Inhibition of cyclic AMP-dependent chloride secretion by PP receptors and alpha 2-adrenoceptors in a human colonic epithelial cell line

Inhibitors of serine/threonine protein phosphatases antagonize the antinociception induced by agonists of alpha 2 adrenoceptors and GABAB but not kappa-opioid receptors in the tail flick test in mice

We previously reported that serine/threonine protein phosphatases (PPs) play a role in the antinociception induced by the mu-opioid receptor agonist morphine. In this study we evaluated the possible involvement of PPs on the antinociception induced by agonists of others G protein-coupled receptors in the tail flick test in mice. The subcutaneous administration of clonidine (0.25-4 mg/kg), baclofen (2-32 mg/kg) or U50,488H (2-16 mg/kg) (agonists of alpha(2) adrenoceptors, GABA(B) and kappa-opioid receptors, respectively) produced dose-dependent antinociception. The antinociceptive effects of clonidine and baclofen were antagonized in a dose-dependent way by the protein phosphatase inhibitors okadaic acid (0.001-10 pg/mouse, i.c.v.) and cantharidin (0.001-10 ng/mouse, i.c.v.), and okadaic acid was 1000 times more potent than cantharidin in producing this effect. The effects of these drugs appear to be specifically due to the blockade of PPs, since L-norokadaone (an analogue of okadaic acid that has no effect on PPs) did not modify clonidine- or baclofen-induced antinociception over the wide range of doses used (0.001-1000 pg/mouse, i.c.v.). On the other hand, the antinociception induced by activation of kappa-opioid receptors with U50,488H was not modified by okadaic acid or cantharidin. In conclusion, our data support the idea that serine/threonine PPs are differentially involved in the antinociceptive effects of several agonists of G protein-coupled receptors in mice.

Control of signalling efficacy by palmitoylation of the rat Y1 receptor

(1) We have investigated the properties of native and haemagglutinin (HA)-tagged neuropeptide Y (NPY) Y(1) receptors after mutation of the palmitoylation site Cys(337) to Ser or Ala. (2) In Chinese hamster ovary cells expressing similar receptor levels, the C337A mutation abolished incorporation of [(3)H]palmitic acid into the HA-Y(1) receptor. (3) Cys(337) substitution did not alter the affinities of Y(1) receptor agonists or antagonists, but it eliminated the ability of guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) to displace [(125)I]PYY-specific binding (compared to approximately 50% inhibition in Y(1) or HA-Y(1) clones). (4) Stimulation of GTPgamma[(35)S] binding by native and HA-Y(1) receptors in standard incubation buffer (100 mM NaCl, 10 micro M GDP) was prevented by Cys(337) mutation. In this assay, the function of Y(1)(C337S) receptors could be partially rescued by reducing the Na(+) concentration, and when overexpressed (B(max): approximately 10 pmol mg(-1)), both HA-Y(1) and HA-Y(1)(C337A) receptors displayed similar responses to NPY and peptide YY (PYY). (5) In stably transfected adenocarcinoma cells expressing Y(1) or Y(1)(C337S) receptors, PYY inhibited anion secretion stimulated by vasoactive intestinal peptide (VIP; measured as short-circuit current, I(SC)) with similar potency (EC(50): 26-53 nM). In contrast to the transient Y(1) receptor-mediated responses observed at maximal PYY concentrations, I(SC) reductions in both Y(1)(C337S) clones were sustained. (6) We conclude that nonpalmitoylation of the Y(1) receptor reduces its coupling efficiency to G proteins, and may also indirectly influence desensitisation processes that depend on the formation of an active agonist-receptor conformation.

Regulation of Cl- secretion by alpha2-adrenergic receptors in mouse colonic epithelium

Previous studies have shown that alpha2 adrenoceptor (alpha2AR) 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 alpha2AR activation on transepithelial anion secretion across isolated murine colonic epithelium. We found that alpha2AR agonists, UK 14,304, clonidine and medetomidine were potent inhibitors of anion secretion, especially in the proximal colon. Short circuit current measurements (Isc) in colonic epithelia from normal and cystic fibrosis (CF) mice showed that alpha2AR 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 microg 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 microM). In the presence of a basolaterally directed K+ gradient, UK 14,304 significantly reduced nystatin-activated Isc indicating that activation of alpha2ARs inhibits basolateral K+ channels. Studies with selective K+ channel inhibitors and openers showed that alpha2AR agonists inhibited KATP 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 KATP channels comprising Kir6.1/SUR2B or Kir6.2/SUR2B subunits. Inhibition of anion secretion by alpha2AR agonists required activation of pertussis toxin-sensitive Gi/o proteins, but did not involve classical second messengers, such as cAMP or Ca2+. In summary, alpha2ARs inhibit anion secretion in colonic epithelia by acting on basolateral KATP channels, through a process that does not involve classical second messengers.

