Electrical membrane responses to secretagogues in parietal cells of the rat gastric mucosa in culture

Ex vivo real-time observation of Ca(2+) signaling in living bone in response to shear stress applied on the bone surface

Bone cells respond to mechanical stimuli by producing a variety of biological signals, and one of the earliest events is intracellular calcium ([Ca(2+)](i)) mobilization. Our recently developed ex vivo live [Ca(2+)](i) imaging system revealed that bone cells in intact bone explants showed autonomous [Ca(2+)](i) oscillations, and osteocytes specifically modulated these oscillations through gap junctions. However, the behavior and connectivity of the [Ca(2+)](i) signaling networks in mechanotransduction have not been investigated in intact bone. We herein introduce a novel fluid-flow platform for probing cellular signaling networks in live intact bone, which allows the application of capillary-driven flow just on the bone explant surface while performing real-time fluorogenic monitoring of the [Ca(2+)](i) changes. In response to the flow, the percentage of responsive cells was increased in both osteoblasts and osteocytes, together with upregulation of c-fos expression in the explants. However, enhancement of the peak relative fluorescence intensity was not evident. Treatment with 18 α-GA, a reversible inhibitor of gap junction, significantly blocked the [Ca(2+)](i) responsiveness in osteocytes without exerting any major effect in osteoblasts. On the contrary, such treatment significantly decreased the flow-activated oscillatory response frequency in both osteoblasts and osteocytes. The stretch-activated membrane channel, when blocked by Gd(3+), is less affected in the flow-induced [Ca(2+)](i) response. These findings indicated that flow-induced mechanical stimuli accompanied the activation of the autonomous [Ca(2+)](i) oscillations in both osteoblasts and osteocytes via gap junction-mediated cell-cell communication and hemichannel. Although how the bone sense the mechanical stimuli in vivo still needs to be elucidated, the present study suggests that cell-cell signaling via augmented gap junction and hemichannel-mediated [Ca(2+)](i) mobilization could be involved as an early signaling event in mechanotransduction.

Genetic dissection reveals unexpected influence of beta subunits on KCNQ1 K+ channel polarized trafficking in vivo

Targeted deletion of the Kcne2 potassium channel β subunit gene ablates gastric acid secretion and predisposes to gastric neoplasia in mice. Here, we discovered that Kcne2 deletion basolaterally reroutes the Kcnq1 α subunit in vivo in parietal cells (PCs), in which the normally apical location of the Kcnq1-Kcne2 channel facilitates its essential role in gastric acid secretion. Quantitative RT-PCR and Western blotting revealed that Kcne2 deletion remodeled fundic Kcne3 (2.9±0.8-fold mRNA increase, n=10; 5.3±0.4-fold protein increase, n=7) but not Kcne1, 4, or 5, and resulted in basolateral Kcnq1-Kcne3 complex formation in Kcne2(-/-) PCs. Concomitant targeted deletion of Kcne3 (creating Kcne2(-/-)Kcne3(-/-) mice) restored PC apical Kcnq1 localization without Kcne1, 4, or 5 remodeling (assessed by quantitative RT-PCR; n=5-10), indicating Kcne3 actively, basolaterally rerouted Kcnq1 in Kcne2(-/-) PCs. Despite this, Kcne3 deletion exacerbated gastric hyperplasia in Kcne2(-/-) mice, and both hypochlorhydria and hyperplasia in Kcne2(+/-) mice, suggesting that Kcne3 up-regulation was beneficial in Kcne2-depleted PCs. The findings reveal, in vivo, Kcne-dependent α subunit polarized trafficking and the existence and consequences of potassium channel β subunit remodeling.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Real time measurements of water flow in amphibian gastric glands: modulation via the extracellular Ca2+-sensing receptor

