Blood glucose metabolism in man during muscular work

Abstract

A resting human subject was given an intravenous injection of a “trace” dose of glucose-U-C14, and blood and respiratory CO2 samples were collected during ensuing rest, exercise, and rest periods. The CO2 was assayed for radioactivity, and the blood was assayed for total and radioactive glucose and lactate. Total O2 uptake and CO2 output were also measured. In the initial rest period, blood glucose and lactate remained constant while their specific activities declined exponentially. Immediately with onset of exercise, blood lactate level and CO2 excretion increased greatly, but their low specific activities in comparison with that of the blood glucose indicated their origin from endogenous substrates, presumably muscle glycogen. During the subsequent recovery period the specific activities of lactate and CO2 rose while the blood lactate and respiratory CO2 excretion dropped to normal levels. Although the level of blood glucose did not change markedly, its specific activity fell more rapidly during work than during rest, indicating a greater “turnover.” However, this increased turnover began some time after commencement of work and continued into the subsequent recovery period. These findings suggest that the stored muscle glycogen represents the immediate fuel for glycolysis and respiration of working muscle. Subsequently, there is an increased uptake of the blood glucose by working muscle, compensated in the present experiments by an increased hepatic glucose output. The data are in accord with previous suggestions that there is a mechanism whereby muscular work stimulates glucose uptake. Submitted on July 27, 1961