Presynaptic histamine H2 receptors modulate the sympathetic nerve transmission in the isolated rat vas deferens; no role for H3-receptors

The modulatory activity mediated by histamine receptors on the sympathetic nerve transmission was investigated in the rat vas deferens. Agonists and antagonists acting at the different histamine receptor subtypes (H₁, H₂ and H₃) were tested on electrically-driven preparationsin vitro. Low-frequency stimulation (0.1 Hz) evoked muscle contractions almost completelysustained by ATP release, while at high-frequency stimulation (5–10 Hz) norepinephrine was mainly involved. The H₁ receptor agonists, pyridilethylamine and 2-(2 aminoethyl)thiazole, enhanced the electrically evoked twitch responses, but not contractions induced by exogenously-applied norepinephrine and ATP. These effects were prevented by the H₁-blocking drugs, mepyramine and phenyramine, but only at high concentrations (10 mol/l). All these H₁-antagonists strongly enhanced muscle response to electrical stimulation. The H₂ receptor agonists, dimaprit, amthamine and impromidine, reduced the contractions evoked by field stimulation, but not by exogenously applied norepinephrine and ATP, the effect being antagonised by H₂-blocking drugs, ranitidine and famotidine. The H₃ receptor agonist,R()-methylhistamine, reduced the electrically evoked muscle contractions, the effect being not modified by the selective H₃-blocking drug, thioperamide, but prevented by famotidine. These data suggest that rat vas deferens contains presynaptic histamine H₂ receptors, able to mediate inhibitory effects on the sympathetic transmission, while histamine H₃ receptors are apparently not involved. On the contrary, the role of H₁ is still unclear, since both agonists and antagonists may have the same effects.     

Species differences in histamine receptors in the vas deferens

Histamine, specific H₁-and H₂-receptor agonists in conjunction with specific H₁-and H₂-receptor antagonists and other types of classical antagonists were used to characterize histamine receptors in the vasa deferentia of mice, rats and guinea pigs. The H₁-receptor mediates contraction while the H₂-receptor produces inhibition. There were marked qualitative and quantitative differences in the distribution of the two types of histamine receptors in the vas deferens of different species. Results indicate that mouse and rat vas deferens contain an inhibitory H₂-receptor, but virtually no excitatory H₁-receptor. In contrast, guinea pig vas deferens contained an excitatory H₁-receptor but was essentially devoid of an inhibitory H₂-receptor. The rank order of relative potencies of various agonists as well as the calculated pA ₂ values of cimetidine in the mouse and rat vas deferens suggest that the two species probably have the same H₂-receptor. High concentrations of histamine and 2-methyl histamine have a stimulant action in the mouse and rat vas deferens which was secondary to release of endogenous noradrenaline rather than to the stimulation of an excitatory H₁-receptor.     

High spatial resolution studies of muscarinic neuroeffector junctions in mouse isolated vas deferens

It is acknowledged that neurotransmission in the mouse vas deferens is predominantly mediated by ATP and noradrenaline (NA) released from sympathetic nerves while cholinergic transmission in the rodent vas deferens is often overlooked despite early literature. Recently we have characterized a cholinergic component of neurogenic contraction of mouse isolated vas deferens. In the present paper, by confocal imaging of Ca²⁺ dynamics we detected acetylcholine (ACh) action at muscarinic cholinergic neuroeffector junctions at high-resolution. Experiments were carried out in the presence of prazosin (100 nM) and α,β methylene ATP (α,β-MeATP) (1 μM) to inhibit responses to NA and ATP respectively. Exogenous ACh (10 μM) elicited Ca²⁺ transients, an effect blocked by the muscarinic receptor antagonist, cyclopentolate (1 μM). Ca²⁺ transients were evoked by electrical stimulation of intrinsic nerves in the presence of the cholinesterase inhibitor neostigmine (10 μM). Stimulation produced a marked increase in the frequency and number of Ca²⁺ transients. Cyclopentolate reduced the frequency of occurrence of spontaneous and evoked events to control levels. The α₂-adrenoceptor antagonist yohimbine (300 nM) did not affect the spontaneous Ca²⁺ transients, but increased the frequency of occurrence of evoked transients, an effect inhibited by cyclopentolate. The postjunctional effects of neuronally-released ACh are limited by the action of cholinesterase. Release of ACh appears to be tonically inhibited by NA released from sympathetic nerve terminals through action at prejunctional α₂-adrenoceptors. Tetrodotoxin (TTX, 300 nM) abolished the nerve-evoked Ca²⁺ events, with no effect on Ca²⁺ transients elicited by exogenous ACh. In conclusion, the presence of spontaneous and evoked cholinergic Ca²⁺ transients in smooth muscle cells of the mouse isolated vas deferens has been revealed. These events are mediated by ACh acting at M₃ muscarinic receptors. This action stands in marked contrast to the lack of effect of neuronally-released NA on smooth muscle Ca²⁺ dynamics in this tissue.     