The mechanisms for the formation of the osmotic gradient driving water movements in the gastric gland and its modulation via the extracellular Ca(2+)-sensing receptor (CaR) were investigated. Real time measurements of net water flux in the lumen of single gastric glands of the intact amphibian stomach were performed using ion-selective double-barreled microelectrodes. Water movement was measured by recording changes in the concentration of impermeant TEA(+) ions ([TEA(+)](gl)) with TEA(+)-sensitive microelectrodes inserted in the lumen of individual gastric glands. Glandular K(+) (K(+)(gl)) and H(+) (pH(gl)) were also measured by using K(+)- and H(+)-sensitive microelectrodes, respectively. Stimulation with histamine significantly decreased [TEA](gl), indicating net water flow toward the gland lumen. This response was inhibited by the H(+)/K(+)-ATPase inhibitor, SCH 28080. Histamine also elicited a significant and reversible increase in [K(+)](gl) that was blocked by chromanol 293B, a blocker of KCQN1 K(+) channels. Histamine failed to induce net water flow in the presence of chromanol 293B. In the "resting state," stimulation of CaR with diverse agonists resulted in significant increase in [TEA](gl). CaR activation also significantly reduced histamine-induced water secretion and apical K(+) transport. Our data validate the strong link between histamine-stimulated acid secretion and water transport. We also show that cAMP-dependent [K(+)](gl) elevation prior to the onset of acid secretion generates the osmotic gradient initially driving water into the gastric glands and that CaR activation inhibits this process, probably through reduction of intracellular cAMP levels.

Distribution, cloning and sequencing of GnRH, its receptor, and effects of gastric acid secretion of GnRH analogue in gastric parietal cells of rats

Our objective was to study the distribution of gonadotropin-releasing hormone (GnRH) and its receptor, cloning and sequencing of GnRH and its receptor gene in cultured gastric parietal cells of rats. The distribution of GnRH and its receptor mRNA were investigated through immunocytochemical ABC methods and in situ hybridization methods in cultured gastric parietal cells of rats. After isolation of the total RNA from the parietal cells, RT-PCR was conducted to obtain GnRH and its receptor cDNA. Then, the products of PCR was purified, digested by the restriction enzyme of Hind III and EcoR I, and DNA fragments of interests were cloned into pUC19 vector. The products of PCR were analyzed by sequencing with Sanger's method after identified by PCR and digestion of restriction enzyme. Gastric parietal cells showed GnRH and its receptor immunoreactivity; positive material was located in cytoplasm other than in nuclei. GnRH and its receptor mRNA hybridized signals were also detected in cytoplasm with negative nuclei. The specific amplified band of GnRH and its receptor sequences were detected through Agarose gel electrophoresis, and GnRH gene sequence is identical to that of GnRH which has been reported in rat hypothalamus and GnRH receptor sequence is identical to that of the pituitary of rat. GnRH analogue (Alarelin) could inhibit the gastric acid secretion both by direct actions on parietal cells and by inhibiting vagous function. Our data suggest that GnRH could be produced by gastric parietal cells of rats and may modulate physiological function of gastric parietal cells of rats through autocrinal and paracrinal way.

Electrophysiological investigation of microdissected gastric glands of bullfrog. I. Basolateral membrane properties in the resting state

In the present experiments we have made a new attempt to characterize the ion transport properties of H(+)-secreting cells of the gastric mucosa using electrophysiological techniques. Individual gastric glands of bullfrog fundus mucosa were manually dissected, mounted in holding pipettes and superfused with various test solutions while individual cells were punctured with conventional or H(+)-sensitive double-barrelled microelectrodes. All measurements were performed in the resting state (0.1 mmol/l cimetidine). In HCO3(-)-containing control Ringer solution the cell membrane potential (Vb) averaged -45.6 +/- 0.9 mV (+/- SEM, n = 54). From the fast initial Vb responses to changing bath K+, Na+, Cl- or HCO3- concentrations we deduced that the basolateral cell membrane contains conductances for K+, Na+, and Cl- but not for HCO3-, and that a Na(+)-HCO3- cotransporter is not present. The K+ conductance was inhibited by Ba2+ (3 mmol/l), but the Cl- conductance was not inhibited by 4,4' diisothiocyanato-stilbene-2,2' disulphonic acid (DIDS, 0.3 mmol/l), nor selectively inhibited by 5-nitro-2-(3)- phenylpropyl-aminobenzoate (NPPB, 10 mumol/l). In a great number of cells the Vb response to Cl- substitution revealed two components: an initial spiking depolarization which reflected conductive Cl- efflux and a secondary slow hyperpolarization, the origin of which was not immediately evident. Since the latter response could be mimicked by CO2-free perfusion, strongly depressed by Ba2+ and eliminated by DIDS, we conclude that it reflects HCO3- uptake into the cells via a DIDS sensitive Cl-/HCO3- exchanger which alkalinizes the cells and stimulates the basolateral K+ conductance. Our results confirm, revise and extend the results of previous, less direct, investigations of gastric cell ion transport.

