Differential effects of extracellular calcium removal and nonspecific effects of Ca2+ antagonists on acid secretory activity in isolated gastric glands

Histamine H2 Receptor in Blood Cells: A Suitable Target for the Treatment of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) consists in a cancer of early hematopoietic cells arising in the bone marrow, most often of those cells that would turn into white blood cells (except lymphocytes). Chemotherapy is the treatment of choice for AML but one of the major complications is that current drugs are highly toxic and poorly tolerated. In general, treatment for AML consists of induction chemotherapy and post-remission therapy. If no further post-remission is given, almost all patients will eventually relapse. Histamine, acting at histamine type-2 (H₂) receptors on phagocytes and AML blast cells, helps prevent the production and release of oxygen-free radicals, thereby protecting NK and cytotoxic T cells. This protection allows immune-stimulating agents, such as interleukin-2 (IL-2), to activate cytotoxic cells more effectively, enhancing the killing of tumor cells. Based on this mechanism, post-remission therapy with histamine and IL-2 was found to significantly prevent relapse of AML. Alternatively, another potentially less toxic approach to treat AML employs drugs to induce differentiation of malignant cells. It is based on the assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment results in tumor reprogramming and the induction of terminal differentiation. There are promissory results showing that an elevated and sustained signaling through H₂ receptors is able to differentiate leukemia-derived cell lines, opening the door for the use of H₂ agonists for specific differentiation therapies. In both situations, histamine acting through H₂ receptors constitutes an eligible treatment to induce leukemic cell differentiation, improving combined therapies.

Heterogeneity of acid secretion induced by carbachol and histamine along the gastric gland axis and its relationship to [Ca2+]i

The gastric glands of the mammalian fundic mucosa are constituted by different cell types. Gastric fluid is a mixture of acid, alkali, ions, enzymes, and mucins secreted by parietal, chief, and mucous cells. We studied activation of acid secretion using LysoSensor Yellow/Blue in conjunction with fluo 3 to measure changes in pH and Ca(2+) in isolated rabbit gastric glands. We evidenced a spatial heterogeneity in the amplitude of acid response along the gland axis under histamine and cholinergic stimulation. Carbachol induced a transitory pH increase before acidification. This relative alkalinization may be related to granule release from other cell types. Omeprazole inhibited the acid component but not the rise in pH. Histamine stimulated acid secretion without increase of lumen pH. We studied the relationship between Ca(2+) release and/or entry and H(+) secretion in glands stimulated by carbachol. Ca(2+) release was associated with a fast and transient components of H(+) secretion. We found a linear relationship between Ca(2+) release and H(+) secretion. Ca(2+) entry was associated with a second slow and larger component of acid secretion. The fast component may be the result of activation of Cl(-) and K(+) channels and hence H(+)/K(+) pumps already present in the membrane, whereas the slow component might be associated with translocation of H(+)/K(+) pumps to the canaliculi. In conclusion, with cholinergic stimulation, gastric glands secrete a mixture of acid and other product(s) with a pH above 4.2, both triggered by Ca(2+) release. Maintenance of acid secretion depends on Ca(2+) entry and perhaps membrane fusion.

Abrogation of DTH response and mitogenic lectin- and alloantigen-induced activation of lymphocytes by calcium inhibitors TMB-8 and BAPTA-AM

In vitro treatment of mouse lymphocytes with an intracellular calcium chelator BAPTA-AM significantly decreased lectin Concanavalin-A (Con-A)-induced and mixed lymphocyte reaction (MLR) mediated, alloantigen-induced lymphocyte activation as indicated by decreased percentage of lymphoblasts among the BAPTA-treated lymphocytes. In vivo treatment of mice with intracellular Ca(2+) antagonist TMB-8 was found to substantially impair delayed type hypersensitivity (DTH; cell mediated immune) response, as indicated by decreased footpad swelling on tuberculin challenge of mice sensitized with BCG, after a single treatment with a low dose of 0.01mg of TMB-8 per mouse. Interestingly, a second injection of a higher dose of TMB-8 (0.1mg per mouse) resulted in very significant (p=0.001) abrogation of DTH as indicated by complete absence of swelling of foot pad after PPD challenge in BCG-primed treated mice. All mice in this group showed fully impaired DTH response. Lymphocytes of allosensitized mice gave a significantly higher (p<0.05) MLR response than the naïve mice. However, a single treatment of allosensitized mice with 0.1mg TMB-8 resulted into a lower MLR response, comparable in magnitude to that of untreated naïve mice.

Gastric acid secretion in L-histidine decarboxylase-deficient mice

BACKGROUND & AIMS

Histamine, gastrin, and acetylcholine are known to be the primary secretagogues of gastric acid secretion, but how the roles are shared among these secretagogues remains to be fully clarified. To evaluate the cooperation between histamine and the other secretagogues, acid secretion responses induced by each secretagogue were measured in L-histidine decarboxylase (HDC)-deficient mice.

METHODS

Acid secretion was measured by the titration of acid under anesthesia. The expression of selected genes involved in acid secretion was determined by Northern blot and/or immunoblot analysis. Histamine-2 (H(2)) receptor binding in the gastric mucosa was investigated using [(3)H]tiotidine.

