Effects of ethylene glycol and calcium on the kinetics of contraction induced by photo-release of low concentrations of ATP in rat psoas muscle fibres

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Abstract
To induce isometric contraction in the absence of Ca2+ (10mm EGTA), low concentrations (130 μM) of ATP were photoreleased from caged ATP in skinned fibres from rat psoas muscle at 15–16° C. The magnitude of contraction was independent of the concentration of EGTA (1–30mm). Each isometric transient (i) was paired with another (s) obtained under the same conditions but with 0.4% muscle stretch to elevate the rigor force before photolysis. The algebraic difference (d) betweeni ands was assumed to represent detachment of the crossbridges. The time course of force development (f) by the reattached crossbridges could be estimated by subtracting an appropriately scaledd fromi (ors). Ethylene glycol (20% in solvent) reduced the magnitude and the rate of rise off, although it scarcely affectedd, suggesting that ethylene glycol inhibited reattachment of the crossbridges but not their detachment. The presence of Ca2+ (50 μM) increased the magnitude off, but did not affect its time course (130 μM ATP). Detachment,d, was not influenced by Ca2+ in terms of both extent and rate. The effect of Ca2+ in the presence of ethylene glycol was indistinguishable from that in its absence. Ethylene glycol did not seem to substantially affect the extent of Ca-regulation on the contractile activity.