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The sensitivity of Cx45 channels to CO2, transjunctional voltage ( V(j)) and inhibition of calmodulin (CaM) expression was tested in oocytes by dual voltage clamp. Cx45 channels are very sensitive to V(j) and close with V(j) preferentially by the slow gate, likely to be the same as the chemical gate. With a CO2-induced drop in junctional conductance ( G(j)), both the speed of V(j)-dependent inactivation of junctional current ( I(j)) and V(j) sensitivity increased. With 40-mV V(j)-pulses, the tau of single exponential I(j) decay reversibly decreased by;40% during CO2 application, and G(j steady state)/G(j peak) decreased multiphasically, indicating that both kinetics and V(j) sensitivity of chemical/slow V(j) gating are altered by changes in [H(+)](i) and/or [Ca(2+)](i). CaM expression was inhibited with oligonucleotides antisense to CaM mRNA. With 15 min CO2, relative junctional conductance ( G(jt)/ G(jt0)) dropped to 0% in controls, but only by;17% in CaM-antisense oocytes. Similarly, V(j) sensitivity was significantly lessened in CaM-antisense oocytes. The data indicate that both the speed and sensitivity of V(j)-dependent inactivation of the junctional current of Cx45 channels are affected by CO2 application, and that CaM plays a key role in channel gating.
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