Properties of gap junction channels formed of connexin 45 endogenously expressed in human hepatoma (SKHep1) cells

We have evaluated the voltage dependence and unitary conductance of gap junctional channels that were recorded in a clone isolated from the hepatoma cell line SKHep1. In this clonal population (designated SKHep1A), Northern blots, immunoprecipitation, and immunohistochemical staining demonstrated the expression of connexin (Cx) 45; no other gap junction protein was identified by these techniques, although weak hybridization with Cx40 was detected. Macroscopic junctional conductance (g(j)) in these cells was low, averaging 1.3 nS, and was steeply voltage dependent. Parameters of voltage sensitivity were as follows: voltage at which voltage-sensitive conductance is reduced by 50%, 13.4 mV; steepness of relation, 0.115 (corresponding to 2.7 gating charges), and voltage-insensitive fraction of residual to total conductance ≃ 0.06. Unitary conductance (γ(j)) of these junctional channels averaged 32 ± 8 pS; although γ(j) was independent of transjunctional voltage (V(j)), at high V(j) values (>50 mV), smaller conductance values were also detected. Open probabilities of the 30-pS channels at various V(j) values closely matched the predicted voltage-dependent component of macroscopic g(j); the residual conductance at high V(j) might be attributable to the smaller conductance events. The voltage dependence of human Cx45 gap junction channels is as steep as that seen for channels formed by Xenopus Cx38 and is much steeper than that previously reported for channels formed of the highly homologous chick Cx45 and for other mammalian connexins expressed either endogenously or exogenously.