CCK2 receptor antagonists: pharmacological tools to study the gastrin–ECL cell-parietal cell axis

doi:10.1016/S0167-0115(99)00008-7

Regulatory Peptides

Volume 80, Issues 1–2, 17 March 1999, Pages 1-12

https://doi.org/10.1016/S0167-0115(99)00008-7 Get rights and content

Abstract

Gastrin-recognizing CCK₂ receptors are expressed in parietal cells and in so-called ECL cells in the acid-producing part of the stomach. ECL cells are endocrine/paracrine cells that produce and store histamine and chromogranin A (CGA)-derived peptides, such as pancreastatin. The ECL cells are the principal cellular transducer of the gastrin-acid signal. Activation of the CCK₂ receptor results in mobilization of histamine (and pancreastatin) from the ECL cells with consequent activation of the parietal cell histamine H₂ receptor. Thus, release of ECL-cell histamine is a key event in the process of gastrin-stimulated acid secretion. The oxyntic mucosal histidine decarboxylase (HDC) activity and the serum pancreastatin concentration are useful markers for the activity of the gastrin-ECL cell axis.

Powerful and selective CCK₂ receptor antagonits have been developed from a series of benzodiazepine compounds. These agents are useful tools to study how gastrin controls the ECL cells. Conversely, the close control of ECL cells by gastrin makes the gastrin-ECL cell axis well suited for evaluating the antagonistic potential of CCK₂ receptor antagonists with the ECL-cell HDC activity as a notably sensitive and reliable parameter.

The CCK₂ receptor antagonists YF476, YM022, RP73870, JB93182 and AG041R were found to cause prompt inhibition of ECL-cell histamine and pancreastatin secretion and synthesis. The circulating pancreastatin concentration is raised, was lowered when the action of gastrin on the ECL cells was blocked by the CCK₂ receptor antagonists. These effects were associated with inhition of gastrin-stimulated acid secretion. In addition, sustained receptor blockade was manifested in permanently decreased oxyntic mucosal HDC activity, histamine concentration and HDC mRNA and CGA mRNA concentrations. CCK₂ receptor blockade also induced hypergastrinemia, which probably reflects the impaired gastric acid secretion (no acid feedback inhibition of gastrin release). Upon withdrawal of the CCK₂ receptor antagonists, their effects on the ECL cells were readily reversible.

In conclusion, gastrin mobilizes histamine from the ECL cells, thereby provoking the parietal cells to secrete acid. While CCK₂ receptor blockade prevents gastrin from evoking acid secretion, it is without effect on basal and vagally stimulated acid secretion. We conclude that specific and potent CCK₂ receptor antagonists represent powerful tools to explore the functional significance of the ECL cells.

Introduction

The ECL cells constitute 65–75% of the endocrine cells in the oxyntic mucosa (fundus) of the rat stomach 1, 2, 3, 4, 5, 6. They are located in the basal third or basal half of the oxyntic mucosa in mammals and contain histamine, the histamine-forming enzyme histidine decarboxylase (HDC), and chromogranin A (CGA)-derived peptides (e.g. pancreastatin). These features make it possible to distinguish ECL cells from other endocrine cell types in the stomach 2, 4, 7. At the ultrastructural level, they have a characteristic appearance with numerous, often quite large, electron-lucent cytoplasmic vesicles and a few (by comparison), small electron-dense granules and small clear microvesicles 4, 8, 9, 10. Their histochemical and ultrastructural properties suggest that they are peptide hormone-producing cells. They operate under the control of gastrin 3, 4, 5. While the peptide hormone produced by the ECL cells remains unidentified, ECL-cell histamine plays a key role in stimulating parietal cells to secrete acid. In the rat, the ECL cells are a major source of circulating pancreastatin 11, 12, 13. This reflects the fact that the ECL cells constitute one of the quantitatively most prominent endocrine cell populations in the body.

In the present review we summarize our findings on the effects of CCK₂ receptor antagonists on the gastrin–ECL cell-parietal cell axis in the rat. The data are from Ding et al. 14, 15, 16, 17, 18, Ding and Håkanson 19, 20, Lindström et al. [21]and Norlén et al. [22].

Section snippets

The gastrin–ECL cell axis

The ECL cells operate under the control of circulating gastrin, constituting the gastrin–ECL cell axis (Fig. 1) 3, 4, 5, 23. Acute activation of ECL cells by gastrin is manifested in secretion of histamine and pancreastatin 4, 5, 23, 24, and then in activation of HDC. These acute responses are followed by hypertrophy and general, diffuse hyperplasia of the ECL cells if the gastrin stimulation is sustained for weeks or months 3, 4, 5, 10, 23, 25. After 1–2 years of hypergastrinemia, the ECL

