Effects of intracellular pH on plateau formation following the action potential of squid giant axons.

When squid giant axons were intracellularly perfused for about 10 min with 100 mM K+ solutions containing Pronase or Protease Type VII at a concentration of 0.1-0.5 mg/ml, the action potential developed a long-lasting plateau. The plateau persisted after switching to an enzyme-free solution at standard pH (7.3 +/- 0.1) as the intracellular perfusate. The plateau was promptly and completely suppressed when the intracellular pH was lowered below 6.5. Upon increase of pH again to the standard level, the plateau promptly reappeared. With alkaline intracellular pH plateau formation was enhanced. Lowering the pH of the extracellular medium to 5 did not influence the plateau. The duration of the action potential under bi-ionic conditions (inside Cs+, outside Ca++) decreased with low intracellular pH and increased with high intracellular pH. The plateau duration of the action potential from axons intracellularly perfused with solutions containing 10 mM tetraethylammonium also decreased with low intracellular pH and increased with high intracellular pH. The underlying mechanisms for the pH effects remain to be explored.