Histamine receptors in the cardiovascular

Compound 48/80 induces endothelium-dependent and histamine release-independent relaxation in rabbit aorta

Compound 48/80 (C48/80) is a synthetic condensation product of N-methyl-p-methoxyphenethylamine with formaldehyde and is an experimental drug used since the 1950s to induce anaphylactic shock through histamine release. This study was carried out to further elucidate the mechanism by which this drug induces nitric oxide (NO) release. Our specific goals were: (a) to verify if C48/80's relaxation occurs through the stimulation of histamine receptors; (b) to evaluate the endothelium-dependent relaxation induced by C48/80; (c) to identify NO as the endothelium-relaxing factor released by C48/80; (d) to identify the NO synthase (NOS) responsible for NO release; and (e) to verify if the relaxation induced by C48/80 is calcium and cyclic guanidine monophosphate (cGMP) dependent. Rabbit aorta segments, with and without endothelium, were suspended in organ chambers (25ml) filled with Krebs solution maintained at 37 degrees C, bubbled with 95% O(2)/5% CO(2) (pH 7.4). Phenylephrine was used to contract the segments. Other protocol drugs included H(1)- and H(2)-receptor antagonists, cyclooxygenase, NOS, guanylyl cyclase and phospholipase C (PLC) inhibitors. Endothelium-dependent relaxation induced by C48/80 was also studied in calcium-free Krebs solution associated with a calcium chelator. In summary, our investigation demonstrated that the C48/80 vasodilating action: (a) does not depend on H(1) and H(2) histamine receptors; (b) is NO endothelium-dependent; (c) is dependent on the endothelial constitutive NOS (NOS-3) isoform activation; (d) is cGMP-dependent; and that NOS-3 activation by C48/80: (a) is independent of PLC up to 25mug/ml and (b) is partially dependent of this lipase in higher doses.

Histamine release and contrast media-induced renal vasoconstriction

RATIONALE AND OBJECTIVES

The authors' purpose was to investigate the role of histamine release causing renal vasoconstriction induced by application of contrast media, an important element in contrast medium-induced nephrotoxicity.

MATERIALS AND METHODS

Isometric contractions in rabbit segmental renal arteries stimulated with KCl and increasing concentrations of the ionic contrast medium diatrizoate and the nonionic agents iomeprol and iodixanol were studied both with and without increasing concentrations of the histamine H1 and H2 blockers diphenhydramine and cimetidine. Histamine concentrations after contrast medium application were determined.

RESULTS

Contrast-induced, dose-dependent, reversible renal artery contractions of 27%, 4.5%, and 5% of the control KCl contraction were found for diatrizoate, iodixanol, and iomeprol respectively. Those induced by the ionic contrast medium were statistically significantly higher (P < .01). Contractions were partially inhibited by diphenhydramine (49%) but not by cimetidine. Significant elevation of histamine concentrations (P < .05) was detected only after stimulation with diatrizoate but not with nonionic agents.

CONCLUSION

Ionic contrast medium induces histamine release leading to renal vasoconstriction, which can be partly blocked by H1 blockers. Histamine has no effect on renal vasospasm induced by nonionic contrast media.

Computer-aided video angiometry in isolated rabbit hearts: a new method assessing epicardial coronary selectivity

The clinical value of coronary vasodilators in antianginal therapy depends on the ratio of their epicardial versus resistance coronary vessel actions. The coronary flow measured in standard isolated heart preparations, however, does not allow any conclusion about the function of epicardial vessels. Thus, we established a new technique assessing the epicardial coronary diameter directly by video angiometry. Pictures from the cardiac surface were taken by a videocamera mounted on a long-distance microscope. The video signal was digitized for computer-aided evaluation. An area of interest (AOI) was laid over the vascular section to be measured. The gray values of the pixels across the epicardial vessel were registered, and a mean curve of distribution was obtained by averaging the gray values from all video lines within the AOI. The inner epicardial coronary diameter resulted from the distance between the points of inflection of this mean curve of distribution. Experiments with NO-vasodilators and adenosine showed that epicardial coronary arteries of isolated perfused rabbit hearts have no appreciable tone. Pretreatment of the hearts with a combination of histamine (10[-6] mol/l), cimetidine (10[-5] mol/l), and adenosine (10[-7] mol/1), however, caused a marked contraction of the conductive vessels. NO-donors selectively dilated epicardial vessels in such pretreated hearts whereas adenosine increased both epicardial coronary diameter and coronary flow, with only a slight tendency toward preferential action on resistance vessels in low concentrations. Simultaneous registration of coronary flow and epicardial coronary diameter in isolated rabbit hearts pretreated with a spasmogenic drug combination (histamine, cimetidine, and adenosine) may be a feasible method assessing epicardial selectivity of coronary vasodilators.

Highlights in cardiovascular effects of histamine and H1-receptor antagonists

Despite numerous studies, the cardiac actions of histamine are still obscure. Yet, histamine could probably be clinically relevant. It is stored in large amounts in human cardiac tissue, where it is contained in the cytoplasmatic granules of mast cells. Mast cells are present in normal human heart tissue; they are more abundant in diseased human heart tissue where they lie in close proximity to blood vessels and between myocytes. The histamine content of human heart mast cells is comparable to the histamine content of lung parenchymal and skin mast cells. Ultrastructural studies confirmed the presence of mast cells around vessels and between myocytes. Consequently, these cells are easily accessible to circulating antigens, drugs and stimuli that activate the cells to release vasoactive mediators which in turn can exert significant cardiovascular effects. Histamine possesses arrhythmogenic effects and once locally released, may enhance automaticity and induce triggering activity resulting in severe tachyarrhythmias. The major arrhythmogenic effects of histamine consist in increasing sinus rate and ventricular automaticity, and in slowing atrioventricular conduction. In addition, histamine may interfere with depolarization and repolarization through its effects on calcium and potassium currents. These effects are mediated by H2-receptor. Therefore direct activation of histamine receptor can induce cardiac arrhythmias. Consequently, the interference of these histaminergic effects may explain, at least in part, the arrhythmogenic effects described for some second-generation antihistamines, such as terfenadine and astemizole. In this brief review we will discuss the cardiac effects of histamine in experimental animal models and in man, and will review data on the safety of the new second-generation antihistamines, focusing on their cardiotoxic effects.

