Streptozotocin-induced diabetes impairs G-protein linked signal transduction in vascular smooth muscle

The present studies were undertaken to examine if the impaired vascular function observed in diabetes is attributed to the altered levels of G-protein. Diabetes was induced in Sprague Dawley rats by a single intraperitoneal injection of streptozotocin (STZ) (60 mg/kg body wt) and after a period of 5 days, the aorta were used for adenylyl cyclase activity determination and protein quantification. A temporal relationship between the expression of Giα proteins and development of diabetes was also examined on day 1, 2, 3, 4 and 5 of injection of STZ. Blood glucose levels were significantly increased from day 1 in STZ-rats as compared to their counterpart control rats and reached to about 20 mM on 3rd day and 30 mM on 5th day. The expression of Giα-2 and Giα-3 proteins as determined by immunoblotting techniques was decreased by about 70 and 50% respectively in aorta from STZ rats compared to the control rats after 5 days of treatment, whereas 40% decrease in Giα-2 and Giα-3 was observed after 3rd day of STZ injection. On the other hand, the expression of Gsα was unaltered in STZ rats. In addition, the stimulatory effect of cholera toxin (CT) on GTP-mediated stimulation of adenylyl cyclase was not different in STZ as compared to the control group. However, the stimulatory effects of isoproterenol, glucagon, NaF and FSK on adenylyl cyclase activity were significantly enhanced in STZ rats as compared to control rats, whereas basal adenylyl cyclase activity was significantly lower in STZ-rats as compared to control rats. In addition, GTPγS inhibited FSK-stimulated adenylyl cyclase activity in concentration-dependent manner (receptor-independent functions of Giα) in control rats which was completely attenuated in STZ-rats. In addition, receptor-mediated inhibitions of adenylyl cyclase by angiotensin II, oxotremorine, atrial natriuretic peptide (ANP99–126) and C-ANP4–23 were also attenuated (receptor-dependent functions of Giα) in STZ-rats. These results indicate that aorta from diabetic rats exhibit decreased levels of cAMP and decreased expression of Giα. The decreased expression of Giα may be responsible for the altered responsiveness of adenylyl cyclase to hormonal stimulation and inhibition in STZ-rats. It may thus be suggested that the impaired adenylyl cyclase-Giα protein signaling may be one of the possible mechanisms responsible for the impaired vascular functions in diabetes.