Maxi K+ channels from the apical membranes of rabbit oviduct epithelial cells

Large conductance (approximately 210 pS), K(+)-selective channels were identified in excised, inside-out patches obtained from the apical membranes of both ciliated and nonciliated epithelial cells grown as monolayers from the primary culture of rabbit oviduct. The open probability of channels showing stable gating was increased at positive membrane potentials and was sensitive to the concentration of free calcium ions at the cytosolic surface of the patch ([Ca2+]i). In these respects, the channel resembled "maxi K+ channels" found in a number of other cell types. The distributions of dwelltimes in the open state were most consistently described by two exponential components. Four exponential components were fitted to the distributions of dwelltimes in the closed state. Depolarizations and [Ca2+]i increases had similar effects on the distribution of open dwelltimes, causing increases in the two open time constants (tau o1 and tau o2) and the fraction of events accounted for by the longer component of the distribution. In contrast, calcium ions and voltage had distinct effects on the distribution of closed dwelltimes. While the three shorter closed time constants (tau c1, tau c2 and tau c3) were reduced by depolarizing membrane potentials, increases in [Ca2+]i caused decreases in the longer time constants (tau c3 and tau c4). It is concluded that oviduct large conductance Ca(2+)-activated K+ channels can enter at least two major open states and four closed states.

Whole-cell K+ current activation in response to voltages and carbachol in gastric parietal cells isolated from guinea pig

Patch-clamp studies of whole-cell ionic currents were carried out in parietal cells obtained by collagenase digestion of the gastric fundus of the guinea pig stomach. Applications of positive command pulses induced outward currents. The conductance became progressively augmented with increasing command voltages, exhibiting an outwardly rectifying current-voltage relation. The current displayed a slow time course for activation. In contrast, inward currents were activated upon hyperpolarizing voltage applications at more negative potentials than the equilibrium potential to K+ (EK). The inward currents showed time-dependent inactivation and an inwardly rectifying current-voltage relation. Tail currents elicited by voltage steps which had activated either outward or inward currents reversed at near EK, indicating that both time-dependent and voltage-gated currents were due to K+ conductances. Both outward and inward K+ currents were suppressed by extracellular application of Ba2+, but little affected by quinine. Tetraethylammonium inhibited the outward current without impairing the inward current, whereas Cs+ blocked the inward current but not the outward current. The conductance of inward K+ currents, but not outward K+ currents, became larger with increasing extracellular K+ concentration. A Ca(2+)-mobilizing acid secretagogue, carbachol, and a Ca2+ ionophore, ionomycin, brought about activation of another type of outward K+ currents and voltage-independent cation currents. Both currents were abolished by cytosolic Ca2+ chelation. Quinine preferentially inhibited this K+ current. It is concluded that resting parietal cells of the guinea pig have two distinct types of voltage-dependent K+ channels, inward rectifier and outward rectifier, and that the cells have Ca(2+)-activated K+ channels which might be involved in acid secretion under stimulation by Ca(2+)-mobilizing secretagogues.

Histamine-induced chloride channels in apical membrane of isolated rabbit parietal cells

The electrical properties of the apical membrane of isolated rabbit parietal cells were studied using the patch-clamp technique. The apical membrane of the parietal cells plated on Matrigel and maintained in culture conditions was identified by lectin-binding studies. Cell-attached and excised inside-out patches from 10(-4) M cimetidine-treated parietal cells infrequently contained Cl- channels (9% of the patches). A single class of voltage-dependent outwardly rectifying Cl- channels with 24 +/- 1-pS conductance was observed in 75% of the patches from cells stimulated (acid secreting) by 10(-4) M histamine. Other anions passed through these channels with a permeability sequence of I- (1.2) greater than Br- (1.1) greater than or equal to Cl- (1.0) greater than NO3- (0.7) greater than SO4(2-) (0.1), but there was a very low permeability for Na+ or K+ (PCl-/PNa+ or PCl-/PK+ greater than 5). In inside-out patch configurations the Cl- channel was insensitive to Ba2+ and stilbene derivatives but was inhibited by diphenylamine-2-carboxylic acid in a manner characteristic of a reversible open-channel blocker. It is concluded that H2-receptor agonist stimulation of acid secretion by rabbit parietal cells activates Cl- channels in the apical cell membrane.

