Fasting increases the plasma glucagon response in the migratory garden warbler (Sylvia borin)

Acute pancreatic hormonal responses to oral glucose loads were investigated in garden warblers during the prolonged fast that follows their autumnal migratory fattening. Plasma glucose, free fatty acids, beta-hydroxybutyrate, insulin (INS), and glucagon (GLN) were measured prior to and 10 min after an oral glucose load in three groups of birds: One had food ad libitum, and the other two were either food restricted or food deprived down to a given (low) body mass level. Ten minutes after the glucose load, plasma glucose levels increased significantly in all three groups (range of mean values: basal, 15.1-16.0; glucose-stimulated, 19.1-23.7 mmol/L). A smaller increase in food-restricted/deprived groups was not statistically significant. Free fatty acid levels (1.0-1.5 mmol/L) after 10 min were unchanged, while beta-hydroxybutyrate decreased to similar levels in food-restricted/deprived and control groups (basal, 3.3-4.2; glucose-stimulated, 1.9-2.4 mmol/L). Insulin increased and glucagon decreased in response to oral glucose loads. However, initial levels and responses of plasma insulin to glucose were lower, and those of glucagon were higher in the food-restricted/deprived groups (INS, both 2.7; DeltaINS, 0.1-0.3 mIU/L; GLN, 2.8-3.3; DeltaGLN, 1.4-2.2 microg/L) than in the control group (means +/- SE; INS, 3.8 +/- 0.8; DeltaINS, 1.4 +/- 0.7 mIU/L; GLN, 2.5 +/- 0.5; DeltaGLN, 0.7 +/- 0.5 microg/L), resulting in similar increases in the insulin:glucagon ratio. Impaired insulin secretion may thus be compensated for by a greater glucagon response and the decreased glucose utilization rates of fasting garden warblers may result from insulin resistance and/or delayed glucose absorption.