Steady-state and kinetic analyses of gap junctional conductance, gi, in salivary glands of Drosophila melanogaster third instar larvae reveal a strong and complex voltage dependence that can be elicited by two types of voltages. Voltages applied between the cells, i.e., transjunctional voltages, Vj, and those applied between the cytoplasm and the extracellular space, inside-outside voltages, Vi,o, markedly alter gj. Alteration of Vi-o while holding Vj = O,i.e., by equal displacement of the voltages in the cells, causes gj to increase to a maximum on hyperpolarization and to decrease to near zero on depolarization. These conductance changes associated with Vi-o are fit by a model in which there are two independent gates in series, one in each series, one in each membrane, where each gate is equally sensitive to Vi-o and exhibits first order kinetics. Vj's generated by applying voltage steps of either polarity to either cell, substantially reduce gj. These conductance changes exhibit complex kinetics that depend on Vi-o as well as Vj. At more positive Vi-o's, the changes in gj have two phases, an early phase consisting of of a decrease in gj for either polarity of Vj and a later phase consisting of an increase in gj on hyperpolarizing either cell and a decrease on depolarizing either cell. At negative Vi-o's in the plateau region of the gj-Vi-o relation, the later slow increase in gj is absent on hyperpolarizing either cell. Also, the early decrease in gj for either polarity of Vj is faster the more positive the Vi-o. The complex time course elicited by applying voltage steps to one cell can be explained as combined actions of Vi-o and Vj, with the early phase ascribable to Vj, but influenced by Vi-o, and the later phase to the changes in Vi-o associated with the generation of Vj. The substantially different kinetics and sensitivity of changes in gj by Vi-o and Vj suggests that the mechanisms of gating by these two voltages are different. Evidently, these gap-junction channels are capable of two distinct, but interactive forms of voltage dependence.
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