Effects of 16,16-dimethyl prostaglandin E2 on glycoprotein and lipid synthesis of gastric epithelial cells grown in a primary culture

We investigated the biosynthesis of phospholipid, neutral lipids, glycoproteins, and DNA in primary cultures of rat oxyntic mucosal cells. In addition, responses of these biosynthetic pathways to the gastric protective agent 16,16-dimethyl prostaglandin E2 (dmPGE2) were studied. Cultured gastric cells under control conditions synthesized glycoprotein in a linear manner over time. The cells responded to dmPGE2 with an increase in glycoprotein synthesis without an effect on DNA synthesis. Investigations of lipid synthesis showed that phospholipid was produced in a linear fashion by these cells, however, no effect of exogenously administered dmPGE2 on its rate of formation was discernible. In contrast, the incorporation of labeled palmitate into neutral lipids revealed that triglyceride biosynthesis was significantly increased by the addition of dmPGE2 to the culture medium, which could be further enhanced by the administration of the phosphodiesterase inhibitor, isobutyl methyl xanthine. Cyclic nucleotide involvement was further suggested by our finding that triglyceride synthesis in cultured gastric mucous cells could be increased a comparable amount by the addition of both dbcAMP and dbcGMP to the medium. The possible relationship between these biochemical alterations and the gastric protective action of dmPGE2 is discussed.

Acid secretagogues induce Ca2+ mobilization coupled to K+ conductance activation in rat parietal cells in tissue culture

Intracellular recordings from cultured parietal cells of the rat gastric fundus showed that carbachol, pentagastrin, histamine (in the presence of isobutylmethylxanthine; IBMX) and dibutyryl cyclic AMP induced hyperpolarizing responses which were sensitive to a K+ channel blocker, quinine. The Ca2+ ionophore, ionomycin, also induced a quinine-sensitive hyperpolarization. Deprivation of extracellular Ca2+ preferentially inhibited the hyperpolarizing responses to histamine (plus IBMX) and to dibutyryl cyclic AMP. Caffeine, oxalate and dantrolene sodium, which are known to affect Ca2+ transport in the endoplasmic reticulum, selectively inhibited the carbachol response. Mitochondrial inhibitors (KCN and carbonylcyanide p-trifluoromethoxyphenylhydrazone) preferentially suppressed the gastrin response. Cytosolic Ca2+ measurements with fura-2 indicated that significant increases in the intracellular concentration of free Ca2+ were induced not only by Ca2+-mediated acid secretagogues (carbachol and gastrin), but also by a cyclic AMP-mediated secretagogue (histamine plus IBMX). Dibutyryl cyclic AMP also increased cytosolic Ca2+ ions. It is concluded that stimulation of receptors to histamine, carbachol and gastrin gives rise to mobilization of Ca2+ ions into the cytoplasm from the different sources, thereby stimulating Ca2+-activated K+ channels in cultured rat parietal cells.

Transport properties of the basolateral membrane of the oxyntic cells in frog fundic gastric mucosa

The conductive properties of the basolateral membrane of oxyntic cells (OC) of frog fundic gastric mucosa were investigated by utilizing the microelectrode technique. By examining the response of the basolateral cell membrane potential difference, Vcs, to sudden ion concentration changes in the serosal bath it was concluded that the basolateral membrane of OC has a high Ba2+-sensitive K+-conductance, and no Cl- -conductance both in resting (cimetidine) and in stimulated (histamine) state. The response of Vcs to serosal Cl- -removal, consisting in a slight hyperpolarization (anomalous Nernst response), could not be explained by possible permeability changes to K+ and Na+ since the potential response to Cl- was essentially preserved by blocking K+-permeability with Ba2+ and replacing all Na+ by choline. Conversely, hyperpolarization of Vcs after Cl- -free perfusion was abolished by exposure to HCO3- -free solution, indicating that HCO3- -ions are required at the serosal bath for Cl- to get his effect. It was investigated wether the effect of Cl- was due to an electrogenic Na+ (HCO3-)n/Cl- exchange mechanism on the basolateral membrane. Experiments showed that the potential response to HCO3- -removal and to Na+-removal, consisting in a depolarization of Vcs, was similar both in presence and in absence of Cl-. Furosemide (0.5 mmol/l) had no effect on steady Vcs and Vt. The electrophysiological analysis of the data led to excluding the involvement of Na-Cl, Na-2Cl and NaK-2Cl cotransports, and to including the existence of an electrogenic Na+(HCO3-)n/Cl- exchange process, while suggests the presence of an electroneutral Cl-/HCO3- exchange mechanism to explain Cl- -transport across the basolateral membrane of OC.

