Activation of the contractile system of insect fibrillar muscle at very low concentrations of Mg2+ and Ca2+

The mechanical properties of single fibres and fibre bundles of glycerinated dorsal longitudinal muscle from lethocerus maximus were investigated in ATP-salt solutions containing only trace concentrations of free Ca²⁺ (pCa>9). A reduction in the magnesium concentration (pMg 7) resulted in an increase in the instantaneous stiffness of glycerinated insect flight muscle fibres, though very little accompanying tension was developed. Stiffness was measured either using small amplitude sinusoidal length changes of high frequency (1 kHz) or rapid rectangular form length changes. The ratio of stiffness to tension in solutions free of added magnesium and calcium was equal to or greater than that obtained from the tissue in the rigor state, and much larger than that obtained in the presence of both magnesium and calcium. Extrapolation of the linear part of the change-tension relationship (obtained during rapid length changes completed within 0.3 ms) back to zero tension indicated that the elastic elements of attached crossbridges were less extended under conditions of Mg²⁺-deprivation than during Ca-activation in Mg²⁺-rich solution. Following a quick stretch a delayed tension development similar to that obtained in the presence of magnesium and calcium ions was observed. The rise in tension was delayed with respect to the accompanying rise in stiffness and reached a peak value after about 2 s. Similar tension transients followed a subsequent release. The possibility that an unusually slow corss-bridge cycle might be responsible for these slow transients was suggested by the finding that the fibres showed a very low ATPase activity under these conditions which could be slightly activated by stretches. On increasing the free Ca²⁺ concentration during magnesium deprivation, the time course of the stretch induced tension transients became faster, while stiffness and the steady state tension rose to reach a high tension state at aboutpCa 6.5.This work was supported by a grant from the Deutsche Forschungs-gemeinschaft (RU 154/12). One of us (P.J.G.) was supported by the Royal Society European Science Exchange Programme