Regulation of glycolysis in muscle

Abstract

The question was raised, what causes the sudden increase in the rate of glycolysis when muscle contracts and the equally rapid decrease in rate when the muscle relaxes. For the purpose of this study, isolated frog sartorii were stimulated electrically while immersed in anaerobic Ringer's solution without glucose. The rate was varied between 3 and 48 shocks per min and the duration was 30 min. Under these conditions, the conversion of glycogen to lactate is the main energy producing reaction. The results indicated that two reactions are of prime importance in the regulation of glycolysis in the working muscle: the formation of glucose-1-P from glycogen and inorganic P through the action of the phosphorylase system and the removal of fructose-6-P through the phosphofructokinase reaction. From the relation of the concentration of hexosemonophosphates to the flow rate over the glycolytic system it was concluded that during stimulation these two enzyme systems increase their activity synchronously and proportionately. This is in contrast to the action of epinephrine which causes a much greater increase in phosphorylase activity than in phosphofructokinase activity and thereby leads to a large accumulation of hexosemonophosphates. From measurements of tissue concentration and rate of efflux of lactate from muscle immediately after cessation of stimulation it was concluded that lactate formation returned to the resting rate within 5 min. On the basis of measurements of the tissue concentrations of ATP, ADP, AMP, and P i it was concluded that changes in the concentrations of these compounds in stimulated muscle were too small to permit the increase in enzymatic rates actually observed in vivo. This applies particularly to the phosphorylase reaction for which in vitro data are available which permit predictions of rates at different concentrations of the above reactants. The concentrations of glucose-6-P, fructose-6-P and fructose-1,6-di-P were likewise not well correlated to the rate increase. The only two compounds that increased proportionately with the frequency of stimulation were end products of anaerobic glycolysis in muscle, namely lactate and α- l-glycero-P. A detailed investigation of the phosphorylase b [ rlhar2] a interconversion in stimulated muscle suggested that the phosphorylase b kinase underwent an activation-inactivation cycle closely connected with the contraction and relaxation phases of muscle. An analysis of results obtained with a strain of mice incapable of forming phosphorylase a during stimulation indicated that this interconversion, while not essential for lactate formation per se, was of importance for the time of onset and the final speed of lactate formation. A hypothesis based on localization and structural organization of the glycolytic chain in muscle is proposed which could explain the results summarized above. A mechanism is suggested which is similar in nature to the activation and inactivation of actomyosin (and perhaps of phosphorylase b kinase) by Ca ++ ions. It contains the idea of access of reactants to the catalyst during contraction and separation from reactants during relaxation.