Effects of ADP and low ATP on the Ca(2+)-sensitive transient contraction upon photolysis of caged ATP in rat muscle fibres: a study on the Bremel-Weber type cooperation.

Caged ATP was photolysed in rat psoas fibres under various conditions to examine whether ADP plays a special role in the 'Bremel-Weber type cooperation', viz. the Ca(2+)-like action of the rigor cross-bridges. Various concentrations of ATP (0.25-1.6 mM) were photoreleased in the presence of various concentrations of ADP (0-0.4 mM) at approximately 8 or 20 degrees C. The Ca2+ concentration was set at around 0.2 microM in order that the ATP-induced contraction 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 Ca(2+)-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 transient contraction. The mechanism by which the ADP-specific, Ca(2+)-independent contraction occurs immediately after the release of ATP remains to be clarified.