ION-SPECIFIC AND GENERAL IONIC EFFECTS ON CONTRACTION OF SKINNED FAST-TWITCH SKELETAL-MUSCLE FROM THE RABBIT

We used single fibers from rabbit psoas muscle, chemically skinned with Triton X-100 nonionic detergent, to determine the salts best suited for adjusting ionic strength of bathing solutions for skinned fibers. As criteria we measured maximal calcium-activated force (F(max)), fiber swelling estimated optically, and protein extraction from single fibers determined by polyacrylamide gel electrophoresis with ultrasensitive silver staining. All things considered, the best uni-univalent salt was potassium methanesulfonate, while a number of uni-divalent potassium salts of phosphocreatine, hexamethylenediamine N,N,N',N'-tetraacetic acid, sulfate, and succinate were equally acceptable. Using these salts, we determined that changes in F(max) correlated best with variations of ionic strength (1/2-SIGMA-c(i) z(i)2, where c(i) is the concentration of ion i, and z(i) is its valence) rather than ionic equivalents (1/2-SIGMA-c(i)\z(i) \). Our data indicate that increased ionic strength per se decreases F(max), probably by destabilizing the cross-bridge structure in addition to increasing electrostatic shielding of actomyosin interactions.