Connexin43 gap junctions exhibit asymmetrical gating properties

A communication-deficient cell line (RIN cells, derived from a rat islet tumour), stably transfected with cDNA coding for rat connexin43 (Cx43), was chosen to further assess the mechanism of voltage gating of Cx43 gap junction channels. The experiments were carried out on preformed cell pairs using a dual whole-cell, voltage-clamp method. The junctional current, I-j, revealed a time- and voltage-dependent inactivation at transjunctional voltages V-j > +/- 40mV. When an asymmetrical pulse protocol was used (in cell 1 the holding potential was maintained, in cell 2 it was altered to establish a Variable V-j), the channels exhibited an asymmetrical gating behaviour: V-j,V-o = -73.7 mV and 65.1 mV for negative and positive V-j, respectively (V-j at which I-j is half-maximally inactivated); g(j) (min) = 0.34 and 0.29 (normalized minimal conductance); tau = 350 ms and 80 ms at V-j = 100 mV (time constant of I-j inactivation). Hence, these parameters were more sensitive to positive V-j values. When a symmetrical pulse protocol was used (the holding potentials in cell 1 and cell 2 were altered simultaneously in steps of equal amplitude but of opposite polarity), the V-j-dependent asymmetries were absent: I-j,I-0 = -60.5 and 59.5; g(j) (min) = 0.27 and 0.29; tau = 64 ms and 47 ms at 100 mV. Putative explanations for these observations are discussed. A possibility is that the number of channels alters with the polarity of V-j.