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

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Abstract
The effects of elevated potassium ion (K + ) concentration on radioactive calcium ( 45 Ca) movements and distribution were investigated in media-intimal strips prepared from rabbit aortic smooth muscle. 45 Ca uptake was measured in either normal (1.5 mM Ca 2+ ) or low-Ca (0.1 mM Ca 2+ ) 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 45 Ca uptake. Exposure of the muscle strips to high K + concentrations during washout of 45 Ca decreased the 45 Ca 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 45 Ca 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 (Sr 2+ ) plus 45 Ca prevented this decrease in 45 Ca efflux. It would appear that the 45 Ca affected by high K + concentrations is at Sr-sensitive sites which are inaccessible to EDTA. If 45 Ca at these sites contributes directly to 45 Ca efflux, a shift of some of this 45 Ca to less superficial cellular sites might be responsible for the decreased efflux of 45 Ca observed in the presence of increased K + concentration.

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