Neurotensin levels in the hepatic-portal circulation are inversely related to the circadian feeding cycle in rats.

To investigate whether the circulating level of neurotensin (NT) in the rat is related to either the 24-h pattern in food consumption or environmental lighting conditions, the plasma level of NT was determined every 4 h in the hepatic-portal vein and the abdominal aorta over the course of 24 h. At each time interval, pooled plasma samples from groups of 4 rats were extracted, lyophilized, reconstituted, and subjected to HPLC. Column fractions were radioimmunoassayed with both N- and C-terminal directed antisera. Animals housed in a 12-h light, 12-h dark cycle and given food and water ad libitum had a significant (P less than 0.05) 24-h variation in the level of chromatographically and immunochemically identified NT in the portal circulation while the level of NT in the systemic circulation remained unchanged. The level of NT in portal blood ranged from 12-38 fmol/ml and was highest in the afternoon, 12-16 h after peak feeding. The level of NT in aortic blood never exceeded 7 fmol/ml. Similar results were obtained from animals exposed to constant illumination for 13-32 h with free access to food and water. The release of NT during the fasting phase of the feeding cycle was dependent upon the prior intake of food, since the level of NT in the hepatic-portal circulation of rats housed in 12-h light, 12-h dark cycle and fasted for 20-24 h was about 2-fold less than that observed in animals allowed free access to food. In summary, these data show that the release and circulation of NT are tightly linked to the circadian pattern of food intake and that the greatest release of NT into the hepatic-portal circulation occurs 5-10 h after the cessation of eating during the fasting phase of the feeding cycle.