Intracellular pH and plateau duration of internally perfused squid giant axons

The effects of changing the intracellular pH on the action potential duration and other electrophysiological properties were studied in squid giant axons perfused intracellularly with TEA(tetraethylammonium)-containing solutions and under Ca-Na bi-ionic conditions. The duration of the action potential plateau, produced by TEA, was markedly decreased with acidic intracellular solutions and increased with alkaline intracellular solutions. The normalized duration, d, was calculated by dividing the plateau duration by that of the standard intracellular pH of 7.3, and plotted against the intracellular pH. The curve could be expressed by the formula log [d/(D-d)] = n(pH-pK') where D is the saturated value of d, n a constant and pK' the value of pH at which d becomes D/2. The values selected for D and n were 2.6 and 1.4, respectively. The pK' was found to be 7.5. Lowering of the extracellular pH to 6.2 only slightly changed the plateau duration. Voltage clamp analysis revealed that acidic intracellular solutions decreased the size of the inward current and the slope conductance which were measured at the late period of the depolarizing clamping pulse. Alkaline intracellular solutions increased the size of the inward current and the slope conductance measured at this late period. Intracellular perfusion with a low pH solution also shortened the duration of the Ca-Na bi-ionic action potential. It is argued that the remarkable generality of the pH effect on the plateau duration implies the existence of a common mechanism of formation of the plateau under widely differing experimental conditions.