Effects of intracellular acidification on membrane currents in ventricular cells of the guinea pig.

The membrane currents of single ventricular cells were measured under whole cell voltage clamp using a giga-sealed patch electrode, and the effects of intracellular acidification were examined by perfusing the electrode pipette with different pH solutions. The plateau of the action potential was shortened when the pH of the pipette solution was lowered from the control of 7.2 to 6, and finally to 5. The pH 6 pipette solution evoked a time-independent outward current at positive potentials and increased the slope conductance near the resting potential. These changes were suppressed by removal of both intra- and extracellular potassium ion, indicating that these currents were carried by potassium ions, but not by protons. Increasing the calcium concentration in the pipette from pCa 8 to pCa 6 induced a time-dependent outward current which had a reversal potential of about -13 mV. This result clearly differed from the changes induced by the acidic pipette solution, suggesting that the calcium-mediated conductance was not involved in the genesis of the acidic effects. The calcium current was not significantly affected by perfusion at pH 6, but was decreased by the more acidic (pH 5) solution. When the calcium current was recorded in sodium- and potassium-free external solution but with a cesium-rich internal solution, however, the calcium current was suppressed even with a weak acidic (pH 6.8) pipette solution. This effect was attributed not to an increased sensitivity of the calcium channel to protons, but to a more extensive intracellular acidification, which might have been caused by a depressed extrusion of proton via a sodium-hydrogen exchange mechanism on the surface membrane.