HEPATIC VAGAL AMINO-ACID SENSORS MODULATE AMINO-ACID INDUCED INSULIN AND GLUCAGON-SECRETION IN THE RAT

To clarify the physiological role of vagal amino acid sensors in the liver, the effect of hepatic vagotomy and/or celiac vagotomy (sectioning of the hepatic branch and/or the celiac branches of the vagus nerve) on the secretion of insulin and glucagon after intraperitoneal injection of neutral (L-alanine, L-leucine, and L-Phenylalanine), acidic (L-glutamate), or nonmetabolized (cycloleucine) acids, was examined in rats. Hepatic vagotomy enhanced both plasma glucose and glucagon concentrations after intraperitoneal injecton of alanine more than those in sham-vagotomized (control) rats, while after intraperitoneal injection of leucine, hepatic vagotomy decreased plasma glucose concentrations and enhanced plasma insulin concentrations more than in control animals. These effects, following both alanine and leucine administration, were blocked by celiac vagotomy. Glutamate, phenylalanine, and cycloleucine stimulation in hepatic-vagotomized rats caused no significant differences in plasma glucose, insulin, or glucagon levels as compared to those in sham-vagotomized rats. Celiac vagotomy alone did not affect plasma glucose, insulin, or glucagon concentrations after stimulation by these five amino acids. The physiological role of alanine and leucine sensors may be to prevent amino acid-induced exaggerated pancreatic hormone secretion and to maintain blood glucose homeostasis, while glutamate, phenylalanine, and cycloleucine have no effect on this pancreatic neuroendocrine system.