When the potassium in Ringer's solution is increased above 25 mM, frog skeletal muscle contracts and then spontaneously relaxes. Elevated external potassium will not produce tension again until the stimulating potassium has been removed and a recovery process has taken place. The kinetics of this recovery reaction have been studied in toe muscles bathed in choline Ringer's solutions in which the levels of Ca and K were varied. In 2.5 mM K the recovery reaction appears to follow first order kinetics and the recovery is slowed by lowering the external Ca. In 1.8 mM Ca the recovery is complete in 60 sec.; in 0.2 mM Ca it is 70 per cent complete. When the external K is increased to 10, 15, or 20 mM, the recovery reaches a maximum at 60 sec. and then declines. As the external K is increased, the maximum recovery achieved declines. At K concentrations above 20 mM there is no recovery. When the external Ca is reduced to 0.2 mM, the area under the contracture tension curve is reduced by 40 ± 3 per cent (mean ± SEM). When 1.6 mM Ni is added to 0.2 mM Ca Ringer's, the contracture area is restored beyond the control value. This addition of the Ni to 0.2 mM Ca Ringer's does not, however, restore the recovery reaction to its rate in 1.8 mM Ca.

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