Structure-activity relationship of histamine H2-receptor agonists

Conclusions As demonstrated by the results the active C-5(4) substituted histamines and N-methyl-histamines are to various extents selective H₂-receptor agonists.In contrast to the ileal H₁-receptors the atrial H₂-receptors appear to be noticeably stereoselective.     

Effect of histamine H2-receptor and β-receptor blockade on histamine-, orciprenaline- and prostaglandin-stimulated frequency of the isolated guinea-pig atrium

For the oxyntic cell of the stomach the hypothesis was forwarded by Grossmann and Konturek [1] that, if one of its three receptors is blocked, the physico-chemical properties of the two others are changed in such a way that they respond less sensitively to their specific stimulation. This hypothesis was tested for the heart by studying the effect of histamine-H₂-receptor- and β-receptor-blockade on the orciprenaline-, histamine-, and prostaglandin E₁-stimulated frequency of the spontaneously beating isolated guinea-pig atrium. Therefore cumulative dose response curves were established for orciprenaline, histamine and prostaglandin E₁ (PGE₁) alone or in the presence of metiamide or pindolol. (1) The β-blocker pindolol inhibited the effect of orciprenaline in a competitive manner, without having an effect on histamine- and PGE₁-stimulation. (2) The histamine H₂-receptor blocker metiamide inhibited the histamine response competitively. (3) In contrast to pindolol, metiamide inhibited the PGE₁-stimulated rise in atrial frequency, most obviously non- or uncompetitively. From these results it is evident that in the heart the particular inhibitors, at least at the receptor site, act rather specifically without affecting neighbouring receptors and that metiamide influences the PGE₁-response in a way different from the receptor site.     

Heterogeneity of histamine H2-receptor mediated responses in the rabbit aorta

Distribution and properties of histamine H₂-receptor mediated responses in segments of rabbit aorta was studied with histamine and H₂-receptor stimulating drugs, dimaprit and impromidine. All agonists produced concentration-dependent tonus decreased in precontracted vascular strips, which were antagonised by selective H₂-receptor antagonists, cimetidine and oxmetidine. Activities of the agonists were segment-dependent, and increased caudally along the aorta. A nonspecific smooth muscle relaxant, papaverine had homogeneous activity along the vessel, suggesting receptor specific nature of the observed heterogeneity.     

A species comparison of the histamine H2-receptor on mast cells and basophils

The histamine H₂-agonist dimaprit was employed in experiments to investigate the role of H₂-receptors in mediator release systems from the rat, guinea-pig and man. Whereas inhibition of histamine release by dimaprit was observed in all 3 species, this effect appeared not always to be related to H₂-receptor occupancy. Although the data in general support previous evidence for the presence of H₂-receptors on human basophils and guinea-pig mast cells, the use of dimaprit as a pure H₂-agonist in these studies is questioned. No evidence for H₂-receptors on rat mast cells was obtained.     

Cholinergic-like effects of the new histamine H2-receptor antagonist ranitidine

The new H₂-receptor blocker ranitidine, together with the effect on histamine H₂-receptors, possesses a series of cholinergic-like actions: it provokes atropine-sensitive contractions of several isolated smooth muscle preparations from different animal species and it potentiates the stimulant effect of acetylcholine. Moreover it contracts human lower esophageal sphincterin vivo, an effect which is completely prevented by small doses of atropine. Finally, ranitidine potentiates the stimulant effect of bethanechol and of carbachol on salivary glands of the rat while leaving unaffected the secretagogue effect of physalaemin which is known to be completely independent of the cholinergic system. In thein vivo experiments the doses of ranitidine capable of eliciting cholinergic-like effects were of the same order of magnitude as those necessary to cause the H₂-receptor blockade.     

Varicosity-Schwann cell interactions mediated by ATP in the mouse vas deferens

Schwann cells, from a variety of sources, are known to possess P2Y purinergic metabotropic receptors. However, it is not known if Schwann cells associated with autonomic nerve terminals possess such receptors and if so whether these receptors are activated by the endogenous release of ATP from the nerve terminals. We show that such Schwann cells in the vas deferens give evoked calcium transients on nerve stimulation. These transients are mediated, at least in part, by the endogenous release of ATP, which acts on Schwann cell P2Y receptors to release calcium from within the cells. This work suggests the possibility that Schwann cells are active participants in the process of junctional transmission in the autonomic nervous system.     

Metiamide — An orally active histamine H2-receptor antagonist

Metiamide has been found to be about 10 times more active than burimamide in vitro in antagonizing histamine H₂-receptors and nearly 5 times more active in vivo as an antagonist of histamine or pentagastrin-stimulated secretion. Effective oral ED₅₀ doses for inhibition have been estimated as 25 mole kg^–1 against basal secretion in rats and 16 mole kg^–1 against maximal histamine-stimulated secretion in dogs. Administration of metiamide orally daily for 90 days with doses of 1,500 mole kg^–1 to rats and 700 mole kg^–1 to dogs produced signs of kidney damage as the dominant lesion in both species. These toxic doses are roughly 60 and 44 times greater than the ED₅₀ doses in the rat and dog, respectively. These results show that metiamide has the degree of activity and safety needed for a compound to be a candidate for through clinical evaluation.     

