Intraventricular calcitonin gene-related peptide inhibits gastric acid secretion

Calcitonin gene-related peptide (CGRP) is a 37 amino acid peptide recently demonstrated to be a peptide expressed by the calcitonin gene in the rat central nervous system. Intracerebroventricular administration of CGRP in pylorus ligated rats resulted in a dose dependent suppression of gastric acid secretion. This effect was also present in acutely vagotomized rats. In addition, CGRP inhibited the stimulation of gastric acid secretion by thyrotropin releasing hormone. CGRP was considerably less potent in its effect on gastric acid than calcitonin, a well known central inhibitor of gastric acid secretion in the rat. This study suggests that CGRP may be a factor in the central regulation of gastric acid secretion in the rat.     

Effect of intracerebroventricular administration of rat calcitonin gene-related peptide (CGRP), human calcitonin and [Asu1,7]-eel calcitonin on gastric acid secretion in rats

The effect of intracerebroventricular injection of rat calcitonin gene-related peptide (CGRP), human calcitonin (CT) and [Asu1,7]-eel CT on the volume and acidity of gastric juice was examined in the pylorus-ligated male rats. These 3 peptides were effective in suppressing both the volume and acidity of secreted gastric juice. Their potency on a molar basis, however, was markedly different; [Asu1,7]-eel CT was most potent, followed by human CT and finally by rat CGRP. These finding suggest that CGRP could not substitute for [Asu1,7]-eel or human CT in exerting the suppressive effect of gastric acid secretion.     

Inhibition of gastric acid secretion by human calcitonin gene-related peptide with picomolar potency in guinea-pig parietal cell preparations

The effect of CGRP on [14C]-aminopyrine accumulation in isolated parietal cell preparations from guinea-pig fundic mucosa was studied. Parietal cells consisted of 60% of the preparations. [14C]-Aminopyrine accumulation was used as an index of physiological response of parietal cells to secretagogues. CGRP dose-dependently (10(-12)-10(-9) M) inhibited parietal cell aminopyrine accumulation stimulated by histamine (10(-4) M), carbachol (10(-4) M), and pentagastrin (5 X 10(-6) M). The concentration of CGRP exerting half-maximal inhibition of [14C]-aminopyrine accumulation was 8.7 X 10(-11) M for histamine, 9.1 X 10(-11) M for carbachol, and 4.7 X 10(-11) M for pentagastrin. The inhibitory effect was much more potent than cimetidine, pirenzepine or benzotript. CGRP but not cimetidine inhibited DBcAMP stimulated aminopyrine accumulation (IC50 = 7.5 X 10(-11) M). These results suggest that CGRP may exert its inhibitory action on gastric acid secretion by a direct action on the parietal cell or the somatostatin-producing D cell.     

Effect of calcitonin on the interdigestive motility and on gastric and pancreatic secretion in humans

Calcitonin given in doses (0.2 and 1 MRC U/kg-1h-1) reproducing the levels observed in medullary carcinoma of the thyroid, induced the appearance of phase III type activity and reduced the duration of the IMC in the small intestine from 123 to 87 min with 0.2 MRC U kg-1h-1 and from 123 to 43 min with 1 MRC U kg-1h-1 but not in the stomach of young volunteers. This increase in phase III-like activity occurred despite a sharp reduction in motilin levels. Only the highest dose of calcitonin reduced significantly acid secretion (by more than 90%) while both doses reduced amylase secretion by respectively 65 and 71% when compared to the control levels. These changes in motility and secretion could partly explain the diarrhea observed in patients with the medullary carcinoma of the thyroid.     

Inhibition of gastric acid secretion by intracerebral injection of calcitonin gene related peptide in rats

Calcitonin gene related peptide (CGRP), a 37 amino acid peptide recently characterized from the rat brain, injected into the cisterna magna or the lateral hypothalamus, inhibited gastric acid secretion and increased serum gastrin levels in conscious pylorus-ligated rats. CGRP suppressed pentagastrin- and histamine-induced stimulation of gastric secretion in urethane-anesthetized gastric fistula rats. Peptide action was dose-dependent, not modified by adrenalectomy and peripheral sympathectomy induced by guanethidine pretreatment and reversed by vagotomy. These results demonstrate that intracisternal or intralateral hypothalamic injection of CGRP inhibits stimulated gastric acid secretion. The mechanism of action is vagal dependent and unrelated to modification of gastrin secretion or activation of sympathetic outflow.     

Stimulation of calcitonin secretion in the pig by calcitonin gene-related peptide

Pig thyroid glands were surgically isolated in situ and perfused with autologous blood to which was added known concentrations of calcitonin gene-related peptide (αCGRP). When thyroids were perfused with measured concentrations of CGRP within the range of 0.6–600 nM, the secretion rate of calcitonin (CT) was stimulated while the release of T₃, T₄, and somatostatin remained unchanged. Specific binding of ¹²⁵I-CGRP to pig thyroid plasma membranes was demonstrated, and binding was inhibited by unlabelled CGRP but not by CT or by other peptides unrelated structurally to CGRP. The findings indicate that the pig thyroid gland contains plasma membrane binding sites for CGRP and that CGRP is capable of stimulating the secretion of CT.     

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/10⁶ cells per h under basal conditions and 14.3 fmol in response to histamine.     

