Displacement of activator thresholds in cardiac muscle by protons and calcium ions.

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Abstract
The Na current threshold in sheep cardiac Purkinje fibers and frog atrium is shifted in a positive direction by protons and Ca2+ ions. The titration curves for Purkinje fibers are consistent with a surface potential of -18 mV at pH 7.4 and 1.8 mM-Ca. In Purkinje fibers, the pacemaker K current activation curve, s.infin., is shifted in a positive direction by Ca2+ ions. The results are consistent with a surface potential of -16 mV in normal physiological solutions. The results on s.infin. during pH changes are unexpected. The voltage shifts are usually in the opposite direction to that expected from titration of external surface negative charges. Acid solutions reduce the magnitude of .hivin.iK2 [pace-maker K current] when fully activated. Alkalinity has little effect on .hivin.iK2. Acidification and alkalinization are both capable of arresting spontaneous activity in Purkinje fibers. Effects of acidity are usually irreversible. Effects of alkalinity are reversible.