Effect of feeding and feed deprivation on plasma concentrations of prolactin, insulin, growth hormone, and metabolites in horses.

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.