RESULTS

HDC-deficient mice showed low basal and high exogenous histamine-stimulated acid secretion. The mutant mice showed hypergastrinemia and did not undergo acid secretion upon treatment with exogenous gastrin. However, carbachol stimulated weak and transient acid secretion in the mutants. The Bmax values for H(2) and the expression of Gs alpha in gastric mucosal membranes were higher in the mutants than in the wild-type mice.

CONCLUSIONS

This study confirms the concept that histamine production is essential for gastric acid secretion induced by gastrin, but not for that induced by carbachol. HDC-deficient mice should be a suitable model for further functional analyses of the correlation between histamine and the other acid secretagogues.

Role of neuropeptide-sensitive L-type Ca(2+) channels in histamine release in gastric enterochromaffin-like cells

Peptides release histamine from enterochromaffin-like (ECL) cells because of elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) by either receptor-operated or voltage-dependent Ca(2+) channels (VDCC). To determine whether VDCCs contribute to histamine release stimulated by gastrin or pituitary adenylate cyclase-activating polypeptide (PACAP), the presence of VDCCs and their possible modulation by peptides was investigated in a 48-h cultured rat gastric cell population containing 85% ECL cells. Video imaging of fura 2-loaded cells was used to measure [Ca(2+)](i), and histamine was assayed by RIA. Cells were depolarized by increasing extracellular K(+) concentrations or by 20 mM tetraethylammonium (TEA(+)). Cell depolarization increased transient and steady-state [Ca(2+)](i) and resulted in histamine release, dependent on extracellular Ca(2+). These K(+)- or TEA(+)-dependent effects on histamine release from ECL cells were coupled to activation of parietal cells in intact rabbit gastric glands, and L-type channel blockade by 2 microM nifedipine inhibited 50% of [Ca(2+)](i) elevation and histamine release. N-type channel blockade by 1 microM omega-conotoxin GVIA inhibited 25% of [Ca(2+)](i) elevation and 14% of histamine release. Inhibition was additive. The effects of 20 mM TEA(+) were fully inhibited by 2 microM nifedipine. Both classes of Ca(2+) channels were found in ECL cells, but not in parietal cells, by RT-PCR. Nifedipine reduced PACAP-induced (but not gastrin-stimulated) Ca(2+) entry and histamine release by 40%. Somatostatin, peptide YY (PYY), and galanin dose dependently inhibited L-type Ca(2+) channels via a pertussis toxin-sensitive pathway. L-type VDCCs play a role in PACAP but not gastrin stimulation of histamine release from ECL cells, and the channel opening is inhibited by somatostatin, PYY, and galanin by interaction with a G(i) or G(o) protein.

Ezrin-calpain I interactions in gastric parietal cells

Gastric ezrin, a membrane-cytoskeletal linker with sequence homology to talin and erythrocyte band 4.1, has been associated with the remodeling of parietal cell apical membrane that occurs with adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase stimulation. Here we examine the interrelationship between parietal cell ezrin and Ca(2+)-dependent protease activity. Addition of Ca2+ to sonicated gastric gland preparations rendered a relatively selective proteolysis of the 80-kDa ezrin, accompanied by the appearance of a 55-kDa breakdown product. Ca(2+)-dependent proteolysis of ezrin was blocked by E64, a cysteine protease inhibitor, or calpastatin, indicating calpain as the responsible protease. Degradation of ezrin in intact gastric glands was achieved by varying extracellular [Ca2+] and [ionomycin]. Ezrin degradation in situ was rapid and relatively selective, although Ca(2+)-dependent degradation of some spectrin-like bands was also observed. The effect of activated calpain I on parietal cell function was assessed by probing the secretory response to histamine stimulation using [14C]aminopyrine uptake, along with parallel measurements of calpain activity, over a wide range of ionomycin. Activation of calpain, as evidenced by loss of parietal cell ezrin, was correlated with decreased AP uptake by stimulated gastric glands, supporting a role for ezrin in the oxyntic secretory process. The calpain-ezrin interaction established here, and the similarities of calpain with talin and erythrocyte band 4.1, suggest a common feature to this family of ezrin/band 4.1/talin proteins that have been implicated in membrane-cytoskeletal association.

Receptor-operated Ca2+ channels in gastric parietal cells: gastrin and carbachol induce Ca2+ influx in depleting intracellular Ca2+ stores

