Evaluation of inhibitory guanine nucleotide regulatory protein Gi function in hepatocyte and liver membranes from obese Zucker (fa/fa) rats and their lean (Fa/?) littermates

Abstract

Previous studies have shown that hepatocyte and liver membranes from insulin resistant animals exhibit an impairment of inhibitory guanine nucleotide binding regulatory protein, Gi function, such that a Gi defect may contribute towards the diabetic syndrome. In the current studies, it is shown that the demonstration of Gi activity in liver and hepatocyte membranes is dependent critically on the membrane preparation technique. A technique is defined that allows functional Gi activity to be demonstrated in liver and hepatocyte membranes from both lean (Fa/?) and obese (fa/fa) Zucker rats. Consequently, previous reports on the loss of Gi function in insulin resistant states require revaluation.

α-Fluoromethylhistidine influences somatostatin content, binding and inhibition of adenylyl cyclase activity in the rat frontoparietal cortex

“…Functional G i activity was examined in frontoparietal cortex membranes of control and a-FMH Table 2 Effect of somatostatin (SS) (10 -4 M) and forskolin (FK) (10 -5 M) on adenylyl cyclase (AC) activity (pmol cAMP/min/mg protein) in frontoparietal cortex membranes from control rats (n = 5) and oc-fluoromethylhistidine-treated rats (n = 5) a-Fluoromethylhistidine due to an alteration at the G i level. In the presence of FK (3 X 10 -6 M), which amplifies basal AC activity, increasing concentrations of Gpp(NH)p produce an inhibitory effect on AC activity that has been used by several investigators as a measure of G i function [35]. The ability of Gpp(NH)p to inhibit FK-stimulated AC activity was similar in frontoparietal cortex membranes from control and a-FMH (6 h)-treated rats (Fig.…”

Effects ofPygeum africanum extract (Tadenan�) on vasoactive intestinal peptide receptors, G proteins, and adenylyl cyclase in rat ventral prostate

“…In addition to stimulatory G-protein (Gs)-mediated stimulation of adenylyl cyclase, inhibitory G-Protein (Gi) function was also assessed at this level by determining the ability of low Gpp[NH]p concentrations to inhibit forskolin-stimulated enzyme activity [33]. As shown in Figure 3, control and castrated rats yielded a typical biphasic response curve at a fixed dose of forskolin (1 M): low nucleotide doses between 0.01-10 nM decreased adenylyl cyclase activity due to Gi activation, whereas higher Gpp[NH]p concentrations (0.1-100 M) resulted in enzyme stimulation through Gs activation.…”

“…The consideration of G proteins other than Gs involved in the adenylyl cyclase pathway led us to apply an accepted test, as in the observation of the inhibitory effect of low Gpp[NH]p concentrations on forskolin-stimulated adenylyl cyclase activity [33]. Gi proteins were functional in all groups studied, with the exception of castrated/testosterone-treated rats that did not present the inhibitory arm on the pattern of enzyme activity.…”

The Functional State of Hormone-Sensitive Adenylyl Cyclase Signaling System in Diabetes Mellitus

“…It may be probably due to the difference in the DM models, duration, and severity of the disease and to the application of different approaches in the study of G i protein functions. For instance, in the early studies, there were no significant differences in the functional activity of G i proteins in the membranes isolated from hepatocytes and the whole liver of the diabetic animals, such as fat Zucker ( fa/fa ) rats and rats with short, one-week, STZ T1DM [150, 151]. …”