Cardiac Na currents and the inactivating, reopening, and waiting properties of single cardiac Na channels.

Abstract

Tetrodotoxin (TTX)-sensitive Na currents were examined in single dissociated ventricular myocytes from neonatal rats. Single channel and whole cell currents were measured using the patch-clamp method . The channel density was calculated as 2/jrn1, which agreed with our usual finding of four channels per membrane patch . At 20°C, the single channel conductance was 20 pS . The open time distributions were fit by a single-exponential function with a mean open time of -., 1 .0 ms at membrane potentials from -60 to -40 mV. Averaged single channel and whole cell currents were similar when scaled and showed both fast and slow rates of inactivation . The inactivation and activation gating shifted quickly to hyperpolarized potentials for channels in cell-attached as well as excised patches, whereas a much slower shift occurred in whole cells. Slowly inactivating currents were present in both whole cell and single channel current measurements at potentials as positive as -40 mV. In whole cell measurements, the potential range could be extended, and slow inactivation was present at potentials as positive as -10 mV. The curves relating steady state activation and inactivation to membrane potential had very little overlap, and slow inactivation occurred at potentials that were positive to the overlap. Slow inactivation is in this way distinguishable from the overlap or window current, and the slowly inactivating current may contribute to the plateau of the rat cardiac action potential. On rare occasions, a second set of Na channels having a smaller unit conductance and briefer duration was observed . However, a separate set of threshold channels, as described by Gilly and Armstrong (1984 . Nature [Loud.] . 309:448), was not found. For the commonly observed Na channels, the number of openings in some samples far exceeded the number of channels per patch and the latencies to first opening or waiting times were not sufficiently dispersed to account for the slowly inactivating currents ; the slow inactivation was produced by channel reopening. A general model was developed to predict the number of openings in each sample . Models in which the number of openings per sample was due to a dispersion of waiting times combined with a rapid transition from an open to

An investigation on the inactivation process of sodium currents in single frog ventricular cells.

“…The model we have selected is essentially the same as that of HORN and VANDENBERG (1984) as described below: The presence of a delay of onset of inactivation obliges us to propose several closed states before the open state and the inactivated state. Experimental evidence to support the notion of the presence of several closed states has been accumulated in Myxicola axon (GOLDMAN, 1976), squid axon (BEZANILLA and ARMSTRONG, 1977), rat pituitary gland (HORN and VANDENBERG,1984), neonatal rat cardiac cells (KUNZE et al, 1985), and canine cardiac Purkinje cells . Another experimental finding to limit the selection of the model for the Na+ channel is that the time constants for development of and recovery from inactivation at the same membrane potential are similar to each other.…”

“…39, No. 3, 1989 h~-curve in the hyperpolarizing direction of 25-50 mV within 15-80 min after the start of the experiment (CACHELIN et al, 1983;KUNZE et al, 1985;. In this experiment, special attention was paid to this curious phenomenon INa's are superimposed on the groups of peak inward currents is as follows: 1) in the case of CP = -20 mV, the steady state current of the conventional INa was set to be zero; 2) in CP= -35 mV, since h~-curve does not become zero (see Fig.…”

“…There have been a number of reports using the voltage clamp method which indicate that the general properties of the cardiac Na + channel are similar to those in nerve cells (BRowN et al, 1981a, b;BODEwEI et al, 1982;BUSTAMANTE and MCDONALD, 1983;CACHELIN et al, 1983;MATSUKI and HERMSMEYER, 1983;BENNDORF et al, 1985;KUNZE et al, 1985;PATLAK and ORTIz, 1985). However, observations by some investigators of subtle differences in the time course of 1.…”

Single ionic channels induced by palytoxin in guinea‐pig ventricular myocytes

“…Although palytoxin-induced single channel is highly selective for Na' in normal Tyrode solution, it is unlikely that the channel is induced by a modification of the voltage-dependence and ion selectivity of the existing Na channel. Unlike the Na channel (Kunze et al, 1985), the palytoxin-induced channel has the following features: (1) the palytoxin-induced channel is insensitive to tetrodotoxin; (2) the amplitude of single channel conductance differs from that 11 1-"'A. (1972); (IV) Hironaka & Narahashi (1977) and Hagiwara et al (1972); (V) Garber & Miller (1987).…”

Quantification of exponential Na+ current activation in N‐bromoacetamide‐treated cardiac myocytes of guinea‐pig.

“…y of 20 pS at 20°C (Kunze, Lacerda, Wilson & Brown, 1985) and an ENa of + 50 mV were used. NPo(t)/Cm is plotted against m(t) in Fig.…”