Multiple-channel conductance states and voltage regulation of embryonic chick cardiac gap junctions

We used the double whole-cell voltage-clamp technique on ventricle cell pairs isolated from 7-day chick heart to measure the conductance of their gap junctions (Gj) and junctional channels (gamma j) with a steady-state voltage difference (Vj) applied across the junction. Currents were recorded from single gap junction channels (ij) as symmetrical rectangular signals of equal size and opposite sign in the two cells, and gamma j was measured from ij/Vj. We observed channel openings at six reproducible conductance levels with means of 42.6, 80.7, 119.6, 157.7, 200.4 and 240.3 pS. More than half of all openings were to the 80- and 160-pS conductance levels. The probability that a high conductance event (e.g., 160 or 240 pS) results from the random simultaneous opening of several 40-pS channels is small, based on their frequency of occurrence and on the prevalence of shifts between small and large conductance states with no intervening 40-pS steps. Our results are consistent with three models of embryonic cardiac gap junction channel configuration: a homogeneous population of 40-pS channels that can open cooperatively in groups of up to six; a single population of large channels with a maximal conductance near 240 pS and five smaller substates; or several different channel types, each with its own conductance. Gj was determined from the junctional current (Ij) elicited by rectangular pulses of applied transjunctional voltage as Ij/Vj. It was highest near 0 Vj and was progressively reduced by application of Vj between 20 and 80 mV or -20 and -80 mV. In response to increases in Vj, Gj decayed in a voltage- and time-dependent fashion. After a 6-sec holding period at 0 Vj, the initial conductance (G(init) measured immediately after the onset of an 80-mV step in Vj was nearly the same as that measured by a 10-mV prepulse. However, during 6-sec pulses of Vj greater than +/- 20 mV, Gj declined over several seconds from G(init)to a steady-state value (Gss). At potentials greater than +/- 20 mV the current decay could be fit with biexponential curves with the slow decay time constant (tau 2) 5-20 times longer than tau 1. For the response to a step to 80 mV Vj, for example, tau 1 = 127 msec and tau 2 = 2.6 sec. The rate of current decay in response to smaller positive or negative steps in Vj was slower, the magnitude of the decline was smaller, and the ratio tau 2/tau 1 decreased.(ABSTRACT TRUNCATED AT 400 WORDS)