Effects of low-chloride solutions on action potentials of sheep cardiac Purkinje fibers.

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Abstract
The rapid repolarization during phase 1 of the action potential of sheep cardiac Purkinje fibers was attributed to a time- and voltage-dependent chloride current. This conclusion was based on experiments that showed a substantial slowing of phase 1 when larger, presumably impermeant, anions were substituted for chloride in Tyrode''s solution. The electrical effects of low-chloride solution were re-examined. Action potentials of sheep cardiac Purkinje fibers in normal Tyrode''s solution and in low-chloride solutions made by substituting sodium propionate, acetylglycinate, methylsulfate, or methanesulfonate for the NaCl of Tyrode''s solution were recorded. Total Ca was adjusted to keep Ca ion activity of test solutions equal to that of control solutions. Propionate gave qualitatively variable results in preliminary experiments; it was not tested further. Low-chloride solutions made with the other anions gave much more consistent results: phase 1 and the notch that often occurs between phases 1 and 2 were usually unaffected, and the action potential duration usually increased. The only apparent change in the resting potential was a transient 3-6 mV depolarization when low-chloride solution was first admitted to the chamber, and a symmetrical transient hyperpolarization when chloride was returned to normal. If a time- and voltage-dependent chloride current exists in sheep cardiac Purkinje fibers, it may play little role in generating phase 1 of the action potential.