Regulation of Metabolism in Working Muscle in Vivo

Abstract

The concentrations of 28 metabolites, including glycogen, trehalose, components of the glycolytic pathway and the Krebs cycle, and amino acids in flight muscle of the blowfly, Phormia regina, were measured concurrently after periods of flight ranging from 5 sec to 1 hour. At the onset of contraction, there was a rapid utilization of trehalose followed by catabolism of thoracic glycogen and then of the polysaccharide stored in the abdomen. Coincident with the decrease in the concentration of trehalose within the muscle was a rapid and transient increase in that of glucose. The concentrations of the hexose monophosphates were constant or declined. The concentration of fructose-1,6-diphosphate rose with the initiation of flight and then fell. The concentrations of triose phosphates, including α-glycerophosphate, increased slightly, but transiently, or were steady during continuous flight. The concentration of pyruvate increased strikingly at the start of exercise. Initially, pyruvate was largely converted to alanine. There were no accumulations of lactate or citrate. The amino group made available for the enormous increase in the concentration of alanine within the muscle after commencement of flight seems to have been derived largely from proline. The concentration of glutamate was lowered slightly during flight; no significant changes in the concentrations of ammonia, aspartate, glutamine, and asparagine within the muscle were found after initiation of flight. The concentration of malate increased sharply at the onset of active contraction, declined slightly for 2 min, and then remained high throughout the entire flight. The concentrations of α-ketoglutarate and oxaloacetate were apparently not altered.

Measurements of coincident changes in metabolites concomitant with the 100-fold increase in glycolytic flux attained on initiation of flight provided an unique system to determine the steps that control glycolysis in vivo, in the transition from a tissue at rest to one performing strenuous work. Three sites of regulation—the phosphorylation of fructose 6-phosphate, the cleavage of trehalose, and the phosphorolysis of glycogen—were established in this system in vivo. Possible additional controls at the oxidations of α-glycerophosphate and of proline are suggested.