THE EFFECTS OF SODIUM ION AND RATE OF STIMULATION ON THE REFRACTORY PERIOD OF ISOLATED RABBIT ATRIAL MUSCLE

¹ Department of Pharmacology, State University of New York, Upstate Medical Center, Syracuse, New York

Variations in sodium ion concentration and rate of stimulation influence the effective refractory period of rabbit atrial muscle. Thus sodium chloride concentrations higher than normal and an increase in rate of stimulation decreased while low NaCl concentrations and a decrease in rate of stimulation increased the effective refractory period of heart muscle. An increase in potassium ion concentration in the bathing solution increased the effective refractory period of rabbit atrial muscle and this increase could be reversed by addition of NaCl or by an increase in heart rate. Addition of sufficient amounts of quinidine or a decrease in the bath temperature from 37.5°C to about 25°C also increased the refractory period of atrial muscle. NaCl addition or an increase in heart rate could not reverse this increase in the effective refractory period. The results obtained have been discussed in the light of the ionic theory of impulse production.