Characterization of gamma- and delta-subunits of Ca2+/calmodulin-dependent protein kinase II in rat gastric mucosal cell populations

We searched for the occurrence of a Ca2+/calmodulin-dependent protein kinase in rat gastric cell types as a likely member in the chain of gastrin- and muscarinic-receptor-mediated signal transmission. A Ca(2+)- and calmodulin-dependent phosphorylation of major 50, 60 and 100 kDa substrates was observed in parietal cell cytosol and a major 60 and 61 kDa protein doublet was found to bind 125I-calmodulin in 125I-calmodulin-gel overlays. A specific substrate of the multifunctional Ca2+/calmodulin-dependent protein kinase II, autocamtide II, was phosphorylated in a calmodulin-dependent manner. The specific inhibitor of this enzyme, KN-62, antagonized protein kinase activity. RNA extracted from gastric mucosal cells was shown to contain sequences of the gamma- and delta- but not alpha- and beta-subunits of the calmodulin-dependent kinase II, and mRNA of both subtypes was demonstrated in highly purified parietal, chief and mucous cells. A calmodulin-dependent kinase II composed of gamma- and delta-subunits is a likely mediator of Ca(2+)-dependent signal transmission in these populations of gastric cells.

Intracellular pH-regulating ion transport mechanisms in parietal cell basolateral membrane vesicles

Na(+)-H+ and Cl(-)-base exchangers on the parietal cell have been demonstrated by several authors. Controversy exists concerning a basolateral Na(+)-HCO3- cotransporter in the parietal cell. To clarify this issue, we prepared highly enriched basolateral membrane (BLM) and apical-tubulovesicular membrane (to serve as negative controls) vesicles from rabbit fundic mucosa. Na(+)-H+ exchange was demonstrated by measuring pH gradient-driven amiloride-sensitive 22Na+ uptake and Na+ gradient-driven proton uptake into voltage-clamped BLM but not into apical-tubulovesicular vesicles. Anion exchange was demonstrated by measuring 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-inhibitable influx of 36Cl- into Cl(-)- or HCO3(-)-loaded voltage-clamped BLM vesicles. Na(+)-HCO3- cotransport was assessed by comparing HCO3(-)-driven 22Na uptake with uptake driven by an identical pH gradient. No significant difference was found between 22Na uptake in the presence and absence of HCO3-; 1 mM amiloride inhibited 22Na uptake > 90% in both conditions, whereas 2 mM DIDS had no effect. In BLM vesicles prepared from rabbit renal cortex, however, a HCO3- gradient stimulated 22Na uptake much more than an equivalent pH gradient, and DIDS inhibited this HCO3- gradient-driven 22Na uptake. This indicates that our experimental setup was suitable to detect a Na(+)-HCO3- cotransporter if present. Our data suggest that the parietal cell BLM contains Na(+)-H+ exchangers and Cl(-)-HCO3- exchangers but no Na(+)-HCO3- cotransporter.

Influence of acid secretory state on Cl(-)-base and Na(+)-H+ exchange and pHi in isolated rabbit parietal cells

Parietal cell apical proton secretion is accompanied by apical Cl- secretion, K+ cycling, and the generation of intracellular base. We examined the ability of the parietal cell to maintain intracellular pH (pHi) during its transformation from the resting to the stimulated state and evaluated the ion transport mechanisms involved in the maintenance of ionic equilibrium. Isolated rabbit parietal cells were loaded with the pH-sensitive fluorescent dye BCECF, and pHi was monitored in maximally stimulated, resting (absence of a secretagogue), and omeprazole-inhibited cells. Although [14C]aminopyrine (AP) accumulation increased up to 30-fold above basal during stimulation, the mean pHi of maximally stimulated cells was not different from that of inhibited cells both immediately and late after stimulation in an extracellular pH range from 6.2 to 7.8 in either HEPES or CO2/HCO3- buffer. When the stilbene DIDS was added 2 min after stimulation of acid formation, pHi rapidly increased (0.09 +/- 0.02 vs. 0.04 +/- 0.02 pH units in unstimulated cells in 6 min), indicating an increased base efflux through a DIDS-sensitive transporter (most likely the Cl(-)-base exchanger). Stimulation of acid secretion did not change the transport capacity, apparent affinity for extracellular Cl-, or dependency of the anion flux rate on pHi of the DIDS-sensitive base exporter. For a given pHi, amiloride-inhibitable proton efflux rates during pHi recovery from an acid load were identical in resting and stimulated cells, suggesting that neither the transport capacity not the pHi set point of the parietal cell Na(+)-H+ exchanger is altered by cAMP-dependent stimulation of acid formation. We conclude that, during the cAMP-mediated stimulation of acid formation in isolated rabbit parietal cells, the pHi remains constant, but an increased base efflux occurs without a change in the transport capacity of the involved base extrusion mechanisms or an inhibition of the parietal cell base loading mechanisms. Whether changes in the intracellular Cl- concentration on stimulation of acid formation initiate the increased base efflux and whether additional ion transporters are involved in the maintenance of ion homeostasis during acid secretion remain to be determined.