Anion and cation channels in the basolateral membrane of rabbit parietal cells

Ion channels in the basolateral membrane of rabbit parietal cells in isolated gastric glands were studied by the patch clamp technique. Whole-cell current-clamp recordings showed that the membrane potential (Em) changed systematically as a function of the chloride concentrations of the basolateral bathing solution ([Cl-]0), and of the pipette (intracellular) solution. The relationship between Em and [Cl-]0 was not affected by additions of histamine, dibutyryl-cAMP, 4-acetoamido-4'-isothiocyanostilbene-2,2'-disulfonic acid and diphenylamine-2-carboxylate. The whole-cell Cl- conductance was insensitive to voltage. In cell-attached and cell-free patch membranes, however, single Cl- channel opening events could not be observed. The value of Em depended little on the basolateral K+ concentration, but inward-rectifier K+ currents were observed in the whole-cell configuration, activated by hyperpolarizing pulses and inhibited by extracellular Ba2+. In cell-attached and cell-free patches, openings of single inward-rectifier K+ channels and non-selective cation channels were infrequently recorded. Neither cAMP nor Ca2+ activated these cation channels. The single K+ channel conductance was about 230 pS under the symmetrical high K+ conditions and was inhibited by intracellular tetraethylammonium ions (TEA). The non-selective cation channel had a voltage-independent single conductance of 22 pS and was not inhibited by TEA.

Pharmacological control of the human gastric histamine H2 receptor by famotidine: comparison with H1, H2 and H3 receptor agonists and antagonists

Histamine 0.1 microM-0.1 mM increased adenylate cyclase activity five- to ten-fold in human fundic membranes, with a potency Ka = 3 microM. The histamine dose-response curve was mimicked by the H3 receptor agonist (R) alpha-MeHA, but at 100 times lower potency, Ka = 0.3 mM. Histamine-induced adenylate cyclase activation was abolished by H2, H1 and H3 receptor antagonists, according to the following order of potency IC50: famotidine (0.3 microM) greater than triprolidine (0.1 mM) thioperamide (2 mM), respectively. Famotidine has no action on membrane components activating the adenylate cyclase system, including the Gs subunit of the enzyme stimulated by forskolin and cell surface receptors sensitive to isoproterenol (beta 2-type), PGE2 and VIP. The Schild plot was linear for famotidine (P less than 0.01) with a regression coefficient r = 0.678. The slope of the regression line was 0.64 and differs from unity. Accordingly, famotidine showed a slow onset of inhibition and dissociation from the H2 receptor in human cancerous HGT-1 cells. The results demonstrate that famotidine is a potent and selective H2 receptor antagonist with uncompetitive actions in human gastric mucosa. Consequently, famotidine might be a suitable drug with long-lasting actions in the treatment of Zollinger-Ellison syndrome. The results also confirm and extend the previous observations that (R) alpha-MeHA and thioperamide are two selective ligands at histamine H3 receptor sites. In the human gastric mucosa, these drugs are respectively 330 and 6700 times less potent than histamine and famotidine on the adenylate cyclase system. The possible involvement of histamine H3 receptors in the regulation of gastric secretion is proposed.

