Role of the Steady-State Na+ Channel Current in Pacemaker Depolarizations in Young Embryonic Chick Ventricular Myocytes

Abstract

ABSTRACT-To assess the age-related changes in kinetic properties of the cardiac Na+ channel, whole-cell voltage-clamp (v-c) experiments were conducted using 3-, 10-and 17-day-old embryonic chick ventricular heart cells. In line with the first-order kinetic model, kinetic parameters for the activation and inactivation of the channel were determined from the v-c results. Simulation studies using kinetic parameters so determined have reproduced the current-voltage relations and the steady-state inactivation characteristics observed in cells in the three age groups. The rate of depolarization of the simulated action potentials was also comparable to that experimentally recorded. In conclusion, the steady-state Na+ conductance can play a significant role in the automatic depolarizations observed in young embryonic ventricular cells.Keywords: Heart, Voltage-clamp, Development, Automaticity, Simulation Depression of cardiac pacemaker activity is a common property shared among all class-1 antiarrhythmic agents (1), but it has been questioned if the major action of these drugs, their Na+ channel blocking action, is responsible for the depression, since the Na+ channels contribute little to the pacemaker depolarizations due to the low channel density as well as their inactivation in the range of pacemaker potentials.The fast Na+ channel properties change during development. Above all, the presence of the automatic activities and slow rate of rise of action potential (AP) in ventricular cells is characteristic only in an early stage of development (see ref. 2 for a review). In the previous voltageclamp (v-c) study using cultured heart cells from 3-, 10-and 17-day-old embyronic chick ventricles (3), we quantified the developmental changes in the activation and inactivation kinetics parameters. In the study, we found that the steady-state (window) conductance of the Na+ channel at ca. -40 mV in the young (3-day) heart was 0.6010 of the maximum conductance (gNa), and suggested that the current in this magnitude was large enough to induce the pacemaker depolarization. To provide evidence for the involvement of the window conductance in automatic depolarizations in this tissue, further v-c experiments were carried out for the three age groups, with special reference to the inactivation kinetics over a whole potential range, to enable the simulation study. We report here that the Na+ window current plays an important role in the slow depolarization during the diastole in ventricular cells of young embryos. MATERIALS AND METHODS Experimental proceduresThe hearts from 3-, 10-and 17-day-old chick embryos were dissected under sterile conditions. The procedures for cell separation and cultivation were similar to the previously reported ones (3 -5). The use of fertilized eggs for the v-c experiments was approved and authorized by the Animal Experiment Committee, Yamaguchi University, School of Medicine. Glass pipette electrodes were fabricated by a two-step pulling of Pyrex glass capillary tubes (o.d. =1.4 mm), and each electrode was ...

An ionic model for rhythmic activity in small clusters of embryonic chick ventricular cells

“…There is voltage-clamp evidence for the existence of INa in reaggregates of 7-to 11-day ventricular cells (22,72), in single 2-to 18-day ventricular cells (29,40,82,83,112), and at the single-channel level in 7-day ventricular cells (64,112). Voltage-clamp studies of 7-day ventricular clusters in our laboratory show a fast inward current upon a depolarizing clamp step from potentials more hyperpolarized than about Ϫ60 mV.…”

“…Very early during development (3 days), the steady-state inactivation curve of I Na is shifted in the depolarizing direction (82); simulations suggest that the window component of I Na might then contribute to diastolic depolarization (83). I st has been reported during diastolic depolarization in spontaneously active single SA node cells (71, but see Ref.…”