The influence of ionic strength upon relaxation from rigor induced by flash photolysis of caged-ATP in skinned murine skeletal muscle fibres

The influence of ionic strength upon relaxation kinetics from rigor in skinned murine extensor digitorum longus (EDL) skeletal muscle fibres was examined using photolysis of caged-ATP at low Ca²⁺. The ionic strength was adjusted with either KMeSO₃ or ethylene glycol bis-(-aminoethyl ether)N,N,N,N-tetraacetic acid, dipotassium salt (K₂EGTA) in the range of /2=65–215 mM, or I.E. 49–194 mM, where I.E. denotes ionic equivalent. Following rigor development at a/2 of 165–215 mM (I.E. 144–194 mM), the liberation of approximately 0.5 mM ATP resulted in an initial 6-to 10-ms detachment phase with a decline in force of approximately 10–20% followed by a 10-to 30-ms reattachment with up to a 60% increase compared to the corresponding rigor level and a final detachment leading to complete relaxation. Interestingly, when similar ATP concentrations were liberated at lower ionic strengths between a /2 of 65 mM and 110 mM (I.E. 60–100 mM), the initial detachment phase was shortened and force decreased by only approximately 5–10%, while the following reattachment phase was lengthened and led to an increased steady-state force of approximately 20–80% without final relaxation. ATP-induced detachment and subsequent reattachment were mainly determined by the currently present ionic strength and were relatively independent of the preceding rigor state which had been developed at higher or lower ionic strengths. The effects of phosphate and apyrase on the force transient suggest that reattachment of ADP- binding crossbridges may contribute to the increase in tension at high and even more at low ionic strengths. The study shows that the kinetics of initial fast relaxation and subsequent redevelopment of force following flash photolysis of similar ATP concentrations are markedly modified by the ionic strength in the narrow range of between 65 mM and 215 mM.