Gap junction permeability is compromised in proliferating vascular smooth muscle cells

Gap junction-mediated communication is necessary for normal growth control. We previously reported that dye coupling between proliferating vs. growth arrested vascular smooth muscle cells (SMC) is compromised, connexin(Cx) 40 expression is increased and Cx43 expression unchanged. We hypothesized that compromised coupling reflected an increased selectivity of the junction resulting from the altered ratio of Cx40:Cx43 expression. To determine whether selectivity of the junction differed in proliferating vs growth arrested cells, we measured junctional conductance (g_j) with three patch solutions that differed in composition of the major current carrying ions: KCl, K-glutamate (K-glut) and TEA-Cl. Relative to the KCl solution, the conductivities of K-glut and TEA-Cl were 74% and 68%, respectively. If the junctions were freely permeable to glutamate and TEA, g_j measured with K-glut and TEA-Cl would be predicted to be 74% and 68% of g_j measured with KCl. In growth arrested cells, conductances measured with the three solutions were as predicted: KCl, 20±2nS; K-glut, 14±lnS (73% of KCl); TEA-Cl, 14±lnS (71% of KCl). However, in proliferating cells the measured conductances were higher than expected: KCl, 20±2nS; K-glut, 18±lnS (92% of KCl); TEA-Cl, 17±lnS (85% of KCl). Equivalent changes in g_j for both the anion (K-glut vs KCl) and cation (TEA-Cl vs KCl) substitution argue for an overall decrease in the large ion permeability in the proliferating state. To determine whether this change in selectivity might reflect the altered ratio of Cx40:Cx43, A7r5 cells were stably transfected with an antisense Cx43 construct. In one of the resultant cell lines, Cx43 expression was elevated such that Cx40:Cx43 ratio mimicked that occurring in growth arrested cells without withdrawal of serum. In these cells, the effect of ion substitution on g_j was predicted by the conductivity of the patch solutions (KCl, 27±3nS; K-glut, 21±lnS (78% of KCl); TEACl, 20±2nS (73% of KCl). These data suggest that the observed decrease in large ion permeability in proliferating SMCs is explained by altered connexin expression.