Effects of ADP and low ATP on the Ca2+-sensitive transient contraction upon photolysis of caged ATP in rat muscle fibres: a study on the Bremel-Webertype cooperation

Caged ATP was photolysed in rat psoas fibres under various conditions to ex amine whether ADP plays a special role in the 'Bremel-Weber type cooperatio n', viz. the Ca2+-like action of the rigor cross-bridges. Various concentra tions of ATP (0.25-1.6 mM) were photoreleased in the presence of various co ncentrations of ADP (0-0.4 mM) at similar to 8 or 20 degrees C. The Ca2+ co ncentration was set at around 0.2 mu M in order that the ATP-induced contra ction was significant but short. Both lower [ATP] and higher [ADP] resulted in a slower detachment of the rigor cross-bridges and a larger contraction after the detachment. ADP did not seem to affect the relationship between the rate of detachment and the size of the Ca2+-sensitive component of the subsequent contraction. It is concluded that there is little evidence that the ADP-bound rigor cross-bridges are more potent than the nucleotide-free ones in the Bremel-Weber type cooperation that is seen in the ATP-induced t ransient contraction. The mechanism by which the ADP-specific, Ca2+-indepen dent contraction occurs immediately after the release of ATP remains to be clarified. (C) Kluwer Academic Publishers.