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

¹ Laboratory of General Physiology, National Veterinary School, University of Nantes, BP 527, F-44026Nantes Cedex, France

Isometric tension of K⁺ contractures, membrane potential, internal Na⁺ activity (aⁱN_a) and intracellular pH (pH_i) 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 pH_i. However the concomitant decrease of aⁱN_a 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.