Effect on mast cell histamine of inhibiting histamine formation in vivo with alpha-fluoromethylhistidine

Gastric acid secretion after depletion of enterochromaffin-like cell histamine. A study with alpha-fluoromethylhistidine in rats

BACKGROUND

Histamine is thought to play a central role in the regulation of gastric acid secretion. In the rat oxyntic mucosa most of the histamine is synthesized and stored in enterochromaffin-like (ECL) cells, and the rest resides in mast cells. The present study examines the role of ECL-cell histamine in the control of acid secretion in the intact, conscious rat.

METHODS

Rats were treated with alpha-fluoromethylhistidine (alpha-FMH) to inhibit histamine synthesis. alpha-FMH was given by continuous subcutaneous infusion (3 mg/kg/h) for up to 9 days. An additional oral dose of alpha-FMH (50 mg/kg) was given 2 h before each acid secretion test. Acid secretion was studied in pylorus-ligated rats and in chronic gastric fistula rats stimulated with histamine, gastrin-17, or insulin after 2-6 days of alpha-FMH infusion.

RESULTS

Treatment with alpha-FMH lowered oxyntic mucosal histamine synthesis by 80%. From previous observations this is thought to reflect depletion of histamine from the ECL cells. The remaining 20% resides in mucosal and submucosal mast cells, which seem to be resistant to alpha-FMH. Basal acid secretion was inhibited by more than 60% after alpha-FMH treatment and by more than 80% by ranitidine. Histamine-stimulated secretion was unaffected by alpha-FMH and abolished by the histamine H2-receptor antagonist ranitidine. The acid response to gastrin-17 was almost abolished in histamine-depleted rats and abolished by ranitidine. Vagally induced acid secretion (provoked by the injection of insulin or by pylorus ligation) was unaffected by alpha-FMH treatment but abolished by ranitidine and by the muscarinic M1-receptor antagonist pirenzepine.

CONCLUSION

The results suggest that gastrin stimulates acid secretion by releasing histamine from ECL cells. Vagally induced acid secretion is also dependent on a histaminergic pathway but not on ECL-cell histamine.

The antiproliferative potency of histamine antagonists correlates with inhibition of binding of [3H]-histamine to novel intracellular receptors (HIC) in microsomal and nuclear fractions of rat liver

Previously, we identified in rat liver microsomes, low (microM) affinity histamine receptors (HIC), associated with antiestrogen binding sites (AEBS). N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine HCl (DPPE), a potent AEBS ligand, is a specific HIC antagonist. Through binding HIC, newly-formed intracellular histamine mediates, and DPPE inhibits, human platelet aggregation. We now provide evidence that histamine, mobilized from cytoplasmic stores, is a mediator of the mitogenic response to concanavalin A in mouse spleen cells. DNA synthesis and intracellular histamine levels are decreased over time by the histidine decarboxylase inhibitor, alpha-fluoromethylhistidine. For DPPE, H1 and H2 antagonists, rank order of potency to inhibit [3H]-histamine binding to HIC in rat liver microsomes correlates with antiproliferative potency. DPPE also competes for [3H]-histamine binding at low and high affinity sites in rat liver nuclei (IC50 approximately 2 microM). Thus, histamine may mediate growth through two intracellular subtypes of HIC.

Differential effects of morphine on histamine metabolism in brain and spinal cord of mice

The effects of morphine on the levels of histamine (HA), its metabolite tele-methylhistamine (t-MH) and on t-MH synthesis rates (thought to be indicative of neuronal HA release) were investigated in brain regions and spinal cords of DBA/2J (DBA) and C57/BL6 (C57) mice, two strains known to differ in their sensitivity to morphine. In DBA (a strain highly sensitive to morphine antinociception), morphine (10 mg/kg, s.c.) had no effect on brain regional t-MH or HA levels, but produced a generalized inhibition of regional t-MH synthesis rates ranging from 11 to 53%. The monoamine oxidase (MAO) inhibitor pargyline (used to estimate t-MH synthesis rates) had no effect on HA or t-MH levels in the DBA or C57 spinal cord, indicating the absence of detectable spinal HA turnover. Morphine (10 mg/kg) had no effect on DBA or C57 spinal cord HA or t-MH levels, but significantly increased t-MH synthesis in the DBA but not in the C57 spinal cord. These results suggest that in DBA mice, antinociceptive doses of morphine inhibit HA release in brain, and promote the release of HA from spinal cord. Neither effect was found in C57 mice, which are resistant to morphine antinociception. The relevance of these actions to previous studies showing the blockade of opiate antinociception by H2 antagonists remains to be established.

Does intracellular histamine mediate mast cell histamine release?

Previously, we demonstrated that through binding a novel intracellular receptor of microM affinity (HIC), histamine mediates, and the HIC antagonist N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine. HCl (DPPE) inhibits, platelet aggregation and serotonin granule secretion; the latter response is dependent upon the same processes that mediate histamine release from mast cell granules. We now show that, as for platelet serotonin release, DPPE blocks concanavalin A-stimulated mast cell histamine release with a potency (IC50 = 30 microM) greater than the H1-antagonist, pyrilamine (IC50 = 150 microM) or the H2-antagonist cimetidine (IC50 = 5 mM), correlating with rank order of potency to inhibit 3H-histamine binding in rat brain membranes and liver microsomes. We postulate that histamine release from mast cells is mediated at HIC by second messenger intracellular histamine. However, unlike platelets, mast cells do not appear to rely on newly synthesized histamine. Rather, as for calcium, histamine may be mobilized from bound stores to mediate histamine secretion.

Alpha-fluoromethylhistidine in the treatment of idiopathic cold urticaria

Alpha-fluoromethylhistidine (alpha-FMH), a new irreversible inhibitor of mammalian histidine decarboxylase, was tested in the treatment of idiopathic cold urticaria in 11 patients. In the initial trial with 50 mg b.i.d., a significant decrease (about 30%) in the total blood histamine level was found after 3 weeks of treatment but clinically there was no improvement in the symptoms of ten cold urticaria patients nor in the responses to the ice-cube test. In the second trial with three patients suffering from severe idiopathic cold urticaria, a higher dose of up to 500 mg b.i.d. of alpha-FMH for 3 weeks resulted in a marked decrease in the total blood histamine level as well as in an apparent inhibition of histamine synthesis in the skin previously exposed several times to cold water. The symptoms of cold urticaria and the responses in the ice-cube tests also decreased simultaneously. No clinical side effects nor changes in laboratory analysis were seen during the treatment with alpha-FMH. These results suggest that alpha-FMH may be useful in the treatment of severe cold urticaria especially in combination with histamine exhaustion of mast cells using cold water.

Inhibitors of histidine decarboxylase decrease basal gastric acid secretion in the rat

We examined the ability of two specific inhibitors of histidine decarboxylase, (s)-alpha-fluoromethylhistidine (FMHd) and (s)-alpha-fluoromethylhistamine (FMHm), to inhibit basal gastric acid secretion. The two highest doses of FMHd administered, 50 and 100 mg/kg, decreased basal gastric acid secretion and total secretion volume but did not affect intraluminal pH. FMHm decreased gastric acid secretion, raised intraluminal pH, and to a lesser degree decreased total secretion volume. Neither compound changed the severity of gastric ulcers produced by cold restraint stress.