Effects of fasting and adrenalectomy on the kinetics of glucose metabolism in granivorous and carnivorous birds.

1. [1-(14)C]-Glucose was used to estimate the rate of glucose replacement and of hexose recycling via 3-carbon compounds in fed, fasted and fasted-adrenalectomized granivorous (chickens) and carnivorous (black vultures) birds. 2. Fed chickens had a larger body glucose mass and 3 to 4-fold higher rates of glucose replacement than fed black vultures. Glucose recycling represents a larger fraction of the overall glucose replacement rate in chickens (60%) than in black vultures (25%). 3. Following a 72-h fast, the body glucose mass of chickens decreased markedly, without appreciable changes in fractional turnover rates but glucose replacement rates were reduced to 48% of fed values. In contrast, these parameters were much less affected by fasting in carnivorous birds, the largest difference being the rate of replacement which was 70% of the fed values. 4. In both species, fasting after adrenalectomy induced a marked reduction of body glucose mass. However, while fractional turnover rates increased in black vultures, they actually decreased in chickens. Consequently, glucose replacement rates were markedly reduced in fasted-adrenalectomized chickens, but did not change or increase in black vultures. Adrenalectomized chickens showed decreased tolerance to intravenous glucose loads. 5. The results indicate that: a) the maintenance of constant blood sugar levels during prolonged fasting in carnivorous birds is due solely to their high gluconeogenic capacity and b) in carnivorous, but not granivorous birds, increased rates of peripheral glucose utilization plus diminished gluconeogenesis contribute to reduce blood sugar levels after adrenalectomy.