NORADRENALINE INCREASES GLUCOSE-TRANSPORT INTO BROWN ADIPOCYTES IN CULTURE BY A MECHANISM DIFFERENT FROM THAT OF INSULIN

Glucose uptake into brown adipose tissue has been shown to be enhanced directly by noradrenaline (norepinephrine) released from sympathetic nerves. In this study we characterized the glucose transport system in cultured brown adipocytes, which responds to noradrenaline as well as insulin, and analysed the mechanism underlying the noradrenaline-indu ced increase in glucose transport. Insulin increased 2-deoxyglucose (d Glc) uptake progressively at concentrations from 10(-11) to 10(-6) M, with maximal stimulation at 10(-7) M. Noradrenaline concentrations ran ging from 10(-8) to 10(-6) M also enhanced dGlc uptake, even in the ab sence of insulin. The effects of noradrenaline and insulin on dGlc upt ake were additive. The stimulatory effect of noradrenaline was mimicke d by the beta(3)-adrenergic agonist, BRL37344, at concentrations two o rders lower than noradrenaline. Dibutyryl cyclic AMP also mimicked the stimulatory effect of noradrenaline, and the antagonist of cyclic AMP , cyclic AMP-S Rp-isomer, blocked the enhancement of glucose uptake du e to noradrenaline. Furthermore Western blot analysis with an anti-pho sphotyrosine antibody revealed that, in contrast with insulin, noradre naline apparently does not stimulate intracellular phosphorylation of tyrosine, suggesting that the noradrenaline-induced increase in dGlc u ptake depends on elevation of the intracellular cyclic AMP level and n ot on the signal chain common to insulin. When cells were incubated wi th insulin, the content of the muscle/adipocyte type of glucose transp orter (GLUT4) in the plasma membrane increased, with a corresponding d ecrease in the amount in the microsomal membrane. In contrast, noradre naline did not affect the subcellular distribution of GLUT4 or that of the HepG2/erythrocyte type of glucose transporter. Although insulin i ncreased V-max. and mas. decreased the K-m value for glucose uptake, t he effect of noradrenaline was restricted to a pronounced decrease in K-m. These results suggest that the mechanism by which noradrenaline s timulates glucose transport into brown adipocytes is not due to transl ocation of GLUT but is probably due to an increase in the intrinsic ac tivity of GLUT, which is mediated by a cyclic AMP-dependent pathway.