Effect of Ca2+ on cross-bridge turnover kinetics in skinned single rabbit psoas fibers: implications for regulation of muscle contraction.
The effect of Ca2+ upon the rate constant of force redevelopment following a period of isotonic shortening with immediate restretch to the starting sarcomere length was studied in rabbit psoas fibers at 5 degrees C. Control experiments support the assumption that the rate constant of force redevelopment represents isometric cross-bridge turnover kinetics (fapp + gapp), where fapp and gapp are the rate constants characterizing the transitions from the non-force-generating states to the force-generating states and back to the non-force-generating states, respectively. Parallel measurements of the rate constant of force redevelopment and of force, stiffness, and fiber ATPase during isometric contraction allow the effect of Ca2+ upon fapp and gapp to be determined. Analysis reveals that Ca2+ has a marked effect upon fapp, while gapp remains approximately unchanged. Furthermore, in the range above 25-30% of maximum Ca2+ activation, regulation of force, stiffness, and ATPase is mediated through changes in fapp. Below this range, however, it cannot be ruled out that, in addition, cross-bridges are also switched in and out of the turnover process ("recruitment"). As a consequence of regulation through turnover kinetics, both Ca2+ sensitivity and the slope of force-pCa (-log[Ca2+]) relations are shown to be affected by the ratio fapp/gapp, which may represent an important mechanism of modulation of contractile function in addition to modulation through changes within the regulatory protein system.
Brenner
B
B
Institute of Physiology II, University of Tübingen, Federal Republic of Germany.
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000465:Algorithms; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D007700:Kinetics; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism*; D011817:Rabbits
D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D002118:Calcium