Force and membrane potential in acetylcholine and potassium contractures of denervated mouse muscles.

Depolarization and contracture force (P) provoked by acetylcholine (ACh) and by K ions were studied in bundles dissected from mouse soleus muscles that had been denervated for 4-7 days. Cl-free solutions were used. The muscle fibres were depolarized by solutions containing 150 mM K or 10 microM ACh to nearly zero mV resulting in maximum P (Pmax). Threshold P was produced when the membrane was depolarized to more than about -60 mV by both agents. 50% Pmax was produced by [K] causing the membrane to depolarize to -42 mV, whereas a potential more positive than -20 mV was required for 50% Pmax to be produced when ACh was used. The rate of depolarization was always higher for ACh than for K. Pretreatment by 0.05 microM ACh (about threshold for P) did not affect the P-[K] relation appreciably showing that ACh did not "stabilize" the membrane. Nearly equal P was provoked by successive applications of just suprathreshold agent concentrations when the order of application was ACh----K but not with the reverse order. Hypertonicity (by addition of 300 mM sucrose to all solutions) caused PACh to decrease and PK to increase. It was concluded that the ACh receptors are located in the surface membrane of the muscle fibres, not in the T-system membranes.