Modulations of changing the extracellular pH on the electrophysiological activity of isolated rabbit atrio-ventricular (AV) nodal cells were investigated using the two microelectrode voltage-clamp technique. Increasing pH from 7.4 to 9.5 enhanced spontaneous activity. The action potential amplitude and the maximum rate of depolarization were decreased. The action potential duration at 50% repolarization and the cycle length were shortened. The maximum diastolic potential was hyperpolarized. The pH elevation increased the maximum conductances for both the slow inward current and the delayed outward current systems. In contrast, a decline of pH from 7.4 to 5.5 inhibited the activity and the ionic currents. The effects on the action potential parameters were reversed. However, both acidification and alkalinization failed to affect the gating kinetics of the channels. These results suggest that H⁺ would modulate the electrical activity of the
AV nodal cells, due not to an alteration of the membrane surface charge, but to a direct
protonation of the ionic channels
