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

Abstract

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.

The dual effect of calcium on the action potential of the frog's heart

“…10-12 were 20-2 R. NIEDERGERKE AND R. K. ORKAND reduced to similar heights in all these cases, between 10 and 20 mV. This result is in good agreement with that of Ware (1961) who found only small variations of the overshoot, within the range of 10-20 mV, when examining the effects of calcium in continuously beating ventricle strips. Other findings.…”

“…To test the hypothesis that progressive accumulation of calcium ions inside the cells tends to reduce the overshoot, the effects of periods of repetitive stimulation were examined using fluids of varying calcium concentrations. A constant stimulus frequency of about 20 shocks/min as previously used by Ware (1961) was chosen for most of the present experiments.…”

“…Although the effects of calcium and sodium ions on the cardiac action potential have been the subject of previous investigations (Ware, 1961;Brady & Woodbury, 1960), it appeared worth while to extend the earlier results using a variety of different experimental conditions. Thus in the present work it was found that the rate at which the heart is stimulated has an important influence on the effects of calcium.…”

Staircase in frog ventricular muscle. Its dependence on membrane excitation and extracellular ionic composition.

“…9). Thus the observations presented here are also in accord with those of Ware,15 who reports that the plateau is lowered on raising Ringer's calcium concentration from 1 to 3 mM. Furthermore, a new steady state of the action potential configuration is attained many beats before that of contractile force (Hirsch and Kavaler, unpublished observations), as in the case of staircase.…”

“…This portion of the complex dependence of action potential configuration on extracellular calcium concentration was described by Ware. 15 Thus the action potential plateau is not raised (i.e., made more inside positive) during staircase but is in fact lowered. The correlation between calcium influx and inside positivity during the action potential, when extracellular calcium concentration is altered, also is not simply attributable to charge transfer, as the varying effects on plateau level in differing ranges of extracellular calcium concentration indicate.…”