[Developmental aspects of electrophysiology in cardiac muscle]

Electrical properties of the cardiac muscles drastically change with development. The changes in the current density of ionic currents of cardiomyocytes are inconsistent among species. In cultured embryonic chick ventricular myocytes, the developmental changes in the fast Na+ channel properties (3- to 17-day-old) are reviewed. The sensitivity to TTX, with a KD as high as 2 nM, remains unchanged. The limiting conductance (GNa) increased by 8-10-fold. The activation kinetics such as the steady-state activation (m infinity) and time constant of activation (tau m) remain unchanged. The voltage-dependence of inactivation kinetics such as the steady state inactivation (h infinity) and time constant (tau h) shift in the hyperpolarizing direction. The window conductance tends to be reduced. On the other hand, the L-type Ca2+ channel is important during the development of rat heart, and also the fe-type current (dihydropyridine-resistant) is important in the fetal stage. In chick embryo cardiomyocytes, the L-type channel exhibits long-lasting opening behavior. The behavior is gradually abolished during development, cAMP-dependent protein kinase enhances the Ca2+ channel current on and after the late fetal/embryonic stage. cGMP-dependent protein kinase markedly inhibits the Ca2+ channel current in the fetal/embryonic stage, compared with adult heart. These changes would play an important role for cardiac functions during development.