Abstract
1. 1. Transmembrane potentials in single cells of the moth myocardium were measured during exposure to physiological bathing solutions in which the chloride content was completely replaced with a series of anions (acetate, nitrate, bromide and sulfate) of varying molecular size known to exhibit varying degrees of penetrability in vertebrate muscle fibers. 2. 2. The degree of penetrability of these anions was assessed according to the transient and long-term effects produced on spontaneous bioelectric activity. 3. 3. Both acetate and sulfate solutions produced cardiac standstill, acetate by hyperpolarization of the membrane and sulfate by depolarization of the membrane. A transient slowing followed by an increase in spontaneous heartbeat frequency resulted from nitrate solution. Bromide solution had little if any effect. 4. 4. This evidence suggests that the order of penetrability of these anions is acetate > nitrate > bromide > chloride. The membrane is impermeable to sulfate.
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