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 bundles was examined before and after phosphorylation of myosin regulatory light chain (RLC). Relaxation was initiated using diazo-2, a photolabile 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 relaxation initiated from 0.27 +/- 0.02, 0.51 +/- 0.03, and 0.61 +/- 0.03 Po were 90 +/- 6, 140 +/- 6, and 182 +/- 9 ms, respectively. After phosphorylation of RLC, the half-times for relaxation from 0.36 +/- 0.03 Po, 0.59 +/- 0.03 Po, and 0.65 +/- 0.02 Po were 197 +/- 35 ms, 184 +/- 35 ms, and 179 +/- 22 ms. This slowing of relaxation rates from steady-state forces less than 0.50 Po was also observed when bundles of fibers were bathed with N-ethylmaleimide-modified myosin S-1, a strongly binding cross-bridge derivative of S1. These results suggest that phosphorylation of RLC slows relaxation, most likely by slowing the apparent rate of transition of cross-bridges from strongly bound (force-generating) to weakly bound (non-force-generating) states, and reduces or eliminates Ca2+ and cross-bridge activation-dependent changes in relaxation rates.
Patel
J R
JR
Department of Physiology, University of Wisconsin, Madison 53706, USA. patel@facstaff.wisc.edu
Diffee
G M
GM
Huang
X P
XP
Moss
R L
RL
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002614:Chelating Agents / Q000494:pharmacology; D003979:Diazonium Compounds; D066298:In Vitro Techniques; D007700:Kinetics; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D009126:Muscle Relaxation / Q000187:drug effects / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D018994:Myosin Light Chains / Q000378:metabolism*; D010642:Phenoxyacetates; D010766:Phosphorylation; D011817:Rabbits; D013314:Stress, Mechanical
D002614:Chelating Agents; D003979:Diazonium Compounds; D018994:Myosin Light Chains; D010642:Phenoxyacetates; C063685:diazo-2; D002118:Calcium