Effects of Low Ionic Strength and Calmodulin on the Decline in Maximal Calcium-Activated Force in Permeabilized Smooth Muscle
Maximal calcium-activated tension (Tmax) was measured from saponin-and alpha-toxin permemabilized strips of rat anococcygeus muscle stimulated repetitively. 50 mins after saponin treatment, Tmax had declined to 20% of the initial value. Calmodulin (1µM) only partially reduced this decline. Lowering the ionic strength of the bathing solution from 0.2M to 0.07M markedly reduced the rate of decline of tension. Addition of calmodulin to the bathing medium of low ionic strength prevented the decline of force still further; 50 mins after permeabilization, Tmax declined to 80% of the original level. Loss of endogenous calmodulin should be prevented by permeabilization with alpha-toxin from Staphylococcus aureus. Nevertheless, Tmax still declined to approximately 50% of the original value after 50mins. However, negligible force reduction was observed if the bathing solution had an ionic strength of 0.07M.