Endothelium-independent contractile and relaxant responses to histamine in the rabbit aorta and common carotid, mesenteric, renal, and femoral arteries

1. The role of the vascular endothelium in the relaxant and contractile responses to histamine of the isolated rabbit aorta; common carotid, mesenteric, renal, and femoral arteries; as well as receptor types mediating these responses were analyzed. 2. Histamine (10(-8) to 10(-4) mol/l) contracted resting rings and caused a further concentration-dependent contraction of rings of the arteries precontracted by phenylephrine. 3. Pyrilamine abolished the contractile response to histamine in resting rings of the arteries, whereas it reversed that response into a concentration-dependent relaxant response in precontracted rings of the arteries. The relaxant effect of histamine was abolished by metiamide, but it was not affected by sotalol and atropine. Moreover, in control experiments, the phenylephrine-induced contractions and acetylcholine-induced relaxations were not changed by pyrilamine and metiamide, respectively. 4. Endothelial removal did not influence the contractile and relaxant responses of the arteries to histamine. 5. These findings indicate that, in the isolated rabbit aorta and common carotid, mesenteric, renal, and femoral arteries, the contractile effect of histamine resulting from the activation of H1 receptors overcomes its relaxant effect resulting from the activation of H2 receptors. The effects of histamine are neither mediated nor modulated by the endothelial cells.

Flush induced by fluoroquinolones in canine skin

The flush induced by two fluoroquinolone antibacterial agents, balofloxacin and ofloxacin, was studied in beagle dogs. Intradermal injection of the fluoroquinolones at concentrations above 10(-5) M produced a localized flushed area. The flush responses to fluoroquinolones were inhibited by co-administration with H2-antagonist(s) (ranitidine or cimetidine), but not with H1-antagonist(s) (mepyramine or chlorpheniramine). Similar inhibitory effects of these H2-antagonists were observed for the response to histamine. The flush responses to fluoroquinolones were inhibited by a local pretreatment with compound 48/80 administered to deplete the local stores of mast cell-bound histamine. When the fluoroquinolones were orally administered at a dose of 400 mg/kg, the concentration of histamine in plasma was increased, being accompanied by systemic erythema. These results indicate that the flush induced by fluoroquinolones is mediated by histamine release from canine cutaneous mast cells and H2-receptor stimulation.

Inhibition by (-)-deprenyl of agonist-evoked contractions in rabbit aorta

The effect of (-)-deprenyl, a relatively selective MAO-B inhibitor, was examined for its ability to inhibit the contractions of rabbit isolated aorta evoked by various agonists and potassium. (-)-Deprenyl (10(-5)-3 x 10(-4) M) antagonized the contractions evoked by noradrenaline (10(-8)-3 x 10(-4) M); pA2: 5.10. The antagonism was reversible. It was attenuated by cocaine (3 x 10(-5) M); pA2: 4.38, unchanged by corticosterone (4 x 10(-5) M); pA2 4.79 and enhanced by cocaine (3 x 10(-5) M) plus corticosterone (4 x 10(-5) M); pA2: 5.48. (+)-Deprenyl (10(-6)-10(-4) M) did not alter the contractions evoked by noradrenaline (3 x 10(-9)-10(-4) M). Clorgyline (10(-5) and 10(-4) M) antagonized the noradrenaline-evoked contractions. Pargyline (10(-4) and 3 x 10(-4) M) had no effect. (-)-Deprenyl (10(-5)-3 x 10(-4) M) antagonized the contractions evoked by phenylephrine (10(-8)-10(-4) M); pA2: 5.10. Removal of the endothelium did not alter the antagonism; pA2: 5.35. (-)-Deprenyl (10(-5)-3 x 10(-4) M) antagonized the contractions evoked by either 5-hydroxytryptamine (3 x 10(-8)-3 x 10(-4) M); pA2: 4.61 or by histamine (10(-6)-3 x 10(-2) M); pA2: 4.84. (-)-Deprenyl (3 x 10(-4) M) caused a noncompetitive antagonism of the contractions evoked by potassium (1.5-5.5 x 10(-2) M). It is concluded that (-)-deprenyl is a weak inhibitor of postjunctional alpha 1-adrenoceptors, 5-hydroxytryptamine (5-HT2) receptors, and histamine (H1) receptors.

Cooling effects on the histaminergic response of rabbit ear and femoral arteries: role of the endothelium

The effects of cooling on the isometric response of rabbit isolated central ear (cutaneous) and femoral (non-cutaneous) arteries to histamine were determined at 37 degrees C and 24 degrees C (cooling). Under resting tension, both types of arteries contracted to histamine (10(-7)-10(-3) M), and the sensitivity of ear arteries, but not of femoral arteries was lower at 24 than at 37 degrees C. Chlorpheniramine (10(-7) M) blocked the contraction of both types of arteries to histamine at both temperatures. In ear arteries, endothelium removal or treatment with the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 10(-5) M) did not affect the contraction to histamine at 37 degrees C, but it reversed the decreased contraction at 24 degrees C. In femoral arteries, endothelium removal or L-NAME (10(-5) M) did not affect the response to histamine at 37 and 24 degrees C. Ear and femoral arteries precontracted with endothelin-1 (10(-8)-10(-7) M) and pretreated with chlorpheniramine (10(-5) M) relaxed to histamine (10(-7)-10(-4) M), and the sensitivity of this relaxation in ear arteries, but not in femoral arteries, increased at 24 degrees C. The relaxation of ear and femoral arteries to histamine was not modified by endothelium removal, L-NAME (10(-5) M) or meclofenamate (10(-5) M), but it was blocked by cimetidine (10(-6) M) at 37 degrees C and 24 degrees C. These results suggest: (1) ear and femoral arteries have contracting H1 and relaxing H2 receptors, probably located on smooth musculature, and (2) cooling reduces the contraction and increases the relaxation of cutaneous arteries to histamine: the reduction of this contraction could be caused by an augmented availability of endothelial nitric oxide, and the increment of this relaxation could be caused by an augmented sensitivity of H2 receptors of smooth musculature induced by cooling. These features do not seem to occur in deep vessels.

Effect of dopexamine hydrochloride on contractions of rabbit isolated aorta evoked by various agonists

The inhibitory affinity of dopexamine hydrochloride to postsynaptic adrenoceptors, cholinoceptors, 5-hydroxytryptamine and histamine receptors was studied in rabbit isolated aorta. Dopexamine (10(-7)-10(-5) M) antagonized competitively the contractions of rabbit aorta evoked by noradrenaline (pA2: 6.60). Neither cocaine plus corticosterone nor cocaine, corticosterone plus propranolol altered the inhibition (pA2: 6.77 and 6.63, respectively). The antagonism of dopexamine against noradrenaline-evoked contractions was the same after 1 and 4 hr of pretreatment with dopexamine. In the presence of cocaine plus corticosterone, dopexamine antagonized the contractions evoked by phenylephrine (pA2: 6.94). Removal of endothelium did not influence this antagonism (pA2: 7.06). Dopexamine (10(-7)-10(-5) M) did not antagonize the contractions of aorta evoked by histamine (3 x 10(-7)-6 x 10(-5) M) and by 5-hydroxytryptamine (3 x 10(-7)-3 x 10(-4) M). Dopexamine (10(-8 and 10(-7) M) did not alter the contractions of endothelium-free aorta evoked by carbachol. Dopexamine (10(-7)-10(-5) M) slightly enhanced the contractions of aorta evoked by potassium (10(-2)-5.5 x 10(-2) M). These results suggest that dopexamine is an alpha 1-adrenoceptor antagonist. Furthermore, dopexamine has no affinity to cholinoceptors, histamine and 5-hydroxytryptamine (5-HT2) receptors and is apparently not a calcium antagonist.

