Inhibition of calmodulin expression prevents low-pH-induced gap junction uncoupling in Xenopus oocytes

The relationship among intracellular pH (pH(i)), -log(10) intracellular Ca2+ concentration (pCa(i)) and gap junctional conductance, the participation of Ca2+ stores, and the role of calmodulin in channel regulation have been studied in Xenopus oocytes, expressing the native connexin (Cx38), exposed to external solutions bubbled with 100% CO2. The time courses of pH(i) [measured with 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorscein (BCECF)], pCa(i) (measured with the membrane-associated fura-C-18) and junctional conductance (measured with a double voltage-clamp protocol) were compared. The data obtained confirm previous evidence for a closer relationship of junctional conductance with pCa(i) than with pH(i). Evidence for an inhibitory effect of intracellularly injected ruthenium red or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) on CO2-induced uncoupling, coupled to negative results with Ca2+-free external solutions, point to a low-pH(i)-induced Ca2+ release from internal stores, likely to be primarily mitochondria. The hypothesis proposing a participation of calmodulin in channel gating was tested by studying the effects of calmodulin expression inhibition by intracellular injection of oligonucleotides antisense to the two calmodulin mRNAs expressed in the oocytes. Calmodulin mRNA was permanently eliminated in 5 h. The oocytes injected with the antisense nucleotides progressively lost the capacity to uncouple with CO2 within 72 h. The effect of CO2 on junctional conductance was reduced by approximate to 60% in 24 h, by approximate to 76% in 48 h and by approximate to 93% in 72 h. Oocytes that had lost gating sensitivity to CO2 partially recovered gating competency following calmodulin injection. The data suggest that lowered pH; uncouples gap junctions by a Ca2+-cacalmodulin-mediated mechanism.