Submitted on November 3, 2006
Accepted on June 20, 2007
Food induced expression of orexin-receptors in rat duodenal mucosa regulates the bicarbonate secretory response to orexin-A
Magnus W Bengtsson1, Kari Makela2, Markus Sjoblom1, Sanna Uotila2, Karl E.O. Akerman3, Karl-Heinz Herzig4, and Gunnar Flemstrom1*
1 Department of Neuroscience, Uppsala University, Uppsala, Sweden
2 A.I. Virtanen Institute for Molecular Sciences, University of Kuopio, Kuopio, Finland
3 Department of Neuroscience, Uppsala University, Uppsala, Sweden; A.I. Virtanen Institute for Molecular Sciences, University of Kuopio, Kuopio, Finland
4 A.I. Virtanen Institute for Molecular Sciences, University of Kuopio, Kuopio, Finland; Department of Internal Medicine, Kuopio University Hospital, Kuopio, Finland; Department of Physiology, Oulu University, Oulu, Finland
* To whom correspondence should be addressed. E-mail: gunnar.flemstrom{at}neuro.uu.se.
Presence of appetite regulating peptides orexin-A and orexin-B in mucosal endocrine cells suggests a role in physiological control of the intestine. Our aim was to characterize orexin-induced stimulation of duodenal bicarbonate secretion and modulation of secretory responses and mucosal orexin receptors by overnight food deprivation. Lewis x Dark Agouti rats were anaesthetized and proximal duodenum cannulated in situ. Mucosal bicarbonate secretion (pH stat) and mean arterial blood pressure were continuously recorded. Orexin-A was administered intra-arterially close to the duodenum, intraluminally or into the brain ventricles. Total RNA was extracted from mucosal specimens, reverse transcripted to cDNA and expression of orexin receptors 1 and 2 (OX1 and OX2) measured by quantitative real-time PCR. OX1 protein was measured by Western blot. Intra-arterial orexin-A (0.24-0.6 nmol/h,kg) increased (p<0.01) the duodenal secretion in fed but not in fasted animals. The OX1-receptor antagonist SB-334867, which was also found to have a partial agonist action, abolished the orexin-induced secretory response but did not affect secretion induced by the muscarinic agonist bethanechol. Atropine, in contrast, inhibited bethanechol but not orexin-induced secretion. Orexin-A infused into the brain ventricles (2-20 nmol/kg,h) or added to luminal perfusate (1.0-100 nM) did not affect secretion, indicating that orexin-A acts peripherally and at basolateral receptors. Overnight fasting decreased mucosal OX1 and OX2 mRNA expression (p<0.01) as well as OX1 protein expression (p<0.05). We conclude that stimulation of secretion by orexin-A may involve both receptor types and is independent of cholinergic pathways. Intestinal OX-receptors and secretory responses are markedly related to food intake.
