EFFECT OF THIOCYANATE ON RADIOCALCIUM UPTAKE DURING POTASSIUM CONTRACTURE OF FROG SARTORIUS MUSCLE

¹ Department of Pharmacology, Schools of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania

The potentiating effect of 10 mM thiocyanate (SCN^-) on potassium contractures of frog sartorius muscle has been studied at external calcium levels of 1.0 and 0.1 mM, using 40 mM potassium chloride (hypertonic) for stimulation. A portion of the radiocalcium taken up during such stimulation appears to become bound to sites of higher binding affinity than the radiocalcium taken up in the resting state, although the total amount entering is not significantly different. At (Ca⁺⁺)o of 1.0 mM, stimulation results in a net calcium loss of 0.15 ± 0.02 µmol/g wet weight of muscle. Lowering (Ca⁺⁺)o by a factor of 10 results in even a greater net loss of calcium (0.26 ± 0.05); the influx is reduced 10-fold and the tension area of the contracture is reduced 3-fold. The data suggest that, in frog fast muscle fibers, sufficient internal calcium is released to cause contraction and that internal reaccumulation of calcium is blocked by SON^-.