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 surf ace, a process which contributes to the protection against acid/pepsin injury. Previously, we have shown that dopaminergic compounds stimula te the duodenal bicarbonate secretion in situ, in the anaesthetized ra t, 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 an d calcium chelation. Two major cell fractions, one mainly from villi a nd the other mainly of crypt origin, were studied. In the villus cell fraction, the activity of alkaline phosphatase was 1.6 +/- 0.2 mumol m g 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 +/- 1 0.5, respectively. Effects of dopamine, two selective dopamine recepto r agonists and vasoactive intestinal peptide (VIP) on intracellular ac cumulation of cAMP were examined by radio-immunoassay (RIA). In the cr ypt 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 vill us cell fraction, maximal responses to VIP (60-24% above control, P < 0.05), did not occur until after 60 min of incubation. In contrast, th ere were no significant differences between villi and crypt enterocyte s 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 accumulati on with a maximal response after 5-15 min of incubation. Quinpirole (1 0(-5) M), decreased (P < 0.05) cAMP accumulation in both crypt and vil li cell fractions. The results suggest that cAMP is involved as a mess enger in dopamine D1 receptor-mediated stimulation of duodenal ulcerop rotective alkaline secretion. The time course of cAMP accumulation ind uced by the potent duodenal secretagogue VIP is different in crypt and villus cells.