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

Abstract

SUMMARYIsometric tension of K+ contractures, membrane potential, internal Na+ activity (aNa) 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 aNa 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.

Actions of perchlorate ions on rat soleus muscle fibres.

“…The influence of Na+-Ca2+ exchange Ca2+ entering depolarized fibres via the Ca2+-Ca2+ exchange mode of the Na+-Ca2+ exchange protein (Philipson, 1985) might contribute to K+ contractures (Curtis, 1988;Noireaud & Leoty, 1988). Although external [Ca2+] was previously reduced to 15 JIm, the [Ca2+] in the T-system might have been higher and Ca2+ may have entered through Ca2+-Ca2+ exchange.…”