Development of Excitation-Contraction Coupling in Cardiomyocytes

Abstract

The excitation-contraction (E-C) coupling system during the development of heart can be investigated because of marked progression in electrophysiology and microfluorescence studies. During the developmental period, Ca²⁺ influx mediating the E-C coupling is mainly through the L-type Ca²⁺ channels. In the fetal period, T-type Ca²⁺ channels and the reverse mode of the Na-Ca exchange system also contribute to Ca²⁺ influx. These contributions probably reduce gradually until adulthood. The contraction of fetal cardiomyocytes has been thought to depend mainly on the Ca²⁺ influx. However, recent studies reveal that immature sarcoplasmic reticulum (SR) already works in the early fetal period. Ca²⁺ spark, a local and unitary movement of Ca²⁺, can be observed in adult cardiomyocytes by the use of a confocal microscope. On the other hand, no Ca²⁺ spark is observed in fetal cardiomyocytes. The frequency of Ca²⁺-spark evocation increases during the postnatal period. Therefore a close distance between the L-type Ca²⁺ channel and the SR Ca²⁺-release channel is essential to the establishment of the Ca²⁺ spark.