Twelve light horse mares were fed a control diet that provided 100% of their maintenance protein and energy requirements for 7 d and were then either continued on the control diet or totally deprived of feed (with access to water) for 3 d . Plasma samples were drawn twice daily throughout the experiment, at 15-min intervals for 9 h beginning 45 h after feed removal, and at 10-min intervals around an exercise bout beginning 73 h after feed removal. Feed deprivation increased (P < or = .06) whole blood beta-hydroxybutyrate and plasma NEFA, urea N, L-lactate, and glucagon concentrations, decreased (P = .02) IGF-I concentrations, and did not change (P > .1) plasma glucose insulin, prolactin, triiodothyronine, and thyroxine concentrations. Exercise increased (P < .05) plasma NEFA, prolactin, and growth hormone (GH) concentrations in all mares. Plasma NEFA concentrations increased (P < .001) after exercise and remained increased in fed mares, but rapidly decreased in deprived mares (time x diet interaction, P = .006). Plasma glucose concentrations following exercise increased in deprived mares but decreased in fed mares (time x diet interaction, P = .07). The plasma prolactin response after exercise also differed between groups (P = .09). Feed-deprived mares had greater (P = .02) plasma GH concentrations before exercise (73 h after feed withdrawal) and had a greater (P < .001) GH peak at 10 min after initiation of exercise. The increase in secretion rate o GH due to feed deprivation in these mares was similar to that reported for other domestic species but was not nearly as great in magnitude.

To test the hypothesis that prolactin mediates the increase in seminal volumes induced by sexual stimulation in stallions, semen was collected from six stallions every other day for 26 d. The last eight collection days were treatment days. For each stallion, four treatments were randomly assigned to the first four of the eight treatment collection days, and then repeated in reverse order on the last four collection days; 1) CONTROL: semen collected per normal procedures; 2) Sexually stimulated: stallions were presented to mares in a chute for 10 min before collection; 3) Bromocriptine (dopamine agonist) plus sexual stimulation: stallions were administered bromocriptine 10 min before 10 min of sexual stimulation prior to collection; and 4) Sulpiride: stallions were administered sulpiride (a dopamine antagonist) 25 min before collection. Prolactin concentrations in plasma were increased by sexual stimulation (P < .01) and sulpiride (P < .001) administration and were decreased (P < .01) when bromocriptine was administered before sexual stimulation. Sexual stimulation alone increased (P < .01) volume of gel-free semen relative to control values (102 vs 81 mL), and bromocriptine prevented this response (89 mL; P < .075 relative to sexual stimulation). Sulpiride had no effect (P > .1) on gel-free volume. Volume of gelatinous material, number of mounts, sperm concentration, motility, pH of gel-free semen, number of spermatozoa per ejaculate, and prolactin concentration in gel-free semen were not affected (P > .1) by treatment. Although the effect of bromocriptine indicated that prolactin may mediate the rise in seminal volume due to sexual stimulation, if it does, it must be in conjunction with other hormone(s) or factor(s), because increasing prolactin concentrations alone did not increase seminal volume in these stallions.

Two experiments were conducted to determine 1) the prolactin response to different kinds of feedstuffs in stallions and 2) the effects of total feed deprivation on prolactin secretion in mares and its interaction with the prolactin response to feeding. Experiment 1 was performed with stallions as a 6 x 6 Latin square: A) no feed; B) pelleted feed fed to meet 82.5% of the horses' CP requirements; C) pelleted feed at 25% of the amount in B; D) pelleted feed as in B plus water ad libitum; E) cracked corn at the weight in B; and F) chopped alfalfa at the weight in B. The positive prolactin responses (P < .05) to feeding were similar for treatments B through F. The insulin response to feeding was highest (P < .05) in stallions fed water with the pelleted feed. In Exp. 2, 72 h of feed deprivation did not affect (P > .1) daily prolactin secretion. Feeding of a meal on the 3rd d of deprivation increased (P < .05) plasma prolactin, insulin, and glucose concentrations similarly in all mares. There was a positive growth hormone response (P < .1) after feeding in feed-deprived mares but not in fed mares. The prolactin response (P < .001) to thyrotropin-releasing hormone was greater (P = .083) for feed-deprived mares than for controls, whereas the response to sulpiride (P < .001) only tended to differ (P = .16) between groups. We conclude that prolactin secretion may be stimulated by aspects of eating other than the feedstuff itself. Total feed deprivation had little effect on the subsequent prolactin response to a meal or to other known secretagogues.

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