Gap junctional conductance between pairs of ventricular myocytes is modulated synergistically by H₊ and Ca⁺⁺

Gap junctional conductance (g_j) 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 (Ca_i), measured by fura-2 fluorescence in cell suspensions, was 148 ± 39 nM (± SEM, n = 6) and intracellular pH (pH_i), 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 pH_i decreased to 5.9-6.0 in medium equilibrated with 100% CO₂. In air-equilibrated low-calcium medium (no added CaCl₂; 2-5 μM Ca⁺⁺), Ca_i was 61 ± 9 nM (n = 13) at pH_i 7.1. Ca_i increased to only 243 ± 42 nM (n = 9) at pH_i 6.0 in CO₂-equilibrated low-calcium medium. Junctional conductance, in most cell pairs, was not substantially reduced by acidification to pH_i 5.9-6.0 in low-calcium medium. Cell pairs could still be electrically uncoupled reversibly by the addition of 100 μM octanol, an agent which does not significantly affect Ca_i. In low-calcium low-sodium medium (choline substitution for all but 13 mM sodium), acidification with CO₂ increased Ca_i to 425 ± 35 nM (n = 11) at pH_i 5.9-6.0 and g_j was reduced to near zero. Junctional conductance could also be reduced to near zero at pH_i 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 g_j when intracellular calcium was low. Increasing intracellular calcium did not appreciably reduce g_j at pH_j 7.0. These results suggest that, although other factors may play a role, H⁺ and Ca⁺⁺ act synergistically to decrease g_j.