DOPAMINE AND VASOACTIVE-INTESTINAL-PEPTIDE STIMULATE CYCLIC ADENOSINE-3',5'-MONOPHOSPHATE FORMATION IN ISOLATED RAT VILLUS AND CRYPT DUODENOCYTES

Secretion of bicarbonate increases the pH at the duodenal mucosal surface, a process which contributes to the protection against acid/pepsin injury. Previously, we have shown that dopaminergic compounds stimulate the duodenal bicarbonate secretion in situ, in the anaesthetized rat, through an action on peripheral dopamine D1 receptors. In order to study the possible involvement of cyclic adenosine-3',5'-monophosphate (cAMP) as an intracellular mediator in enterocytes isolated from rat duodenum, cells were collected by a combination of enzyme treatment and calcium chelation. Two major cell fractions, one mainly from villi and the other mainly of crypt origin, were studied. In the villus cell fraction, the activity of alkaline phosphatase was 1.6 +/- 0.2 mumol mg protein-1 min-1 and that of sucrase 98.8 +/- 16.4 nmol mg protein-1 min-1. In the crypt fraction, activities were 0.7 +/- 0.1 and 28 +/- 10.5, respectively. Effects of dopamine, two selective dopamine receptor agonists and vasoactive intestinal peptide (VIP) on intracellular accumulation of cAMP were examined by radio-immunoassay (RIA). In the crypt cell fraction, VIP (10(-7) M) caused an increase in cAMP which was maximal after 5 min (78 +/- 28% above control, P < 0.01). In the villus cell fraction, maximal responses to VIP (60-24% above control, P < 0.05), did not occur until after 60 min of incubation. In contrast, there were no significant differences between villi and crypt enterocytes in respect to effects of dopamine, the dopamine D1-receptor agonist SKF-38393 and the D2-receptor agonist quinpirole. Dopamine (10(-5) M), as well as SKF-38393 (10(-5) M), increased (P < 0.05) cAMP accumulation with a maximal response after 5-15 min of incubation. Quinpirole (10(-5) M), decreased (P < 0.05) cAMP accumulation in both crypt and villi cell fractions. The results suggest that cAMP is involved as a messenger in dopamine D1 receptor-mediated stimulation of duodenal ulceroprotective alkaline secretion. The time course of cAMP accumulation induced by the potent duodenal secretagogue VIP is different in crypt and villus cells.