We have previously shown that in highly enriched rat gastric parietal cells the intestinal peptide hormones oxyntomodulin and glucagon-like peptide-2 (GLP-2) compete for receptor-binding with glucagon-like peptide-1 (GLP-1), a potent cAMP-dependent stimulus of H⁺ production in vitro. It is, however, unknown whether oxyntomodulin and GLP-2 elicit a biological response by interacting with the GLP-1 receptor. Therefore, we used enriched rat parietal cells to investigate the effects of both hormones on the production of cAMP and H⁺ ([¹⁴C]aminopyrine accumulation). Both parameters were stimulated by oxyntomodulin in a concentration-dependent manner. EC50 values were 6.2-10^-8 and 2.5·10^-7M oxyntomodulin for stimulation of H⁺ and cAMP production, respectively. The maximally effective concentrations for stimulation of [¹⁴C]aminopyrine accumulation and cAMP production were l·10^-6 and 1 · 10^-5M oxyntomodulin, respectively. At these concentrations oxyntomodulin was nearly as effective as 10^-4Mhistamine and equally effective as 10^-8 MGLP-1 (7-36)NH₂. In the enriched parietal cell preparation there was no immunocytochemical evidence of contaminating D cells. Accordingly, the responses to oxyntomodulin and GLP-1 (7-36)NH2 were not augmented by incubating the cells in the presence of a polyclonal anti-somatostatin antibody. [¹⁴C]Aminopyrine accumulation in response to oxyntomodulin was inhibited by the GLP-1 (7-36)NH₂ receptor antagonist, exendin (9-39)NH₂, but not by the H₂-receptor antagonist, ranitidine. Oxyntomodulin and carbachol acted additively to stimulate [¹⁴C]aminopyrine accumulation. GLP-2 (10^-7 to 10^-5M) was without effect on basal H⁺ and cAMP production; however, at 10^-5M GLP-2 markedly inhibited oxyntomodulin-stimulated [¹⁴C]aminopy-rine accumulation. It is concluded that, by interacting with parietal cell receptors for GLP-1 (7-36)NH₂, oxyntomodulin, but not GLP-2, directly stimulates H⁺ production by activating the adenylate cyclase.

The intestinal peptide hormone glucagon-like peptide-1 (GLP-1) (7-36) amide is a potent stimulus of H+ production in isolated rat parietal cells, suggesting the presence of specific GLP-1-receptors on this cell type. Our aim was to characterize these receptors. Enzymatically isolated rat gastric mucosal cells (F0) were fractionated by counterflow elutriation, resulting in five fractions (F1-F5) according to increasing cell diameter and parietal cell content (3, 5, 4, 27, 81%). Additional density gradient centrifugation of F4 yielded enriched chief cells (74%; parietal cells: 1%; F6), whereas density gradient centrifugation of F5 almost purified parietal cells (97%; chief cells: 1%; F7). Northern blot of total cellular RNA from F0-F7 with a probe specific for the GLP-1-(7-36) amide receptor revealed two RNA species of 2.7 and 3.6 kb. These messages were present to some extent in small cells (F1, F2), much more pronounced in F5, abundant in F7, barely detectable in F3 and F4, and absent from F6. Cross-linking of 125I-labeled GLP-1-(7-36) amide to parietal cell membranes revealed a single 59-kDa band that was abolished by unlabeled GLP-1-(7-36) amide. Throughout fractions F1-F7 specific binding of 125I-GLP-1-(7-36) amide was correlated with parietal cell content (r = 0.99; P < 0.01) and H+ production ([14C]aminopyrine accumulation) in response to GLP-1-(7-36) amide or histamine (r = 0.96; P < 0.01). Binding was maximal in purified parietal cells (F7), whereas almost no binding was detectable in enriched chief cells (F6). In F7, Scatchard analysis revealed a single class of high-affinity binding sites (KD = 2.8 +/- 0.6 x 10(-10) M, Bmax = 6.8 +/- 1.4 fmol/10(6) cells, 4,096 +/- 793 receptors/parietal cells). The following half-maximal inhibition values were found for GLP-1-(7-36) amide and (1-37) and (1-36) amide: 6.6 +/- 0.9 x 10(-10), 1.4 +/- 0.7 x 10(-7), and 2.6 +/- 0.4 x 10(-7) M, respectively. Pancreatic glucagon, GLP-2, and oxyntomodulin, products of the proglucagon gene, were 3-4 log units less potent displacers while gastric inhibitory peptide, vasoactive intestinal peptide, and secretin were ineffective. We conclude that rat parietal cells are equipped with specific high-affinity receptors for GLP-1-(7-36) amide, which, in addition, are present in as yet unidentified small cells (F1, F2) but not in chief cells.