Physiology and pharmacology of the parietal cell

The parietal cells possess the unique capacity to produce large quantities of acid at a high concentration, and this is reflected in unique properties at the cellular level. The cells are comparatively large, and they are equipped with secretory canaliculi, a multitude of mitochondria, and cytoplasmic tubulovesicles. During secretion many of the tubulovesicles merge with the secretory canaliculi, which then expand. In the process H+, K+-ATPase is transferred from the tubulovesicular membrane to the secretory membrane. This enzyme catalyses the final step in the production of HCl. Parietal cell activity is regulated through receptors on the basolateral cell surfaces. In the isolated gland and in the isolated parietal-cell fractions, stimulation of receptors for histamine evokes higher secretion than receptor stimulation with cholinergic compounds or with gastrin. In these experimental models, specific inhibitors are required to block acid secretion; for example histamine H2-receptor antagonists will block histamine-induced secretion but will be inactive when secretion is evoked by gastrin or by cholinergic stimulation. These stimuli cause a more or less marked increase in the intracellular levels of Ca2+, which acts as a second messenger, leading to the activation of phosphokinases and, ultimately, to morphological transformation of the parietal cells and acid secretion. Another such intracellular messenger is cAMP, which is formed in response to histamine stimulation only; prostaglandins may prevent this process and block acid secretion. The final step in the production of acid requires K+ and Cl- channels in the secretory membrane and the H+, K+-ATPase-catalysed exchange of K+ for H+ across this membrane. This reaction consumes large amounts of energy and depends on the aerobic production of ATP by the parietal cells. Substituted benzimidazoles, such as omeprazole, accumulate in the acid compartments of the parietal cells and inhibit the H+, K+-ATPase, thereby blocking acid production.

Effect of gastric secretagogues on the formation of inositol phosphates in isolated gastric cells of the rat

The effects of compounds affecting gastric acid secretion were studied on the formation of inositol phosphates after prelabelling with [3H]-inositol in enriched gastric parietal cells of the rat, prepared by isopycnic centrifugation with Percoll. In cell preparations with 60 to 70% parietal cells, carbachol (10(-6)-10(-2) M) enhanced the accumulation of [3H]-inositol monophosphate ([3H]-IP1), [3H]-inositol bisphosphate ([3H]-IP2) and [3H]-inositol trisphosphate ([3H]-IP3) in a concentration-dependent manner, an effect which was antagonized by 10(-8) M atropine. Li+ (0.5-30 mM) enhanced the basal and carbachol-induced accumulation of all three [3H]-inositol phosphates, the formation of [3H]-IP1 being more sensitive to Li+ than those of [3H]-IP2 and [3H]-IP3. The concentration of Ca2+ in the incubation medium did not affect the relative stimulation of the accumulation of [3H]-inositol phosphates by carbachol, although the basal formation was higher in the presence of Ca2+ in the medium. In the absence of added Ca2+, the incorporation of [3H]-inositol into phospholipids was increased--an effect which was further enhanced by the addition of EGTA to the medium. Gastrin and pentagastrin (10(-8)-10(-5) M) enhanced the formation of [3H]-inositol phosphates, although they were clearly less effective than carbachol. Histamine (10(-6)-10(-3) M) had no effect of its own, but slightly attenuated the effect of carbachol. Cholecystokinin octapeptide (10(-9)-10(-6) M) slightly increased the formation of [3H]-inositol phosphates. Indomethacin (10(-4) M) had no consistent effect on the basal and carbachol-induced accumulation of [3H]-inositol phosphates, nor did prostaglandin E2 (10(-5) M) modify it. Adrenaline (10(-3) M), 5-hydroxytryptamine (10(-3) M), forskolin (10(-5) M), vasopressin (10(-5) M), angiotensin II (10(-5) M) and bombesin (10(-9)-10(-6) M) were all without effect. We suggest that the hydrolysis of inositol phospholipids may be involved in the signal transduction mechanism by which the activation of the muscarinic and gastrin receptors on the parietal cells leads to Ca2+ mobilization and the stimulation of hydrogen ion secretion.

Regulation of K+ channels in the basolateral membrane of Necturus oxyntic cells

Patch-clamp methods were used to study single-channel events in isolated oxyntic cells and gastric glands from Necturus maculosa. Cell-attached, excised inside-out and outside-out patches from the basolateral membrane frequently contained channels which had conductances of 67 +/- 21 pS in 24% of the patches and channels of smaller conductance, 33 +/- 6 pS in 56% of the patches. Channels in both classes were highly selective for K+ over Na+ and Cl-, and shared linear current-voltage relations. The 67-pS channel was activated by membrane depolarization, whereas the activity of the 33-pS channel was relatively voltage independent. The larger conductance channels were activated by intracellular Ca2+ in the range between 5 and 500 nM, but unaffected by cAMP. The smaller conductance channels were activated by cAMP, but not Ca2+. The presence of K+ channels in the basolateral membrane which are regulated by these known "second messengers" can account for the increase in conductance and the hyperpolarization of the membrane observed upon secretagogue stimulation.

