Hepatic Glucagon Clearance During Insulin Induced Hypoglycemia

Summary

Studies concerning the importance of glucagon secretion in hypoglycemic counterregulation have assumed that peripheral levels of glucagon are representative of rates of pancreatic glucagon secretion. The measurement of peripheral levels of this hormone, however, may be a poor reflection of secretion rates because of glucagon's metabolism by the liver. Therefore, in order to understand the relationship between pancreatic glucagon secretion and levels of glucagon in the peripheral blood during hypoglycemia, we evaluated hepatic glucagon metabolism during insulin induced hypoglycemia. Four dogs received an insulin infusion to produce glucose levels less than 50 mg/dl for 45 minutes. In response to this, the delivery of glucagon to the liver increased from 36.7±5.9 ng/min in the baseline to 322.6±6.3 ng/min during hypoglycemia. Hepatic glucagon uptake increased proportionally from 13.6±7.2 ng/min to 103.1±28.3 ng/min and the percentage of delivered hormone that was extracted did not change (30.8±13.8% vs 32.9±11.6%). The absolute amount of glucagon metabolized by the liver was dependent on the rate of delivery and was not directly affected by plasma glucose level per se. To directly study the effect of hypoglycemia on hepatic glucagon metabolism, five dogs were given an exogenous infusion of somatostatin followed by an infusion of glucagon and then administered insulin to produce hypoglycemia. The percent of glucagon extracted by the liver (19.5±4.9% and 21.3±6.4%) was not affected by a fall in the plasma glucose level. We conclude 1) Hepatic glucagon uptake is directly dependent on hepatic glucagon delivery during euglycemic and hypoglycemic conditions, 2) the percentage of glucagon extracted by the liver is not changed by the glucose level or rate of glucagon delivery and 3) in an individual animal, peripheral glucagon levels during hypoglycemia may be used as a reflection of the rate of glucagon secretion, just as they are under euglycemic conditions.

Key-Words

Glucagon Metabolism - Hypoglycemia