The use of caged adenine nucleotides and caged phosphate in intact skeletal muscle fibres of the mouse

The effects of 1‐(2‐nitrophenyl)ethyl ester of ATP (NPE‐caged ATP), NPE‐caged ADP, NPE‐caged phosphate (P i ) and desoxybenzoinyl phosphate (desyl‐caged P i ) on mouse skeletal muscle function were studied. All these caged compounds, when microinjected into intact single mouse muscle fibres, reduced the myoplasmic calcium during a tetanus (tetanic [Ca 2+ ] i ) and reduced force. Flash photolysis partially reversed this reduction of tetanic [Ca 2+ ] i and force. In fibres fatigued by repeated tetani, flash photolysis of NPE‐caged ATP, ADP and P i also caused a transient recovery of tetanic [Ca 2+ ] i and force. Because photolytic release of ATP, ADP and P i produced comparable effects it seems that the mechanism of action is the reduction in concentration of the caged compound rather than the release of the biologically active molecule. Experiments on mechanically skinned rat skeletal muscle fibres with intact T‐tubular/sarcoplasmic reticulum coupling showed that 1 m M NPE‐caged ATP had no effect on depolarization‐induced force. This result suggests that the depressant effects of the NPE‐caged compounds are neither on voltage‐activated Ca 2+ release from the sarcoplasmic reticulum nor on myofibrillar function. Thus all the caged compounds tested inhibit excitation–contraction coupling in muscle by an unknown mechanism and this limits their value as sources of biologically important molecules. This inhibitory effect was smallest for desyl‐caged P i and under conditions of maximal activation photolytic release of P i caused a direct inhibition of the contractile proteins. This inhibition amounted to a 1% reduction of maximum force with an increase of [P i ] of about 0.3 m M . The mean rate of force decline under these conditions was 55 s –1 , which reflects the rate of cross‐bridge cycling during a maximal tetanus.