The Roles of Calcium in Excitation-Contraction Coupling in Various Muscles of the Frog, Mouse, and Barnacle

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Abstract
In rectus abdominis muscles of the frog the active shortening provoked by 15–40 mM K was supported by Ca, Sr, and Ba, but not by Ni, Co, Mn, Cd, or Zn ions. Addition of the latter cations to a solution containing Ca decreased the responses in a manner suggesting competitive inhibition. The shortening of the rectus muscle found in divalent cation-free, low K solutions is abolished by Ni, Co, Mn, and Mg. In rat muscles a transient increase in the contractural responses to elevated potassium was observed when Ni was applied following partial washout of Ca. In single muscle fibers of the barnacle, development of tension was supported by Ca and Sr, and the other divalent cations were without effect. Retention of Ca45 in barnacle resting muscle fibers soaked in solutions containing 10 mM Ca for 2 min and subsequently washed for 10 min was 60 ± 3.1 mμM Ca/g, whereas retention of Ca45 in contracting muscles similarly exposed to Ca45 was 156 + 17 mμM Ca/g of fresh muscle. The results are compatible with the idea that activation of contraction in some types of muscles is due to entry of extracellular Ca.