Effect of divalent cations on the potassium contracture of slow muscle fibers ofRana temporaria

Abstract

K contractures of single slow muscle fibers ofRana temporaria were measured isometrically in the presence of normal, reduced, and increased Ca2+ concentrations at 18 to 20°C. At normal Ca2+ concentration (1.8mm) contracture tension decreased from its peak value of 35.4±8.2 N/cm2 to 59.4±23.9% within one minute, and to 48.3±27% within two minutes (30 fibers). Peak tension was virtually unaffected by changes of the Ca2+ concentration, but maintenance of tension was impaired by low (0.2mm), and improved by high (10mm) Ca2+ concentrations. When Ca2+ was added during the K contracture, there was practically no restoration of lost tension. Effects similar to those of Ca2+ were observed upon addition of foreign divalent cations to the medium. Co2+, Ni2+, and Cd2+ were slightly more effective than Ca2+ and Mn2+; the smallest effects were obtained with Mg2+, Sr2+, and Ba2+. The effects of foreign divalent cations were independent of the presence of Ca2+. It is concluded that in slow fibers ofRana temporaria maintenance of contracture tension is not due to an influx of Ca2+ ions. Instead, binding of divalent cations to superficial sites seems to be essential.