Altered response to histamine in brain tumor vessels: the selective increase of regional cerebral blood flow in transplanted rat brain tumor

The authors studied the effect of intracarotid administration of histamine on the regional cerebral blood flow (rCBF) in transplanted rat C6 glioma by the hydrogen clearance method. Histamine infusion at doses of 1 and 10 micrograms/kg/min produced an increase of rCBF in the tumor (24.6% +/- 16.4%, p < 0.002, and 37.6% +/- 18.2%, p < 0.0001, respectively) and also in brain surrounding the tumor (26.8% +/- 16.2%, p < 0.002, and 34.9% +/- 9.2%, p < 0.0001, respectively) without any significant changes in the ipsilateral hemisphere. Intravenous administration of pyrilamine (H1 antagonist) and cimetidine (H2 antagonist) reduced blood flow responses to histamine; cimetidine was a more effective blocking agent than pyrilamine. Intracarotid infusion of histamine (1 and 10 micrograms/kg/min) with intravenous injection of Evans blue dye disclosed the selective extravasation of dye in the tumor and the brain surrounding the tumor. These results indicated that brain tumor vessels could respond to histamine differently than normal brain capillaries. The mechanism of selective response to histamine could be explained either by increased permeability or by altered characteristics of histamine receptors in the tumor vessels.

Effect of the histamine antagonist cimetidine on infarct size in the rat

We determined whether the histamine H2 antagonist cimetidine would reduce infarct volume in a model of photochemically-induced thrombotic infarction. Rats were pretreated with either vehicle (n = 6), 5.0 mg/kg cimetidine (n = 6), or 20.0 mg/kg (n = 6) cimetidine 30.0 min prior to infarct formation. Cortical infarction was produced by irradiating the brain with green light (560 nm) through the intact skull for 4 min following the systemic injection of rose bengal. Five days after infarct induction, rats were perfusion-fixed and processed for routine histopathologic analysis. With computer-assisted planimetry, infarct areas and volumes were determined using multiple coronal sections spanning the anterior-posterior extent of the infarct. Morphologic analyses of infarct volume demonstrated no differences between the vehicle (56.0 +/- 6 mm3), 5.0 mg/kg cimetidine (50.0 +/- 8 mm3), or 20.0 mg/kg cimetidine (53.0 +/- 7 mm3) treated groups. In this cortical infarct model, pretreatment with a histamine antagonist fails to reduce infarct size. It is concluded that photochemically-induced microvascular thrombosis results in too severe an insult for cimetidine to chronically protect.

Role of the endothelium and smooth muscle tone in the dilator response of the rabbit coeliac artery to histamine

The role of the endothelium and vascular tone was studied on histamine-induced relaxation and contraction of the rabbit coeliac artery. Histamine contracted the tissue via the release of noradrenaline (NA), an action blocked by prazosin and mepyramine but not influenced by endothelial removal. Tachyphylaxis readily developed to histamine-induced contractions. After a moderate tone (40-55%) was induced with NA, histamine relaxed the tissues concentration-dependently. This relaxation was absent when the endothelium was removed indicating that the receptors involved are located on the endothelium. When the tone was increased to 80-85% with NA, relaxation could only be demonstrated after blocking H1-receptors. Removal of the endothelium did not influence this response. The relaxant effect of histamine in both preparations, however, was blocked by metiamide indicating that it is H2-receptor mediated. In the rabbit coeliac artery, endothelial H2-receptors are readily activated at moderate tone while muscular H2-receptors are stimulated at high tone after blocking H1-receptors. The predominance of H2-receptors in this artery may serve the physiological function of vasodilation in the blood vessels of the stomach.

Amine-induced responses of pial and penetrating cerebral arteries: evidence for heterogeneous responses

Middle cerebral arteries (MCAs) of rabbits were compared with two types of small branches (less than 100-microns outer diameter), penetrating arteries (PAs) and surface arteries (SAs), by determining their mechanical reactivity to several amines and standard contractile agents. Two techniques were employed: (a) measurement of isometric tension of 1-mm rings (MCA, PA, or SA); (b) measurement of perfusion pressure of segments consisting of essentially MCA or essentially PA. Both techniques revealed similar reactivity of the different types of vessel to acetylcholine, i.e., relaxations to a maximum of 52-78%, and similar strong contractile responses to histamine, although the MCA was more sensitive. Under H1 blockade, histamine dilated the PA (both techniques) and the MCA (perfusion technique), but not the SA. Relatively weak contractile responses to serotonin were observed in the MCA (both techniques) and the PA (perfusion technique), but not the SA (isometric tension only); no dilative responses could be elicited. Responses to noradrenaline varied with the vessel considered: The MCA contracted only, whereas the PA weakly contracted or relaxed at basal tone, and many preparations relaxed after precontraction with uridine triphosphate; the SA did not react. Relaxation of precontracted PA by noradrenaline occurred at relatively low concentrations and was antagonized by propranolol at 3 x 10(-7) or 3 x 10(-6) M. These results reveal very significant differences in the segmental reactivity to amines and suggest that noradrenaline released from sympathetic fibers might have opposing actions in the major pial arteries and the smaller penetrating branches.

Interspecies comparisons in toxicology: the utility and futility of plasma concentrations of the test substance

A classical dilemma in toxicology is how the dose administered relates to the dose delivered to the target site. Plasma concentrations of the test substance may be misleading since the concentration of any given substance in the plasma may not be representative of its concentration in tissues. Furthermore, a given tissue concentration of a xenobiotic can evoke responses which are highly species-dependent. While evaluating toxicity data within one species, plasma concentrations reflect the effects of route of administration, bioavailability, dose level, multiple dosing, age, gender, etc. However, when toxicity data is compared across species, the relevance of plasma concentrations depends on the nature of the toxicity. Reversible, pharmacodynamic effects often correlate with plasma concentrations, although there may be marked interspecies differences in dose-response relationships. Irreversible effects, if pharmacodynamic in origin, often correlate better with the intensity/duration of the pharmacodynamic response, rather than with plasma concentration. On the other hand, irreversible effects, if chemically mediated, may not correlate at all with plasma concentration, the lesions being caused by reactive metabolites of fleeting existence, which rarely survive long enough to leave their site of synthesis. They cannot be measured in the plasma nor predicted from plasma concentrations of the parent xenobiotic. The limitations of plasma concentrations in interpreting the toxicology of substances which are tissue-sequestered, which are subject to pharmacogenetic factors, or which show plasma concentrations that are not proportional to dose are also discussed. Mention is made of possible alternatives to plasma concentrations in assessing exposure in toxicology studies.

