Stability of circadian and minor cycles of exocrine pancreatic secretion in atropine- and MK-329-infused rats
Maouyo, D.; Guan, D.; Rivard, N.; Adelson, J.W.; Morisset, J.
American Journal of Physiology 268(2 Pt 1): G251-G259
1995
ISSN/ISBN: 0002-9513 PMID: 7864122 Document Number: 454672
We previously demonstrated the existence of a circadian rhythm of pancreatic secretion of fluid and digestive enzymes, which was superimposed on by a regular 1.84-h minor cycle of exocrine pancreatic secretion of fluid and total protein, amylase, and chymotrypsinogen (25). Direct control of these daily and hourly rhythms of pancreatic secretory function has not been addressed. Cholinergic and cholecystokinin (CCK)-associated influences on these two rhythms of exocrine pancreatic secretion were investigated in rats provided with pancreatic, biliary, duodenal, and jugular vein cannulas, allowing separate drainage of bile and pure pancreatic juice, as well as intravenous infusions of atropine sulfate and/or MK-329. Rats were kept in restraint cages under controlled temperature and humidity, with a regular 12-h light cycle, and divided into five groups. The first group of fed rats was constantly infused with 200 mu-g cntdot kg-1 cntdot h-1 atropine, the second with 0.5 mg cntdot kg-1 cntdot h-1 MK-329, and the third with both. In the group where both drugs were simultaneously infused, 500 mu-g cntdot kg-1 cntdot h-1 atropine was intraperitoneally administered, whereas MK-329 was infused by intravenous cannula. Two groups consisted of fasted rats, of which one was also given atropine (100 mu-g cntdot kg-1 cntdot h-1). Three-day experiments were performed separately with fed rats, and 2-day experiments were performed with fasted rats; atropine and/or MK-329 infusion, starting on day 2, was constant over 48 h in both fed and fasted rats. Atropine decreased fluid, total protein, and amylase outputs, whereas chymotrypsinogen output increased; in MK-329-infused rats, significant decreases were observed for all secretory parameters. When both antagonists were simultaneously infused in fed rats, total mean outputs for all secretory parameters decreased after the MK-329-induced tendency. In fasted rats, atropine infusion decreased fluid, total protein, and amylase mean outputs, but chymotrypsinogen output significantly increased. Even though total mean outputs significantly changed under drug infusion, pancreatic secretion rose in the dark period and fell in the light period in fasted as well as in ad libitum-fed rats. Atropine and MK-329 did not significantly alter the overall pattern of minor cycles of pancreatic secretion even though total mean outputs markedly changed. The stability of the rhythm of minor-cycle peak intervals in antagonist-infused rats suggests that the 1.8-h cycle of pancreatic secretion is independent of both cholinergic and CCK-related mechanisms, even though these antagonists have clear effects on overall secretory outputs.