Relaxation of muscle fibers with adenosine 5'-[gamma-thio]triphosphate (ATP[gamma S]) and by laser photolysis of caged ATP[gamma S]: evidence for Ca2+-dependent affinity of rapidly detaching zero-force cross-bridges.
The relationship between the mechanical and biochemical states of the muscle cross-bridge cycle and the control of contraction were investigated by using the nucleotide analogs adenosine 5'-[gamma-thio]triphosphate (ATP[gamma S]) and caged ATP[gamma S] [the O-1(2-nitrophenyl)ethyl P3-ester of ATP[gamma S]]. ATP[gamma S] interacts with actomyosin in a manner similar to ATP but is hydrolyzed (by a factor of 500) more slowly. Generation of ATP[gamma S] by photolysis of caged ATP[gamma S] within a permeabilized fiber in rigor in the absence of Ca2+ relaxed tension and stiffness as occurs with ATP. The transient rise in tension prior to final relaxation observed with photolysis of caged ATP was absent with caged ATP[gamma S]. This result suggests that following detachment of a cross-bridge, ATP is normally hydrolyzed before force generation. In the presence of Ca2+, photolysis of caged ATP[gamma S] within rigor fibers caused tension to relax fully but significant stiffness remained. Stiffness also developed without concomitant tension when Ca2+ concentration was raised from less than 1 nM to 30 microM in the presence of ATP[gamma S]. The amplitude of the tension response to ramp stretches in the presence of Ca2+ and ATP[gamma S] increased with ramp stretch velocity, suggesting that the cross-bridges have detachment rate constants extending into the 10(3) s-1 range. The results provide evidence that the Ca2+-regulatory system can directly control attachment of cross-bridges into states before the power stroke.
Dantzig
J A
JA
Department of Physiology, University of Pennsylvania, Philadelphia 19104.
Walker
J W
JW
Trentham
D R
DR
Goldman
Y E
YE
D000205:Actomyosin / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D007834:Lasers; D009119:Muscle Contraction / Q000187:drug effects*; D009126:Muscle Relaxation / Q000187:drug effects*; D010782:Photolysis; D011817:Rabbits; D013997:Time Factors
C022571:adenosine 5'-O-(3-thiotriphosphate); D000255:Adenosine Triphosphate; D000205:Actomyosin; D002118:Calcium