Glucagon challenge in the rat: A robust method for the in vivo assessment of Glycogen phosphorlyase inhibitor efficacy

INTRODUCTION

Glycogen phosphorlyase inhibitors (GPi) act on the glycogenolytic pathway decreasing hepatic glucose output, making them potential candidates for Type 2 diabetes treatment. We established a robust in vivo method to assess GPis efficacy utilising glucagon-stimulated glycogenolysis.

METHODS

Blood glucose was monitored in both male AP Wistar and AP Zucker rats using tail prick samples pre- and post intraperitoneal or subcutaneous glucagon administration. The effect of glycogen phosphorylase inhibitors GPi296 (6-60 mg kg(-1) po) and DAB (5 mg kg(-1) po) upon glucose response to subcutaneous glucagon were examined in both strains.

RESULTS

In the Wistar rat glucagon induced dose related increases in blood glucose, with the maximum increase occurring 20 min post dose (4.0+/-0.88 mmol l(-1), intraperitoneal; and 2.8+/-0.72 mmol l(-1), subcutaneous, ns). Intraperitoneal glucagon administration produced shorter duration blood glucose elevation than observed with the subcutaneous route of administration. In the Zucker rat, no differences were observed between the 10 and 13 week old rats in response to glucagon (3-200 microg kg(-1) subcutaneous). The maximum blood glucose increase was lower in the Wistar rat compared to the Zucker rats (2.9+/-0.20 vs 7.7+/-1.22 mmol l(-1), P<0.0000018). GPi296 and DAB both produced similar inhibition in each strain.

DISCUSSION

Subcutaneous glucagon administration induced more sustained increases in blood glucose than intraperitoneal administration. Blood glucose response to glucagon was higher in the Zucker rat compared to the Wistar rat; there was no difference in inhibition mediated by either GPi296 or DAB between the two strains. We believe that subcutaneous glucagon administration produces a robust model for the assessment of GPis in either rat strain.