Effect of change in length of day on the insulin sensitivity of the rat heart.

Abstract

GALACTOSE, a stereoisomer of glucose which is not metabolized by peripheral tissues1,2, is transported in the isolated perfused rat heart by an insulin-sensitive carrier mechanism2. The effect of insulin, which is to increase markedly the rate of movement of the sugar across the muscle cell membrane, is most clearly demonstrated in the efflux of the sugar from the tissue. Hearts loaded with galactose by perfusion for 20 min. with a Krebs–bicarbonate medium containing the hexose (1 gm./100 ml.) are transferred to a sugar-free perfusate, and the effluent collected in a timed sample. The fractional fall in the heart galactose content during the fourth minute after transfer is used as a measure of efflux-rate. It has been shown3 that the dose-response curves of rate of galactose efflux against insulin concentration show a marked seasonal variation; the sensitivity being greatest in summer and least in winter (Fig. 1). The dose-response curves are Michaelis–Menten in form; and the change with season suggests the presence of a competitive inhibitor of insulin during the winter months.