title: Rapid activation by photolysis of nitr-5 in skinned fibres of the striated adductor muscle from the scallop.
abstract: Photolysis of nitr-5, a caged calcium molecule, has been used for rapid activation of skinned fibre bundles of a myosin-regulated muscle, the striated adductor of the scallop, Pecten maximus. Chemically skinned fibre bundles (diameter 70-200 microns) were equilibrated in solutions containing 1-3 mM nitr-5 (pCa 6.1) and then activated by ultraviolet laser pulse (25 ns). Pulse energies of 60-95 mJ gave contractions of over 90% maximum tension and a mean half-time for tension rise of 43 ms (n = 4) at 12 degrees C. Electrically stimulated bundles of intact fibres develop a tetanus with a rise half-time of 60.2 ms at 10 degrees C (n = 5) (Rall, J.A. (1981) J. Physiol. 321, 287-295, and personal communication). At lower pulse energies the skinned fibres gave smaller amplitude contractions with slower rates of rise (up to 260 ms half-time). In addition, a slower component of tension development (mean rise half-time 13.3 s) was often observed. In ATP-free solutions containing hexokinase and glucose, rigour tension developed with a delayed onset. Rapid release of ATP (0.47-0.59 mM) from photolysis of caged ATP (2 mM) at pCa 4.5 then caused a rapid contraction with a mean half-time for tension development of 17 ms (n = 4). The fast activation rates obtained by the photorelease of Ca2+ from nitr-5 are similar to those obtained with skinned skeletal fibres of actin-regulated muscle. The results imply that the rate-limiting step in excitation-contraction coupling of the scallop muscle is not the increase in sarcoplasmic Ca2+, but rather the activation of the muscle in response to this increase. The half-times of ATP-induced contractions at pCa 4.5 suggest that in a contraction activated by a rapid Ca2+ jump the process comprising ATP hydrolysis and cross-bridge cycling occurs at a somewhat faster rate than the Ca2(+)-dependent activation process which precedes it.
authors.affiliation: University Laboratory of Physiology, Oxford, U.K.
authors.forename: T J
authors.initials: TJ
authors.lastname: Lea
authors.forename: M J
authors.initials: MJ
authors.lastname: Fenton
authors.forename: J D
authors.initials: JD
authors.lastname: Potter
authors.forename: C C
authors.initials: CC
authors.lastname: Ashley
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology / Q000031:analogs & derivatives; D007700:Kinetics; D007834:Lasers; D008974:Mollusca / Q000502:physiology*; D009119:Muscle Contraction / Q000187:drug effects; D009132:Muscles / Q000187:drug effects / Q000502:physiology*; D009210:Myofibrils / Q000502:physiology; D010782:Photolysis
chemical_list: C052455:nitr 5; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D002118:Calcium