Evidence for involvement of cyclic nucleotides in intrinsic factor secretion by isolated rabbit gastric mucosa

A micromethod for the assay of cellular secretory physiology: application to rabbit parietal cells

A micromethod for investigating secretory physiology in isolated cells was evaluated. The method utilized a specially designed polycarbonate incubation chamber to provide constant oxygenation to cells incubating in a 96-well microtiter plate. Cells were rapidly separated from media by vacuum filtration. Isolated parietal cells were utilized to demonstrate the versatility of the method for assay of intracellular accumulation of [14C]-aminopyrine, secretion of intrinsic factor into the medium, and assay of intracellular cAMP. Histamine stimulated the uptake of [14C]aminopyrine and intrinsic factor secretion in a sustained and linear fashion. At the end of the 2-h period uptake of aminopyrine and secretion of intrinsic factor were increased 17- and 5-fold, respectively. This response to histamine was accompanied by a rapid and sustained 3-fold rise in intracellular cyclic AMP. In contrast, carbamylcholine caused a transient increase in [14C]aminopyrine accumulation and intrinsic factor secretion which was most pronounced during the first 10 min and had almost ceased by 30 min. Carbamylcholine had no effect on intracellular cAMP levels. This new method, which can handle 400 replicates using parietal cells from the fundic mucosa of a single rabbit, is suitable for studying the time course of intracellular events which accompany general secretory processes.

cAMP- and calcium-mediated modulation of intrinsic factor secretion in isolated rabbit gastric glands

We have studied the role of cAMP-mediated and calcium-mediated secretagogues in stimulating secretion of intrinsic factor (IF) from parietal cells in isolated rabbit gastric glands. The magnitudes of IF release caused by maximally stimulating doses of the following agents were compared: histamine (5 x 10(-5) M), forskolin (10(-5) M), 8-bromo-cAMP (10(-3) M), pentagastrin (10(-7) M), carbachol (10(-5) M), and the calcium ionophore A23187 (10(-3) M). Each agent was tested simultaneously in gastric glands prepared from the same animal so as to minimize the effects of variations between preparations. Gastric glands were incubated 30 min alone (unstimulated) or with each of the secretagogues. IF release into the culture medium was measured using an assay based on the binding of IF to 57Co-cyanocobalamin. Results were expressed as percentages IF release above unstimulated levels. Three cAMP-mediated agents significantly (P less than 0.05) stimulated IF secretion above unstimulated levels: histamine (439 +/- 33%), forskolin (769 +/- 385%), and 8-bromo-cAMP (1483 +/- 362%). Two agents thought to act through calcium-dependent mechanisms significantly (P less than 0.05) stimulated IF release: carbachol (79 +/- 25%) and pentagastrin (16 +/- 6%). A23187 did not increase IF release above unstimulated levels (P greater than 0.05). In this study, cAMP-mediated secretagogues stimulated significantly (P less than 0.05) more IF release than carbachol or pentagastrin. This study supports the hypothesis that both cAMP- and calcium-mediated mechanisms participate in regulating IF release.

Prostaglandin E analogue inhibition of intrinsic factor release

In many mammalian species (including humans) the parietal cell secretes both acid and intrinsic factor (IF). The aim of this study was to examine the direct effect of prostaglandin E analogues on basal and stimulated IF release in isolated, enriched rabbit parietal cells. The effects of graded concentrations (10(-12) to 10(-6) M) of 16,16-dimethyl prostaglandin E2 (DMPGE2) and 16-methyl,16-hydroxyl prostaglandin E1 (misoprostol) on submaximal histamine-stimulated (10(-6) M) secretion were tested. Both analogues failed to alter basal release of IF or aminopyrine accumulation (indirect measure of acid secretion). Increasing concentrations of DMPGE2 resulted in a dose-dependent inhibition of IF release (22 +/- 8% decrease at 10(-9) M; p less than 0.05) and a maximal effective response at 10(-7) M (54 +/- 9%; p less than 0.005). Dose-dependent inhibition of IF secretion was also observed with increasing concentrations of misoprostol, with a 22 +/- 7% decrease at 10(-9) M (p less than 0.05) and maximal effective inhibition at 10(-6) M (47 +/- 8%; p less than 0.01). Misoprostol and DMPGE2 inhibition of acid secretion paralleled IF release. Prostaglandin analogues appear to block IF release potently in histamine-stimulated parietal cells.

Membrane receptors in the gastrointestinal tract

This review focusses on the roles that membrane receptors and their transducers play in the physiology and pathology of the gastrointestinal tract. The multifactorial regulation of [correction] mucosal growth and function is discussed in relation to the heterogeneity of exocrine and endocrine populations that originate from progenitor cells in stomach and intestine.

