THE EFFECT OF FEED RESTRICTION AND OF A BETA-ADRENERGIC AGONIST ON PLASMA MELATONIN CONCENTRATIONS IN DOMESTIC PIGS

The objective of the present study was to determine the effect of feed restriction and a beta-adrenergic agonist (isoproterenol) on plasma melatonin concentrations in domestic pigs. At the vernal equinox, eight prepubertal Yorkshire X Landrace gilts (68.9 +/- 1.3 kg) were transferred to a barn with natural lighting. Supplementary lighting (minimum of 150 1x) was provided from 07:00 to 16:00 daily. During an 18-d adaptation period, gilts were fed to appetite twice daily on a ration containing 16% crude protein. The feed intake of four gilts was then reduced to 30% of the to-appetite level for the next 7 d. Blood samples were taken from all gilts hourly for 8 h from 21:00 (dark) and again from 09:00 (light) for melatonin analysis. Gilts were then given an i.v. infusion of 0.5 mg kg(-1) isoproterenol at 20:00 and 21:00. Blood samples were taken hourly from 21:00 to 24:00 and again at 10:00 and 11:00 the following morning. Plasma melatonin concentrations were higher during the night than during the day in both feed-restricted gilts (P = 0.06) and gilts fed to appetite (P = 0.18). There was no significant effect of feed intake on plasma melatonin during the night (44 +/- 2 vs. 38 +/- 3 pg mL(-1) for restricted and ad libitum gilts, respectively, P = 0.6), but daytime concentrations of the hormone were higher in feed-restricted gilts than in their counterparts fed to appetite (29 +/- 2 vs. 22 +/- 1 pg mL(-1), P = 0.03). Melatonin concentrations tended to be higher at night following isoproterenol treatment in feed-restricted gilts than in the samples taken 24 h earlier (72 +/- 12 vs. 47 +/- 6 pg mL(-1), P = 0.12) but did not differ in gilts fed to appetite (50 +/- 14 vs. 40 +/- 12 pg mL(-1), P = 0.60). These data indicate that plasma melatonin concentrations in the domestic pig may be influenced by nutrition and that the physiological systems governing melatonin secretion in the pig do not differ fundamentally from those of other species.