Pituitary hormone and insulin responses to infusion of amino acids and N-methyl-D,L-aspartate in horses

Thirty-nine adult light horse mares, geldings, and stallions were used in t wo experiments to assess the pituitary hormone and insulin responses to inf usions of arginine, aspartic acid, lysine, glutamic acid, and N-methyl-D,L- aspartate (NMA). In Exp. 1, 27 horses were assigned to one of three infusio n treatments: 1) physiological saline (1 L); 2) 2.855 mmol of arginine/kg B W in 1 L of water; or 3) 2.855 mmol of aspartic acid/kg BW in 1 L of water. In Exp. 2, 12 horses were assigned, in a multiple-square 4 x 4 Latin squar e design, to one of four infusion treatments: 1) 2 mt of saline/kg BW; 2) 2 .855 mmol of lysine/kg BW in water; 3) 2.855 mmol of glutamic acid/kg BW in water; or 4) 1 mg of NMA/kg BW in water. In Exp. 1, an acute (within 20 mi n) release of growth hormone (GH) was induced (P = 0.002) by aspartic acid. In contrast, acute release of prolactin (P = 0.001) and insulin (P = 0.002 ) was induced only by arginine; moreover, the arginine effect on insulin wa s present only in mares (P = 0.011). In Exp. 2, an acute release of GH was induced (P = 0.001) by glutamic acid and NMA. In males, the glutamic acid-i nduced GH release was greater than that of NMA; in mares, the NMA-induced G H release was greater than that of glutamic acid (P = 0.069). Both lysine a nd glutamic acid induced (P = 0.001) acute release of prolactin, whereas an acute release of insulin was elicited (P = 0.002) only by lysine. The NMA- induced LH response was due almost entirely to the response in mares and st allions (P = 0.016), and the NMA-induced FSH release was due almost entirel y to the response in mares (reproductive status effect; P = 0.004). In the horse, aspartic acid, glutamic acid, and NMA seem to stimulate GH release; arginine and lysine seem to stimulate prolactin and insulin release; and NM A seems to stimulate LH and FSH release. It seems that N-methyl-D-aspartate glutamate receptors are involved in controlling GH, LH, and FSH secretion, whereas other mechanisms are involved with prolactin secretion. These resu lts also indicate that gonadal steroids interact with amino acid-induced pi tuitary hormone release in adult horses.