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

1. The Na current threshold in sheep cardiac Purkinje fibres and in frog atrium is shifted in a positive direction by protons and Ca2+ ions. The titration curves for Purkinje fibres are consistent with a surface potential of -18 mV at pH 7.4 and 1.8 mM-Ca. 2. In Purkinje fibres, the pacemaker K current activation curve, s infinity, is shifted in a positive direction by Ca2+ ions. The results are consistent with a surface potential of -16 mV in normal physiological solutions. 3. The results on s inifinity during pH changes are unexpected. As also shown by Van Bogaert, Vereecke & Carmeliet (1975) the voltage shifts are usually in the opposite direction to that expected from titration of external surface negative charges. 4. Acid solutions reduce the magnitude of iK2 when fully activated. Alkalinity has little effect on iK2. 5. Acidification and alkalinization are both capable of arresting spontaneous activity in Purkinje fibres. The effects of acidity are usually irreversible. The effects of alkalinity are reversible.