The mechanism whereby gastrin-type receptor and muscarinic M3-type receptor regulate free intracellular Ca2+ concentration ([Ca2+]i) was studied in rabbit gastric parietal cells stimulated by either gastrin or carbachol. Both agonists induced a biphasic [Ca2+]i response: a transient [Ca2+]i rise, followed by a sustained steady state depending on extracellular Ca2+. Gastrin and carbachol also caused a rapid and transient increase in Mn2+ influx (a tracer for bivalent-cation entry). Pre-stimulation of cells with one agonist drastically decreased both [Ca2+]i increase and Mn2+ influx induced by the other. Neither diltiazem nor pertussistoxin treatment had any effect on agonist-stimulated Mn2+ entry. Thapsigargin, a Ca(2+)-pump inhibitor, induced a biphasic [Ca2+]i increase, and enhanced the rate of Mn2+ entry. Preincubation of cells with thapsigargin inhibits the [Ca2+]i increase as well as Mn2+ entry stimulated by gastrin or by carbachol. Thapsigargin induced a weak but significant increase in Ins(1,4,5)P3 content, but this agent had no effect on the agonist-evoked Ins(1,4,5)P3 response. In permeabilized parietal cells, Ins(1,4,5)P3 and caffeine caused an immediate Ca2+ release from intracellular pools, followed by a reloading of Ca2+ pools which can be prevented in the presence of thapsigargin. We conclude that (i) gastrin and carbachol mobilize common Ca2+ intracellular stores, (ii) Ca2+ permeability secondary to receptor activation involves neither a voltage-sensitive Ca2+ channel nor a GTP-binding protein from the G1 family, and (iii) agonists regulate common Ca2+ channels in depleting intracellular Ca2+ stores.

Muscarinic responses of gastric parietal cells

Isolated rabbit gastric glands were used to study the nature of the muscarinic cholinergic responses of parietal cells. Carbachol (CCh, 100 microM) stimulation of acid secretion, as measured by the accumulation of aminopyrine, was inhibited by the M1 antagonist, pirenzepine, with an IC50 of 13 microM; by the M2 antagonist, 11,2-(diethylamino)methyl-1 piperidinyl acetyl-5,11-dihydro-6H-pyrido 2,3-b 1,4 benzodiazepin-6-one (AF-DX 116), with an IC50 of 110 microM; and by the M1/M3 antagonist, diphenyl-acetoxy-4-methylpiperidinemethiodide (4-DAMP), with an IC50 of 35 nM. The three antagonists displayed equivalent IC50 values for the inhibition of carbachol-stimulated production of 14CO2 from radiolabeled glucose, which is a measure of the turnover of the H,K-ATPase, the final step of acid secretion. Intracellular calcium levels were measured in gastric glands loaded with FURA 2. Carbachol was shown to both release calcium from an intracellular pool and to promote calcium entry across the plasma membrane. The calcium entry was inhibitable by 20 microM La3+. The relative potency of the three muscarinic antagonists for inhibition of calcium entry was essentially the same as for inhibition of acid secretion or pump related glucose oxidation. Image analysis of the glands showed the effects of carbachol, and of the antagonists, on intracellular calcium were occurring largely in the parietal cell. The rise in cell calcium due to release of calcium from intracellular stores was inhibited by 4-DAMP with an IC50 of 1.7 nM, suggesting that the release pathway was regulated by a low affinity M3 muscarinic receptor or state; Ca entry and acid secretion are regulated by a high affinity M3 muscarinic receptor or state, inhibited by higher 4-DAMP concentrations (greater than 30 nM), suggesting that it is the steady-state elevation of Ca that is related to parietal cell function rather than the [Ca]i transient. Displacement of 3H N-methyl scopolamine (NMS) binding to purified parietal cells by CCh showed the presence of two affinities for CCh, but only a single affinity for 4-DAMP and lower affinity for pirenzepine and AFDX 116, providing further evidence for the parietal cell location of the [Ca]i response. Elevation of steady-state [Ca]i levels with either ionomycin or arachidonic acid did not replicate M3 stimulation of acid secretion or glucose oxidation, hence elevation of [Ca]i is necessary but not sufficient for acid secretion.

Second messengers in the gastric gland: a focus on calcium

The rabbit gastric gland model was used to study the nature of the muscarinic cholinergic and gastrin responses of parietal cells. Carbachol (100 microM) stimulation of acid secretion, as measured by the accumulation of aminopyrine, was inhibited by the M1 antagonist pirenzepine with an IC50 of 13 microM; by the M2 antagonist 11,2-(diethylamino)methyl-1-piperidinyl acetyl-5,11-dihydro-6H-pyrido 2,3-b 1,4-benzodiazepin-6-one (AF-DX 116) with an IC50 of 110 microM; and by the M3 antagonist diphenylacetoxy-4-methylpiperidinemethiodide (4-DAMP) with an IC50 of 35nM. The three antagonists displayed similar IC50 values for the inhibition of carbachol-stimulated production of 14CO2 from radiolabeled glucose, which is a measure of the turnover of the H(+)-H(+)-ATPase. Intracellular calcium levels wer measured in gastric glands loaded with FURA2. Carbachol was shown both to release calcium from an intracellular pool and to promote calcium entry across the plasma membrane. The calcium entry was inhibitable by 20 microM La3+. The relative potency of the three muscarinic antagonists for inhibition of calcium entry was essentially the same as for inhibition of acid secretion or metabolism. However, the rise in cell calcium due to release of calcium from intracellular stores was inhibited by 4-DAMP with an IC50 of 1.7 nM. Image analysis confirmed that the effect of carbachol and of the antagonists on intracellular calcium was occurring in the partial cell. In particular, the high-affinity inhibition of calcium release by 4-DAMP occurs in the parietal cell. Accordingly, it appears that the secretory receptor of the parietal cell is of the M3 type, and acid secretion depends on the entry of calcium rather than on calcium release from intracellular stores. In parallel experiments gastrin (G-17-sulfated) produced a dose-dependent increase in intracellular calcium (EC50, 0.14 +/- 0.013 microM). No stimulation of acid secretion was observed, but pepsinogen secretion was stimulated dose-dependently (EC50 = 1.17 +/- 0.21 microM).

