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 m elatonin concentrations in domestic pigs. At the vernal equinox, eight prepubertal Yorkshire X Landrace gilts (68.9 +/- 1.3 kg) were transfe rred 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 adap tation period, gilts were fed to appetite twice daily on a ration cont aining 16% crude protein. The feed intake of four gilts was then reduc ed to 30% of the to-appetite level for the next 7 d. Blood samples wer e 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. i nfusion of 0.5 mg kg(-1) isoproterenol at 20:00 and 21:00. Blood sampl es 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 dur ing 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 ef fect 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 fe ed-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-restri cted 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 s ecretion in the pig do not differ fundamentally from those of other sp ecies.