CCK₂ receptor antagonists

The peptide hormones gastrin and CCK share the carboxy-terminal pentapeptide amide sequence which carries the biological message. The amino-terminal part provides the tag for receptor recognition, enabling gastrin and CCK to bind to specific receptors on their target cells, thereby activating separate intracellular signalling pathways. The receptors for the gastrin/CCK family of peptides are classified as CCK₁ and CCK₂ (formerly CCK-A and CCK-B) as recommended by the nomenclature subcommittee

CCK₂ receptor blockade suppresses ECL-cell HDC activity

Measurement of HDC activity is a convenient and simple method to assess the functional activity of the ECL cells 19, 23, 46, 90, 91. The CCK₂ receptor antagonists YF476, YM022, RP73870, JB93182, AG041R, L-740,093 and L-365,260 were found to inhibit gastrin-induced HDC activation in a dose-dependent manner 15, 16, 19(Fig. 3). The ID₅₀ values for these compounds to antagonize gastrin-evoked HDC activation are shown in Table 2. High doses of YF476, YM022 and RP73870 produced a rightward shift of

CCK₂ receptor blockade suppresses stimulated but not basal histamine and pancreastatin release from ECL cells

The ECL cells are rich in pancreastatin 2, 11, 13, 92, 93, 94. In the rat, the circulating pancreastatin level reflects the activity of the ECL cells and serves as a marker of the activity of the gastrin–ECL cell axis 11, 12, 13. Any treatment that raises the circulating gastrin concentration (gastrin infusion, treatment with omeprazole and re-feeding after fasting) without blocking the CCK₂ receptors can be expected to stimulate the ECL cells to release pancreastatin, resulting in elevated

CCK₂ receptor blockade elevates serum gastrin

A single dose of either of the two antisecretory drugs omeprazole or ranitidine induces a 4–5-fold rise in the serum gastrin concentration of fasted rats [11]. In contrast, intravenous infusion of YM022 or YF476 at maximally effective doses (antagonizing gastrin-induced HDC activation) did not raise serum gastrin in fasted rats 15, 16, 17. However, in freely fed rats a sustained CCK₂ receptor blockade (induced by continuous subcutaneous infusion of YM022 or RP73870) promptly raised the serum

Time-course of deactivation of ECL cells in response to CCK₂ receptor blockade

CCK₂ receptor blockade deactivates ECL cells effectively and promptly. When infused to normal freely fed rats, RP73870 and YM022 caused a prompt and dramatic decline in the serum pancreastatin concentration and the oxyntic mucosal HDC activity and HDC and CGA mRNA concentrations. The serum pancreastatin concentration was reduced by 40% after 6 h and the reduction was maximal after 2–3 days. The oxyntic mucosal HDC activity, which started to decline after 1–2 h, was reduced by 60–70% after 6 h

Effect of CCK₂ receptor blockade on gastric acid secretion

CCK₂ receptor antagonists abolished gastrin-evoked acid secretion in the perfused rat stomach and in the chronic gastric fistula rat (Fig. 6), but did not affect the acid response to either histamine or vagal stimulation 18, 20, 71, 76. Does gastrin stimulate acid secretion by a direct action on the parietal cells, or indirectly via the mobilization of ECL-cell histamine? Ever since the discovery of the anti-secretory effects of histamine H₂-receptor antagonists it has been speculated that

Reversibility of CCK₂ receptor stimulation and blockade

Long term hypergastrinemia (evoked by for instance omeprazole) is known to cause activation and growth of the ECL cells. These effects were reversed after stopping administration of the drug 98, 99. The serum gastrin concentration and the ECL-cell HDC activity returned to pretreatment levels within 3–5 days after withdrawal of the anti-secretory drug (e.g. omeprazole), while longer times (weeks) were required for normalization of the oxyntic mucosal histamine content and the ECL cell density

The gastrin–ECL cell-parietal cell axis

The cellular basis for the role of gastrin in acid secretory regulation has been the subject of much speculation revolving around the question of whether gastrin acts directly to stimulate the parietal cells or indirectly, through the release of ECL-cell histamine. The latter hypothesis gained support following the discovery that specific histamine H₂ receptor blockers inhibited gastrin-stimulated acid secretion. Other reports, on the other hand, support a direct effect of gastrin on parietal

Conclusions

The two benzodiazepine derivatives YM022, YF476 and the ureidoacetamide compound RP73870 are potent and effective CCK₂ receptor antagonists in vivo. By the use of these compounds it has been confirmed that rat stomach ECL cells are under the close control of circulating gastrin. The response of the ECL cells to gastrin was dose-dependently suppressed by CCK₂ receptor antagonists. CCK₂ receptor blockade impaired the ability of the ECL cells to maintain their activity in normal rats and prevented

Acknowledgements

The work described has been supported by grants from the Swedish MRC (04X-1007), RmC (2542-B91-02XB), the A. Påhlsson Foundation, Ferring A/S, and the Medical Faculty, University of Lund, Sweden.

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ReviewCCK2 receptor antagonists: pharmacological tools to study the gastrin–ECL cell-parietal cell axis

Abstract

Introduction

Section snippets

The gastrin–ECL cell axis