Regulation of Cl- secretion by 2-adrenergic receptors in mouse colonic epithelium

Abstract

Previous studies have shown that α(2) adrenoceptor (α(2)AR) agonists inhibit electrolyte secretion in colonic epithelia, but little is known about the molecular mechanisms involved in this process. In this study we examined the effect of α(2)AR activation on transepithelial anion secretion across isolated murine colonic epithelium. We found that α(2)AR agonists, UK 14,304, clonidine and medetomidine were potent inhibitors of anion secretion, especially in the proximal colon. Short circuit current measurements (I(sc)) in colonic epithelia from normal and cystic fibrosis (CF) mice showed that α(2)AR agonists inhibited basal cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl(−) secretion but had no effect on CFTR activation by cAMP-dependent phosphorylation. Apical administration of an ionophore, nystatin (90 μg ml(−1)), was used to investigate the effect of UK 14,304 on basolateral K(+) transport. The Na(+)–K(+)-ATPase current, measured as ouabain-sensitive current in the absence of ion gradients, was unaltered by pretreatment of the tissue with UK 14,304 (1 μm). In the presence of a basolaterally directed K(+) gradient, UK 14,304 significantly reduced nystatin-activated I(sc) indicating that activation of α(2)ARs inhibits basolateral K(+) channels. Studies with selective K(+) channel inhibitors and openers showed that α(2)AR agonists inhibited K(ATP) channels that were tonically active in mouse colonic epithelia. RT-PCR and pharmacological studies suggested that these channels could be similar to vascular smooth muscle K(ATP) channels comprising Kir6.1/SUR2B or Kir6.2/SUR2B subunits. Inhibition of anion secretion by α(2)AR agonists required activation of pertussis toxin-sensitive G(i/o) proteins, but did not involve classical second messengers, such as cAMP or Ca(2+). In summary, α(2)ARs inhibit anion secretion in colonic epithelia by acting on basolateral K(ATP) channels, through a process that does not involve classical second messengers.