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

Isometric tension of K + contractures, membrane potential, internal Na + activity ( a i N a ) and intracellular pH (pH i ) have been measured in vitro under conditions which modify sarcolemma-bound calcium and the activity of the Na + -Ca 2+ 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 pH i . However the concomitant decrease of a i N a was related to a Na + -Ca 2+ exchange across the sarcolemmal membrane. The data suggest that in soleus muscle, a sarcolemmal calcium pool is involved during the development of K + contractures.