Phosphate Exchange Between High‐Energy Phosphate Compounds in Resting Crustacean Muscle

Abstract

Single giant muscle fibres from Megabalanuiiis psittactis were microinjected with ATP-32P or internally perfused with a solution containing 32P-PArg in an attempt to demonstrate, in the absence of inhibitors, ATP utilization during mechanical activity with negative results. The rate constant of transfer of 32p from microinjected 32P-PArg to endogenous ATP was studied at two different temperatures, obtaining a Q1o of 108+0 12. It is concluded that the rate limiting step in the process of transference of phosphate in the resting muscle from balanus is the intracellular diffusion of PArg and not the rate at which the equivalent of Lohman's reaction in invertebrates proceeds.It is generally accepted that adenosine triphosphate (ATP) and phosphocreatine (PCr) are split during one of the steps of the contraction-recovery cycle in muscle from vertebrates [Wilkie, 1968a, b;Mommaerts, 1969;Mommaerts and Wallner, 1967;Woledge, 1971]. It is also thought that the ADP generated is rephosphorylated from PCr by the Lohman reaction so that no net change in concentration of ATP can be detected except when the Lohman reaction is blocked by 1-fluoro-2, 4-dinitrobenzene (FDNB;) [Cain and Davies, 1962]. In general ATP and PCr splitting reactions are thought to take place during the initial part of the mechanical events. In the measurement of the changes in ATP and PCr level in muscle one encounters some difficulties; chief among them are (1) arresting the chemical changes in the muscle by sufficiently rapid freezing and (2) making an extract of the frozen muscle containing all the substances of interest and analysing it without producing further chemical changes. Furthermore, the results are obtained by comparison of the substitute level in two muscles which, as a rule, are initially different. Although the method presented in the preceding paper resolves this last difficulty [Nassar-Gentina and Rojas, 1980], the rates of phosphate transfer under resting conditions have to be determined accurately before attempting to measure the effects of mechanical activation of the fibres.