Thin filament activation and unloaded shortening velocity of rabbit skinned muscle fibres.
The unloaded shortening velocity of skinned rabbit psoas muscle fibres is sensitive to [Ca2+]. To determine whether Ca2+ affects the unloaded shortening velocity via regulation of crossbridge kinetics or crossbridge number, the shortening velocity was measured following changes in either [Ca2+] or the number of active thin filament regulatory units. The native troponin C (TnC) was extracted and replaced with either cardiac TnC (cTnC) or a mixture of cTnC and an inactive mutant cardiac TnC (CBMII TnC). The unloaded shortening velocity of the cTnC-replaced fibres was determined at various values of [Ca2+] and compared with different cTnC:CBMII TnC ratios at a saturating [Ca2+]. If Ca2+ regulates the unloaded shortening velocity via kinetic modulation, differences in the velocity-tension relationship between the cTnC fibres and the cTnC:CBMII TnC fibres would be apparent. Alternatively, Ca2+ control of the number of active crossbridges would yield similar velocity-tension relationships when comparing the cTnC and cTnC:CBMII TnC fibres. The results show a decline in the unloaded shortening velocity that is determined by the relative tension, defined as the level of thin filament activation, rather than the [Ca2+]. Furthermore, at lower levels of relative tension, the reduction in unloaded shortening is not the result of changes in any cooperative effects of myosin on Ca2+ binding to the thin filament. Rather, it may be related to a decrease in crossbridge-induced activation of the thin filament at the level of the individual regulatory unit. In summary, the results suggest that Ca2+ regulates the unloaded shortening velocity in skinned fibres by reducing the number of crossbridges able to productively bind to the thin filament without affecting any inherent property of the myosin.
Morris
Carl A
CA
Department of Physiology, School of Medicine, University of California, Los Angeles, CA 90095, USA. camorris@mail.med.upenn.edu
Tobacman
Larry S
LS
Homsher
Earl
E
D008841:Actin Cytoskeleton / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D005260:Female; D006652:Histological Techniques; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D009154:Mutation; D016658:Psoas Muscles / Q000502:physiology*; D011817:Rabbits; D011930:Reaction Time / Q000187:drug effects; D019209:Troponin C / Q000235:genetics / Q000494:pharmacology
D019209:Troponin C; D002118:Calcium