Some properties of the smooth muscle of mouse vas deferens.

1. Contractions of the mouse vas deferens in response to electrical stimulation differ form those recorded form the guinea-pig vas deferens in that they are abolished by tetrodotoxin. 2. Changes in membrane potentials were recorded form the smooth muscle of both preparations in response to stimulation with current pulses applied by an intracellular electrode and by alrge extracellular plate electrodes. 3. Both preparations behaved similarly in response to intracellular stimulation. Electrotonic potentials in response to extracellular current pulses spread in a longitudinal direction in the guinea-pig vas deferens in accordance with the cable-like properties of this preparation. In contrast, no longitudinal spread of eletrotonus was observed in the mouse vas deferens. 4. Responses to nerve stimulation differed in the two preparations. In the guinea-pig, single stimuli caused excitatory junction potentials (e.j.p.s) which gave rise to action potentials. Some cells from the mouse vas deferens showed similar e.j.p.s and action potentials, although the threshold for the initiation of action potentials was lower and more variable. 5. The majority of cells in the mouse vas deferens failed to show action potentials in response to a single stimuli even though the amplitude of e.j.p.s was from 35 to 40 mV. This was probably due to the large resting membrane potentials of these cells, as all-or-nothing action potentials could be evoked if successive e.j.p.s were allowed to sum with each other or if a depolarizing current pulse was applied at the peak of an e.j.p. 6. The nature of the response to nerve stimulation recorded from differnt cells in the mouse vas deferens could be correlated with the amplitude and time course of the response of the same cell to intracellular stimulation. 7. It is concluded that individual smooth muscle cells in both preparations are probably coupled electrically but that there are few, if any, low resistance pathways in the longitudinal direction in the mouse vas deferens.     

Pharmacological characterization of histamine receptors in the mouse seminal vesicle: Sensitivity to H1- and H2-receptor agonists and antagonists

Neither histamine nor the more specific H₁- or H₂-receptor agonists (0.2–220 M) produced a contraction of the mouse seminal vesicle. However, all these agonists decreased resting tone and caused a dose-related inhibition of the electrically evoked twitch responses in these concentrations. The slopes of the percentage response versus log molar concentration curves for all agonists did not differ significantly from that of histamine. The rank order and relative potency of the compounds tested were: histamine (100%)>dimaprit (65%)4-methylhistamine (36%)>2-methylhistamine (4.5%)>2-(2-thiazolyl) ethylamine (1.7%). Inhibition of the twitch response by histamine was antagonized by cimetidine (3.5–140 M) but not by mepyramine (0.1–1.0 M). The slopes of Schild plots for cimetidine against histamine and dimaprit did not differ from unity, and the calculated pA₂ values of cimetidine were 5.70 and 5.55, respectively. Histamine (2.2 and 6.5 M) inhibited the contraction elicited by a submaximal concentration of exogenous noradrenaline (1 M); this inhibition could also be selectively antagonized by cimetidine. These results demonstrate that histamine and selective H₁- and H₂-receptor agonists have an inhibitory action on the mouse seminal vesicle, and that this action is mediated via a post-junctional H₂-receptor. The calculated pA₂ values of cimetidine against histamine and dimaprit also suggest that the receptor in the mouse seminal vesicle is of the conventional H₂-type. The results further indicate that an H₁-receptor is absent in the mouse seminal vesicle.     

Studies on the mode of action of histamine H1- and H2-receptor antagonists on gastric histamine methyltransferase

Histamine methyltransferase from pig antrum mucosa was inhibited by 33 H₁-receptor antagonists, by the H₂-receptor antagonists burimamide and metiamide and by the burimamide analogue 5-methylburimamide, which did neither act on H₁-nor on H₂-receptors. Whereas all H₁-receptor blocking agents as well as metiamide and 5-methylburimamide inhibited the enzyme in a competitive manner, the type of inhibition found for burimamide was a mixture of non-competitive and uncompetitive with respect to histamine, which is similar to that one observed with 1-methylhistamine, the product of the histamine methylation reaction. Furthermore, all compounds tested — with the only exception of burimamide — activated the gastric histamine methyltransferase in lower concentrations, the most potent activator being piprinhydrinate (180% increase of the enzyme activity). This enhancement of 1-methylhistamine formation by antihistaminic drugs was not due to a true activation of the enzyme by increasingV _max, but was caused by partially abolishing the inhibition of histamine methyltransferase by so-called optimum concentrations of histamine. Two explanations were given for the different mode of action of burimamide compared with that of metiamide and the other antihistaminic drugs: (1) The change in the type of inhibition from burimamide to metiamide seemed to be due to the introduction of a methylgroup into position 5 of the imidazole ring. (2) Burimamide and 1-, 2- and 3-methylhistamine were the only compounds tested in which the imidazole nucleus was substituted at position 4, but not at position 5, and which thus probably produced substrate or product inhibition.supported by a grant of the Deutsche Forschungsgemeinschaft (Lo 199/3).