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 muscl e 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 differen ce was not due to contamination of the anterior bundles with faster fi ber types, as histological examination showed that all bundles contain ed more than 98.9 % red fibers. Further, maximum velocities of shorten ing (V-max) at 20 degrees C were nearly identical, 5.37 ML s(-1) (wher e ML is muscle length) for the anterior musculature and 5.47 ML s(-1) for the posterior musculature, suggesting that the difference in relax ation times was not due to a difference in the crossbridge detachment rates associated with different myosin isoforms. The possibility of di fferences in the Ca2+ pumping rate influencing relaxation rate was exp lored using cyclopiazonic acid (CPA), a sarcoplasmic reticulum (SR) Ca 2+-ATPase inhibitor. The concentration of CPA could be adjusted to slo w the relaxation rate of an anterior muscle to that of a posterior mus cle. However, SDS gels showed no difference in the intensity of SR Ca2 +-ATPase protein bands between muscle positions. These results suggest that differences in the Ca2+ pumping could account for the observed d ifference in relaxation rate, but do not support the simplest hypothes is that the difference in relaxation rates is due to differences in nu mbers of Ca2+ pumps. Other possible mechanisms for this difference are explored.