The Effects of External Sodium and Potassium Concentration on the Membrane Potential of Atrioventricular Fibers of the Toad

Intracellular records were made from fibers in the A-V conducting system of isolated toad hearts. The A-V region was perfused with Ringer's solution of various K and Na concentrations. Resting potential in 2.8 mM [K](o) was about 60 mv. Over the range 0.28 to 28 mM, resting potential diminished with increasing [K](o). Spontaneous action potentials appeared when [K](o) was increased to 11.2 mM, and when resting potential had fallen to about 40 to 50 mv. Changes in [Na](o) over the range 22 to 110 mM had a little effect on resting potential, but there was a linear relation between the peak value of the action potential and log [Na](o) Wenckebach periodicity was observed when [Na](o) was lowered.