The nature of ABH blood group antigens in human gastric secretion

The origin of blood group ABH activity in human gastric content was investigated. Dialyzed and lyophilized samples of ten individual gastric secretions were assayed for ABH antigen under various conditions. The native activity persisted in delipidated residue of the respective secretions, but was completely missing in the lipid extracts of the analyzed samples. The alkaline degradation of the native and delipidated samples led to total loss of blood group activity of the analyzed materials, but no effect on A-active glycosphingolipid was evolved. Purified glycolipid portion of the lipid extract was lacking ABH activity and was shown to have distinct composition. This fraction contained only glyceroglucolipids and neither sphingosine nor other carbohydrates were present. On the basis of blood group activity assays of the native, delipidated, alkaline degraded samples and also on glycolipid analysis it was established that the ABH blood group activity of stomach secretion originated entirely from the glycoprotein portion of these samples.     

The calcium-sensing receptor acts as a modulator of gastric acid secretion in freshly isolated human gastric glands

Gastric acid secretion is activated by two distinct pathways: a neuronal pathway via the vagus nerve and release of acetylcholine and an endocrine pathway involving gastrin and histamine. Recently, we demonstrated that activation of H(+)-K(+)-ATPase activity in parietal cells in freshly isolated rat gastric glands is modulated by the calcium-sensing receptor (CaSR). Here, we investigated if the CaSR is functionally expressed in freshly isolated gastric glands from human patients undergoing surgery and if the CaSR is influencing histamine-induced activation of H(+)-K(+)-ATPase activity. In tissue samples obtained from patients, immunohistochemistry demonstrated the expression in parietal cells of both subunits of gastric H(+)-K(+)-ATPase and the CaSR. Functional experiments using the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5-(and 6)-carboxyfluorescein and measurement of intracellular pH changes allowed us to estimate the activity of H(+)-K(+)-ATPase in single freshly isolated human gastric glands. Under control conditions, H(+)-K(+)-ATPase activity was stimulated by histamine (100 microM) and inhibited by omeprazole (100 microM). Reduction of the extracellular divalent cation concentration (0 Mg(2+), 100 microM Ca(2+)) inactivated the CaSR and reduced histamine-induced activation of H(+)-K(+)-ATPase activity. In contrast, activation of the CaSR with the trivalent cation Gd(3+) caused activation of omeprazole-sensitive H(+)-K(+)-ATPase activity even in the absence of histamine and under conditions of low extracellular divalent cations. This stimulation was not due to release of histamine from neighbouring enterochromaffin-like cells as the stimulation persisted in the presence of the H(2) receptor antagonist cimetidine (100 microM). Furthermore, intracellular calcium measurements with fura-2 and fluo-4 showed that activation of the CaSR by Gd(3+) led to a sustained increase in intracellular Ca(2+) even under conditions of low extracellular divalent cations. These experiments demonstrate the presence of a functional CaSR in the human stomach and show that this receptor may modulate the activity of acid-secreting H(+)-K(+)-ATPase in parietal cells. Furthermore, our results show the viability of freshly isolated human gastric glands and may allow the use of this preparation for experiments investigating the physiological regulation and properties of human gastric glands in vitro.     

F.9 embryonal carcinoma cell calcitonin autocrine system: correlation between immunoreactive calcitonin secretion and calcitonin receptor number

Mouse teratocarcinoma cells in culture offer an in vitro system to study the initial steps of embryogenesis. It has been suggested that, at such early stages, cell functions are regulated by an autocrine process in which embryonic cells produce factors that in turn act on themselves. F.9 cells possess specific membrane receptors for calcitonin (CT) (120 fmol/mg of protein, Ka, = 3.5 X 10(8) M-1). These cells produce CT detected by heterologous radioimmunoassay in serum-free culture-conditioned medium (75 pg/10(7) cells/12 h). When F.9 cells are incubated in serum-free medium, CT binding and secretion concomitantly drop by 50% within the first 2 h, then increase progressively to an upper plateau after the sixth hour. Preincubation with 10(-6) M CT leads to disappearance of CT receptors and CT secretion in the culture medium up to 6 h. Avoiding accumulation of CT in the medium by a continuous flow rate for 6 h leads to a progressive decrease of CT receptors. In addition, retinoic acid treatment of cells induces a parallel progressive decrease of CT receptor number and of total CT synthesis. These results suggest a reciprocal regulation of CT receptors and CT secretion, or a close relationship between their regulations.     

A model for integrative study of human gastric acid secretion.

We have developed a unique virtual human model of gastric acid secretion and its regulation in which food provides a driving force. Food stimulus triggers neural activity in central and enteric nervous systems and G cells to release gastrin, a critical stimulatory hormone. Gastrin stimulates enterochromaffin-like cells to release histamine, which, together with acetylcholine, stimulates acid secretion from parietal cells. Secretion of somatostatin from antral and corpus D cells comprises a negative-feedback loop. We demonstrate that although acid levels are most sensitive to food and nervous system inputs, somatostatin-associated interactions are also important in governing acidity. The importance of gastrin in acid secretion is greatest at the level of transport between the antral and corpus regions. Our model can be applied to study conditions that are not yet experimentally reproducible. For example, we are able to preferentially deplete antral or corpus somatostatin. Depletion of antral somatostatin exhibits a more significant elevation of acid release than depletion of corpus somatostatin. This increase in acid release is likely due to elevated gastrin levels. Prolonged hypergastrinemia has significant effects in the long term (5 days) by promoting enterochromaffin-like cell overgrowth. Our results may be useful in the design of therapeutic strategies for acid secretory dysfunctions such as hyper- and hypochlorhydria.