Calcium oscillations and morphological transformations in single cultured gastric parietal cells

Calcium is an important regulator of cellular activities including HCl secretion by parietal cells. With cholinergic agonists, a role for calcium is established; however, with histamine, at least two signaling pathways may be involved including calcium and adenosine 3',5'-cyclic monophosphate (cAMP). Because chelation of medium and/or cellular calcium has pronounced inhibitory effects on cholinergic but lesser effects on histamine-stimulated acid secretory responses in cell populations, the calcium pathway may not be of central importance for HCl secretion regulated by histamine. We have used digitized video imaging of fura-2 fluorescence ratios and cellular morphology to determine more precisely the relationship between cellular calcium signaling mechanisms and acid secretion in single cultured rabbit parietal cells. Calcium signaling patterns were found to exhibit striking differences with histamine as compared with the cholinergic agonist carbachol. Maximal doses of histamine initiated repetitive oscillations in intracellular calcium ([Ca2+]i) in approximately 50% of cells, whereas the maximal carbachol response was characterized by a typical initial spike followed by a sustained elevation in [Ca2+]i. Oscillations in response to carbachol were detected only at doses below the half-maximal concentration for initiation of acid secretion. Correlation of gradual expansion of acidic vacuoles with increases in [Ca2+]i in the same cells indicated that approximately 20% of cells increased acid secretory-related activities in response to histamine with no detectable rise in [Ca2+]i. These data suggest two possibilities: 1) a rise in [Ca2+]i is not necessary for histamine-stimulated HCl secretion, or 2) heterogeneous receptor-coupling mechanisms exist in parietal cell populations with either calcium or cAMP mechanisms predominating in different subpopulations. The ability to assess simultaneously acid secretory-related responses and calcium signaling patterns allows, for the first time, correlation of "physiological" and biochemical responses in single parietal cells. This methodology is expected to provide new insight into second messenger control mechanisms that are not possible either in cell populations or acutely isolated parietal cells that do not exhibit morphological transformations detectable at the light microscope level.

Gastric H+,K(+)-ATPase as a therapeutic target in peptic ulcer disease

The presence of unbuffered acid appears to be an essential contributory factor in the pathogenesis of peptic ulcer disease. Treatment has concentrated therefore on the reduction of acidity, and the last decade has seen the widespread and effective use of H2 antagonists. They are, at low doses, more successful in improving the natural history of duodenal ulcer disease than of gastric or esophageal ulceration. The H2 receptor plays a central role in activation of parietal cell acid secretion, and antagonists at this receptor block most (but not all) of the acid secretion due to even gastrinergic or muscarinic (vagal) stimulation. In hypergastrinemic states such as Zollinger-Ellison syndrome, or where acid secretion has to be inhibited by more than 20% over a 24-hr period, such as for treatment of esophagitis, NSAID damage, or gastric ulcers, the dose and frequency of administration of the currently available antagonists must be increased to achieve reliable therapy. This has led to a search for an alternative target for acid inhibitory drugs, such as the gastric acid pump, the H+,K(+)-ATPase. This article focuses on the function of this ATPase and suggests that inhibition of this pump will provide a more efficacious means of reduction of acid secretion by the stomach, hence improving and simplifying therapy of acid related diseases.

HCl secretion and [Ca2+]i in cultured parietal cells

Gastric parietal cells in primary culture have been tested to determine their utility as a model for the study of the role of intracellular calcium ([Ca2+]i) in the control of HCl secretion. Changes in [Ca2+]i were measured in single cells on a microscope stage using the cell-permeant form of the fluorescent calcium probe, fura-2. Simultaneous images of cell fluorescence and morphology were acquired using a digitized video image analysis system and two video cameras, one for low-light level fluorescence detection and one for high resolution DIC/transmitted light images. Both histamine and carbachol, which are known stimulants of HCl secretion, increased [Ca2+]i and stimulated dramatic changes in morphology in these cultured cells. Changes in morphology were accompanied by an increased uptake of the weak base, [14C]-aminopyrine (AP), and a shift from green to red fluorescence of another weak base, acridine orange. These results indicate that cultured parietal cells, maintained under controlled conditions on a microscope stage, retain viability and secretagogue responsiveness. Thus, this cellular model appears to be suitable for correlation of changes in [Ca2+]i and activation of HCl secretion.