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We investigated the effect of glucagon-like peptide 1 (GLP-1)-(7-36) amide and its molecular variants GLP-1-(1-37) and GLP-1-(1-36) amide on enzymatically dispersed enriched rat parietal cells using [14C]aminopyrine accumulation as a measure of H+ production. GLP-1-(7-36) amide was 100 times more potent than GLP-1-(1-37) and GLP-1-(1-36) amide in stimulating [14C]aminopyrine accumulation. At their maximally effective concentrations, GLP-1-(7-36) amide (10(-8) M), GLP-1-(1-37) (10(-6) M), and GLP-1-(1-36) amide (10(-6) M) reached 80-90% of the response to 10(-4) M histamine. However, the peptides were 100-10,000 times more potent than histamine, which induced maximal [14C]aminopyrine accumulation at 10(-4) M. Stimulation by GLP-1 was dependent on the presence of a phosphodiesterase inhibitor and was not altered by pertussis toxin. Ranitidine failed to affect the response to the GLP-1 variants. Stimulation of H+ production by GLP-1 was accompanied by an increase in the formation of adenosine 3',5'-cyclic monophosphate (cAMP) but not by changes in phosphoinositol breakdown. In stimulating [14C]aminopyrine accumulation, the GLP-1 variants acted additively to threshold but not to maximal concentrations of histamine, suggesting that histamine and GLP-1 activate the same cAMP pool. In contrast, in anesthetized rats GLP-1-(7-36) amide (10-500 ng.kg-1.h-1) had no effect on basal and pentagastrin-stimulated acid secretion in vivo. We conclude that GLP-1 exerts a direct stimulatory effect on rat parietal cells. This potent effect is mediated by cAMP and is independent of H2 receptors. In vivo direct stimulation by GLP-1 of the parietal cells might be counterbalanced by indirect inhibitory mechanisms that are excluded in the in vitro cell system.

The influence of postprandial-like plasma insulin levels on intestinal calcium absorption (CaA) was studied in 9 healthy men. On separate occasions, they received either an i.v. infusion of 40 mU/m2 minute synthetic human insulin as well as a variable glucose infusion in order to clamp the plasma glucose at the baseline level (= glucose clamp), or insulin- and glucose-free vehicle infusions (= vehicle). During these infusions, an oral load containing 326 mg Ca in the form of Ca chloride was administered and CaA was determined thereafter with a 47Ca/85Sr double tracer method. During glucose clamp, mean plasma insulin was 172 +/- (1 SEM) 10 as compared to 6 +/- 1 microU/ml during vehicle infusions. During the clamp, 3-hour cumulative CaA rose significantly by 14% as compared to vehicle (39.2 +/- 2.5 vs. 34.4 +/- 2%, P less than 0.02). AT the same time, serum potassium and phosphorus dropped significantly, whereas serum parathyroid hormone (PTH) and 1,25(OH)2D levels were unchanged as compared to vehicle. The urinary excretions of potassium, sodium, and inorganic phosphorus, as well as the urinary specific activity of 47Ca, dropped significantly during glucose clamp, whereas the urinary excretion of cAMP was unchanged as compared to vehicle. The results suggest that, under the conditions of euglycemic hyperinsulinemic clamp, insulin stimulates CaA of healthy humans in a PTH- and 1,25(OH)2D-independent manner. Insulin may thus possibly be regarded as a factor participating in the regulation of CaA in humans.