INHIBITION OF CYCLIC-AMP-DEPENDENT CHLORIDE SECRETION BY PP RECEPTORSAND ALPHA(2)-ADRENOCEPTORS IN A HUMAN COLONIC EPITHELIAL-CELL LINE

The effects of a number of agonists which inhibit intestinal chloride secretion were investigated in Colony-1 (Col-1) cells, a subpopulation derived from the HCA-7 human adenocarcinoma cell line. Neither peptid e yy (pyy) or somatostatin 14-28 (SRIF) reduced short-circuit current (SCC) in Col-1 epithelial layers stimulated with vasoactive intestinal polypeptide (VIP), suggesting that their respective receptors are eit her absent in this cell line, or are not functionally coupled. A secon d member of the neuropeptide Y family, pancreatic polypeptide (PP), de creased VIP-elevated SCC with an EC(50) of 25.6 nM. Maximal PP respons es were unaffected by prior addition of PYY, indicating that Col-1 cel ls may express a PP specific, Y-4-like receptor. The alpha(2)-adrenoce ptor agonist clonidine also attenuated VIP-stimulated SCC (EC(50) 342 nM) through the alpha(2A) receptor subtype, since clonidine responses were inhibited by yohimbine and rauwolscine but not altered by previou s addition of prazosin. Col-1 cells responded to both apical and basol ateral addition of VIP or clonidine; to an extent, this lack of sidedn ess reflects the ability of drugs to permeate through the Col-1 epithe lial layers. Both PP and clonidine also inhibited SCC in unstimulated Col-1 cells or those pretreated with 3-isobutyl-1-methylxanthine (IBMX ) or a submaximal concentration of forskolin, agents which both direct ly elevate intracellular cAMP. After a maximal concentration of forsko lin (10 mu M), which increased SCC to a significantly greater extent t han either VIP or IBMX, the effects of both agonists were negligible. The absence of PP and clonidine responses under these conditions may h ave implications for the mechanisms by which these agonists inhibit ch loride secretion in Col-1 epithelia. In addition carbachol reduced SCC stimulated by 10 mu M forskolin, in contrast to control carbachol res ponses which consisted of a rapid decrease followed by a transient ele vation in SCC; this observation suggests that Col-1 cells may also be a useful model for studying the interactions between Ca2+- and cAMP-de pendent mechanisms involved in epithelial ion transport.