ION-SPECIFIC PROTEIN DESTABILIZATION OF THE CONTRACTILE PROTEINS OF CARDIAC-MUSCLE FIBERS

We investigated the inhibitory effects of increased salt concentration s on maximal calcium-activated force (F-max) of rabbit cardiac papilla ry muscle bundles skinned with Triton X-100. While other studies have reported a lack of ion-specific effects on F-max of cardiac muscle, we clearly demonstrated the presence of such effects when a wider variet y of salts was investigated. In addition, like skeletal muscle, cardia c muscle was found to be sensitive to ionic strength and not to ionic equivalence. In support of our hypothesis that the ion-specific effect s are due to protein destabilization, we found that a protein stabiliz er (trimethylamine N-oxide, TMAO) completely abolished the ion-specifi c effects on F-max. The ion-specific effect is probably due to binding of ions to the contractile proteins. The general ionic effect is most likely due to electrostatic shielding that remains in the presence of TMAO. Neither 300 mM sucrose nor TMAO significantly altered F-max at physiological ionic strength indicating that the ion-specific depressi on of F-max was not due to a colligative/osmotic effect. Furthermore, adding sucrose to solutions with a supraphysiological ionic strength c aused a further decrease in F-max indicating that certain osmolytes ca n alter F-max if the contractile proteins are initially destabilized.