EFFECT OF MANGANESE IONS ON EXCITATIONCONTRACTION COUPLING IN FROG SARTORIUS MUSCLE

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Manganese ions are known to inhibit the calcium spike in crayfish and barnacle. In frog sartorius muscle, we found that Mn++ inhibits the peak tension of twitch, tetanus, and potassium contracture and also the rate of rise and relaxation of tension. The inhibiting effect of Mn++ increased with increasing concentration (1-10 mM). The magnitude of resting potential and the amplitude of action potential were not significantly affected, but the negative afterpotential was enhanced by 10 mM Mn++. At this concentration the peak tension of twitch, tetanus, and potassium contracture was almost completely inhibited. In 10 mM Mn++, the steep initial rise of tension in caffeine contracture was abolished, but the subsequent slow rise in tension was not affected. The peak tension was reduced by about 30 %. Mn++ actually activated the Mg++-activated ATPase of myofibrils from rabbit skeletal muscle and decreased the extra Ca-induced splitting of ATP by the sarcoplasmic reticulum. These results indicate that manganese ions inhibit excitation-contraction coupling by acting on the transverse tubular system and possibly terminal cisternae of the sarcoplasmic reticulum.
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