Alternate energy transduction routes in chemically skinned rabbit psoas muscle fibres: a further study of the effect of MgATP over a wide concentration range.

Complex stiffness data were studied over an extended range of MgATP concentrations (3 muM-5 nM) in single fibres of Ca2+-activated, chemically skinned adult rabbit psoas. The data were analysed in terms of a model involving three exponential processes, the presence of which was previously observed in fully activated muscles. As fibres were transferred from a rigor solution into solutions of gradually increasing MgATP concentration, the three processes appeared sequentially, each with a unique Km. The order of appearance as MgATP increase is (1) the slowest of three processes [designated process (A)], (2) the fastest of the three processes [designated (C)], and (3) process (B), which occupies the middle range of frequencies; the KmS are approximately 10 muM, 0.2 mM, and 0.8 mM, respectively. The single phase advance [process (A)] remaining at very low substrate concentrations was found to be better described by a distribution of rate constants than by a single rate constant. The influence of substrate concentration on these processes is examined and two parallel hydrolysis routes are suggested as a possible mechanism.