Y. Shimizu et al., NORADRENALINE INCREASES GLUCOSE-TRANSPORT INTO BROWN ADIPOCYTES IN CULTURE BY A MECHANISM DIFFERENT FROM THAT OF INSULIN, Biochemical journal, 314, 1996, pp. 485-490
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.