Effects of calcium deficiency and excess on transmembrane potentials in frog heart

¹ Physiology-Biophysics Branch, School of Aerospace Medicine, Brooks Air Force Base, San Antonio, Texas

The effects of severe calcium deficiency and calcium excess on transmembrane potentials in isolated frog ventricular strips have been investigated. Resting potential rose about 5 mv above normal during perfusion with three times normal calcium and fell about 4 mv below normal during exposure to calcium-free Clark's solution. Mean overshoot rose about 3 mv during calcium lack, but was unaffected by excess calcium. Maximum depolarization rate increased about 20% during calcium deprivation and fell a similar amount during high calcium perfusion. However, the membrane potential at the moment of maximum depolarization rate was unchanged from normal by either experimental solution. High calcium augmented the "spike" and "plateau" during repolarization, while calcium deficiency abolished the spike, producing "hump-backed" action potentials with prolonged membrane reversal. These results are discussed, especially in relation to possible permeability changes during upstroke of the action potential.