Glucagon resistance as a case of hypertriglyceridaemia

Glucagon Receptor Signaling and Glucagon Resistance

Hundred years after the discovery of glucagon, its biology remains enigmatic. Accurate measurement of glucagon has been essential for uncovering its pathological hypersecretion that underlies various metabolic diseases including not only diabetes and liver diseases but also cancers (glucagonomas). The suggested key role of glucagon in the development of diabetes has been termed the bihormonal hypothesis. However, studying tissue-specific knockout of the glucagon receptor has revealed that the physiological role of glucagon may extend beyond blood-glucose regulation. Decades ago, animal and human studies reported an important role of glucagon in amino acid metabolism through ureagenesis. Using modern technologies such as metabolomic profiling, knowledge about the effects of glucagon on amino acid metabolism has been expanded and the mechanisms involved further delineated. Glucagon receptor antagonists have indirectly put focus on glucagon's potential role in lipid metabolism, as individuals treated with these antagonists showed dyslipidemia and increased hepatic fat. One emerging field in glucagon biology now seems to include the concept of hepatic glucagon resistance. Here, we discuss the roles of glucagon in glucose homeostasis, amino acid metabolism, and lipid metabolism and present speculations on the molecular pathways causing and associating with postulated hepatic glucagon resistance.

Sustained effect of zinc-protamin-glucagon in hyperlipidaemic patients

Is is well known from laboratory investigations and acute intervention studies that glucagon is involved in lipid metabolism. The present study was undertaken in order to evaluate a possible sustained effect of zinc-protamin-glucagon in moderately hyperlipidaemic patients. 4 patients were investigated after an acute bolus dose of glucagon 5 mg intravenously and 16 patients were given zinc-protamin-glucagon 5 mg subcutaneously, b.i.d. for 5 days. A similar degree of decrease of plasma cholesterol and triglycerides was obtained in the acute and chronic study, while the increase in fasting blood sugar and plasma insulin seen in the acute study was not present after 5 days treatment. The effect on plasma triglycerides was much more variable than the effect on plasma cholesterol. Five days after treatment the lipid concentrations had returned to the pretreatment values. The effect could not be predicted from the pretreatment lipid concentrations, neither by the type of hyperlipidaemia present. The decrease in plasma cholesterol and triglycerides were not correlated. The drug might prove useful in the long term research of lipid metabolism and the various complex hormonal interactions.

Metabolic effects of glucagon in endogenous hypertriglyceridemia

The metabolic effects of glucagon, administered i.v. in doses of 1 microgram/kg, were evaluated in two groups of patients with endogenous hypertriglyceridemia (Types IV and V according to Fredrickson) with normal and reduced glucose tolerance and in a control group. Glucagon had a lipolytic effect, evaluated as the plasma increase of free fatty acids (FFA) during the first 20 min in normal subjects, but not in the two hyperlipemic groups. A negative correlation was observed between fasting IRI level and FFA mobilization. The ketogenic and hypotriglyceridemic effects of glucagon were demonstrated in normal and hyperlipemic groups. It would seem, therefore, that at the pharmacological doses injected, there is no resistance to the hypotriglyceridemic effect of glucagon in endogenous hypertriglyceridemia.

The effect of induced hyperglucagonaemia on the Zucker fatty rat

The effect of chronic treatment with a long-acting glucagon preparation on liver glucagon and insulin receptors, adenylate cyclase and plasma lipids has been examined in Zucker fatty rats (fa/fa) and their lean littermates (Fa/-). Liver insulin and glucagon receptors were examined using radioreceptor assay techniques. Neither fatty nor lean rats showed any change in insulin receptors after glucagon treatment. Glucagon receptors of the fatty rats showed a 33% drop in the number of the glucagon receptors after glucagon treatment, whilst there was no such change in the lean group. Plasma membranes of the treated fatty rats and their controls bound only 50% as much insulin per mg of liver membrane protein as those of the treated lean rats and their controls. Glucagon treatment raised plasma NEFA in lean rats and reduced them in fatty ones. Plasma cholesterol levels were reduced in both groups of animals as were plasma triglycerides, though to a lesser degree in fatty than in lean animals. Glucagon treatment increased basal and stimulated adenylate cyclase activity in the lean rats and even more so in the fatty ones. The data lend no support to the concept that hypertriglyceridaemia in fatty Zucker rats is a consequence of abnormal glucagon responsiveness.