Proline metabolism in enterocytes of neonatal pigs

Abstract

Sow's milk is remarkably deficient in arginine (an raphics. essential amino acid for neonates) but is rich in proline and glutamine plus glutamate. In neonatal pigs, enterocytes play an important role in synthesizing citrulline and arginine from dietary proline via proline oxidate. However, little is known about the regulation of this metabolic pathway in animals. On the basis of the previous studies with hepatic proline oxidate in rats, the author hypothesized that lactate, codicil and adrenocorticotrophic hormone (ACTH) may be important physiological regulators of intestinal proline catabolism in piglets. To test this hypothesis, two experimental series were conducted. In Experiment 1, jejunal enterocytes were prepared from l4-day-old pigs, and incubated at 370C in the sequence of 2 my Iu-l4clproline, 2 AM glutamine 'and 0.0-10.0 my lactate. Amino acids were quantified by high-performance liquid chromatography. Mitochondria proline oxidate activity was determined in the presence of 0.5-20 my proline and 0.0-15 my lactate. A double reciprocal plot of 1/IS) vs IN revealed noncompetitive inhibition of proline oxidizes by lactate. As a result, lactate inhibited the intestinal synthesis of citrulline and arginine from proline in a concentration-dependent manner. This finding helps elucidate the mechanism responsible for hypocitrullinemia and hypoargininemia in neonates with hyperladacidemia. In Experiment II, 7-day-old nursing pigs received daily intramuscular injections of hydrocortisone 21-acetate (25 mg/kg body weight), ACTH (1.0 mg/kg body weight), or vehicle solution (saline, 1 ml) for 7 days. At l4-days of age, jugular venous blood was withdrawn, and intestinal segments were obtained for metabolic studies and histologists examination. Cortisol and ACTH administration markedly increased plasma cortical concentrations 7.4-and 2.2-fold, respectively. Plasma ACTH concentrations increased in AcTH-treated piglets 3.l-fold but decreased in cortisol-treated piglets by 72%. Cortisol stimulated intestinal growth, proline transport, and synthesis of citrulline and arginine from proline, but ACTH had opposite embeds. ACTH decreased Vmax values of intestinal proline oxidate but had no effect on its Km values for proline, suggesting a decrease in enzyme concentrations. Cortisol or ACTH increased treatment plasma concentrations of branched-chain amino acids, lysine and ornithine, and ACTH also increased plasma concentrations of histidine and other neutral amino acids including proline. These results suggest the differential regulation of intestinal proline catabolism and whole body amino acid metabolism by cortical and ACTH in neonatal pigs. Cortisol, but not ACTH, may be beneficial in preventing the normal decline in the intestinal synthesis of citrulline and arginine in suckling pigs compared with newborn pigs, and in advancing intestinal maturation for successful early weaning of piglets.