Pharmacological characterization of histamine receptors in the human temporal artery

1. The subtypes of histamine-receptors which mediate dilatation of small human temporal arteries have been characterized in vitro using 'selective' agonists and antagonists. 2. Dilatory responses were studied after preconstriction with prostaglandin F2 alpha since contraction was not seen at histamine concentrations up to 10(-4) M. Histamine caused a concentration-related relaxation of cerebral vessels with an IC50 value of 2.8 +/- 0.6 X 10(-7) M. 3. Cimetidine caused a parallel shift to the right of the histamine concentration-response curve whereas mepyramine was without observable effect. This suggests the presence of histamine H2-receptors only. However, combined treatment with mepyramine and cimetidine caused a more marked displacement of the concentration-response curve to the right. Schild analysis indicated that in situations of near complete blockade of the histamine H1-receptor subtypes, simple competitive antagonism at H2-receptors can be revealed with a pA2 value of 6.58 for cimetidine. The apparent pA2 value for mepyramine was 8.58. 4. The 'selective' H1-receptor agonists pyridylethylamine, 2-methylhistamine and thiazolylethylamine, and the H2-receptor agonists dimaprit, impromidine and 4-methylhistamine all mimicked the histamine response, but all except impromidine were less potent than histamine. The order of potency was impromidine greater than thiazolylamine greater than 4-Me-histamine greater than 2-Me-histamine greater than dimaprit greater than pyridylethylamine greater than tele-Me-histamine. 5. These results indicate that the histamine-induced dilatation in small human temporal arteries is mediated by both H1- and H2-receptors and that the latter subtype of histamine receptors predominates.

Endothelium-dependent relaxations to histamine in the rat common carotid, renal and cranial mesenteric artery

The effect of histamine on the isolated rat common carotid, renal and cranial mesenteric arteries was examined. Histamine (10(-8)-10(-4) M) caused concentration-dependent relaxations of the arteries during contractions induced with phenylephrine (10(-8)-10(-7) M). Removal of the vascular endothelium inhibited the histamine-induced relaxations. Pyrilamine (6 X 10(-6) M), but not metiamide (10(-6) M), abolished the relaxant effect of histamine. Moreover, pyrilamine (6 X 10(-6) M) did not affect endothelium-dependent relaxations of the arteries produced with acetylcholine. These results indicate that histamine causes endothelium-dependent relaxations of the rat peripheral large conduit arteries, which appeared to be mediated via H1-histaminergic receptors.

Relaxation of rabbit middle cerebral arteries in vitro by H1 histaminergic agonists is inhibited by indomethacin and tranylcypromine

The H1-histaminergic agonists 2-pyridylethylamine (2-PEA) and 2-methylhistamine relaxed potassium-constricted, perfused, rabbit middle cerebral arteries at low concentrations (3 x 10(-11) to 3 x 10(-8) M) and constricted them at high concentrations (3 x 10(-7) to 3 x 10(-4) M). The relaxation and the contraction were not antagonized by propranolol (up to 3 x 10(-6) M) given 30 min before, suggesting that beta-adrenergic mechanisms were not involved. When 2-PEA was tested on arteries constricted with uridine triphosphate (UTP), similar results were obtained. In the UTP-constricted arteries, the 2-PEA-induced responses were competitively antagonized by 3 x 10(-9) M mepyramine. Together with previous work (Ea Kim et al., 1986), these results are compatible with the hypothesis that H1-receptors were responsible for both the relaxation and the contraction observed. When either indomethacin (10(-8), 3 x 10(-7), or 10(-5) M), dexamethasone (10(-5) M), or tranylcypromine (10(-5) or 10(-4) M) were tested on the response to 2-PEA or 2-methylhistamine, these inhibitors suppressed the relaxation or reversed it to a contraction. Furthermore, they potentiated the contraction induced by these agonists. These results favour the hypothesis that the H1-mediated relaxation in rabbit cerebral arteries may in part involve the release of prostaglandins, especially prostacyclin. The participation of such a prostanoid in histaminergic relaxation seems exclusively an H1-mediated mechanism, since the relaxation induced by the H2-agonist dimaprit (in the presence of mepyramine) was not antagonized by either indomethacin (3 x 10(-7) M) or tranylcypromine (10(-4) M).

Dual effects of histamine on spontaneous activity in isolated bovine mesenteric lymphatics

The mode of action of histamine on spontaneous contractions is isolated bovine mesenteric lymphatics was investigated by recording isometric tensions. Histamine at lower concentrations between about 5 x 10(-8) and 10(-6) M caused a dose-dependent deceleration of the rhythm of spontaneous contractions. Higher concentrations of histamine (more than about 5 x 10(-6) M) produced a dose-related acceleration of the rhythm in association with a slight elevation of basal tone in 115 of 173 preparations. In 58 of 173 lymphatic preparations, histamine at concentrations ranging from about 5 x 10(-8) to 10(-5) M caused only the positive chronotropic effect. The histamine-induced positive and negative chronotropic effects were unaltered by pretreatment with alpha- and beta-adrenergic antagonists but were dose-dependently antagonized by pretreatment with H1- or H2-blockers (diphenhydramine or cimetidine). The specific H1- and H2-agonists, 2-pyridylethylamine (2PEA) and dimaprit caused dose-related positive and negative chronotropic effects, respectively, on spontaneous contractions of isolated bovine mesenteric lymphatics. The effect of 2PEA was significantly blocked by pretreatment with 10(-6) M diphenhydramine, whereas the effect of dimaprit was suppressed by 10(-6) M cimetidine. These results suggest that both H1- and H2-receptors are located on the plasma membrane of smooth muscle cells in bovine mesenteric lymphatics, and that the excitations of H1- and H2-receptors respectively produce an acceleration and a deceleration of the rhythm of spontaneous contractions in lymphatic smooth muscles.

