Easy as 1, 2, 3? Histamine receptors and gastric acid

It is over 80 years since the stimulatory effect of histamine on gastric acid was reported.¹ The observation that the conventional antihistamines (subsequently shown to be H₁ antagonists) failed to block the acid stimulatory action² ultimately led to the discovery and availability of H₂ antagonists.³ These were not only effective drugs but tools to dissect acid secretory physiology, and develop our continually evolving paradigm of histamine as the major paracrine stimulant of gastric acid.⁴

In the gastric mucosa, histamine is found within enterochromaffin-like (ECL) cells and mast cells, the relative proportion of the two cell types being species and site dependent. Histamine is formed by the decarboxylation of histamine by histidine decarboxylase (HDC). After release histamine is enzymatically deactivated by two pathways. The majority is methylated onto one of the nitrogen atoms in the imidazole ring by imidazole-N-methyltransferase, and a smaller proportion is degraded by oxidative deamination to imidazole acetic acid. A further potential methylation site is on the terminal nitrogen of the side chain, producing N-alpha-methyl-histamine (NAMH). In 1935, soon after it was first chemically synthesised, NAMH was shown to be a potent stimulant of canine acid secretion.⁵ NAMH was detected in canine gastric juice following histamine stimulation, and was more than twice as potent as histamine in stimulating acid secretion.⁶ The acid stimulatory action was sensitive to H₂ blockers. Although a broad specificity mammalian enzyme capable of catalysing side chain as well as ring methylation was subsequently described,⁷ there has never been any evidence that NAMH was a physiological product.

It is likely that the gastric effects of NAMH would have been sidelined if it were not for two independent discoveries in the early 1980s. The first was the description of Helicobacter pylori, as we now know it, and the subsequent interest in gastric physiology. The second was the description of a novel high affinity histamine receptor type (H₃).⁸ Initially described as receptors inhibiting histamine release in rat brain, many studies followed characterising the receptor. It was soon appreciated that H₃ receptor agonists could inhibit acid secretion in vivo,⁹ inhibit histamine secretion from ECL cells in vitro,¹⁰ and possibly regulate gastrin and somatostatin secretion.⁹ NAMH was found to be a high affinity H₃ receptor agonist and it came to be used widely as a ligand to investigate the H₃ receptor.⁹

Courillon-Mallet et al brought these two strands together; NAMH and N-alpha-methylating activity were detected by biochemical means in H pylori positive mucosa and cultures of H pylori, but not in H pylori negative mucosa. Binding studies suggested bacterially produced NAMH was occupying mucosal H₃ receptors. This occupation seemed to be correlated with suppression of both HDC activity and mucosal somatostatin.¹¹

NAMH became an attractive putative mediator of the abnormalities of gastric secretion in H pylori infection. NAMH stimulated acid secretion in cultured isolated parietal cells¹² and gastrin secretion from cultured G cells¹³: these effects were blocked by H₂ antagonists. No evidence for NAMH acting on H₃ receptors inhibiting either parietal cell acid secretion or D cell somatostatin secretion were found.^12,14

In this issue of Gut, Saitoh et al have examined the interaction of NAMH with the H₂ receptor in an attempt to clarify these disparate observations [see 786].¹⁵ They utilised a Chinese hamster ovary cell line stably transfected with, and expressing, the human H₂ receptor gene. Histamine and NAMH displaced tiotidine (a H₂ antagonist) from the receptor but the archetypal H₃ selective agonist (R)-alpha-methyl-histamine did not. Functional activation of the H₂ receptor by both histamine and NAMH was illustrated by dose dependent cAMP generation. This was blocked by the H₂ antagonist famotidine but not by the H₃ antagonist thioperamide. NAMH demonstrated greater potency in terms of cAMP generation, with greater maximal response and lower EC₅₀, while histamine exhibited more affinity for the receptor, as demonstrated by radioligand displacement. Transfectant studies are a powerful means of characterising receptors and these studies have confirmed the inferences drawn from previous investigations; that NAMH is a potent agonist at both H₂ and H₃ receptors.

Saitoh et al studied coupling to adenylate cyclase, which is believed to be the acid stimulatory pathway in parietal cells, as the marker for receptor activation. It is now known that H₂ receptors can also directly couple to the phosphoinositide signalling pathway and activate the protein kinase C, mitogen activated protein kinase, and p70 S6 kinase pathways.^16,17 Activation of these pathways may be involved in H₂ receptor dependent regulation of growth and differentiation. In view of the proliferative effects of H pylori infection and the suggestion that H₂ receptor agonism stimulates growth of gastric carcinoma cells,¹⁸ it would have been interesting if Saitoh et al had studied these interactions.

Two notes of caution must be raised when considering the results of this study. Although in retrospect the source of canine gastric NAMH was likely to have been Helicobacter rather than canine metabolism, the potential role of NAMH in human pathophysiology requires further assessment. Only small numbers have been studied and the data are inconsistent. NAMH was detected in the gastric juice of 5/7 H pylori positive and 0/9 H pylori negative subjects using gas chromatography-mass spectrophotometry,¹⁹ but in contrast with Courillion-Mallet and colleagues,¹¹ it was not detected in either H pylori positive gastric biopsies or cultures of H pylori.

Secondly, the physiological role of histamine receptors in the stomach does not appear to be the simple balance outlined by Saitoh et al. While activation of H₂ receptors by histamine and NAMH explains the acid stimulatory actions, data concerning potentially inhibitory actions of the H₃ receptor are conflicting. The majority of in vivo studies have confirmed that (R)-alpha-methyl-histamine inhibits acid secretion. The data are most consistent with inhibitory H₃ receptors located on ECL cells and cholinergic or intramural neurones,^9,14 although increased acid secretion secondary to reduced somatostatin secretion has been reported.²⁰ At present it is not clear whether these differences reflect species or methodological variation. Application of knowledge from the cloning of the H₃ receptor gene should clarify this situation.²¹

The recent description of a fourth receptor type²² emphasises the fact that we still have much to learn about the biology of this deceptively simple molecule.