CIRCADIAN-RHYTHM OF EXOCRINE PANCREATIC-SECRETION IN RATS - MAJOR ANDMINOR CYCLES

The circadian variations of exocrine pancreatic secretion were studied in conscious rats provided with pancreatic, biliary, duodenal, and pe ritoneal cannulas and kept in restraint cages under controlled conditi ons, with a regular 12-h light cycle. Rats were divided into fed and f asted groups, and experiments were performed separately. During a 4-da y postsurgical recovery period, rats were fed ad libitum. During the e xperiment, fed rats had free access to food and water. Food, but not w ater, was denied fasted rats 10 h before the experiment and for its 48 -h duration. During the experiment, pancreatic juice was continuously collected for 4 and 2 days from fed and fasted rats, respectively. Eve ry 30 min, a 20-mul aliquot of sampled pancreatic juice was removed fo r total protein, amylase, and chymotrypsinogen assays. The remainder w as mixed with bile collected simultaneously, and the mixture was recir culated into the duodenum. Over the 4- and 2-day periods there was a c lear circadian rhythm of 24-h duration; for all measured parameters, s ecretory rates increased in the dark period and decreased during the l ight period. This major circadian rhythm was unexpectedly found to be superimposed on by a remarkably constant neurosecretory-like minor cyc le of 2-h duration present in both fed and fasted states. The amplitud e of the minor cycle was diminished by fasting. The outputs of fluid, total protein, and amylase were found to be only modestly correlated w ith each other, whereas chymotrypsinogen output was virtually complete ly independent of the others. The results suggest that the spontaneous major increase of exocrine pancreatic secretion in the dark was at le ast partially independent of food intake. The cyclic pattern of the ci rcadian rhythm of exocrine pancreatic secretion in rats seems to refle ct better the actual rhythm of the secretory processes in the whole gl and than the monotonous pattern until now reported; the independence o f the fluid and individual enzyme outputs reveals the exocrine pancrea s to be a far more flexible and regulated organ than heretofore though t.