DIRECT STIMULATION OF MYOCARDIAL GLUCOSE-TRANSPORT AND GLUCOSE TRANSPORTER-1 (GLUT1) AND GLUT4 PROTEIN EXPRESSION BY THE SULFONYLUREA GLIMEPIRIDE

Freshly isolated and primary cultured cardiac myocytes from adult rats were used to elucidate acute and chronic effects of the sulfonylurea drug glimepiride on basal and insulin-stimulated glucose transport and on expression of the transporter isoforms glucose transporter-1 (GLUT1) and GLUT4. A 30-min incubation with glimepiride (100 mu M) was unable to modify the initial rates of 3-O-methylglucose transport in freshly isolated cardiocytes, both in the absence or presence of insulin (10(-7) M). Cells were then kept in serum-free culture for 20 h in the presence of glimepiride (10 mu M) and a physiological insulin dose. Under these conditions, the sulfonylurea induced an increase in a-deoxyglucose uptake to 186% of control. This drug effect was dose dependent and could also be demonstrated in the absence of insulin during the culture period. The acute action of insulin on glucose transport was additive to the effect of glimepiride, and the insulin responsiveness of glucose transport remained unaltered in sulfonylurea-treated cultures. Western blot analysis of crude membrane fractions obtained from cultured cardiocytes showed that glimepiride increased the expression of both GLUT1 and GLUT4 to 164% +/- 21% and 148% +/- 5% of control, respectively. We concluded that glimepiride increases cardiac glucose uptake by an insulin-independent pathway, probably involving an increased protein expression of GLUT1 and GLUT4. The increased expression of GLUT4 may have a therapeutic impact on the treatment of insulin-resistant states.