Role of glucagon as a contributor to glucose intolerance in acute and chronic uremia

Hepatic glucagon sensitivity and fasting glucose concentration in normal dogs

We assessed hepatic glucagon sensitivity in overnight-fasted, conscious dogs. Six pancreatic replacement protocols were performed in each of five animals. Somatostatin was infused to inhibit endogenous insulin and glucagon, insulin was replaced intraportally at 200 microU.min-1.kg-1, and glucagon was infused intraportally at 0, 0.6, 1, 2, 5, or 20 ng.min-1.kg-1. One intravenous glucose tolerance test was also performed in each animal for measurement of insulin sensitivity (SI). During hormone replacement at a given glucagon dose, plasma glucose differed substantially among animals (P = 0.003). Therefore the dose required for restoration of euglycemia ("glucagon requirement") varied nearly sevenfold among animals, suggesting appreciable differences in glucagon sensitivity (GS). The latter was quantitated in individual animals as the initial slope of integrated glucose output vs. glucagon concentration. GS varied from 0.22 to 3.9 mg.kg-1.pg-1.ml among various animals and was inversely and significantly related to glucagon requirement. SI varied less (approximately 4-fold) and was not associated with glucagon requirement. These observations suggested that interanimal differences in glucose during hormone replacement were the result of substantial differences in GS. In addition, we found the GS of a given animal to be highly associated (P = 0.01) with its fasting glucose level. We conclude that GS varies substantially, and as such may be an important determinant of the fasting glucose level in normal animals.

The effects of chronic uremia on glucagon binding and action in isolated rat hepatocytes

Increased liver sensitivity to glucagon has been proposed to play a role in the complex metabolic state of chronic uremia. In order to assess this possibility at the cellular level, we studied basal and glucagon-stimulated alpha-aminoisobutyric acid (AIB) uptake, glucagon binding, and glucagon degradation in isolated hepatocytes from chronic uremic and pair-fed and ad libitum-fed control rats. The uremic rats were euglycemic and hyperglucagonemic when compared with their controls. The basal rate of AIB uptake was enhanced in hepatocytes from both the uremic and pair-fed control rats. Hepatocytes from ad libitum-fed control animals responded significantly to glucagon at concentrations from 1 X 10(-11) to 1 X 10(-7) mol/L, and those from pair-fed control animals at concentrations from 1 X 10(-8) to 1 X 10(-7) mol/L. Hepatocytes from the uremic rats were unresponsive to glucagon with regard to AIB uptake. 125I-labeled glucagon binding was increased in the uremic rats. This increase of glucagon binding appears to be the results of an increase in the number of binding sites rather than a consequence of a change in binding affinity or decreased glucagon degradation. In conclusion, our data are not supportive of the hypothesis that there exists in uremia an increased sensitivity to glucagon in the liver. The uremic liver is resistant to glucagon with regard to AIB uptake. Despite the high level of circulating immunoreactive glucagon, hepatocytes from uremic rats did not show the expected "down regulation" of their 125I-labeled glucagon binding sites. These studies emphasize the primary role of post-binding events in the regulation of glucagon action and binding.