Transient contraction of muscle fibers on photorelease of ATP at intermediate concentrations of Ca2+.
We isometrically activated skinned fibers in rigor by flash photolysis of caged ATP at various [Ca2+] at 8 degrees C. On release of ATP, tension initially decreased with the same time course at all [Ca2+]. At high [Ca2+] (pCa < or = 5.8), tension rose to the steady-state plateau after the brief relaxation. When the [Ca2+] was intermediate (7.0 < or = pCa < or = 6.0), tension temporarily overshot the final steady-state level. The half-time during this tension transient was longer at higher [Ca2+]. The transient contractions could be simulated by a simple kinetic model: R + ATP-->Q, and X<-->Q<-->A, where R, X, and A are the rigor, relaxed, and active-tension states, respectively; Q is a "pre-active" state where tension is very low; and Ca2+ affects only the X-Q transition. This scheme was also useful for predicting the tension transients in Ca(2+)- and P(i)-jump experiments at various [Ca2+]. ADP enhanced the Ca2+ sensitivity of the ATP-induced transient contraction, which was not in the scope of the model.
Department of Physiology, Oita Medical University, Japan.
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Horiuti
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Kagawa
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Yamada
D000244:Adenosine Diphosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000502:physiology* / Q000528:radiation effects; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000494:pharmacology*; D066298:In Vitro Techniques; D008954:Models, Biological; D009119:Muscle Contraction / Q000187:drug effects* / Q000502:physiology*; D010782:Photolysis; D016658:Psoas Muscles / Q000502:physiology; D051381:Rats
D000244:Adenosine Diphosphate; C029273:P(3)-1-(2-nitro)phenylethyladenosine 5'-triphosphate; D000255:Adenosine Triphosphate; D002118:Calcium