Inhibition of cyclic AMP-dependent chloride secretion by PP receptors and α2-adrenoceptors in a human colonic epithelial cell line

Abstract

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 peptide 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 either absent in this cell line, or are not functionally coupled. A second member of the neuropeptide Y family, pancreatic polypeptide (PP), decreased VIP-elevated SCC with an EC₅₀ of 25.6 nM. Maximal PP responses were unaffected by prior addition of PYY, indicating that Col-1 cells may express a PP specific, Y₄-like receptor. The α₂-adrenoceptor agonist clonidine also attenuated VIP-stimulated SCC (EC₅₀ 342 nM) through the α_2A receptor subtype, since clonidine responses were inhibited by yohimbine and rauwolscine but not altered by previous addition of prazosin. Col-1 cells responded to both apical and basolateral addition of VIP or clonidine; to an extent, this lack of sidedness reflects the ability of drugs to permeate through the Col-1 epithelial 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 directly elevate intracellular cAMP. After a maximal concentration of forskolin (10 μM), which increased SCC to a significantly greater extent than either VIP or IBMX, the effects of both agonists were negligible. The absence of PP and clonidine responses under these conditions may have implications for the mechanisms by which these agonists inhibit chloride secretion in Col-1 epithelia. In addition carbachol reduced SCC stimulated by 10 μM forskolin, in contrast to control carbachol responses which consisted of a rapid decrease followed by a transient elevation in SCC; this observation suggests that Col-1 cells may also be a useful model for studying the interactions between Ca²⁺- and cAMP-dependent mechanisms involved in epithelial ion transport.