Stimulatory and inhibitory actions of carbachol on chloride secretory responses in human colonic cell line T84

The muscarinic agonist carbachol (CCh) was shown to elicit both stimulatory and inhibitory actions on Cl- secretion in T84 cells. These effects were observed in cells that had been prestimulated with adenosine 3',5'-cyclic monophosphate (cAMP) agonists. The addition of CCh to cells treated with submaximal concentrations (1-10 nM) of the receptor-mediated agonist prostaglandin (PG) E2 resulted in a biphasic effect on short-circuit current (Isc) involving a transient synergistic rise followed by a slower and sustained attenuation of the PGE2-activated Isc response. In contrast at higher PGE2 concentrations (greater than 10 nM) or with nonreceptor-mediated cAMP agonists (forskolin, dibutyryl cAMP) CCh elicited a prolonged synergistic response. Both effects of CCh could be reproduced by selective activation of the Ca2+ pathway. Increasing cytosolic Ca2+ with ionophore A23187 partially mimicked the "early" stimulation of secretion, though there was evidence that a combination of A23187 and the protein kinase C activator phorbol 12,13-dibutyrate (PDB) was required for full expression of the secretory response. In contrast, treatment with PDB alone closely mimicked the CCh-induced inhibition of PGE2-stimulated Isc. The antisecretory effects of CCh were associated with a marked attenuation of cAMP production in response to receptor-mediated agonists, including PGE2 and vasoactive intestinal peptide. This effect could also be closely mimicked by PDB. Pretreatment with pertussis toxin partially inhibited the ability of CCh to inhibit cAMP production. CCh potentiated the cAMP response to the nonreceptor agonist forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)