Inositol phosphate formation and [Ca2+]i in secretagogue-stimulated rabbit gastric mucous cells

The formation of inositol phosphates and the changes in free intracellular Ca2+ ([Ca2+]i) in isolated rabbit gastric mucous cells during cholinergic stimulation were examined and the potential role of inositol phosphate turnover and [Ca2+]i in gastric mucus secretion evaluated. Rabbit chief and parietal cells were studied for comparison. The formation of [3H]inositol phosphates in mucous, chief, and parietal cells was stimulated in a time- and concentration-dependent fashion by acetylcholine (ACh). The ACh-induced initial [Ca2+]i peak was maximally (10(-4) M ACh) 199 +/- 8% of basal in mucous cells, 427 +/- 20% in chief, and 455 +/- 31% in parietal cells and was followed by a lower-level plateau in mucous and parietal cells but by a more rapid decline in chief cells. As in parietal and chief cells, the initial [Ca2+]i peak occurred in mucous cells in the absence of external Ca2+. ACh stimulated a mucous cell membrane Ca2(+)-entry mechanism in addition to release of Ca2+ from intracellular stores. The concentration-response relationships for the production of [3H]-inositol phosphates, the initial rise in [Ca2+]i, and the stimulation of glycoprotein secretion by ACh were virtually identical. Suppression of the [Ca2+]i rise by the intracellular Ca2(+)-chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) abolished the secretory response. As with many other secretory cells, gastric mucous cells possess cholinergic receptors that upon stimulation mediate the hydrolysis of phosphoinositides, a release of Ca2+ from intracellular stores, and a stimulation of Ca2+ influx through the plasma membrane.

Ca2+-dependent and -independent secretagogue action on gastric mucus secretion in rabbit mucosal explants

The secretion of high-molecular weight glycoprotein was studied in rabbit antral and fundic explants in response to acetylcholine (ACh), the calcium ionophore A23187, the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), forskolin, histamine, pentagastrin, and prostaglandin E2 (PGE2). Glycoprotein secretion was measured in pulse-chase experiments by incorporation of [14C]N-acetyl-D-glucosamine into explants and quantification of the secreted high-molecular weight-labeled glycoprotein. All tested agents except histamine and pentagastrin stimulated glycoprotein secretion in a concentration-dependent manner. The response to A23187 was abolished by removal of extracellular Ca2+ and greatly reduced by the calmodulin antagonist N-(6-amino-hexyl)-l-naphthalenesulfonamide (W7). The response to ACh was abolished by equimolar concentrations of atropine and by depletion of intracellular Ca2+ stores, an effect which was reversible on addition of Ca2+, and reduced by W7. The response to forskolin and PGE2 was not significantly affected by either extracellular or intracellular Ca2+ depletion. The combination of ACh with forskolin and A23187 with TPA had a synergistic effect on secretion. The absolute dependence of cholinergic stimulation on the presence of intracellular Ca2+ and the Ca2+-independent stimulatory effect of forskolin and PGE2 suggest that gastric mucus secretion can be elicited by at least two distinct pathways.

Effect of CO2 on pHi in rabbit parietal, chief, and surface cells

We investigated the pH recovery mechanisms in rabbit parietal, chief, and surface cells during pH shifts induced by introduction or removal of exogenous CO2-HCO3-. Intracellular pH (pHi) was measured using the fluorescent dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescin (BCECF). Gastric cells were highly purified by density gradient centrifugation and elutriation. When cells suspended in N-2-hydroxyethylpiperazene-N'-2-ethanesulfonic acid (HEPES)-100% O2, extracellular pH (pHo) 7.4, were exposed to 24 mM HCO3- -5% CO2, pHo 7.4, all cells quickly acidified by 0.3-0.4 pH units. Almost complete pH-recovery occurred within 15 min. In parietal cells, 70% of this recovery was dependent on the presence of extracellular Na+ (Nao+) and was blocked by 1 mM amiloride. The Na+-independent recovery was blocked by intracellular Cl- depletion or by 0.4 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). In chief cells and surface cells no recovery occurred in the absence of NaO+, and 1 mM amiloride blocked pH recovery in Na+-containing buffer. On removal of HCO3- -CO2, the cells alkalinized, and subsequent pH recovery was fast, substantially extracellular Cl- (ClO-) and DIDS inhibitable in parietal cells but slow and ClO- -independent in chief and surface cells. These results suggest that during intracellular acidification the Na+-H+ exchanger is the major pH regulator in these three gastric cell types even in the presence of HCO3-. During alkalinization the Cl- -HCO3-(OH-) exchanger is the predominant pH recovery mechanism in parietal, but not in chief and surface cells. In parietal cells, this exchanger is also involved in recovery from acidification.

