EFFECT OF EXTERNAL SODIUM SUBSTITUTION ON POTASSIUM CONTRACTURES OF MAMMALIAN MUSCLES: POSSIBLE INVOLVEMENT OF SARCOLEMMA‐BOUND CALCIUM AND Na+‐Ca2+ EXCHANGE

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Abstract
Isometric tension of K+ contractures, membrane potential, internal Na+ activity (.alpha.Nai) and intracellular pH (pHi) have been measured in vitro under conditions which modify sarcolemma-bound calcium and the activity of the Na+-Ca2+ exchange of normal mammalian soleus muscle. In the absence of external Na+ and for a given high external K+, the maximum amplitude of the contracture was increased (Na+ replaced by TEA+) or decreased (Na+ replaced by Li+ or Cs+) compared with that obtained in the presence of Na+. Replacement of external Na+ by another monovalent cation (TEA+) did not induce any change of pHi. However the concomitant decrease of .alpha.Naa was related to a Na+-Ca2+ exchange across the sarcolemmal membrane. The data suggest that in soleus muscle, a sarcolemmal calcium pool is involved during the development of K+ contractures.

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