Characterization of histamine receptors in isolated human cerebral arteries

1. The subtypes of histamine-receptors which mediate dilatation of small human cerebral arteries have been characterized in vitro using 'selective' agonists and antagonists. 2. Dilator responses were studied after preconstriction with prostaglandin F2 alpha, since contraction was not seen with histamine concentrations up to 10(-4) M. Histamine caused a concentration-related relaxation of cerebral vessels with an IC50 value of 5.2 +/- 1.6 x 10(-8) M. 3. Mepyramine caused a parallel shift to the right of the histamine concentration-response curve whereas cimetidine was without observable effect. This suggests the presence of histamine H1-receptors only. However, combined treatment with mepyramine and cimetidine caused a more marked displacement of the concentration-response curve to the right. Schild analysis indicated that in situations of near complete blockade of either of the histamine receptor subtypes, simple competitive antagonism both at H1- and H2-receptors can be revealed with a pA2 value of 8.64 for mepyramine and a pA2 value of 6.52 for cimetidine. 4. The 'selective' H1-receptor agonists pyridylethylamine, 2-methylhistamine (2-Me-histamine) and thiazolylethylamine, and the H2-receptor agonists dimaprit, impromidine and 4-methylhistamine (4-Me-histamine) all mimicked the histamine response, but were less potent than histamine. The order of potency was thiazolylethylamine greater than dimaprit greater than impromidine greater than 2-Me-histamine greater than pyridylethylamine greater than 4-Me-histamine. 5. These results indicate that the histamine-induced dilatation in small human cerebral arteries is mediated by both H1- and H2-receptors and that the former subtype of histamine receptor predominates.

5-Hydroxytryptamine-induced tachycardia in the pig: possible involvement of a new type of 5-hydroxytryptamine receptor

1. The mechanism of 5-hydroxytryptamine (5-HT)-induced tachycardia is species-dependent and is mediated directly or indirectly either by '5-HT1-like' (cat), 5-HT2 (rat, dog) or 5-HT3 (rabbit) receptors, or by an action similar to tyramine (guinea-pig). The present investigation is devoted to the analysis of the positive chronotropic effect of 5-HT in the pentobarbitone-anaesthetized pig. 2. Intravenous bolus injections of 5-HT (3, 10 and 30 micrograms kg-1) in pigs resulted in dose-dependent increases in heart rate of 24 +/- 2, 38 +/- 3 and 51 +/- 3 beats min-1, respectively (n = 39). Topical application of a high concentration of 5-HT (150 micrograms kg-1 in 5 ml) on the right atrium was also followed by tachycardia (38 +/- 6 beats min-1, n = 4). 3. A number of drugs which antagonize responses mediated by different 5-HT receptors--phenoxybenzamine, methiothepin, metergoline, methysergide and mesulergine ('5-HT1-like' and 5-HT2 receptors), ketanserin, cyproheptadine, pizotifen and mianserin (5-HT2 receptors), and MDL 72222 and ICS 205-930 (5-HT3 receptors)--did not attenuate the chronotropic responses to 5-HT. 4. The 5-HT-induced tachycardia was also not affected by antagonists at alpha- and beta-adrenoceptors, muscarinic, nicotinic, histamine and dopamine receptors, and calcium channels. 5. Selective inhibitors of 5-HT-uptake, indalpine and fluvoxamine, themselves increased porcine heart rate and facilitated 5-HT-induced tachycardia both in magnitude and in duration. 6. A number of putative selective agonists at '5-HT1-like' receptors or their possible subtypes (5- carboxamidotryptamine (5-CT), 8-hydroxy-24di-N,N-n-propylamino) tetralin (8-OH-DPAT), BEA 1654 and RU 24969), or at 5-HT3 receptors (2-methyl-5-HT), elicited no or only a weak tachycardiac response in the pig. RU 24969, but not 8-OH-DPAT, seemed to potentiate the responses to 5-HT, whereas 5-CT slightly inhibited these responses. 7. It was concluded that the tachycardia induced by 5-HT in the pig does not involve the receptors for some common neurotransmitter substances but may be mediated by a new 5-HT receptor type that is clearly different from '5-HT1-like', 5-HT2 or 5-HT3 receptors.

Capsaicin-sensitive nerves and the cutaneous allergy reaction in man. Possible involvement of sensory neuropeptides in the flare reaction

The effects of local capsaicin pretreatment on the cutaneous triple response reaction induced by allergen exposure or anti-IgE were studied in man. Acute exposure of the human skin to capsaicin caused a burning sensation and a clearcut flare reaction but no wheal response. Upon repeated administration these local reactions to capsaicin disappeared. The flare component and the subjective itching sensation of the cutaneous allergy reaction to rat antigen in sensitized persons or anti-IgE in non-allergic persons were then markedly reduced. Two weeks after capsaicin pretreatment the flare response to allergen was not significantly changed compared to the control reaction, suggesting a reversible effect of capsaicin treatment. The wheal component of the allergy or anti-IgE reaction was, however, not influenced by capsaicin pretreatment, indicating that the wheal and flare components are caused by different mechanisms. It is concluded that capsaicin sensitive sensory nerves are of importance for the human cutaneous triple response reaction induced by allergen exposure. Thus, secondary release of mediators, such as CGRP or tachykinins from sensory nerve branches, may contribute to the flare component of this reaction. Furthermore the itching sensation seems to be dependent to a large extent on capsaicin-sensitive nerves. However, sensory nerves seem to have less importance for the wheal reaction, i.e. the protein extravasation response.

The effects of histamine on contraction frequency, sodium influx, and cyclic AMP in cultured rat heart cells

Histamine has been shown to have both positive inotropic and chronotropic effects. To evaluate the chronotropic effects, spontaneously contracting monolayers of cultured rat myocardial cells were treated with histamine, 10(-7) M-10(-4) M. This resulted in a dose-dependent increase in contraction frequency reaching a maximum in 10(-5) M histamine. Contraction frequency (mean +/- SEM) increased from a control of 121 +/- 5 contractions per minute to 153 +/- 4.5, 181 +/- 9, 212 +/- 4, and 216 +/- 1 in 10(-7) M, 10(-6) M, 10(-5) M, and 10(-4) M histamine, respectively (for each n = 10, p less than 0.001). The effect was time-dependent, taking 30 minutes to develop fully. Changes in contraction frequency were accompanied by parallel dose- and time-dependent increases in the verapamil-sensitive sodium influx. Verapamil-sensitive sodium influx (pmol/cm2/sec) increased from a control of 10.45 +/- 1.44 (mean +/- SEM) to 24.34 +/- 2.41 and 32.57 +/- 2.35 at 10- and 30-minute treatment with 10(-6) M histamine (n = 5, p less than 0.001). These data fit the previously described relation between verapamil-sensitive sodium influx and contraction frequency in these cells. Cimetidine (10(-4) M) but not diphenhydramine (10(-4) M) abolished both the contraction frequency and sodium influx response to histamine. Subsequent studies showed a dose- and time-dependent elevation of cyclic adenosine monophosphate (cAMP) with histamine treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronotropic cardiac effects of histamine in the conscious dog with chronic atrioventricular block: interactions with the autonomic nervous system

