title: Effects of phosphate and ADP on shortening velocity during maximal and submaximal calcium activation of the thin filament in skeletal muscle fibers.
abstract: The effects of added phosphate and MgADP on unloaded shortening velocity during maximal and submaximal Ca2+ activation of the thin filament were examined in skinned single skeletal fibers from rabbit psoas muscle. During maximal Ca2+ activation, added phosphate (10-30 mM) had no effect on unloaded shortening velocity as determined by the slack-test technique. In fibers activated at submaximal concentrations of Ca2+ in the absence of added phosphate, plots of slack length versus duration of unloaded shortening were biphasic, consisting of an initial high velocity phase of shortening and a subsequent low velocity phase of shortening. Interestingly, in the presence of added phosphate, biphasic slack-test plots were no longer apparent. This result was obtained in control fibers over a range of submaximal Ca2+ concentrations and in maximally Ca2+ activated fibers, which were first treated to partially extract troponin C. Thus, under conditions that favor the appearance of biphasic shortening (i.e., low [Ca2+], troponin C extraction), added phosphate eliminated the low velocity component. In contrast, in fibers activated in the presence of 5 mM added MgADP, biphasic slack-test plots were apparent even during maximal Ca2+ activation. The basis of biphasic shortening is not known but it may be due to the formation of axially compressed cross-bridges that become strained to bear a tension that opposes the relative sliding of the myofilaments. The present findings could be explained if added phosphate and MgADP bind to cross-bridges in a strain-dependent manner. In this case, the results suggest that phosphate inhibits the formation of cross-bridges that bear a compressive strain. Added MgADP, on the other hand, may be expected to detain cross-bridges in strong binding states, thus promoting an increase in the population of cross-bridges bearing a compressive strain. Alterations in the population of strained cross-bridges by added phosphate and MgADP would alter the internal load within the fiber and thus affect the speed of fiber shortening.
authors.affiliation: Department of Physiology, University of Michigan School of Medicine, Ann Arbor 48109, USA. metzgerj@umich.edu
authors.forename: J M
authors.initials: JM
authors.lastname: Metzger
mesh_terms: D000205:Actomyosin / Q000502:physiology; D000244:Adenosine Diphosphate / Q000494:pharmacology*; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects* / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000502:physiology; D010710:Phosphates / Q000494:pharmacology*; D016658:Psoas Muscles / Q000187:drug effects / Q000502:physiology; D011817:Rabbits; D014336:Troponin / Q000502:physiology; D019209:Troponin C
chemical_list: D010710:Phosphates; D014336:Troponin; D019209:Troponin C; D000244:Adenosine Diphosphate; D000205:Actomyosin; D002118:Calcium