Dissociation of gastric acid and pepsinogen secretion in response to mercaptomethylimidazole--a new secretory compound

Mercaptomethylimidazole (MMI), a potent antithyroid drug of the thionamide group, induces both acid and pepsinogen secretion independently in control and pylorus ligated mice. The effect is dose dependent and the drug is more effective than histamine, carbachol or isoproterenol when administered by an intraperitoneal route. MMI-stimulated pepsinogen secretion could be dissociated from the acid secretion by the use of cimetidine and omeprazole which effectively block the acid secretion without affecting the pepsinogen output. Neither acid nor pepsinogen secretion by MMI is inhibited by atropine indicating a lack of muscarinic receptor involvement in both of the processes. Nifedipine and verapamil, the calcium antagonists, by inhibiting the MMI-induced acid secretion can also dissociate pepsinogen secretion from the acid secretion. Clonidine, an alpha 2-agonist, and hexobarbital, a membrane active barbiturate, also inhibit acid secretion without affecting the pepsinogen output. These data indicate that MMI induces pepsinogen secretion independent of acid secretion. Furthermore, MMI-stimulated acid secretion is not additive with that of the histamine indicating same site (H2-receptor) of action while its synergistic effect in presence of carbachol (muscarinic receptor) indicates different site of interaction of the two compounds. On the other hand, an additive effect of MMI and carbachol on pepsinogen secretion indicates that while the carbachol effect is mediated through the muscarinic receptor, MMI stimulates pepsinogen secretion through some still unknown mechanism.

Carbachol-induced potentiation and inhibition of acid secretion by guinea pig gastric gland

The effects of muscarinic ligands on acid secretion were examined by estimating the accumulation of [14C]aminopyrine in gastric glands isolated from guinea pigs. The accumulation of [14C]aminopyrine in the presence of 0.1 mM histamine was potentiated by 1 microM carbachol but suppressed by 1 mM. These two effects of carbachol were abolished by atropine, pirenzepine and AF-DX 116. Assuming that the binding of carbachol to one site (Site 1) increases [14C]aminopyrine accumulation but its binding to the other site (Site 2) reduces [14C]aminopyrine accumulation, we analysed the dose-response curves for the carbachol effects in the absence and presence of different concentrations of atropine, pirenzepine and AF-DX 116. The dissociation constants determined for these ligands at Sites 1 and 2 were as follows: carbachol, 0.28 and 7.1 microM; atropine, 0.28 and 0.54 nM; pirenzepine, 45 and 560 nM; and AF-DX 116, 380 and 4400 nM, respectively. The binding of [3H]N-methylscopolamine to the gastric glands indicated the presence of two populations of binding sites with different affinities for the above ligands, other than atropine. The apparent dissociation constants, which were estimated by analysing the displacement curves for [3H]N-methylscopolamine binding, were as follows: carbachol, 0.18 microM (10%) and 31 microM (90%); atropine, 1.24 nM; pirenzepine, 15 nM (16%) and 220 nM (84%); and AF-DX 116, 370 nM (10%) and 2970 nM (90%). These results suggest that there are two kinds of muscarinic acetylcholine receptors in the guinea pig gastric gland, one potentiating and the other inhibiting the acid secretion induced by histamine.

Calcium involvement in the muscarinic response of the gastric parietal cell

The influence of extracellular Ca2+ on the mediation of carbachol stimulation in isolated rabbit gastric parietal cells was studied. Removing Ca2+ from extracellular medium caused a 42% decrease of the aminopyrine accumulation due to carbachol with the same EC50 value (approximately 5 microM). A short time depletion in extracellular calcium suppressed the carbachol-dependent Ca2+ influx without affecting Ca2+ release from internal stores (fura-2 measurements). Similarly, the production of inositol phosphates under cholinergic stimulation was reduced by 29%. A rapid increase in Ins(1,4,5)P3 was obtained 5 s after carbachol stimulation, and this increase was not changed in Ca2(+)-depleted medium. In contrast, a 20 min incubation with carbachol caused a 50% reduction in both basal and carbachol-stimulated inositol phosphate accumulations. In conclusion, phospholipase C activation, intracellular Ca2+ release and aminopyrine accumulation were sequentially observed following carbachol stimulation of the isolated gastric parietal cell and extracellular calcium contributed to sustain this acid secretory response.

Histamine methyltransferase in dispersed cells from rabbit fundic mucosa

The cellular distribution of histamine N-methyltransferase was studied in rabbit gastric mucosa. The fundic mucosa was dispersed by collagenase treatment in Hanks' or calcium-free medium. In calcium-free medium, the number of dispersed cells/g wet tissue, as well as their viability was increased; histamine N-methyltransferase recovery was up to three-fold larger than in cells prepared in Hanks' medium. Furthermore, the calcium-free medium led to a greater acid secretory response, whereas the cellular pepsinogen content tended to be lower. Histamine N-methyltransferase activity was found in all cell fractions but was higher in the larger cell types. The enzyme activity showed only a partial correlation with either oxyntic or chief cells. These results indicate that the use of calcium-free medium to disperse and isolate rabbit mucosal cells improves cell quality. Histamine N-methyltransferase in the rabbit fundic mucosa, is found in more than one cell type, primarily the oxyntic and chief cells.