Chronotropic effects of histamine and dimaprit were studied in the conscious dog with chronic atrioventricular block. Histamine at 0.2-5 micrograms/kg and dimaprit at equimolar doses (i.e. 0.25-6.25 micrograms/kg) increased atrial rate dose-relatedly. Blockade of muscarinic receptors reduced these effects and simultaneous blockade of muscarinic and beta-adrenergic receptors abolished them. Histamine and dimaprit moderately increased ventricular rate. Blockade of muscarinic receptors did not modify these effects, but blockade of beta-adrenoceptors with or without simultaneous blockade of muscarinic receptors suppressed them. After blockade of beta-adrenoceptors, histamine and more rarely dimaprit sometimes decreased atrial and ventricular rates. These effects were prevented by additional muscarinic blockade. Histamine and dimaprit lowered mean blood pressure to the same degree before and after each antagonist. The positive chronotropic effects of histamine and dimaprit, at these doses, are probably reflex responses to their hypotensive effects. The negative chronotropic effects of histamine after pindolol are due to muscarinic receptor activation. No evidence was found to implicate histamine-specific receptors in any of the chronotropic effects of histamine and dimaprit.

Binding of [3H]ICIA 5165, an H2-receptor antagonist to guinea pig gastric mucosa

ICIA 5165, 2-guanidino-4-[4-(2-cyano-3-methylguanidino)butyl] thiazole, a selective histamine H2-receptor antagonist was radiolabelled with tritium to a specific activity of 50.8 Ci/mmol for use in binding studies. Radiolabelling did not impair bioactivity. Binding characteristics of [3H]ICIA 5165 to guinea pig gastric mucosa were determined. Ligand binding was rapid, reaching equilibrium within five minutes at 0 degrees C, reversible and saturable. Specific [3H]ICIA 5165 binding had an equilibrium dissociation constant of 1.29 X 10(-8) M, determined by Scatchard plot analysis, and of 1.02 X 10(-8) M, calculated from the ratio of the dissociation to association rate constants. A Hill number, nH, of 1.02 was determined for the specific binding component. Specific binding of [3H]ICIA 5165 to gastric mucosal supernatant was not inhibited by methapyrilene, diphenhydramine, mepyramine, d-chlorpheniramine or l-chlorpheniramine (all at 10(-7) M), or by atropine or propranolol (both at 10(-6) M). Specific [3H]ICIA 5165 binding was inhibited in a concentration dependent manner by non-radioactive ICIA 5165 and tiotidine, as well as by a variety of other agents, with H2 agonist or H2 antagonist properties. In competition experiments, however, difficulties encountered in accurately defining the degree of specific binding indicate some reservation should be observed in interpreting these results.

Histamine is released from skin by substance P but does not act as the final vasodilator in the axon reflex

We have explored in man the hypothesis that histamine released from dermal mast cells by neurotransmitters from afferent nerves contributes to vasodilatation of the axon reflex. The ability of substance P to release histamine from human skin in vivo, and the effects of a histamine H1-receptor antagonist on capsaicin-induced axon reflex flares were studied. Intradermal injections of substance P (50 pmol) produced a weal and flare response which was associated with increased histamine concentration in blood draining the site (mean plasma histamine concentration before injection 0.17 +/- 0.02 ng ml-1 (+/- s.e.mean), concentration one minute after injection 1.26 +/- 0.28 ng ml-1, n = 6). Terfenadine, an H1-receptor antagonist, had no effect on the flare response to intradermal injection of capsaicin at a dose which inhibited by more than 60% the flare response to exogenous histamine and to histamine released from dermal mast cells by substance P. Substance P releases histamine from human skin in vivo. However, whatever the nature of the neurotransmitter released from afferent nerves during the axon reflex, it does not produce vasodilatation through release of histamine from dermal mast cells. Histamine may still contribute to the flare by initiation of the reflex.

Effect of in vivo antiestrogen pretreatment on rabbit atrial chronotropic response to histamine

The chronotropic response (delta rate) to histamine (1.4 to 18 X 10(-6) M) of isolated atria from antiestrogen (tamoxifen)-pretreated immature female rabbit was investigated. Tamoxifen treatment (1.0 and 10.0 mg/kg/day for 14 days) had no significant effect on the delta rate. The Rmax and D1/2max were not significantly different in the two tamoxifen-treated groups compared to the oil-treated (1.0 ml/kg/day for 14 days) control group. Cimetidine (2.8 X 10(-7) M) inhibited the delta rate to histamine in all groups: control, 27%; tamoxifen (1.0 mg/kg), 38%; and tamoxifen (10.0 mg/kg), 28%. Only the low dose of tamoxifen was found to be estrogenic (uterotropic). We conclude that tamoxifen pretreatment, both at estrogen-agonist and estrogen-antagonist doses, is without effect on atrial chronotropic response to histamine.

Age-related decrease in endothelium-dependent dilator response to histamine in rat mesenteric artery

The effect of aging on the vasodilator responses to histamine, 2-pyridylethylamine and 4-methylhistamine of ring segments of rat mesenteric arteries were investigated. The maximal extent of histamine-induced dilatation of the arteries previously contracted with norepinephrine was greatest for arteries from rats aged 2 and 8 weeks. The maximal response decreased progressively with an increase in age to 13 and 56 weeks. Arteries from 99 week old rats scarcely responded to histamine. Under these conditions, the dilatation induced by papaverine showed no change with age. The vasodilatation caused by 2-pyridylethylamine and 4-methylhistamine also decreased age dependently. The dilatation of the arteries induced by these agents was inhibited by the H1-antagonist chlorpheniramine, but not by the H2-antagonist cimetidine. Removal of the endothelium completely abolished the vasodilator effect of histamine, leaving the effect of papaverine unaffected. Hydroquinone and methylene blue reversed the dilatation induced by histamine, without affecting that caused by papaverine. These results suggest that the age-related decrease in dilatation of rat mesenteric artery in response to histamine is mainly due to a decrease in the ability of the endothelium to liberate a mediator(s).

Pharmacological characterization of histamine receptors in the rabbit renal artery

Histamine receptors were characterized pharmacologically in the rabbit renal artery. Histamine concentration-dependent contractions were blocked by chlorpheniramine (pA2 = 8.25 +/- 0.16) and mepyramine (pA2 = 9.80 +/- 0.22). A low concentration of metiamide (0.5 microM) enhanced significantly the effect of histamine. Higher concentrations either had no effect or reduced the contractile response to histamine. Impromidine, dimaprit and, in the presence of chlorpheniramine or mepyramine, histamine (at the concentrations that produced contractions) relaxed the precontracted vessel. The relaxant effect of histamine was quantitatively greater in the presence of mepyramine than it was in the presence of chlorpheniramine. Endothelial removal did not influence the contractile response to histamine but enhanced the relaxation suggesting the presence of histamine receptors mediating contraction in the intima. Relaxation responses to histamine and dimaprit were blocked competitively by cimetidine (pA2 = 6.65 +/- 0.10 and 6.60 +/- 0.05 respectively). It is concluded that there are two types of histamine receptors in the renal artery of the rabbit; H1-receptor mediating contraction and H2-receptor mediating relaxation. Both types of receptors are present in the media: some excitatory receptors may be present in the intima.

