Contraction kinetics of red muscle in scup: mechanism for variation in relaxation rate along the length of the fish.

We studied possible mechanisms for the twofold difference in red muscle relaxation times between the posterior (207.2 ms) and anterior (98.4 ms) musculature of scup Stenotomus chrysops, which has been shown to have a large effect on power generation during swimming. This difference was not due to contamination of the anterior bundles with faster fiber types, as histological examination showed that all bundles contained more than 98.9% red fibers. Further, maximum velocities of shortening (Vmax) at 20 degrees C were nearly identical, 5.37 MLs-1 (where ML is muscle length) for the anterior musculature and 5.47 MLs-1 for the posterior musculature, suggesting that the difference in relaxation times was not due to a difference in the crossbridge detachment rates associated with different myosin isoforms. The possibility of differences in the Ca2+ pumping rate influencing relaxation rate was explored using cyclopiazonic acid (CPA), a sarcoplasmic reticulum (SR) Ca(2+)-ATPase inhibitor. The concentration of CPA could be adjusted to slow the relaxation rate of an anterior muscle to that of a posterior muscle. However, SDS gels showed no difference in the intensity of SR Ca(2+)-ATPase protein bands between muscle positions. These results suggest that differences in the Ca2+ pumping could account for the observed difference in relaxation rate, but do not support the simplest hypothesis that the difference in relaxation rates is due to differences in numbers of Ca2+ pumps. Other possible mechanisms for this difference are explored.