PHOSPHORYLATION OF MYOSIN REGULATORY LIGHT-CHAIN ELIMINATES FORCE-DEPENDENT CHANGES IN RELAXATION RATES IN SKELETAL-MUSCLE

The rate of relaxation from steady-state force in rabbit psoas fiber b undles was examined before and after phosphorylation of myosin regulat ory light chain (RLC). Relaxation was initiated using diazo-2, a photo labile Ca2+ chelator that has low Ca2+ binding affinity (K-Ca = 4.5 x 10(5) M-1) before photolysis and high affinity (K-Ca = 1.3 x 10(7) M-1 ) after photolysis, Before phosphorylating RLC, the half-times for rel axation initiated from 0.27 +/- 0.02, 0.51 +/- 0.03, and 0.61 +/- 0.03 P-o were 90 +/- 6, 140 +/- 6, and 182 +/- 9 ms, respectively. After p hosphorylation of RLC, the half-times for relaxation from 0.36 +/- 0.0 3 P-o, 0.59 +/- 0.03 P-o, and 0.65 +/- 0.02 P-o were 197 +/- 35 ms, 18 4 +/- 35 ms, and 179 +/- 22 ms. This slowing of relaxation rates from steady-state forces less than 0.50 P-o was also observed when bundles of fibers were bathed with N-ethylmaleimide-modified myosin S-1, a str ongly binding cross-bridge derivative of S1. These results suggest tha t phosphorylation of RLC slows relaxation, most likely by slowing the apparent rate of transition of cross-bridges from strongly bound (forc e-generating) to weakly bound (non-force-generating) states, and reduc es or eliminates Ca2+ and cross-bridge activation-dependent changes in relaxation rates.