Role of Ca2+ in H+ transport by rabbit gastric glands studied with A23187 and BAPTA, an incorporated Ca2+ chelator

The role of Ca2+ in stimulation of H+ gastric secretion by cAMP-dependent and -independent secretagogues was studied in isolated rabbit glands using Ca2+ ionophore, A23187, and an intracellular Ca2+ chelator (BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) incorporated as its acetoxymethyl ester (BAPTA-AM). Acetylcholine (ACh), tetragastrin (TG), histamine and forskolin induced a transitory increase of intracellular Ca2+ concentration, [Ca2+]i, measured in gastric glands loaded with Ca2+-sensitive dye fura-2, and provoked an acid secretory response evaluated with aminopyrine accumulation ratio (AP ratio). The Ca2+-ionophore A23187 also induced an increase in [Ca2+]i and in AP ratio. cAMP-dependent secretagogues were more potent stimulants of acid secretion than cAMP-independent secretagogues. cAMP analogue, 8-bromo-adenosine 3',5'-cyclic monophosphate (8-BR-cAMP) induced an increase in AP ratio without modifying [Ca2+]i. BAPTA-AM (5-25 microM) induced a transient decrease of resting [Ca2+]i which returned to basal level due to extracellular Ca2+ entry. Increases in [Ca2+]i produced by ACh and TG were abolished by BAPTA and those produced by Ca2+ ionophore A23187 were partially buffered. BAPTA inhibited in a dose-dependent manner H+ secretion induced by cholinergic and gastrinergic stimulants in the presence of cimetidine. A23187 increased the AP ratio to values similar to those obtained with ACh or TG and was not inhibited by BAPTA. BAPTA partially inhibited (40%) the increase in AP ratio induced by forskolin and histamine inspite of the complete inhibition of the Ca2+ response. BAPTA did not inhibit the response to 8-BR-cAMP. BAPTA inhibition of forskolin stimulation was reversed by A23187 and the response was potentiated. These results indicate that ACh and TG response are completely dependent on an increase of [Ca2+]i. The response to cAMP-dependent agonists histamine and forskolin depend both on Ca2+ and cAMP. For forskolin stimulation the response may be the result of a potentiation between Ca2+ and cAMP.

Fura 2 analysis of cytosolic calcium regulation in elutriated rat gastric parietal cells

The calcium probe, Fura 2, is used to establish and partially characterize histamine-, carbachol-, and forskolin-induced calcium transients in enriched parietal cell populations prepared by centrifugal elutriation of dispersed rat fundic mucosa cell isolates. The magnitude of the maximal carbachol response, which is blocked by atropine but not cimetidine, is nearly five times that of histamine or forskolin. Time to peak responses for carbachol, forskolin, and histamine are approximately 7, 17, and 28 sec, respectively. Carbachol-, histamine-, and forskolin-induced increases in Fura 2 fluorescence appear dependent upon extracellular calcium, since these responses are attenuated in low calcium media and blocked by EGTA in low-calcium media or by lanthanum in high- or low-calcium medium. Trifluoperazine and fenoctimine, at concentrations that inhibit secretion, have no effect on either carbachol- or histamine-induced increases in cytosolic calcium. Seven major calcium/EGTA-sensitive phosphoproteins are identified by SDS-PAGE electrophoresis of ATP 32P-labeled cell sonicates. We conclude that cytosolic calcium in enriched rat gastric parietal cell populations is regulated by secretagogue receptor-controlled calcium channels. We postulate that these channels may be controlled by cyclic AMP-dependent phosphorylation, since neither changes in cyclic AMP nor calcium alone mediate the effects of secretagogues entirely, but the interplay between these two second-messenger systems potentiates the actions of these agents. The role of cytosolic calcium as a second messenger in secretagogue action appears similar to that of cyclic AMP in that a specific cellular concentration must be reached to initiate acid secretion.

Histamine H2-receptor mediated stimulation of gastric acid secretion by mercaptomethylimidazole

Intraperitoneal administration of mercaptomethylimidazole (methimazole), a potent antithyroid drug belonging to the thionamide group, caused a significant increase in gastric secretion both in control and pylorus-ligated mice. The drug also induced significant stimulation of gastric acid and pepsinogen secretion in both the animal systems studied. The dose-response curve indicated a nearly 10-fold increase in acid output by injection of 0.55 mg mercaptomethylimidazole per 25 g body weight. The duration profile of the drug response at the dose mentioned showed acid secretion almost at a linear rate up to 2.5 hr, after which the response decreased to some extent. Of the other antithyroid drugs of the same family, only thiourea activated acid secretion but the response was much smaller than mercaptomethylimidazole. Histamine, one of the physiological secretagogues of gastric acid secretion, was found to be less active than mercaptomethylimidazole. Mercaptomethylimidazole-induced stimulation of acid secretion could be effectively blocked by prior administration of cimetidine and completely by omeprazole and not by atropine. Verapamil and nifedipine had also some inhibitory effect. These observations indicate that mercaptomethylimidazole stimulates HCl secretion through the involvement of H2-receptor and through the functioning of the H+-K+-ATPase of the parietal cells. The bulk movement of water during increased HCl secretion was partially sensitive to cimetidine and omeprazole and was also associated with an increased secretion of Na+ and K+ in the gastric juice. This indicates that mercaptomethylimidazole also induced water transport through a separate mechanism.

