Blood Glucose Concentrations Are Not Increased by Chronic IP Glucagon Administration in Leptin-Treated Type 1 Diabetic Rats

Abstract

The overall objective of this research is to increase our understanding of how central leptin administration normalizes blood glucose concentrations independent of insulin in type 1 diabetic rats. This research may lead to new insulin-independent treatments for diabetes. It is widely accepted that leptin administration into the brain of previously uncontrolled diabetic animals can normalize blood glucose concentrations, independent of insulin. The mechanism by which central leptin administration normalizes blood glucose concentrations in diabetic animals is not understood. Some studies have suggested that leptin acts by decreasing the serum concentration of glucagon. However, we hypothesize that leptin decreases the responsiveness of glucagon by inhibiting cyclic adenosine monophosphate (cAMP) signaling in the liver. This would decrease gluconeogenesis in the liver, resulting in a reduction in hepatic glucose output, normalizing blood glucose concentrations. To test this hypothesis, we performed a study to directly examine whether intracerebroventricular (ICV) leptin administration blocks the ability of chronically high doses of glucagon (delivered via an intraperitoneal (IP) osmotic pump) to increase blood glucose concentrations in streptozotocin (STZ)-induced diabetic rats. Two cohorts of rats were used. Four groups of diabetic rats were examined within each cohort: 1) leptin-treated (ICV), glucagon-treated (IP), 2) leptin-treated (ICV), vehicle-treated (IP), 3) vehicle-treated (ICV), glucagon-treated (IP), and 4) vehicle-treated (ICV), vehicle-treated (IP). The change in blood glucose concentration of the four groups was determined on a daily basis and during three different conditions. The three conditions were 1) at various times over the circadian cycle, 2) during an 8-hour fast, and 3) following the IP injection of pyruvate (to determine the rat’s gluconeogenic capacity). As we have seen previously, leptin treatment normalized blood glucose concentrations in diabetic rats. Our new findings showed that chronic glucagon treatment did not increase blood glucose concentrations of leptin-treated diabetic rats. This was true whether based on the concentration of daily blood glucose, the blood glucose concentration around the circadian cycle, the blood glucose concentration during an 8-hour fast, or the blood glucose concentration in response to an injection of pyruvate. This lack of difference was observed despite the fact that serum glucagon concentrations were 4-9-fold greater in glucagon-treated rats as compared to vehicle-treated rats. Leptin treatment decreased phosphoenolpyruvate carboxykinase (PEPCK)/beta-actin content in the liver. We also saw a decrease in hepatic total cAMP-response element binding protein (CREB) with leptin treatment, but not in phospho-CREB (Ser133). This suggests that chronic leptin treatment decreases the cAMP signaling pathway in the liver, resulting in a decrease in gluconeogenesis. Our data also suggests that the serum glucagon concentration does not have to be reduced in order for leptin to normalize blood glucose concentrations. This supports the hypothesis that leptin treatment acts by decreasing the responsiveness to glucagon, rather than by decreasing circulating glucagon concentrations.