Effects of Added or Substituted Potassium Ion on 45 Ca Movements in Rabbit Aortic Smooth Muscle

Abstract

The effects of elevated potassium ion (K⁺) concentration on radioactive calcium (⁴⁵Ca) movements and distribution were investigated in media-intimal strips prepared from rabbit aortic smooth muscle. ⁴⁵Ca uptake was measured in either normal (1.5 mM Ca²⁺) or low-Ca (0.1 mM Ca²⁺) solutions. In low-Ca solutions, elevating the K⁺ concentration either by substituting equimolar (165.4 mM) K⁺ for sodium (Na⁺) (substituted-K solution) or by increasing K⁺ concentration to 80 mM (added-K solution) increased the ⁴⁵Ca uptake. Exposure of the muscle strips to high K⁺ concentrations during washout of ⁴⁵Ca decreased the ⁴⁵Ca efflux into either zero-Ca or low-Ca bathing solutions. Moreover, increased K⁺ concentration elicited its effects in the presence or the absence of Na⁺. The decrease in ⁴⁵Ca efflux induced by substituted-K solution was also obtained in the presence of ethylenediaminetetraacetic acid (EDTA), but prior incubation of muscles with 1.5 mM strontium (Sr²⁺) plus ⁴⁵Ca prevented this decrease in ⁴⁵Ca efflux. It would appear that the ⁴⁵Ca affected by high K⁺ concentrations is at Sr-sensitive sites which are inaccessible to EDTA. If ⁴⁵Ca at these sites contributes directly to ⁴⁵Ca efflux, a shift of some of this ⁴⁵Ca to less superficial cellular sites might be responsible for the decreased efflux of ⁴⁵Ca observed in the presence of increased K⁺ concentration.