Neuropeptide Y, Y1, Y2 and Y4 receptors mediate Y agonist responses in isolated human colon mucosa

1. The aim of this study was to provide a pharmacological characterization of the Y receptor types responsible for neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) effects upon electrogenic ion transport in isolated human colonic mucosa. 2. Preparations of descending colon were voltage-clamped at 0 mV in Ussing chambers and changes in short-circuit current (I(sc)) continuously recorded. Basolateral PYY, NPY, human PP (hPP), PYY(3 - 36), [Leu(31), Pro(34)]PYY (Pro(34)PYY) and [Leu(31), Pro(34)]-NPY (Pro(34)NPY) all reduced basal I(sc) in untreated colon. Of all the Y agonists tested PYY(3 - 36) responses were most sensitive to tetrodotoxin (TTX) pretreatment, indicating that Y(2)-receptors are located on intrinsic neurones as well as epithelia in this tissue. 3. The EC(50) values for Pro(34)PYY, PYY(3 - 36) and hPP were 9.7 nM (4.0 - 23.5), 11.4 nM (7.6 - 17.0) and 14.5 nM (10.2 - 20.5) and response curves exhibited similar efficacies. The novel Y(5) agonist [Ala(31), Aib(32)]-NPY had no effect at 100 nM. 4. Y(1) receptor antagonists, BIBP3226 and BIBO3304 both increased basal I(sc) levels per se and inhibited subsequent PYY and Pro(34)PYY but not hPP or PYY(3 - 36) responses. The Y(2) antagonist, BIIE0246 also raised basal I(sc) levels and attenuated subsequent PYY(3 - 36) but not Pro(34)PYY or hPP responses. 5. We conclude that Y(1) and Y(2) receptor-mediated inhibitory tone exists in human colon mucosa. PYY and NPY exert their effects via both Y(1) and Y(2) receptors, but the insensitivity of hPP responses to either Y(1) or Y(2) antagonism, or to TTX, indicates that Y(4) receptors are involved and that they are predominantly post-junctional in human colon.

Multiple Y receptors mediate pancreatic polypeptide responses in mouse colon mucosa

A functional study has been performed to characterise the Y receptors responsible for NPY, PYY and PP-stimulated responses in mouse colonic mucosal preparations. Electrogenic ion secretion was stimulated with VIP following which NPY, PYY and PP analogues were, to varying degrees, inhibitory. PYY(3-36), hPP, Gln(23)hPP and rPP were effective but less potent than full length PYY, NPY or their Pro(34)-substituted analogues, while the Y(5) agonist Ala(31), Aib(32)hNPY was the least active peptide tested. The Y(1) antagonists, BIBP3226 and BIBO3304 virtually abolished Pro(34)PYY and PYY responses while PYY(3-36) responses were selectively inhibited by the Y(2) antagonist, BIIE0246. A combination of BIBO3304 and BIIE0246 also partially attenuated hPP responses, leaving residual effects that were most probably Y(4)-mediated. Thus we conclude that Y(1), Y(2) and Y(4) receptors attenuate ion secretion in mouse colon.

Constitutive neuropeptide Y Y(4) receptor expression in human colonic adenocarcinoma cell lines

1. Three human adenocarcinoma cell lines, Colony-24 (Col-24), Col-6 and Col-1 have been studied as confluent epithelial layers able to transport ions vectorially in response to basolateral vasoactive intestinal polypeptide (VIP) and pancreatic polypeptides (PP). 2. Different species PP stimulated responses in Col-24 with Y(4)-like pharmacology. Bovine (b)PP, human (h)PP and porcine (p)PP were equipotent (EC(50) values 3.0--5.0 nM) while rat (r)PP, avian (a)PP and [Leu(31), Pro(34)]PYY (Pro(34)PYY) were significantly less potent. PYY was inactive. The PP pharmacology in Col-1 was comparable with Col-24. However, Col-6 cells were different; pPP had an EC(50) intermediate (22.0 nM) between that of bPP (3.0 nM) and hPP (173.2 nM), with aPP and rPP being at least a further fold less potent. 3. Deamidation of Tyr(36) in bPP (by O-methylation or hydroxylation) or removal of the residue resulted in significant loss of activity in Col-24. 4. GR231118 (1 microM) had no PP-like effects. In Col-24 and Col-1, GR231118 significantly attenuated bPP (30 nM) or hPP (100 nM) responses, but it did not alter bPP responses in Col-6. BIBP3226 and GR231118 both inhibited Y(1)-mediated responses which were only present in Col-6. 5. RT--PCR analysis confirmed the presence of hY(4) receptor mRNA in Col-24 and Col-1 epithelia but a barely visible hY(4) product was observed in Col-6 and we suggest that an atypical Y(4) receptor is expressed in this cell line.

