The role of magnesium adenosine triphosphate in the contractile kinetics of insect fibrillar flight muscle.

The changes in tension produced by small step or sinusoidal changes of length have been measured for chemically skinned flight muscle fibres of the giant tropical water bug Lethocerus at MgATP concentrations in the range 0.1-10 mM. In the presence of calcium ion concentrations of about 20 microM, the rates of the rapid mechanical processes observed were found to increase with increasing MgATP, exhibiting saturation with an apparent half-saturation constant lying between 0.1 and 1 mM MgATP, depending upon the conditions used. Under the same conditions, an increase in MgATP concentration was found to lead to a slight decrease in the isometric tension generated by the preparations. The results are discussed with reference to some current crossbridge models of muscle contraction.