Circadian variation of portal, arterial and venous blood levels of melatonin in pigs and its relationship to food intake and sleep

Circadian levels of melatonin were determined in the hepatic portal vein, c ranial vena cava, and the lower aorta of ten juvenile pigs. Blood was sampl ed every hour for a total of 24 hr via temporary cannulas introduced into b lood vessels under anesthesia. No peak levels of melatonin were found in th e mid-scotophase, but hepatic portal concentrations peaked at 06.00 hr. Ove rall levels of melatonin were highest in the hepatic portal vein (range 35- 65 pg/mL), followed by an artery (range 30-55 pg/mL) and the vena cava (ran ge 25-35 pg/mL). Levels of melatonin exhibit strong variation between indiv idual pigs, but generally the average levels from all three sources follow each other's time course. However, on occasion, melatonin levels in the hep atic portal vein varied independently from the levels in the vena cava. Lar ge portal peaks were usually preceded by a feeding period and were associat ed with a subsequent period of sleep. The data indicate that: 1) there is n o clear circadian rhythm of melatonin in the peripheral blood of pigs, 2) r elatively little melatonin is metabolized during the first liver passage, 3 ) food intake may elevate melatonin levels in the hepatic portal vein, and 4) increased levels of melatonin originated in the gastrointestinal tract m ay induce sleep.