No abstract

Three experiments were conducted to determine 1) the relationship between prolactin and growth hormone (GH) secretion in mares and the response 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 acute exercise. In Exp. 1, 10-min blood samples were collected from 12 mares for 8 h, and GHRH (0, 45, 90, or 180 micrograms) was administered at 6 h. In Exp. 2, 15-min blood samples were collected for 4 h from 10 mares, stallions, and geldings. In Exp. 3, eight horses of each sexual status were administered GHRH at 0900; later that day, each horse was exercised for 5 min. Blood samples were collected every 10 min around each 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 increased secretion during that time; there was no correlation between these episodes and those in GH secretion. Prolactin concentrations were not affected (P > .1) by GHRH. In Exp. 2, average concentrations of GH were 2.4, 8.6, and 8.5 ng/mL for mares, stallions, and geldings, respectively; males differed from females (P < .05). Stallions and geldings had more (P < .05) peaks in GH concentrations and greater (P < .05) amplitude of peaks than mares. In contrast, prolactin concentrations were 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 mares or geldings.(ABSTRACT TRUNCATED AT 250 WORDS)

Thirty mares in late gestation were used in a 3-yr study to assess effects of the tall fescue endophyte Acremonium coenophialum on serum prolactin (PRL) and progesterone. Two paddocks of each treatment, 0 or 100% infected 'Kentucky 31' tall fescue, were grazed by the mares for 21 d. Blood was collected three times per week until parturition. At 7-d intervals, mares were challenged with thyrotropin-releasing hormone (TRH) while grazing and blood was collected postinjection. Mares grazing 100% infected tall fescue (E+) had decreased serum PRL compared with mares grazing the 0% infected tall fescue (E-) in 2 of 3 yr. Within 8 d postgrazing, serum PRL for E+ mares equaled or surpassed values of the E- mares. Serum PRL was not different during the 3rd yr. In response to TRH, serum PRL rate of increase was similar between treatments but remained elevated (P less than .01) in the E+ mares at the 180-, 240-, and 300-min sample times. Serum progesterone was lowered (P less than .05) by E+ but increased to control values within 10 d postgrazing. It is concluded that serum PRL and progesterone in the gravid mare were decreased by the presence of A. coenophialum in 'Kentucky 31' tall fescue grass but normal levels were reestablished within 2 to 3 wk.

Prepubertal Quarter horse colts were immunized at 6 mo of age with either estrone-17-oxime-bovine serum albumin (n = 4; treated) or with albumin only (n = 5; controls). All colts received booster injections of the appropriate antigen at 8, 10, 12, 16 and 20 mo of age. Blood samples were drawn every 20 d from 6 to 26 mo of age; body weights were determined monthly. Immunization against estrone-albumin resulted in increased binding of [3H]-estradiol in serum within 40 d that was maintained through 24 mo of age. Antisera from treated colts crossreacted equally well with estrone and estradiol and moderately with other estrogens; androgens, progesterone and glucocorticoids all cross-reacted less than .005%. Body weights were not affected by treatment. Concentrations of testosterone were generally higher (P less than .05) in estrogen-immunized colts compared with controls after immunization. Concentrations of luteinizing hormone were not affected by treatment, whereas concentrations of follicle-stimulating hormone were initially increased (P less than .05) in treated colts after immunization. At castration at 27 mo of age, estrogen-immunized colts had greater (P less than .05) testicular and parenchymal weights and produced more (P approximately equal to .055) spermatozoa per horse than did control colts. Seminal characteristics immediately before castration were not affected by treatment. It appears that estrogens are involved in the regulation of several reproductive traits in the colt. Moreover, active immunization against estrogen in the prepubertal colt may be a useful method of increasing testicular size and sperm production rates in the stallion after puberty.