Intracellular Ca requirements for stimulus-secretion coupling in parietal cell

The role of cytosolic free Ca (Cai) in stimulating acid production by the parietal cell in response to the secretagogues carbachol (carb), histamine (hist), and dibutyryl adenosine 3',5'-cyclic monophosphate plus 3-isobutyl-1-methylxanthine (DBcAMP + IBMX) was evaluated. Microfluorimetry with fura-2 was used to measure Cai in single parietal cells within intact rabbit gastric glands. Acid production was determined in parallel experiments by monitoring the accumulation of [14C]aminopyrine (AP) in suspensions of glands. Carb increased peak Cai levels to 1 microM in a dose-dependent manner [concentration for half-maximal response (K0.5) = 8 microM] that correlated well with the dose dependence of carb-stimulated AP accumulation (K0.5 = 18 microM). The Ca chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was used to attenuate secretagogue-induced Cai increases. Incubating glands for 10 min with 1 and 10 microM BAPTA/AM caused resting Cai to decrease from 119 to 93 and 80 nM, respectively. BAPTA/AM, 1 and 10 microM, blocked carb-stimulated increases in Cai by 60 and 90% and AP accumulation by 50 and 90%. Hist, which increases cytosolic cAMP, caused small and relatively infrequent increases in Cai. Even so, hist-stimulated AP accumulation was inhibited 8 and 40% by 1 and 10 microM BAPTA. DBcAMP had no effect on Cai, and AP accumulation caused by DBcAMP was unaffected by the concentrations of BAPTA tested. These data suggest that carb requires an increase in Cai as a secretory signal. Hist also exhibited some Cai dependence, which may be attributable to either a small increase in Cai or the necessity of having a specific basal level of Cai. A Cai requirement for DBcAMP-stimulated acid secretion was not detected. Thus the parietal cell possesses both Ca-dependent and Ca-independent stimulatory pathways, and at least one secretagogue (hist) may utilize both pathways.

Effects of verapamil on gastric acid secretion in vitro and in vivo

The activity of H,K-ATPase, the gastric acid producing enzyme, was concentration dependently inhibited by verapamil in the mM range. Verapamil concentration dependently inhibited acid formation in gastric glands, measured as [14C]aminopyrine accumulation or oxygen consumption. The IC50 values were in the microM range. No inhibition of acid secretion by verapamil was observed in Heidenhain-pouch dogs and stomach-lumen-perfused rats. However, in pylorus-ligated rats an inhibition was observed, this effect is related to its cardiovascular effectiveness. To understand the action of verapamil, its physicochemical properties were considered. Verapamil is a highly lipophilic base with a pKa of 8.7. It accumulates in membranes and in the acidic spaces of the parietal cell. We suggest that the inhibition of vesicular bound H,K-ATPase is dependent on a non-specific accumulation of verapamil in the membrane (detergent effect) and that inhibition of acid production in vitro is due to an additional accumulation of the drug in acidic compartments, leading to an impaired function of the proton pump. Verapamil does not decrease acid secretion in vivo by this mechanism as the required dose would be higher than the dose that causes a strong depression of the cardiovascular system.

Relationships of secretagogue-induced cAMP accumulation and acid secretion in elutriated rat gastric parietal cells

Centrifugal elutriation was adapted and analyzed as a method to separate rat gastric parietal cells from other fundic mucosal cells. Elutriated parietal cell fractions provided sufficient purity by morphological criteria, and fluorescence activated cell sorting analyses, yield, reproducibility and maintenance of functional responses. These characteristics allowed the study and comparison of the kinetics of histamine, isoproterenol, and forskolin-induced cyclic AMP and 14C-amino-pyrine accumulations in the presence of 1-methyl-3-isobutyl xanthine (IBMX) in parietal-cell-rich and parietal-cell-poor fractions. All three acid secretagogues studied produced the same maximal rate of acid secretion as judged by 14C-aminopyrine accumulation. Each secretagogue action peaked at different times and had different accumulation kinetics. For each agonist, cyclic AMP accumulation preceded secretion. However, the rate, extent, and temporal changes of cyclic AMP accumulation were independent of aminopyrine accumulation. HPLC separation and drug inhibition studies of cyclic nucleotide phosphodiesterase activities indicated the presence of multiple, high affinity (Type IV), but not lower affinity (Types I and II), enzyme forms in gastric mucosal cells. IBMX did not distinguish between the two forms, but SQ 65442 and RO 20-1724 were selective inhibitors. Inhibition constants of IBMX for phosphodiesterase hydrolysis agreed closely with its EC50 for histamine-stimulated acid secretion (2-8 microM). Elutriated parietal cells maintained their responses to selective receptor antagonists in the micromolar concentration range.

