The majority of potassium ions in muscle cells is adsorbed on beta- and gamma-carboxyl groups of myosin: potassium-ion-adsorbing carboxyl groups on myosin heads engage in cross-bridge formation during contraction.

High-molecular weight poly(ethylene glycol) (PEG-8000) in the bathing medium prolongs the survival of 2-mm-wide frog muscle segments with open ends. In a PEG-8000-containing medium Rb+, K+, and Na+ in the muscle segments reached new diffusion equilibrium in 2-4 hours. At this new equilibrium, the cell's preference of K+ over Na+ was preserved but very much weakened. Studies of the influence of pH on the equilibrium distribution of labelled Na+ in 2-mm-wide muscle segments confirmed the prediction that beta- and gamma-carboxyl groups, carried respectively on aspartic and glutamic acid residues of intracellular proteins, adsorb K+, Na+ and other monovalent cations. These carboxyl groups have a characteristic pKa between 3.65 and 4.25. A pKa of 3.85 was observed. These findings, when seen in the light of other relevant information available, led to the conclusion that beta- and gamma-carboxyl groups on myosin molecules adsorb--in a close contact one-ion-one-site fashion--the majority (67% to 80%) of K+ in resting muscle cells. Other evidence suggests that in muscle contraction, the K(+)-adsorbing beta- and gamma-carboxyl groups on myosin heads form salt linkages with cationic sites on actin, displacing and releasing the adsorbed K+. Present and earlier findings together offer support for an earlier suggestion that the formation and dissociation of these salt-linkages may underlie the force-generating, cyclic formation and dissociation of cross-bridges during muscle contraction.