The Influence of Ionic Strength on Potassium Contractures and Calcium Movements in Frog Muscle

Abstract

ABST R ACT Frog toe muscles were bathed in isotonic, sodium-free Tris chloride, methanesulfonate, or sulfate solutions containing sucrose or mannitol and varying in ionic strength from 0.006 to 0.291. By decreasing the ionic strength the curve relating the peak tension of the K contractures to the log [K] was reversibly shifted to lower [K]. Increasing the [Ca] from 1 to 4 mM almost abolished this effect. The resting uptake of 4Ca was increased more than two times by decreasing the ionic strength from 0.125 to 0.039. It was not increased significantly by raising [Ca] from 1 to 4 mM at low or normal ionic strength. The additional uptake of 4Ca during contractures provoked by 120 m K was not significantly different at the two levels of ionic strength. The rate of emergence of 45Ca from muscles loaded with 46Ca at reduced ionic strength, was decreased. The effects of low ionic strength are discussed in terms of changes in the potential difference across a membrane with fixed negative charges on the outer surface. The Influence of Ionic Strength on Potassium Contractures and Calcium Movements in Frog MuscleIn the course of studying the effect of changes in extracellular pH on the threshold concentration of potassium necessary to produce contractures in a frog toe muscle, the responses of the muscle were found to be dependent on the composition of the solutions used. When the threshold at pH 7 and 9 was measured with solutions consisting mainly of choline chloride and having the same ionic strength at both pH levels (about 0.125), the threshold was not decreased in the alkaline solution (Lorkovi, 1967). However, if Tris-methanesulfonate solutions were used, so that the ionic strength at pH 9 was about one-third that at pH 7, the threshold concentration of potassium was lower at pH 9. To examine this, the effect on the K contracture threshold of variations in ionic strength from 0.006 to 0.291, at constant pH, was investigated. The results showed that the threshold concentration of potassium is reduced 883

THE ANTAGONISM OF Ca2+ BY Na+ AND OTHER MONOVALENT IONS AT THE FROG NEUROMUSCULAR JUNCTION

“…It could then be argued that only in low Ca2+ is the ratio of Na+ to Ca2+ bound to the receptors sufficient to reveal the inhibitory action of Na+. Lorkovic [1967] has attributed the reduction in the K+ threshold for contracture of muscle. which occurs at low but not high Ca2+ concentrations when the extra-cellular ionic strength is reduced, to saturation of negatively charged groups in the membrane by relatively low concentrations of divalent ion.…”