Role of calcium and calmodulin in activation of the oxyntic cell by histamine and carbamylcholine in the guinea pig

Identification and functional assay of an extracellular calcium-sensing receptor in Necturus gastric mucosa

In mammals and amphibians, increases in extracellular Ca2+ can activate bicarbonate secretion and other protective functions of gastric mucosa. We hypothesized that the recently cloned extracellular Ca(2+)-sensing receptor (CaR) is functioning in the gastric mucosa. In Necturus maculosus gastric mucosa, reverse transcription-polymerase chain reaction using primers based on previously cloned CaR sequences amplified a 326-bp DNA fragment that had 84% nucleotide sequence identity with the rat kidney CaR. Immunohistochemical localization of the CaR using specific anti-CaR antiserum revealed its presence on the basal aspect of gastric epithelial cells. In microelectrode studies of Necturus antral mucosa, exposure to elevated Ca2+ (4.8 mM) and the CaR agonists NPS-467 and neomycin sulfate resulted in significant hyperpolarizations of basal membrane electrical potentials and increases in apical-to-basal membrane resistance ratios. Circuit analysis revealed that these changes reflected specific decreases in basolateral membrane resistance. Inhibition of prostaglandin synthesis using indomethacin significantly attenuated these effects. We conclude that the CaR is present and functioning in Necturus gastric antrum.

A calmodulin antagonist inhibits histamine-stimulated acid production by isolated rat parietal cells

The role of calmodulin in the regulation of histamine-stimulated parietal cell function was studied in isolated rat parietal cells using [14C]aminopyrine uptake as a quantitative index of acid production. In enriched (77-87%) intact parietal cells the calmodulin antagonist naphthalene sulfonamide W 7 dose-dependently inhibited the response to 10(-4) M histamine (IC50: 2 X 10(-6) M). The mechanism of this inhibition was examined further with two other stimuli of H+-production: forskolin which directly activates the parietal cell adenylate cyclase without interacting at the histamine H2-receptor and dbcAMP which mimics the biological action of cAMP without preceding activation of adenylate cyclase. W 7 effectively inhibited the responses to 10(-4) M forskolin (IC50: 6 X 10(-7) M), 10(-3) M dbcAMP (IC50: 10(-6) M) and to 10(-2) M K+ (IC50: 3 X 10(-6) M). The action of W 7 followed non-competitive kinetics since the antagonist reduced the entire range of the concentration-response curves without shifting them rightwards towards higher concentrations of the respective stimulants. The effect of W 7 was reversed by washing the cells. ATP-induced [14C]aminopyrine uptake into digitonin-permeabilized oligomycin-inhibited parietal cells reflects H+-production independent of oxidative phosphorylation and was also inhibited by W 7 (IC50: 10(-5) M). Inhibition of K+-stimulated H+/K+-ATPase activity required even higher W 7-concentrations (IC50: 1.4 X 10(-4) M). Our data suggest that calmodulin might be involved in the intracellular mediation of the response to histamine. Between histamine-induced cAMP-generation and the H+-secreting tubulovesicular system W 7 seems to inhibit an intracellular step that finally activates the H+/K+-ATPase. Yet, direct inhibition of the ATPase requires W 7 concentrations of questionable specificity and is unlikely to be the mechanism behind the action of W 7 on the parietal cell response to histamine.

Rat gastric secretion "in vitro": interaction between histamine and various antisecretagogues acting by different mechanisms

The interaction between histamine and various antisecretagogues acting by different mechanisms has been investigated in the isolated fundus from the rat stomach. Histamine evoked a concentration-dependent stimulatory effect which was competitively antagonized by the H2-receptor antagonist, ranitidine and non competitively by the H+/K+-ATPase inhibitor, omeprazole. The histamine induced secretion was highly resistant to the action of the calcium entry blocker verapamil, somatostatin and KSCN, but some inhibition was obtained with the calmodulin antagonist, trifluoperazine. Removal of calcium ions from the bathing media (both mucosal and serosal) greatly enhanced histamine-induced gastric secretion. The results suggest that the relationship between receptor stimulation and the intracellular events leading to acid secretion is far from being elucidated.

Action of gastrin in guinea pig oxyntic cells. Studies using quantitative cytochemistry

The mechanism of action of gastrin was investigated using cytochemical quantitation of hydroxyl ion production (HIP) in guinea pig gastric oxyntic mucosa. The reaction depends upon the trapping of OH ions produced during gastric stimulation and is blocked by the benzimidazole, Hassle 149/94, which inhibits the K+ + H+-ATPase and by acetazolamide, an inhibitor of carbonic anhydrase activity. It is thus a measure of hydroxyl ions produced during stimulation of the oxyntic cell and reflects upon hydrogen ion production. Gastrin (2.5 X 10(-16) -2.5 X 10(-12) M) caused a linear dose-dependent stimulation of HIP in the oxyntic cells. The response was biphasic, with an early peak at 90 s and a secondary rise at 240 s, which persisted for 10 min. Natural human gastrin (sulfated and nonsulfated) and the active COOH-terminal octapeptide fragment of gastrin stimulated HIP, whereas the biologically inert NH2-terminal (1-13) fragment of gastrin had no effect. The activation of oxyntic cell HIP by gastrin was neutralized by an antiserum directed towards the COOH-terminus of gastrin and not by nonimmune serum. Cimetidine (10(-5) M) blocked 25% and atropine (10(-5) M) had no effect on gastrin-stimulated HIP. EGTA (10(-3) M) and LaCl3 (10(-3) M) inhibited the action of gastrin by 67 and 52%, respectively. The calmodulin antagonists, trifluoperazine (10(-5) M), pimozide (10(-5) M), and the naphthalene sulfonamides, W-7 and W-13 (10(-5) M), inhibited gastrin-stimulated HIP by 45.6 38.5, 42.3, and 37.2%, respectively. Higher doses of W-7 and W-13 (10(-4) M) inhibited gastrin-stimulated HIP by 83 and 67%. The Ca2+ ionophore, A23187 (10(-4) M), stimulated HIP. Thus, it appears that gastrin stimulation of HIP is complex. 25% of its action is via a histamine-dependent pathway. 45% of its action is dependent upon extracellular Ca2+. Its action is also in part dependent upon a Ca2+/calmodulin mechanism.

Cytochemical quantification of physiologic regulation of oxyntic cell carbonic anhydrase

We have studied the physiologic factors regulating oxyntic cell activity using cytochemical quantification of carbonic anhydrase (CA) activity. Gastrin (10 (-16) to 10(-12)M), histamine (10(-17) to 10(-13) M), and carbamylcholine (10(-13) to 10(-8) M) caused a dose-dependent increase in CA in the oxyntic cells in guinea pig gastric fundus, maximal at 90 sec. The stimulation of CA by all three secretagogues was inhibited by the CA inhibitor, acetazolamide. The agonist activities were selectively blocked by respective antagonists. The benzimidazole derivative compound Hassle 149/94 (10(-3)M) abolished the actions of all agonists. Thus, histamine, gastrin and carbamylcholine have independent actions on oxyntic cell CA. The inhibition of the activity of all three secretagogues by H149/94 suggests a close link between CA activity and the functioning of the proton pump H+ + K+-ATPase.