Gap junctional conductance (gj) between cardiac ventricular myocyte pairs is rapidly, substantially, and reversibly reduced by sarcoplasmic acidification with CO2 when extracellular calcium activity is near physiological levels (1.0 mM CaCl2 added; 470 µM Ca++). Intracellular calcium concentration (Cai), measured by fura-2 fluorescence in cell suspensions, was 148 ±39 nM (±SEM, n = 6) and intracellular pH (pHi), measured with intracellular ion-selective microelectrodes, was 7.05 ± 0.02 (n = 5) in cell pair preparations bathed in medium equilibrated with air. Ca i increased to 515 ± 12 nM (n = 6) and pHi decreased to 5.9-6.0 in medium equilibrated with 100% COs2 In air-equilibrated low-calcium medium (no added CaCl2; 2-5 µM Ca++ Cai was 61 ± 9 nM (n = 13) at pHi 7.1. Cai increased to only 243 ± 42 nM (n = 9) at pHi 6.0 in CO2-equilibrated low-calcium medium. Junctional conductance, in most cell pairs, was not substantially reduced by acidification to pHi 5.9-6.0 in lowcalcium medium. Cell pairs could still be electrically uncoupled reversibly by the addition of 100 µM octanol, an agent which does not significantly affect Cai In low-calcium low-sodium medium (choline substitution for all but 13 mM sodium), acidification with CO2 increased Cai to 425 ± 35 nM (n = 11) at pHi 5.9-6.0 and gj was reduced to near zero. Junctional conductance could also be reduced to near zero at pHi 6.0 in low-calcium medium containing the calcium ionophore, A23187. The addition of the calcium ionophore did not uncouple cell pairs in the absence of acidification. In contrast, acidification did not substantially reduce gj when intracellular calcium was low. Increasing intracellular calcium did not appreciably reduce gj at pHi 7.0. These results suggest that, although other factors may play a role, H+ and Ca++ act synergistically to decrease gj.
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