Role of calcium and crossbridges in modulation of rates of force development and relaxation in skinned muscle fibers.
The influence of Ca2+ and force generating crossbridges on the kinetics of force development and relaxation was examined in skinned muscle fibers activated by photolytic release of Ca2+ from a caged calcium or inactivated by photolytic uptake of Ca2+ by a caged Ca2+ chelator. In frog fibers at 10 degrees C, decreasing the Ca2+ released from caged calcium to an extent that resulted in 50% of maximum force development produced an approximate seven-fold decrease in the rate of contraction. In contrast decreasing the number of force generating crossbridges by partial extraction of troponin C (TnC) or addition of vanadate caused only minor changes in contraction rate. Thus the rate of force development decreases dramatically with decreases in the Ca2+ level which suggests that a step in the crossbridge cycle may be Ca2+ dependent. The kinetics of relaxation induced by photolysis of diazo-2 was: a) slowed by stabilization of the sarcomeres by repeated releases and re-stretches during contraction and b) accelerated when the amplitude of force development was decreased by decreasing the [Ca2+] which induced a steady contraction. The half time of relaxation decreased by approximately two- to three-fold, when 50% of maximum force was developed. One interpretation of these results is that decreasing the number of force generating crossbridges may speed relaxation by inducing a decreased affinity of TnC for Ca2+ and thus accelerating the Ca2+ dissociation rate from TnC and thereby increasing relaxation rate.
Rall
J A
JA
Department of Physiology, Ohio State University, Columbus, USA.
Wahr
P A
PA
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002614:Chelating Agents; D003979:Diazonium Compounds; D007700:Kinetics; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology* / Q000648:ultrastructure; D010642:Phenoxyacetates; D010782:Photolysis; D011896:Rana temporaria; D019209:Troponin C / Q000502:physiology
D002614:Chelating Agents; D003979:Diazonium Compounds; D010642:Phenoxyacetates; D019209:Troponin C; C063685:diazo-2; D002118:Calcium