Interlobular ducts were isolated from the rat pancreas and maintained in short-term tissue culture. Fluid secretion from these isolated ducts was measured using micropuncture techniques, intracellular calcium concentration ([Ca2+]i) by fura-2 microspectrofluorimetry, and cyclic AMP by radioimmunoassay. 2. Applying secretin and ACh simultaneously to ducts caused either a stimulation or an inhibition of fluid secretion depending on the doses employed. 3. The inhibitory effect of secretin and ACh could be relieved by atropine, and by the protein kinase C (PKC) inhibitors staurosporine and 1-(5-isoquinolinylsulphonyl)-2-methyl-piperazine (H-7). 4. Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) and phorbol 12, 13-dibutyrate (PDBu) inhibited secretin-evoked fluid secretion. 5. ACh and TPA also inhibited fluid secretion stimulated by the adenylate cyclase activator, forskolin. 6. Neither secretin nor the PKC activators and inhibitors had any effect on either the increase in [Ca2+]i evoked by ACh or the increase in intracellular cyclic AMP evoked by secretin and forskolin. 7. We conclude that the inhibitory effect of combined doses of secretin and ACh on ductal fluid secretion is probably mediated by PKC at a point in the secretory mechanism distal to the generation of intracellular messengers.
