GROWTH-HORMONE AND PROLACTIN CONCENTRATIONS IN PLASMA OF HORSES - SEX-DIFFERENCES AND THE EFFECTS OF ACUTE EXERCISE AND ADMINISTRATION OF GROWTH HORMONE-RELEASING HORMONE

Three experiments were conducted to determine 1) the relationship betw een prolactin and growth hormone (GH) secretion in mares and the respo nse to GH-releasing hormone (GHRH), 2) whether plasma GH and prolactin concentrations differed among mares, stallions, and geldings, and 3) whether sexual differences existed after administration of GHRH and ac ute exercise. In Exp. 1, 10-min blood samples were collected from 12 m ares for 8 h, and GHRH (0, 45, 90, or 180 mu g) was administered at 6 h. In Exp. 2, 15-min blood samples were collected for 4 h from 10 mare s, stallions, and geldings. In Exp. 3, eight horses of each sexual sta tus were administered GHRH at 0900; later that day, each horse was exe rcised for 5 min. Blood samples were collected every 10 min around eac h event. In Exp. 1, prolactin concentrations decreased (P < .01) over the 8-h period, and there was an average of 2.9 +/- .5 episodes of inc reased secretion during that time; there was no correlation between th ese episodes and those in GH secretion. Prolactin concentrations were not affected (P > .1) by GHRH. In Exp. 2, average concentrations of GK were 2.4, 8.6, and 8.5 ng/mL for mares, stallions, and geldings; resp ectively; males differed from females (P < .05). Stallions and gelding s had more (P < .05) peaks in GH concentrations and greater (P < .05) amplitude of peaks than mares. In contrast, prolactin concentrations w ere greater (P < .02) in mares and stallions than in geldings. In Exp. 3, GH response to GHRH was greater (P < .03) in stallions than in mar es or geldings. Geldings had greater (P < .01) GH concentrations than mares and stallions 10 and 20 min after onset of exercise. The prolact in responses to exercise were similar among groups.