Inhibition of acid formation in rabbit parietal cells by prostaglandins is mediated by the prostaglandin E2 receptor

The affinities of seven natural and synthetic prostaglandins [PGE2; 16,16-dimethyl PGE2; iloprost (stable prostacyclin analogue); PGF2 alpha; PGD2; BW245c (stable PGD2 analogue); and U46619 (stable thromboxane analogue)] to the PGE2 binding site of rabbit gastric mucosa were determined by measuring [3H]PGE2 displacement from its high-affinity plasma membrane binding sites. In parallel, the potency of each prostaglandin in inhibiting acid generation in vitro was determined by measuring the inhibition of histamine-stimulated [14C]aminopyrine accumulation in rabbit parietal cells prepared by enzymatic dispersion and enriched by counterflow elutriation. All seven prostaglandins displaced [3H]PGE2 and inhibited histamine-stimulated [14C]aminopyrine accumulation in a concentration-dependent manner. For all tested prostaglandins, the IC50 values were in excellent agreement for both variables measured. It is concluded that (a) a PGE2 receptor is localized on the parietal cell and mediates inhibition of acid formation by all prostaglandins and (b) the different in vitro antisecretory potencies of prostaglandins can be attributed to their different affinities to this PGE2 receptor.

Effect of PCO2 on intracellular pH in in vitro frog gastric mucosa

Steady-state intracellular pH (pHi) in 0, 5, and 10% CO2-buffered Ringer solution in sheets of in vitro frog gastric antral or fundic mucosa has been measured using the pH-sensitive fluorescent dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). In tissues perfused with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-100% O2 buffer [extracellular pH (pHo) = 7.14], steady-state pHi in antral surface cells was 7.08 +/- 0.06 (n = 8), in fundic oxynticopeptic cells 6.91 +/- 0.03 (n = 13), in the muscularis mucosa 7.58 +/- 0.06 (n = 4). In mucosae perfused with 17.8 mM HCO3- -95% O2-5% CO2 buffer (pHo = 7.14), steady-state pHi in antral surface cells was 6.97 +/- 0.02 (n = 22), in fundic oxynticopeptic cells 7.00 +/- 0.04 (n = 18), and in fundic muscularis mucosa 7.39 +/- 0.05 (n = 8). In fundic oxynticopeptic cells perfused with 35.6 mM HCO3- -90% O2-10% CO2 (pHo = 7.14) steady-state pHi was 6.77 +/- 0.07 (n = 4). In tissues equilibrated initially with 100% O2 and changed to 5% CO2, antral surface cells acidified by 0.21 pH units and fundic oxynticopeptic cells by 0.10 pH units, with restoration of pHi to resting levels within 30 and 10 min, respectively. Exposure of tissues initially equilibrated with 5% CO2 to 100% O2 alkalinized antral surface cells by 0.22 pH units and fundic oxynticopeptic cells by 0.23 pH units, with only partial recovery of pHi by 30 min. These data suggest that steady-state pHi is equivalent in surface and oxynticopeptic cells and is lower than in the muscularis mucosa.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of endogenous and exogenous prostaglandins on glycoprotein synthesis and secretion in isolated rabbit gastric mucosa

We studied gastric glycoprotein synthesis and secretion in organ culture before and during cyclooxygenase inhibition and replacement with exogenous prostaglandins (16,16-dimethyl prostaglandin E2 and prostaglandin F2 alpha). Isolated rabbit antral and fundic mucosal explants incorporated [14C]N-acetylglucosamine and [3H]leucine in a linear fashion and steadily secreted labeled proteins and glycoproteins during the 24-h incubation period. On sepharose 4B, greater than 90% of the secreted protein-bound [14C]N-acetylglucosamine was found in the high molecular weight peak. Incorporation of tracer was not influenced by cyclooxygenase inhibition with indomethacin or the addition of exogenous prostaglandins. Secretion of newly formed glycoprotein, however, was significantly inhibited by indomethacin and stimulated by both tested prostaglandins in a concentration-dependent manner. 16,16-Dimethyl prostaglandin E2 caused significant stimulation in concentrations that are well in the physiologic range for endogenous prostaglandin E2, whereas prostaglandin F2 alpha stimulated in 100 times higher concentrations. We conclude that in the isolated gastric mucosa both endogenous and exogenous prostaglandins stimulate mucus secretion. For prostaglandin E2, but not prostaglandin F2 alpha, a role in the physiologic regulation of gastric mucus secretion is probable.