Role of Ca2+ and cross-bridges in skeletal muscle thin filament activation probed with Ca2+ sensitizers.
Thin filament regulation of contraction is thought to involve the binding of two activating ligands: Ca2+ and strongly bound cross-bridges. The specific cross-bridge states required to promote thin filament activation have not been identified. This study examines the relationship between cross-bridge cycling and thin filament activation by comparing the results of kinetic experiments using the Ca2+ sensitizers caffeine and bepridil. In single skinned rat soleus fibers, 30 mM caffeine produced a leftward shift in the tension-pCa relation from 6.03 +/- 0.03 to 6.51 +/- 0.03 pCa units and lowered the maximum tension to 0.60 +/- 0.01 of the control tension. In addition, the rate of tension redevelopment (ktr) was decreased from 3.51 +/- 0.12 s-1 to 2.70 +/- 0.19 s-1, and Vmax decreased from 1.24 +/- 0.07 to 0.64 +/- 0.02 M.L./s. Bepridil produced a similar shift in the tension-pCa curves but had no effect on the kinetics. Thus bepridil increases the Ca2+ sensitivity through direct effects on TnC, whereas caffeine has significant effects on the cross-bridge interaction. Interestingly, caffeine also produced a significant increase in stiffness under relaxing conditions (pCa 9.0), indicating that caffeine induces some strongly bound cross-bridges, even in the absence of Ca2+. The results are interpreted in terms of a model integrating cross-bridge cycling with a three-state thin-filament activation model. Significantly, strongly bound, non-tension-producing cross-bridges were essential to modeling of complete activation of the thin filament.
Wahr
P A
PA
Department of Physiology, University of Michigan, Ann Arbor, Michigan 48109-0622, USA. pwahr@umich.edu
Metzger
J M
JM
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D015764:Bepridil / Q000494:pharmacology; D055592:Biophysical Phenomena; D001703:Biophysics; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley
D002110:Caffeine; D015764:Bepridil; D000255:Adenosine Triphosphate; D002118:Calcium