Regional differences in the pacemaker activity of the heart

点击翻译按钮获取中文摘要

The SA node is an inhomogeneous structure in terms of structure, properties and function. We have investigated the possible regional deferences in the role of various membrane currents in the pacemaker activity in rabbit SA node preparations. Block of L type Ca^<2+> current (I_<Ca>) by 2muM nifedipine abolished spontaneous action potentials in the center, but not in the periphery, in the intact SA node and small ball-like specimens of the SA node. In contrast, block of Na^+ current (I_<Na>) by 20muM TTX had no effect on the electrical activity in small balls from the center, but it slowed the spontaneous acivity in balls from the periphery of the SA node. An application of 2 mM Cs^+,1muM UL-SF 49 or 3muM ZD7288, which has been reported to block hyperpolarization-activated current (I_f), decreased pacemaker slope and spontaneous rate of small balls of the SA node. The decrease was largest in balls from the periphery and least in balls from the center. These results suggest that I_<Ca> Plays an obligatory role in pacemaking in the center, whereas I_<Na> Plays a major role in pacemaking in the periphery, and that the contribution of I_f to spontaneous activity is greater in the periphery than in the center of the SA node. 4-aminopyridine (4-AP), which is known to block a population of K^+ current, caused a marked prolongation of action potential and a slowing of spontaneous rate in the periphery, but it accelerated spontaneous activity in the center of the SA node. This suggests that 4-AP-sensitive currents are also involved in the heterogeneity of the electrical activity within the SA node.