INCREASED EXPRESSION OF GLUCOSE-TRANSPORTER GLUT-4 IN BROWN ADIPOSE-TISSUE OF FASTED RATS AFTER COLD-EXPOSURE

Cold exposure has been shown to increase glucose uptake specifically i n brown adipose tissue (BAT), the major site for sympathetically contr olled metabolic heat production. In this study, the relationship betwe en glucose uptake and glucose transporters (GLUT) was examined in rats exposed to cold for various periods. To minimize the stimulatory effe ct of circulating insulin, all animals were starved for 20-24 h before the measurements. Acute (4 h) cold exposure had no effect on protein and mRNA levels of GLUT-4, the predominant isoform of GLUT in BAT, des pite a significant increase in cellular glucose uptake. Prolonged (1-1 0 days) cold exposure produced a parallel increase in GLUT-4 expressio n and glucose uptake in BAT. In contrast, cold exposure had no noticea ble effect on GLUT-1, another isoform of GLUT in BAT, and on GLUT-4 in other insulin-sensitive tissues such as white adipose tissue and musc les. The increased glucose uptake and GLUT-4 expression were completel y abolished after surgical sympathetic denervation. These findings sug gest that cold exposure increases glucose uptake in BAT by at least tw o distinct mechanisms, both of which are dependent on sympathetic nerv e: 1) an increase in the amount of GLUT-4 due to the stimulation of it s de novo synthesis, and 2) an increase without stimulation of GLUT sy nthesis, probably due to the change in the transport activity of GLUT- 4 and/or its translocation from an intracellular pool to the plasma me mbrane.