Antagonistic effect between histamine and norepinephrine on isolated guinea-pig right atrium

Norepinephrine (NE) and histamine (His) cause a dose related increase in the rate of the guinea pig isolated right atria. The rate increase induced by both amines has been shown to be mediated by pharmacologically different receptors. His shifted the NE CEC to the right in a non parallel fashion. Cimetidine (10(-4) mol/l) abolished this antagonism. Norepinephrine (3 X 10(-7) mol/l) shifted to the right the His CEC also in a non parallel fashion. Practolol (10(-4) mol/l) abolished this antagonism. Theophylline (10(-4) mol/l) reverted both: the antagonistic action of His related to the NE chronotropic effect and the antagonistic action of NE related to the His chronotropic effect. The reversal of the antagonism could be the result of the block of purinergic P1-receptors by theophylline. This block impairs the chronotropic effect of adenilic compounds which can function as a modulater of cardiac rate through a negative feed back mechanism.

Effects of long-term treatment with low dose cimetidine on allergen-induced airway responses and selected immunological parameters in atopic asthmatics

Twenty asymptomatic atopic asthmatics were treated with either cimetidine 100 mg orally (13 patients) or placebo (7 patients) once a day for 4 weeks. Bronchial challenges were performed with the pertinent allergen immediately before and 2 and 4 weeks after the initiation of treatment and, finally, 4 weeks after the cessation of treatment. Before each challenge blood was drawn for the determination of specific IgE antibody levels (RAST procedure) and total IgE (PRIST), allergen- and anti-IgE-induced basophil histamine release, and mitogen-induced lymphocyte (3H)-thymidine incorporation. Patients treated with cimetidine were found to be significantly (P less than 0.05) less responsive to bronchial allergen challenge during the treatment than before it; patients treated with placebo were more reactive (P less than 0.05) 14 days after the initiation of treatment. The difference in responsiveness to treatment between the placebo and the cimetidine groups was significant 14 days (P less than 0.01) and 4 weeks (P less than 0.05) after the initiation of treatment; no significant difference in allergen responsiveness was recorded between the groups 1 month after cessation of treatment. No clear-cut changes in specific IgE antibody or total IgE levels, histamine release capacity, or mitogen-induced lymphocyte responsiveness were observed in either group, except that lymphocytes from cimetidine-treated patients tended to show an increased ratio of PHA- to PMA-induced thymidine incorporation. Thus, it was found that the treatment of asymptomatic atopic asthmatics with low-dose cimetidine reduced their allergen sensitivity in bronchial provocation tests by a mechanism which remains to be elucidated.

On the presence of H1-receptors in various sections of guinea-pig heart: a correlation between binding and functional studies

The binding of 3H-mepyramine in different sections of guinea-pig heart was examined. 3H-mepyramine binds to a single class of binding sites to guinea-pig ventricular membranes and to right atrial suspension with an apparent dissociation constant (Kd) of 4.35 nM and 14.90 nM respectively. When treated as those obtained from the right atrium, the left atrial suspensions do not seem to bind 3H-mepyramine specifically.

The positive inotropic action of isoprenaline is associated with the release of noradrenaline from rabbit, guinea-pig and rat atria

The actions of isoprenaline (1 microM) were investigated in spontaneously beating isolated paired atria from rabbits, guinea-pigs and rats and in paced left atria from rabbits and rats. Isoprenaline, when applied for a 10 min period to paired atrial from rabbits, guinea-pigs and rats, produced a short-lived release of noradrenaline which appeared to be associated with the positive inotropic actions, rather than with the positive chronotropic actions. Exposures of 3 min to isoprenaline in paired atria in the three species produced increases in force and release of noradrenaline (% of tissue content) in the order: rabbit greater than guinea-pig greater than rat. The increases in rate did not differ between species. Rat atria had the highest resting rates followed by guinea-pig and rabbit atria. In rabbit left atrial preparations which were electrically paced at resting rates of approximately 61, 158 and 334 beats per min there were progressive decreases in release of noradrenaline and positive inotropic actions produced by isoprenaline with increasing rates of pacing. In rat left atrial preparations which were electrically paced at a slower rate, which was similar to that of rabbit paired atria, a greater release of noradrenaline associated with a greater positive inotropic effect was induced by isoprenaline than in rat paired atria. In rabbit paced left atria, an increase in pacing frequency alone produced small increases in force and release of noradrenaline. Verapamil (1 microM) significantly reduced the positive inotropic effect and release of noradrenaline induced by isoprenaline without affecting the positive chronotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution and localization of histamine in bovine and rabbit eye

Histamine (HI) is present in various structures of the bovine and rabbit eye (retina, choroid, sclera) and in the optic nerve of both species. The amine levels in particular structures of either cow or rabbit are highly differentiated, as well as profile of HI distribution which differs markedly between both species, except only the retina structure (HI levels were between 70-80 ng per g tissue). In the bovine retina HI is stored in non mast cell compartment, while in the optic nerve at least 50% of the amine is of mast cell origin. Approximately 90% of the retinal HI was recovered in the P1 subcellular fraction. HI in the bovine retina is metabolized by methylation. The data are discussed in terms of a possible physiological role of HI in the retina.

Distribution of the histaminergic neuron system in the central nervous system of rats; a fluorescent immunohistochemical analysis with histidine decarboxylase as a marker

The distribution of histidine decarboxylase-like immunoreactivity (HDCI) in the rat central nervous system was studied by the indirect immunofluorescence technique. HDCI cell bodies were concentrated in the posterior hypothalamic area, such as in the tuberal magnocellular nucleus, caudal magnocellular nucleus, posterior hypothalamic nucleus and lateral hypothalamus just lateral to the fasciculus mammillothalamicus at the level of the posterior hypothalamic nucleus. Extensive networks of HDCI fibers of various densities were found in many areas of the brain; they were particularly dense in the hypothalamus but were also found in the following areas: rostrally in the cerebral cortex, olfactory nuclei, medial amygdaloid nucleus, n. tractus diagonalis, and bed nucleus of the stria terminalis, and caudally in the central gray matter of the midbrain and pons, auditory system, n. vestibularis medialis, n. originis nervi facialis, n. parabrachialis, n. commissuralis, n. tractus solitarii, and n. raphe dorsalis.