Capsaicin pretreatment reduces the gastric acid secretion elicited by histamine but does not affect the responses to carbachol and pentagastrin

Gastric acid secretion was studied following stimulation with a subcutaneous injection of histamine (0.1, 0.5 or 5 mg/kg), carbachol (4, 40 or 160 micrograms/kg) or pentagastrin (25 or 250 micrograms/kg) in conscious rats pretreated with either capsaicin or the vehicle. The secretory response to histamine (0.5 or 5 mg/kg) was greatly reduced in the capsaicin-treated rats, while the slight effect of 0.1 mg/kg histamine and the increase of secretion in response to carbachol and pentagastrin were not affected. The basal secretion was also normal in the capsaicin-treated rats. It is suggested that unlike the effects of carbachol and pentagastrin, the increase of acid secretion elicited by histamine involves a capsaicin-sensitive mechanism. Since capsaicin is a specific neurotoxin for the peptide-containing primary sensory neurons, the present results may indicate that the neurons contribute essentially to the effect of histamine on gastric acid secretion.

Microelectrode measurements from oxyntic cells in intact Necturus gastric mucosa

The electrical properties of oxyntic cells were measured in intact isolated Necturus fundic mucosa by dissecting away the serosal muscle and connective tissue and impaling the oxyntic cells across their basolateral membranes. Their properties under resting [i.e., not secreting acid (10(-4) M serosal cimetidine)] and stimulated (10(-4) M histamine) conditions were compared with those of surface cells impaled across their apical membranes in a separate set of experiments. Histamine hyperpolarized the transepithelial potential by 6-10 mV and reduced the transepithelial resistance by approximately 40%. The basolateral membrane potential (Vcs) of both cell types was significantly hyperpolarized by histamine, that of oxyntic cells from a resting value of -50 to -59 mV (P less than 0.001) and that of surface cells from -50 to -54 mV (P less than 0.05). Histamine also hyperpolarized the apical membrane potential (Vmc) of the oxyntic cells; however, the Vmc of surface cells was significantly depolarized. The ratio of the apical to basolateral cell membrane resistances Ra/Rb (delta Vmc/delta Vcs resulting from transepithelial current pulses) of resting oxyntic cells was 1.1 and that of surface cells was 3.6. Stimulation did not affect the Ra/Rb of either cell type. A tenfold increase in serosal K+ concentration depolarized Vcs and increased Ra/Rb of resting and stimulated oxyntic cells, indicating a significant basolateral K+ conductance. The results are consistent with a purely passive role for surface cells and indicate that stimulation results in a simultaneous decrease of both the apical and basolateral membrane resistances of the oxyntic cells.

Membrane potentials of individual cells of isolated gastric glands of rabbit

Individual glands of rabbit gastric mucosa were prepared for measurements of cell membrane potentials. In the first experiments a collagenase isolation technique was used which produced gland fragments that were fixed on agarose. In later experiments a microdissection technique was used which allowed whole glands to be isolated that were held in suction pipettes. Individual parietal or chief cells could be recognized and impaled with microelectrodes, however, the yield of reliable recordings was small and the distinction from artifacts sometimes difficult. In acceptable recordings the membrane potentials of both cell types varied between around -20 and -35 mV or exceptionally -50 mV in both preparations, with mean values being around -26 mV. The significance of the recordings was tested by ion substitution experiments. Substitution of all chloride by sulfate increased the membrane potential to values ranging up to -60 and -80 mV that are commonly observed in other cells.

HeLa cells have histamine H1-receptors which mediate activation of the K+ conductance

HeLa cells responded to exogenous histamine with a transient hyperpolarization due to increased membrane conductance to K+. After successive applications of histamine, the cell membrane became virtually unresponsive (desensitized). The responses were blocked by pyrilamine but not by cimetidine. Thus, it appears that HeLa cells have H1-receptors which mediate an increase in the K+ conductance.