G-protein-mediated regulation of the insulin-responsive glucose transporter in isolated cardiac myocytes

Isolated muscle cells from adult rat heart were used to study the involvement of G-proteins in the regulation of the glucose transporter by insulin and isoprenaline. Efficient modification of G-protein functions was established by measuring isoprenaline-stimulated cyclic AMP production, viability and ATP content after treating the cells with cholera toxin and pertussis toxin for 2 h. Under these conditions cholera toxin decreased the stimulatory action of insulin on 3-O-methylglucose transport by 56%, but pertussis toxin had no effect. Basal transport was not affected by toxin treatment. Isoprenaline increased 3-O-methylglucose transport by 63%. This effect was not mimicked by dibutyryl cyclic AMP, but was completely blocked by cholera toxin. Streptozotocin-diabetes abolished isoprenaline action and decreased stimulation of transport by 64%. Concomitantly, cholera-toxin sensitivity of glucose transport was lost in cells from diabetic animals. This was paralleled by a large decrease (87 +/- 4%) in mRNA expression of the insulin-regulatable glucose transporter, as shown by Northern-blot analysis of RNA isolated from cardiomyocytes of diabetic rats. These data suggest a functional association between the insulin-responsive glucose transporter and a cholera-toxin-sensitive G-protein mediating stimulation by insulin and isoprenaline.