PYY preference is a common characteristic of neuropeptide Y receptors expressed in human, rat, and mouse gastrointestinal epithelia

This investigation describes the relative potencies of four peptide agonists, namely, peptide YY (PYY), [Leu31,Pro34]PYY (Pro34PYY), neuropeptide Y (NPY), and [Leu31,Pro34]NPY (Pro34NPY), as antisecretory agents in human, rat, and mouse gastrointestinal preparations. The inhibition of agonist responses by the Y1-receptor antagonist BIBP 3226 was also tested in each preparation. An unexpectedly pronounced preference for PYY and Pro34PYY was observed in functional studies of two human epithelial lines stably transfected with the rat Y1 receptor (Y1-7 and C1Y1-6). NPY and Pro34NPY were at least an order of magnitude less effective than PYY in these functional studies but were only marginally less potent in displacement binding studies using membrane preparations of the same clonal lines. The orders of agonist potency obtained in Y1-7 and C1Y1-6 epithelia were compared with those obtained from a single human colonic adenocarcinoma cell line (Colony-6, which constitutively expresses Y1 receptors) and also from mucosal preparations of rat and mouse descending colon. Similar peptide orders of potency were obtained in rat and mouse colonic mucosae and Colony-6 epithelia, all of which exhibited PYY preference (although less pronounced than with Y1-7 and C1Y1-6 epithelia) and significant sensitivity to the Y1 receptor antagonist, BIBP 3226. We have compared the pharmacology of these five mammalian epithelial preparations and provide cautionary evidence against the reliance upon agonist concentration-response relationships alone, in the characterization of NPY receptor types.Key words: Y receptors, neuropeptide Y, gastrointestinal epithelia, ion transport.

Modulation of chloride, potassium and bicarbonate transport by muscarinic receptors in a human adenocarcinoma cell line

1. Short-circuit current (I(SC)) responses to carbachol (CCh) were investigated in Colony 1 epithelia, a subpopulation of the HCA-7 adenocarcinoma cell line. In Krebs-Henseleit (KH) buffer, CCh responses consisted of three I(SC) components: an unusual rapid decrease (the 10 s spike) followed by an upward spike at 30 s and a slower transient increase (the 2 min peak). This response was not potentiated by forskolin; rather, CCh inhibited cyclic AMP-stimulated I(SC). 2. In HCO3- free buffer, the decrease in forskolin-elevated I(SC) after CCh was reduced, although the interactions between CCh and forskolin remained at best additive rather than synergistic. When Cl- anions were replaced by gluconate, both Ca2+- and cyclic AMP-mediated electrogenic responses were significantly inhibited. 3. Basolateral Ba2+ (1-10 mM) and 293B (10 microM) selectively inhibited forskolin stimulation of I(SC), without altering the effects of CCh. Under Ba2+- or 293B-treated conditions, CCh responses were potentiated by pretreatment with forskolin. 4. Basolateral charybdotoxin (50 nM) significantly increased the size of the 10 s spike of CCh responses in both KH and HCO3- free medium, without affecting the 2 min peak. The enhanced 10 s spike was inhibited by prior addition of 5 mM apical Ba2+. Charybdotoxin did not affect forskolin responses. 5. In epithelial layers prestimulated with forskolin, the muscarinic antagonists atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, both at 100 nM) abolished subsequent 10 microM CCh responses. Following addition of p-fluoro hexahydro-sila-difenidol (pF-HHSiD, 10 microM) or pirenzepine (1 microM), qualitative changes in the CCh response time-profile also indicated a rightward shift of the agonist concentration-response curve; however, 1 microM gallamine had no effect. These results suggest that a single M3-like receptor subtype mediates the secretory response to CCh. 6. It is concluded that CCh and forskolin activate discrete populations of basolateral K+ channels gated by either Ca2+ or cyclic AMP, but that the Cl- permeability of the apical membrane may limit their combined effects on electrogenic Cl- secretion. In addition, CCh activates a Ba2+-sensitive apical K+ conductance leading to electrogenic K+ transport. Both agents may also modulate HCO3- secretion through a mechanism at least partially dependent on carbonic anhydrase.