MECHANICAL RESPONSE TO PHOTOLYTIC ATP PULSES OF SKINNED MUSCLE-FIBERS PRE-ACTIVATED WITH A SMALL PULSE OF ATP

(1) Skinned fibres from rat psoas muscle were placed in oil and activated at 10-degrees-C by pulse photolysis of caged ATP. From the force and stiffness transients on small ATP pulses, we estimated the concentration and rate of ATP hydrolysis of the crossbridges in the fibres. They were 0.14 mM and 0.16 mm s-1, respectively, and thus the turnover rate was 1.1 s-1. (2) To examine the mechanical properties of the physiological ADP-bound crossbridges, we took a double-pulse protocol: the skinned fibre was first activated with a conditioning ATP pulse (0.2-0.5 mM). Several (3-5) seconds later, when almost all the ATP should have been hydrolysed into ADP + Pi, we applied another ATP pulse (0.2-1 mm) to test the fibre. (3) An analysis of the transients on the test pulses indicated that the conditioning pulse slowed the rate of crossbridge detachment. The time course of force development was thus expected to be also delayed, but was not. (4) We suggested that the rigor crossbridge might differ from the nucleotide-free intermediate in the physiological reaction cycle.