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

Abstract

SUMMARY1. Using ventricle strips of the frog's heart stimulated at the low rate of about 1 shock/min intracellular action potentials were recorded under conditions of varying calcium concentrations.2. Overshoots of action potentials were increased by about 18*3 mV as a result of a 10-fold increase, within the range of 0-1-5 mm, of the calcium concentration.3. A similar effect was obtained by strontium, but magnesium was ineffective.4. The increase of the overshoot by high calcium was associated with an increased rate of rise of the potential during the later part of its ascending phase. The initial fast upstroke remained unaltered.5. Another effect, a depression of the overshoot, developed during periods of repetitive stimulation, at the rate of 20/min, and this was followed by a gradual recovery during subsequent periods of rest.6. The depression of the overshoot increased with increasing calcium concentrations reaching values of over 40 mV.7. High concentrations of strontium and low concentrations of sodium also induced depression of the overshoot, but high magnesium was ineffective.8. A tentative hypothesis has been proposed attributing these two effects: (a) to an entry of calcium through the excitable membrane thus contributing to the ionic inward current, and (b) to a resulting accumulation of calcium in some cellular store.

Potassium efflux in heart muscle during activity: extracellular accumulation and its implications.

“…2A; see also Niedergerke & Orkand, 1966a). On cessation of stimulation, membrane potential slowly repolarized to its resting level ( Fig.…”

“…In fact, the delayed contractile response (t* -3*0 sec) to a step change in extracellular Ca concentration (Chapman & Niedergerke, 1970) corresponds well to the theoretical predictions of diffusion half times (2.3 sec) based on electron microscopic examination of the same tissue (Page & Niedergerke, 1972). In the past two decades, evidence has been accumulated which suggests that the small interfibrillar spaces underneath the sheath not only act as local ionic pools, but also their ionic content during activity may be sufficiently altered as to affect the state of membrane polarization and permeability (Weidmann, 1956;Carmeliet & Lacquet, 1958; Niedergerke & Orkand, 1966a;Maughan, 1973).…”

Effects of Osmolarity Change on the Excitation-Contraction Coupling of Bullfrog Ventricle

“…It is unlikely that changes in the membrane rectifier systems (instantaneous as well as delayed) are the main cause since we could not find any difference in the current-voltage relationship measured by the constant current method between the iso-and hypertonic conditions (unpublished observations). It is also unlikely that the entry of activator calcium, which is thought to directly contribute to the plateau phase (NIEDERGERKE and ORKAND, 1966;HAGIWARA and NAKAJIMA, 1966), was decreased by hypertonic solution because the rate of force development remained practically unchanged during the hypertonic perfusion (Figs. 1 A and 3b).…”

A down then up staircase in frog ventricle due to altered excitation—contraction coupling

“…This negative staircase is accompanied by a shortening of the duration of the cardiac action potential and a loss of its plateau; it appears therefore to result from a decreased excitatory event rather than from decreased excitation-contraction coupling (Niedergerke, 1956). The changes in the action potential probably result from an accumulation of calcium within the ventricular cells (Niedergerke & Orkand 1966).…”