Freshly isolated and primary cultured adult rat cardiomyocytes were used to elucidate the mechanism of action of the new oral antidiabetic agent (+/-)-5-[4-(6-hydroxy-2, 5, 7, 8-tetramethyl-chroman-2-yl-methoxy)benzyl]-2,4-thiazolidinedione (troglitazone) on the heart. Interaction with protein kinase C (PKC) and regulation of glucose transport were evaluated as possible sites of drug action. Acute treatment (30 min) of cardiomyocytes with troglitazone did not affect the phorbolester-induced membrane association of PKC-delta and PKC-epsilon, which represent the major isoforms present in these cells. However, under these conditions the phorbolester-mediated increase in membrane associated PKC activity was inhibited by 43 +/- 4% (n = 4) without affecting the basal distribution of PKC activity. In contrast to these findings, troglitazone had no acute effect on basal or insulin-stimulated glucose transport in freshly isolated cardiomyocytes; even after 120 min treatment an unaltered release of lactate was determine in the presence of the drug. After 20 h in serum-free culture troglitazone induced a dose-dependent increase in 2-deoxyglucose uptake reaching a 40-fold stimulation at 5 mumol/l. This was paralleled by a dose-dependent increase of glucose transporter-1 (GLUT1) and GLUT4 protein expression to 320 +/- 80 and 156 +/- 15% of control, respectively. In addition, chronic exposure to troglitazone increased the GLUT4 abundance in a plasma membrane fraction about twofold. These data show that troglitazone exerts multiple effects on cardiomyocytes involving inhibition of PKC and regulation of glucose transporter expression and distribution. We suggest that an increased glucose supply may be beneficial for the diabetic heart and that modulation of PKC-activity could be relevant for improving insulin action in muscle tissue.
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 microM) 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 microM) and a physiological insulin dose. Under these conditions, the sulfonylurea induced an increase in 2-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.
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 microM) 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 microM) and a physiological insulin dose. Under these conditions, the sulfonylurea induced an increase in 2-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.
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