the present results revealed that, large changes in the inotropic responses to nifedipine and ryanodine occurs during the late fetal period and during the second postnatal week (Fig.…”

Intrasarcomere [Ca2+] gradients and their spatio‐temporal relation to Ca2+ sparks in rat cardiomyocytes

“…The rapid increase in Ca¥ concentration at the restricted space between the T_tubular and junctional SR membranes during the initial phase of excitation-contraction coupling may allow activation of various Ca¥ sensitive processes in that region when the Ca¥ concentration around the contractile proteins is still low. In the mammalian ventricular myocardium, the dihydropyridine binding sites (L-type Ca¥ channels; Wibo, Bravo & Godfraind, 1991), as well as the Na¤-Ca¥ exchanger (Frank, Mottino Reid Molday & Philipson, 1992;Chen, Mottino, Klitzner, Philipson & Frank, 1995), are reported to be localized along the T-tubules. Rapid inactivation of the trans-sarcolemmal Ca¥ influx through L_type Ca¥ channels and activation of inward current via the Na¤-Ca¥ exchange system could be explained by rapid increase in Ca¥ concentration in the restricted area (AdachiAkahane, Cleeman & Morad, 1996).…”

Developmental aspects of cardiac Ca2+signaling: interplay between RyR- and IP3R-gated Ca2+stores

“…Excitationcontraction coupling in neonatal cardiomyocytes appears to be regulated by Ca 2ϩ influx through the sarcolemma, with the L-type Ca 2ϩ channel being the main source for trans-sarcolemmal Ca 2ϩ flux (2,7,14,34,55). However, although RyRs are functional in neonatal cardiomyocytes, Ry only reduces Ca 2ϩ transients by ϳ15% in the day 1 neonatal rat heart versus 88% in juvenile cardiomyocytes, suggesting an increased reliance on RyR as a function of age (10,11).…”