Extrinsic neural innervation mediates absorption of water and electrolytes in canine proximal colon in vivo

Acute denervation alters the epithelial response to adrenoceptor activation through an increase in alpha1-adrenoceptor expression on villus enterocytes

Loss of sympathetic input due to intestinal denervation results in hypersensitivity and increased intestinal secretion. It is unknown whether denervation-induced alterations in intestinal epithelial physiology are the result of changes in adrenoceptors on enterocytes (ENTs). The purpose of this study was to examine adrenoceptor distribution and pharmacology on small intestinal ENTs following acute intestinal denervation. Lewis rats underwent small bowel transplantation (SBT) or sham operation and proximal small intestinal segments were harvested 1, 2 and 4 weeks postoperatively. Intestinal electrolyte movement was assessed using short-circuit current (Isc) measurements of stripped epithelial sheets following stimulation with phenylephrine (PE), an alpha(1)-adrenoceptor agonist. The presence of adrenoceptor subtypes on separated villus and crypt ENTs was assessed using flow cytometry. Alpha(1)-adrenoceptors were found on approximately 27% of jejunal villus ENTs, but not crypt ENTs, following acute extrinsic denervation. ENTs from the Lewis rat have few beta-adrenoceptors. Alpha(1)-adrenoceptor stimulation of acutely denervated intestinal epithelial sheets decreased Isc by -13.45%. This effect was mediated by a reduction in chloride (Cl(-)) secretion; the absence of Cl(-) reversed the Isc to +13.79%. In conclusion, loss of sympathetic innervation to the gastrointestinal epithelium causes acute upregulation of alpha(1)-adrenoceptors on villus ENTs, leading to inhibition of Cl(-) secretion at the villus tip. The increase in adrenoceptors may reflect a compensatory mechanism to combat the increased secretory state of the bowel due to the loss of the sympathetic innervation and tonic control over intestinal secretion.

Peripheral opioidergic regulation of the tracheobronchial mucociliary transport system

We hypothesized that, in the airway mucosa, opioids are inhibitory neural modulators that cause an increase in net water absorption in the airway mucosa (as in the gut). Changes in bidirectional water fluxes across ovine tracheal mucosa in response to basolateral application of the opioid peptides beta-endorphin, dynorphin A-(1-8), and [d-Ala(2), d-Leu(5)]-enkephalin (DADLE) were measured. beta-Endorphin and dynorphin A-(1-8) decreased luminal-to-basolateral water fluxes, and dynorphin A-(1-8) and DADLE increased basolateral-to-luminal water flux. These responses were electroneutral. In seven beagle dogs, administration of aerosolized beta-endorphin (1 mg) to the tracheobronchial airways decreased the clearance of radiotagged particles from the bronchi in 1 h from 34.7 to 22.0% (P < 0.001). Naloxone abrogated the beta-endorphin-induced changes in vitro and in vivo. Contrary to our hypothesis, the opioid-induced changes in water fluxes would all lead to a predictable increase in airway surface fluid. The beta-endorphin-induced increases in airway fluid together with reduced bronchial mucociliary clearance may produce procongestive responses when opioids are administered as antitussives.

Postprandial absorptive augmentation of water and electrolytes in the colon requires intraluminal glucose

Postprandial absorptive augmentation of water and electrolytes occurs in the small intestine and colon. The role of intraluminal nutrients in this response is poorly understood. Our aim was to determine whether postprandial absorptive augmentation of water and electrolytes in the colon requires the presence of intraluminal glucose. Four adult dogs underwent enteric isolation of a 50 cm segment of proximal colon. An ileal-like electrolyte solution (Na(+), 130 mEq/L; K(+), 10 mEq/L; Cl(-), 115 mEq/L; and HCO(3)(-), 25 mEq/L), alone or containing glucose (10 mmol/L), was infused at 4 ml/min into the colonic segment. Experiments were performed during fasting and postprandially after a 400 Kcal mixed-nutrient meal. Effluent was collected in 60-minute intervals after steady state was achieved. Net absorptive flux of water was increased in the presence of intraluminal glucose during the fasted state (11 +/- 0.8 vs 7.4 +/- 0.9 microl/min/cm, P < 0.01). The net absorptive flux of water and electrolytes increased postprandially only in the presence of intraluminal glucose (P < 0.05). Our finding that glucose augments both baseline and postprandial absorption of water and electrolytes in the proximal colon suggests that luminal factors have a role in postprandial absorptive augmentation. Whether this is specific to glucose or occurs with other nutrients remains to be determined.