Metabolism in sea raven (Hemitripterus americanus) hepatocytes: the effects of insulin and glucagon.

The metabolism of the sea raven, Hemitripterus americanus, hepatocyte preparation was studied, emphasizing the roles of insulin and glucagon on carbohydrate status. Sea raven hepatocyte glycogen was depleted throughout the preincubation and 2-hr incubation period in the presence of either glucose or serine. Bovine glucagon stimulated glycogen loss and increased glucose levels and serine flux to glucose. Porcine insulin prevented glycogen depletion at least over 1.5 hr of incubation, but did not affect glucose levels in the hepatocytes. It also significantly increased serine flux to glucose, glycogen, and protein, and alanine flux to glucose, CO2, and protein. Teleost insulin did not alter the pattern of hepatic glycogen depletion, while it did increase glucose levels and serine flux to glucose, glycogen, and lipids, and alanine flux to CO2 and glucose. Both glucagon and porcine insulin increased glucose flux to glycogen, but neither altered glucose conversion to CO2, lactate, or protein. The teleost insulin had no effect on glucose conversion to any product tested. Teleost insulin had an additive effect on the glucagon-induced increases in total glucose production and gluconeogenesis from serine, while glucagon offset the insulin stimulation of serine flux to glycogen and CO2. The results demonstrate that glucagon functions to increase glucose production from gluconeogenic precursors and glycogen in sea raven hepatocytes, while insulin demonstrates anabolic effects through gluconeogenic precursors. It is suggested that insulin functions in sea raven hepatocytes to increase glycogen stores through increased amino acid utilization and/or to increase glucose production for transport to, and storage in, glucose-utilizing tissues (e.g., muscle). An antagonism between insulin and glucagon on the glycolytic/gluconeogenic pathways as is found in mammalian livers is not as clear in sea raven hepatocytes. These findings are consistent with the carnivorous diet of the sea raven and a preferentially gluconeogenic role for the liver of this species.