STUDIES ON SECRETION OF GASTRIC INTRINSIC FACTOR IN MAN

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.

Low gastric hydrochloric acid secretion and mineral bioavailability

Young infants and approximately 30% of the elderly have low secretion of hydrochloric acid by gastric parietal cells. It has been established that low hydrochloric acid secretion can lead to decreased absorption of ferric iron. Conflicting results have been obtained in clinical studies of the effects of intraluminal gastric pH values on calcium absorption. The results of an in vitro study suggest that the chemical form of the ingested calcium and the presence of protein may influence whether high intraluminal gastric pH values affect resultant calcium solubilities in the small intestine. The effects of low hydrochloric acid secretion on zinc absorption have not been ascertained. The results of an in vitro study indicate that high intraluminal gastric pH values would not affect resultant zinc solubilities in the small intestine following pancreatin digestion of soy protein isolate supplemented with calcium and/or zinc. Considering that the diets of many elderly contain primarily plant foods and that soy protein isolate formulas are commonly fed to infants, further research is especially needed to determine the effects of low hydrochloric acid secretion on mineral bioavailabilities from high fiber and phytate containing plant foods.

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.

Intrinsic factor secretion from isolated human gastric mucosal cells

Human gastric mucosal cells were isolated from the resected fundic mucosa of peptic ulcer patients. The intracellular content and secretion of intrinsic factor were estimated by binding to cyano[57Co]cobalamin. The content was maximal in the enriched parietal cell fraction which also displayed the highest H+ production as measured by amino[14C] pyrine uptake. Secretagogues evoked full response after 15 min of incubation: pentagastrin (181% of basal secretion), carbachol (208%), histamine (250%) and dibutyryl cyclic adenosine monophosphate (304%). The phosphodiesterase inhibitor isobutylmethylxanthine was slightly more effective even than dibutyryl cAMP. The response to histamine was abolished by ranitidine, indicating activation of adenylate cyclase via histamine H2 receptors, but remained unaffected by atropine, which in turn blocked the carbachol effect, whereas ranitidine was ineffective. The mean formation rate was 8.4 fmol intrinsic factor/10(6) cells per h under basal conditions and 14.3 fmol in response to histamine.

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.

Cellular origin and release of intrinsic factor from isolated rat gastric mucosal cells

The cellular content and secretion of intrinsic factor was measured by [57Co]cyanocobalamin binding using isolated rat gastric mucosal cells. The intrinsic factor/R-protein ratio was above 9:1 as evaluated by specific anti-intrinsic factor antibodies. In unfractionized cells with 23 +/- 1.3% parietal cells the intrinsic factor content of 148 +/- 47 fmol/10(6) cells remained almost unchanged over 3 h, whereas basal secretion rose up to 57 +/- 10. In fractionized cells (Percoll) with 3-85% parietal cells most intrinsic factor was found in the parietal cell-depleted fraction (content: 441 +/- 30, secretion/3 h: 139 +/- 16, mean formation/h: 50 +/- 12 fmol/10(6) cells). The intrinsic factor content of the different cell fractions correlated with that of pepsin. [14C]Aminopyrine uptake, an indirect measure of parietal cell H+ production, was inversely related. Carbachol (1 X 10(-6)-10(-3) mol/l) stimulated intrinsic factor secretion, 1 X 10(-3) mol/l being maximally effective (90 +/- 8% above basal). This response was inhibited by atropine and pirenzepine, but not by prostaglandin E2 (PGE2) and somatostatin. Dibutyryl cyclic adenosine monophosphate (dibutyryl cAMP, 43 +/- 7%) and hexoprenaline (24 +/- 5%) enhanced intrinsic factor secretion less effectively and pentagastrin like histamine lacked any stimulatory effect. We conclude that in the rat intrinsic factor is produced and released from chief cells mainly under cholinergic control.

Histamine H2 receptors and intrinsic factor secretion

Intrinsic factor (IF) secretion in healthy male subjects was studied in response to pentagastrin stimulation with and without cimetidine, and to impromidine, a histamine H2 receptor agonist. Peak and total IF output were reduced by cimetidine, but the concentration was unchanged and the response was unaffected by administration of the compound for 1 week. The IF secretory response to impromidine was similar to that to pentagastrin. It is suggested that the acid and IF components of the parietal cell secretory response are mediated via different intracellular pathways, that histamine H2 receptors do not fulfill and obligatory role in the secretion of IF and that synthesis of IF is probably not altered by cimetidine.

Use of radioisotope techniques in the clinical evaluation of patients with megaloblastic anemia

Because virtually all cases of vitamin B12 deficiency seen in this country are due to malabsorption, the availability of radioactive vitamin B12 for direct measurement of absorption of this essential nutrient has proved to be of great clinical value. These tests are useful not only in demonstrating vitamin B12 malabsorption but also often in defining the pathophysiological mechanism responsible for this abnormality. The urinary excretion test of Schilling remains the most useful test for vitamin B12 absorption. Minor precautions and modifications in technique make the test results more reliable and easier to interpret. The 8-hr plasma test for vitamin B12 absorption can no longer be considered acceptable. Some patients with vitamin B12 malabsorption have results in the normal range when studied by this method. Serum vitamin B12 assays utilizing radioactive vitamin B12 and the isotope dilution principle are not widely used and are useful screening tests. Low normal or borderline results observed in patients with clinical evidence suggestive of vitamin B12 deficiency should be interpreted with caution or confirmed by radioactive vitamin B12 absorption studies. Radioactive vitamin B12 can also be used for rapid, reliable assay of gastric intrinsic factor, antibody to intrinsic factor and unsaturated vitamin B12 serum. Methods using radioactive folate compounds for similar in vivo and in vitro studies are not yet applicable for routine use in nuclear medicine laboratories.