Epidermal growth factor (EGF) inhibits both intrinsic factor secretion and acid secretion in histamine-stimulated isolated gastric glands

Epidermal growth factor (EGF) is a polypeptide present in mammalian salivary glands which has been shown to have mitogenic and gastric acid inhibitory properties in vivo. The mechanisms of action of EGF at the level of the parietal cell are not clear. In the present study, we have examined the effects of EGF on both acid and macromolecular (intrinsic factor, IF) secretion stimulated by the cyclic AMP-mediated agonist histamine using the rabbit isolated gastric gland model. Acid secretion was assessed by the accumulation of [14C]aminopyrine (AP) in glands and IF in the supernatants by the binding of [57Co]cyanocobalamin. Histamine (10(-6) to 5 x 10(-5) M) resulted in a 4-6 fold increase in [14C]AP and IF (P less than 0.05). EGF alone (10(-8) M, 10(-7) M) had no significant effect on basal [14C]AP accumulation or IF secretion (P less than 0.05). EGF (10(-7) M) significantly inhibited the histamine dose-response curve for [14C]AP and IF, but a relatively greater inhibition was observed at higher histamine concentration. These data demonstrate that EGF inhibits both acid and IF secretion in vitro at concentrations consistent with those observed in vivo. The observations further support the hypothesis that EGF may play a role in the regulation of parietal cell secretion.

Effects of cyclic nucleotides, betazole hydrochloride and acetylcholine on pepsinogen secretion from isolated rabbit gastric mucosa

The effects of dibutyryl cyclic AMP (db-cAMP), dibutyryl cyclic GMP (db-cGMP), betazole hydrochloride (betazole) and acetylcholine (ach.) on pepsinogen secretion from isolated rabbit gastric mucosa were studied using an organ culture system. The 10(-3) M db-cAMP, but not db-cGMP of any concentration, produced a significant enhancement of pepsinogen secretion into the culture medium. In the presence of aminophylline (phosphodiesterase inhibitor), betazole stimulated pepsinogen secretion at concentrations of 10(-8), 10(-6) and 10(-4) M. The 10(-4) M betazole stimulated pepsinogen secretion most strongly (208% of control) and 10(-6) M betazole induced submaximal secretion (137% of control). Ach. stimulated pepsinogen secretion at 10(-8), 10(-6), 10(-4) and 10(-2) M. The peak secretion occurred at 10(-4) M ach. (303% of control). Betazole (with aminophylline) against a background of 10(-6) M ach. (submaximal stimulation dose), produced an intense stimulation of pepsinogen secretion at the concentrations of 10(-8), 10(-6) and 10(-4) M, and the secretion rate expressed as percent of control were 277, 350 and 329%, respectively. These responses were not considered the additive action by betazole and ach. but the potential interaction between the two agents in pepsinogen secretion. From these findings, we conclude that betazole-ach. interdependency exists in in vitro pepsinogen secretion.

Intrinsic factor secretion from isolated gastric mucosal cells of rat and man--two different patterns of secretagogue control

Intrinsic Factor (IF) secretion was studied using isolated gastric mucosal cells from rat and man. In the rat, IF was localized to the chief cells and its secretion responded most efficaciously to carbachol. DbcAMP and hexoprenaline were less powerful, whereas histamine and pentagastrin lacked any effect. In man, IF secretion derived from the parietal cells and was increasingly enhanced by hexoprenaline, pentagastrin, carbachol, histamine and dbcAMP. In both species, IF secretion differs with respect to its cellular origin and the pattern of secretagogue control: IF release from rat chief cells is due to muscarinic receptor excitation, whereas IF release from human parietal cells responds predominantly to histamine-H2-receptor activation and seems to be mediated by the cAMP system.

Secretion of intrinsic factor from dispersed mucosal cells isolated from guinea pig and rabbit stomach

In dispersed mucosal cells prepared from rabbit and guinea pig stomach, the secretion of intrinsic factor was constant (0.3-0.4%/min) for at least 30 min incubation at 37 degrees C. Histamine or isobutyl methylxanthine increased cyclic AMP and intrinsic factor secretion in both cells preparations. Isobutyl methylxanthine potentiated and cimetidine competitively inhibited (ki = 5.10-7 M) both effects of histamine. Dibutyryl cyclic AMP (1.0 mM), also caused a 3-fold increase in intrinsic factor secretion. These results suggest that in rabbit and guinea pig histamine interacts with H2-receptors to increase cyclic AMP which mediates the rise in the rate of intrinsic factor secretion.