The effects of sustained exercise on gluconeogenesis, glycogenolysis and glycogen synthesis in Merino sheep

Abstract

The ability of sheep to maintain an elevated rate of glucose metabolism in the splanchnic tissues and skeletal muscle during long-term exercise was studied using isotope dilution and arteriovenous difference techniques. Comparisons were made between Merino ewes at rest or walking at 4.5 km h-1 on a 0' incline (30% VO2max) for 4 h or 9¦ incline (60% VO2max.) for 2 h. Sheep were close to exhaustion at the completion of the later rate of exercise. Exercise elicited an initial hyperglycemia with a return to euglycemia at the end of exercise. The entry rate of glucose increased 2-6 and 3.4 times during exercise at 30 and 60% VO2max respectively. Similar increases in the rate of glucose release by the liver, were seen; the increases were maintained throughout the exercise period. Assuming complete conversion of gluconeogenic substrates extracted by the liver gluconeogenesis could have accounted for 90% and 60% of the glucose release during exercise at 30% and 60% VO2max. Hepatic glycogenolysis was also substantial, and when summed with gluconeogenesis 120-150% of the glucose release was accounted for. The ability of sheep to replace glycogen depleted owing to exercise was significant in liver and type IIA muscle (m. semimembranosis) and represented rates 3-6 times those previously reported in sheep and cattle. Providing sheep do not suffer hyperthermia, they are well equipped to endure long periods of exercise owing to an ability to maintain carbohydrate balance.