The response to prednisolone in atrophic gastritis: a possible effect on non-intrinsic factor-mediated vitamin B 12 absorption

The administration of prednisolone did not affect absorption of vitamin B(12) and gastric secretion of intrinsic factor in nine subjects with normal gastric mucosa, or in five with chronic superficial gastritis. No gastric morphological changes were observed in either group. In contrast, four of seven patients with pernicious anaemia showed improvement in absorption of vitamin B(12) during prednisolone administration for periods of two to 10 months. No consistent changes in gastric mucosal morphology were noted. An additional five patients were studied intensively during one month of prednisolone administration. In three, absorption of vitamin B(12) was restored to within the normal range between eight and 13 days after the commencement of the drug. The improvement was not paralleled by enhancement of secretion of intrinsic factor or other vitamin B(12) binders. Suppression of intrinsic factor antibodies in serum and gastric juice did not occur consistently in those patients showing a functional response.

Localization of intrinsic factor and complement fixing intrinsic factor--intrinsic factor antibody complex in parietal cell of man

In an attempt to localize intrinsic factor in the human parietal cell, and to study its intracellular union with the intrinsic factor antibody and complement, intrinsic factor antibody was separated from coexisting parietal cell antibody in pernicious anaemia sera by gel filtration. Intrinsic factor antibody of both `binding' and `blocking' type was also produced in rabbits by immunization with semi-purified human intrinsic factor–[⁵⁷Co]B₁₂ complex.

Intrinsic factor antibody obtained from both sources produced fluorescence in the human parietal cells in the indirect Coons' test in the presence of fluoresceinated anti-human IgG. The fluorescence was localized peripherally, at the cell membrane. When instead of the fluoresceinated anti-human IgG a fluoresceinated anti-human complement (C) serum and normal complement containing serum were used, intrinsic factor antibody from both sources produced fluorescence of the entire parietal cell cytoplasm of the human mucosa.

Thus, intrinsic factor was localized at highest concentration at the membrane of the parietal cell in man, the intrinsic factor antibody–intrinsic factor complex was demonstrated within the human parietal cell, and evidence was obtained that this antigen–antibody complex fixes complement (C). The possible role of the intrinsic factor–intrinsic factor antibody–complement complex in the development of gastric atrophy in pernicious anaemia has been considered.

Comparison of the effects of food and carbachol on the absorption of vitamin B 12 after partial gastrectomy

Absorption of radioactive vitamin B(12) was measured by the Schilling procedure in 14 patients who had had a partial gastrectomy. Injection of carbachol (250 mug) before the administration of the test dose increased absorption to a much smaller degree than the ingestion of a light meal containing a minimal quantity of the natural vitamin. The results support the belief that food may improve absorption of vitamin B(12) after partial gastrectomy by stimulating the secretion of a small residual amount of intrinsic factor.

Gastric secretory function in polycythaemia vera

Maximal acid output (M.A.O.) and intrinsic factor (I.F.) of the stomach were measured in 16 patients with polycythaemia vera, 4 of whom had a duodenal ulcer and 2 pernicious anaemia. It was found that, in general, the M.A.O. was significantly less than normal unless an active duodenal ulcer was present when the M.A.O. would reach the usual levels for duodenal ulcer subjects. The gastric I.F. output showed a significant correlation with the M.A.O. These results are discussed in the light of the increased incidence of peptic ulcer in polycythaemia vera and the occasionally reported cases of polycythaemia vera and pernicious anaemia.

Immunoassay of gastric intrinsic factor and the titration of antibody to intrinsic factor

A modification of the charcoal method for the detection and titration of antibody to intrinsic factor in serum and for the assay of intrinsic factor in human gastric juice is described. The antibody was detected in a titre greater than 5 ng units/ml in 52% of the sera of 162 patients with Addisonian pernicious anaemia. False positive results may occur if the serum is withdrawn within 24 hr of the parenteral administration of normal therapeutic doses of vitamin B₁₂.

In the analysis of human gastric juice, the charcoal method of immunoassay clearly distinguishes between intrinsic factor and other vitamin B₁₂ binding substances. A good correlation was observed between the gastric acid secretion, the ability to absorb oral radioactive vitamin B₁₂ in the Schilling test, and the assay of intrinsic factor in the gastric juice. Intrinsic factor as an antigen is stable in acid and alkali media but is destroyed by incubation at 56°C for 30 min. A secretion of less than 100 ng units intrinsic factor in the gastric juice during the post-histamine hour is not compatible with adequate vitamin B₁₂ absorption and is indicative of Addisonian pernicious anaemia.

A good correlation was observed between the immunoassay and the biological activity of preparations of hog intrinsic factor. To correct the absorption of oral vitamin B₁₂ in the Schilling test in patients with pernicious anaemia 300–600 ng units hog intrinsic factor are required. It is suggested that hog intrinsic factor preparations should be defined in terms of nanogram units intrinsic factor per milligram.

Gastrointestinal lesions causing macrocytic anemia

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