Studies on the effect of histamine in isolated human pulmonary arteries and veins

The effect of histamine (0.01-200 microM) was studied in isolated human pulmonary vessels. Histamine induced concentration dependent contractions in both arteries and veins. In veins the maximal response to histamine was lower than in arteries. Histamine and 2-methyl-histamine had a dual action in both arteries and veins clearly demonstrated in vessels precontracted with potassium. In these vessels histamine and 2-methyl-histamine induced relaxation at low concentrations and contractions at high concentrations. Veins were more sensitive to the relaxant effect of histamine than arteries. Mepyramine eliminated the dual action of 2-methyl-histamine and histamine and unveiled a mepyramine resistant relaxation at the highest histamine concentrations used which was resistant to the effect of cimetidine and metiamide. The H2 receptor agonist dimaprit (10-400 microM) induced a slight relaxation in both arteries and veins that could be eliminated by metiamide (100 microM). The results show that histamine has a dual action in human pulmonary vessels which includes a contractile effect mediated via H1 receptors and a relaxant response partly mediated through H1 receptors and partly via unspecific mechanisms. However, an H2 mediated relaxant effect cannot be excluded.

Effects of histamine on mechanical performance and biochemical and electrical activity in the heart of monkeys (Macaca fuscata)

The properties of the cardiac effects of histamine on the isolated heart muscles of the Japanese monkey (Macaca fuscata) were investigated. Histamine had a concentration-dependent positive inotropic effect on left atria and papillary muscles and a positive chronotropic effect on right atria. Histamine increased the levels of cyclic AMP and shortened the duration of the action potential in Purkinje fibers, measures of its biochemical and electrophysiological effects. All of these effects of histamine were blocked by 10(-5) M cimetidine. These results indicate that histamine H2-receptors mediate the cardiac effects of histamine on the monkey heart. Histamine (10(-5) M) also restored the action potentials and contractility of K+-depolarized preparations. These effects were inhibited by verapamil (10(-6)-10(-5) M) but not by TTX (10(-5) M), suggesting that, in the monkey heart, histamine may act by increasing the slow inward current.

Influence of prostaglandin-synthesis inhibitors on carbachol- and histamine-induced vasodilatation in perfused rat hindquarters

In perfused rat hindquarters, in which vascular tone was maintained by norepinephrine, carbachol-induced dilatations were blocked by atropine (10(-7) M), while histamine dilatations were inhibited as well by mepyramine (10(-6) M) as by cimetidine (10(-5) M) indicating a histamine effect through both H1- and H2-receptors. This double-receptor histamine effect was confirmed by the observation that specific H1- and H2-receptor agonists, respectively PEA (2-pyridyl-ethyl-aminedihydrochloride) and dimaprit also produced a vasodilatation. Carbachol- and histamine-induced dilatations were also inhibited by ETYA (5,8,11,14-eicosatetraynoic acid) and quinacrine but not by indomethacin. The inhibition of the histamine vasodilatation appeared to rest on an interference with the H1-receptor mechanism. It is concluded that metabolites of arachidonic acid possibly mediate the dilating effect of carbachol, acting through muscarine receptors, and of histamine, acting through H1-receptors.

Specific binding of [3H]mepyramine to histamine H1-receptors in vascular smooth muscle membranes

The antagonist-sensitive binding of [3H]mepyramine to beef aortic membranes was as expected for binding to histamine H1-receptors. [3H]mepyramine binds rapidly and in saturable fashion to the specific receptor sites, specific binding reaching equilibrium in 3 min at 37 degrees C. SCATCHARD's analysis of the binding data gave a dissociation constant of 3.0 nM for the radioligand-receptor complex and maximal number of binding sites: 31 fmol/mg protein. In the competition studies histamine H1-antagonists are more potent inhibitors of radioligand binding than H2-antagonist. They inhibit [3H]mepyramine binding in the following order: mepyramine greater than triprolidine greater than promethazine much greater than cimetidine. Binding data are in correlation with the previous pharmacological studies.

Structure-activity relationships for some substance P-related peptides that cause wheal and flare reactions in human skin

Substance P (6.25-25 p-mole) produced dose-dependent flare and wheal responses when injected intradermally into the volar surface of the human forearm. The maximum flare response was obtained within the first 3 min of injection and declined thereafter. The wheal response reached a maximum after 12 min following the injection. Only those peptides having one or more basic residues in the N-terminal region were effective in producing a flare reaction. Eledoisin-related peptide and SP1-9 were 17 and 7 times less active than substance P respectively, whilst [D-pro2, D-phe7, D-trp9]SP1-11 was twice as active. The N-terminal tetrapeptide, SP1-4 and eledoisin were inactive in the dose range tested. Wheal-producing activity was not dependent on the presence of basic residues and the rank order of relative potencies was: physalaemin (2.0): [D-pro2, D-phe7, D-trp9]SP1-11 (1.1): SP1-11 (1.0): SP4-11 (0.4): SP1-9 (0.15): eledoisin-related peptide (0.08): eledoisin (0.06). The N-terminal tetrapeptide failed to produce a wheal response in the dose range tested. Substance P was approximately equi-active with poly-L-arginine in the production of wheal and flare and both of these agents were about 10 times more potent than histamine. Adenosine triphosphate (25-400 n-mole) produced dose-dependent wheal and flare responses and was 10,000 times less potent than substance P. Pre-treatment of the subjects with the H1 histamine antagonist, chlorpheniramine, (20 mg I.V.) reduced the wheal and flare responses to substance P. Local anaesthetic injection into the skin reduced the spread of the flare response but did not affect the development of the wheal response. Pre-treatment of the skin with capsaicin reduced the flare but not the wheal response to intradermal injection of histamine. The results are discussed in relation to the mechanism of the 'axon reflex' vasodilatation in skin. This is thought to involve mast cells in addition to substance P-containing primary afferent neurones.

Role of the endothelium in the vasodilator response of rat thoracic aorta to histamine

Despite their potent vasodilating action in vivo, acetylcholine and histamine often show a vasoconstricting action in vitro. As the endothelium has an important role in the vasodilating effect of acetylcholine, we investigated the possible role of the endothelium in the vasodilating effect of histamine in comparison to acetylcholine. Experiments were done on ring segments of rat thoracic aorta mounted for isometric tension measurements. We demonstrated that relaxation by histamine and acetylcholine of pre-contracted rat aorta segments required the presence of endothelial cells. Acetylcholine acting on muscarinic receptors, and histamine acting on H1-receptors seemed to initiate the production of mediator(s) from the endothelial cells, which leads to relaxation of the vascular smooth muscle cells. This production appeared to be depressed by ETYA and hydroquinone, and under hypoxic conditions.