THE EFFECT OF POTASSIUM ON NICOTINE-INDUCED CONTRACTURE AND Ca⁴⁵ MOVEMENTS IN FROG SARTORIUS MUSCLE

¹ Department of Pharmacology, Medical College of Virginia, Richmond, Virginia

The contracture and three-fold increase in Ca⁴⁵ uptake induced by nicotine (1.25-5.0 mM) in frog sartorius muscle are inhibited by prior exposure of the muscle to Ringer's solution containing 80 mM K⁺. The inhibition of increased Ca⁴⁵ uptake by K⁺ also occurs at concentrations of nicotine (0.415-0.83 mM) too low to elicit recordable contractures. The rate of Ca⁴⁵ uptake is maintained for a full 10-min incubation period at lower nicotine concentrations (0.415 and 0.83 mM), whereas more than five-sixths of the increased Ca⁴⁵ uptake at 2.50 mM nicotine occurs during the first 5 min of a 10-min incubation period. Resting Ca⁴⁵ uptake is also inhibited by prior administration of K⁺. Prolonged washout in O-Ca-Ringer's solution containing 4 mM EDTA does not prevent a nicotine-induced contracture which is accompanied by an increased Ca⁴⁵ efflux. Under these conditions, 80 mM K⁺ inhibits the contracture but not the increased Ca⁴⁵ efflux. These observations suggest that the inhibition of nicotine-induced Ca⁴⁵ uptake and contracture by 80 mM K⁺ is a result of prior displacement by K⁺ of that store of bound Ca⁺⁺ normally released by nicotine. The actions of nicotine may occur at different calcium sites from those affected by other agents which induce contracture in frog sartorius muscle.