Enhanced Permeability to Sugar Associated with Muscle Contraction : Studies of the role of Ca⁺⁺

When contractures were induced in isolated frog sartorius muscles with 4 mM caffeine, there was an increase in permeability of the muscle cells to 3-methylglucose. This observation suggests that the changes in permeability to sugar that are known to occur in electrically stimulated muscles may not be intimately related to the depolarization phase of the tissue response. Contractures that were elicited by exposing the muscles to a high concentration of K⁺ were also associated with an increased permeability to sugar. As the concentration of ⁴⁵Ca in the medium was raised, more ⁴⁵Ca entered the muscles during potassium contractures, and the contractures lasted longer, in agreement with the observations of other investigators. There was also a greater change in permeability to sugar when potassium contractures were elicited in the presence of higher concentrations of Ca⁺⁺. The possibility that the enhanced permeability to sugar may be related to changes in the intracellular concentration of Ca⁺⁺ is discussed.