Effect of physical activity and fasting on gut and liver proteolysis in the dog

The aim of this study was to determine how gut and liver protein kinetics adapt to acute exercise in the 18-h-fasted dog (n = 7) and in dogs glycogen depleted by a 42-h fast (n = 8). For this purpose, sampling (artery and portal and hepatic veins) and infusion (vena cava) catheters and Doppler flow probes (portal vein and hepatic artery) were implanted with animals under general anesthesia. At least 16 days later, an experiment, consisting of a 120-min equilibration period, a 30-min basal sampling period, and a 150-min exercise period, was performed. At the start of the equilibration period, a constant rate infusion of [1-13C]leucine was initiated. Gut and liver leucine appearance and disappearance rates were calculated in these studies by combining a novel stable isotopic method and arteriovenous difference methods. In the determination of tissue leucine kinetics the tissue inflow of both alpha-[13C]ketoisocaproic acid and [13C]leucine was taken into account. The results of this study show that 1) the splanchnic bed (liver plus gut) contributes approximately 40% to the whole body proteolytic rate in the basal state and during exercise in dogs fasted for either 18 or 42 h, 2) the contributions of the gut and liver to splanchnic bed proteolysis is about equal in the basal state in both 18- and 42-h-fasted dogs, and 3) exercise in the 18-h-fasted dog leads to a greater emphasis on gut proteolysis and a lesser emphasis on hepatic proteolysis. These studies highlight the important contribution of gut and hepatic proteolysis to whole body proteolysis and the ability of the gut to acutely adapt to changes in physical activity.