Protection by the calcium antagonist Wy-47,037 against stress ulceration in the rat

Wy-47,037, a novel compound with both intracellular and extracellular calcium-blocking properties, was evaluated for its effects on cold/restraint stress induced ulceration in the rat. Wy-47,037 dose-dependently inhibited ulcerogenesis; with an ED50 of 8 mg/kg, PO, it was approximately twice as potent as nitrendipine (ED50 = 15 mg/kg). Wy-47,037 also reduced basal gastric acid secretion (ED50 = 7 mg/kg) and gastrointestinal motility (ED50 = 16 mg/kg). It is thus possible that Wy-47,037's alteration of basal gastric acid secretion and/or of gastrointestinal motility may contribute to its therapeutic efficacy against stress induced ulcer formation.

Intracellular Ca regulation during secretagogue stimulation of the parietal cell

Intracellular [Ca] ([Ca]i) was measured following secretagogue stimulation of rabbit gastric glands loaded with the Ca-sensitive dye fura-2. Glands were mounted on cover slips and placed either in a perfused cuvette (for spectrofluorimetric measurements on whole glands) or in a chamber on the stage of a microscope (for microspectrofluorimetric measurements on single parietal cells within a gland). In parietal cells resting [Ca]i = 91 nM. Either histamine or carbachol caused [Ca]i to increase (spike) rapidly (within 5 s) to greater than 425 nM by releasing Ca from an intracellular store. The two hormones acted on the same store, with carbachol being the more potent releaser. The spike occurred equally well in solutions containing the Ca channel blockers verapamil, nifedipine, Co, or La. After the spike, [Ca]i decreased to a plateau level (130-200 nM) that was dependent on the presence of secretagogue but was independent from the release of the intracellular store. This plateau persisted (up to 40 min) until the addition of antagonist or until the removal of either extracellular Ca or the agonist. Histamine and carbachol were specifically blocked by the H2-antagonist cimetidine and the muscarinic antagonist atropine, respectively. Histamine often induced repeated increases in [Ca]i. A combination of dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) and the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) had no effect on [Ca]i, but if cells were pretreated with histamine, DBcAMP + IBMX would cause [Ca]i to increase. Because exposure to DBcAMP + IBMX stimulates acid secretion yet does not affect [Ca]i, two independent pathways to acid secretion may exist.

Release of intracellular Ca2+ and elevation of inositol trisphosphate by secretagogues in parietal and chief cells isolated from rabbit gastric mucosa

The fluorescent intracellular Ca2+ indicator, fura2/AM, was used to determine the effects of carbachol, cholecystokinin octapeptide (CCK-8), gastrin and histamine on intracellular Ca2+ ([Ca2+]i) in parietal cells from rabbit gastric mucosa enriched to more than 95% purity by a new Nycodenz gradient/centrifugal elutriation technique. Changes in [Ca2+]i in response to the same agonists were also measured in enriched chief cells. Carbachol, histamine, gastrin and CCK-8 increased parietal cell [Ca2+]i with the response to carbachol greater than CCK -8 = histamine = gastrin. Prestimulation with msximal doses of carbachol blocked histamine-induced increases in [Ca2+]i. In chief cells, carbachol increased [Ca2+]i but to a lesser degree than CCK-8, while histamine had no significant effect on [Ca2+]i. Neither removal of extracellular Ca2+ coupled with acute addition of 1 mM EGTA nor addition of the Ca2+-channel blocker nicardipine prevented agonist-induced changes in [Ca2+]i in either cell type. In the presence and absence of 10 mM LiCl2, carbachol and CCK-8 were found to increase inositol trisphosphate (IP3) content in both parietal and chief cells while histamine had no significant effect on this phosphoinositide hydrolysis product. From these results and previous observations with gastric glands (Chew, C.S. (1986) Am. J. Physiol. 13, G814-G823) we conclude that: carbachol, CCK-8, gastrin and histamine increase parietal cell [Ca2+]i initially by release of Ca2+ from the same intracellular store(s); the release of [Ca2+]i in response to carbachol and CCK-8 in both chief and parietal cells appear to be mediated by IP3; however, other mechanisms may be involved in histamine-induced release of parietal cell Ca2+.

Muscarinic stimulation of inositol phosphate accumulation and acid secretion in gastric fundic mucosal cells

The muscarinic agonist, carbachol (CCh), was shown to stimulate the production of inositol phosphates (IP) in isolated cells from rabbit fundic mucosa. This stimulatory effect was time- and dose-dependent: EC50 values for IP1, IP2 and IP3 accumulation were not statistically different. The mean value was 30 +/- 8 microM (n = 6). The corresponding maximal stimulation (% of basal value) observed after 20 min incubation in the presence of 100 microM CCh was 160 +/- 15%. CCh-induced IP accumulation was abolished by atropine (Ki = 0.32 +/- 0.18 nM (n = 3)). The CCh concentrations leading to half-maximal inhibition of N-[3H]methylscopolamine binding and half-maximal IP accumulation were similar. The half-maximal value for CCh-induced aminopyrine accumulation was 8-times lower. These results indicate that IP3-mediated mobilization of intracellular Ca2+ might be involved in CCh-induced acid secretion by parietal cells.