Loss of Ca2+-dependent regulation in glycerinated skeletal muscle contraction

Glycerinated muscle fiber from rabbit psoas muscle often lost Ca2+-dependent regulation of its contraction with long-term extraction in a 50% glycerol solution containing 5 mM EGTA at -20 degrees C, designated as Ca2+-insensitive muscle fiber (CaIS-fiber). About 30 or 40% of glycerinated muscle fibers were CaIS-fibers after 1 to 3 months in the glycerol solution. We investigated the cause of the loss of Ca2+-sensitivity of the glycerinated muscle fiber by tension mechanogram and SDS polyacrylamide gel electrophoresis. This natural CaIS-fiber showed a new band of 30K daltons peptide on SDS gels. On the other hand, Ca2+-sensitive fiber (CaS-fiber) changed to CaIS-fiber by trypsin digestion for 40 sec. The tryptic CaIS-fiber had no troponin C and 30K daltons peptide bands in the electrophoretograms. Incubating with 2 mM CaCl2 for 40 hr at 25 degrees C, CaS-fiber changed easily to CaIS-fiber which had 30K daltons peptide and faint troponin T and I bands, as in natural CaIS-fiber. All CaIS-fibers could recover their Ca2+-dependent regulation by incubating with native tropomyosin from rabbit skeletal muscle for 2 days at 4 degrees C. These results indicate that the loss of Ca2+-dependent regulation of glycerinated muscle fiber is due to degradation of regulatory protein system by endogenous Ca2+-activated proteolytic enzymes.