Effects of Fasting, Feeding, and Insulin on Enhancing Effect of GTP on cAMP Binding in Rat Hepatic Cytosol

GTP, in physiologic concentration, enhances the binding of cAMP to a protein in the hepatic cytosol that may be the regulatory subunit of protein kinase II. Ingestion of carbohydrate suppresses hepatic gluconeogenesis and glycogenolysis, two processes that are stimulated by cAMP. In this study, we have examined the possibility that carbohydrate inhibits these processes partly by decreasing the sensitivity of the GTP-responsive cAMP-binding protein to the effect of GTP. We found that 100 μM GTP was much less effective in enhancing cAMP binding in the hepatic cytosol of rats given 15% glucose for 2 days than in the cytosol of fasted rats [21 ± 3% (mean ± SE) increase vs. 67 ± 6%, P < .01]. Corresponding results were noted in diethylaminoethyl (DEAE)-cellulose extracts of the hepatic cytosol of these rats. GTP stimulation of cAMP binding was also diminished in the hepatic cytosol of diabetic rats treated for 7 days with insulin compared with that of untreated diabetic rats (29 ± 10 vs. 81 ± 11% increase, P < .01), but this could have been due to increased food intake in the treated rats. We conclude that GTP stimulation of hepatic cAMP binding is decreased in the carbohydrate-fed state and that this effect may be mediated by the increase in plasma insulin induced by carbohydrate. Our observations suggest that some of the cellular effects of cAMP may be regulated by modulation of the stimulatory effect of GTP on the GTP-responsive cAMP-binding protein.