Histamine-induced vasodilations mediated by H1- and H2-receptors in isolated rat common carotid arteries

Using the cannula inserting method, the vasodilatory effects of histamine were analysed employing selective histamine H1- and H2-receptor agonists and antagonists in isolated, perfused rat common carotid arterial preparations which were preconstricted by a continuous infusion of phenylephrine with propranolol. Histamine, 2-pyridylethylamine (2-PEA) (a selective H1-agonist) and dimaprit (a selective H2-agonist) produced a vasodilation in a dose-related manner. The order of potency was histamine greater than dimaprit greater than 2-PEA. Histamine-induced dilations were significantly inhibited by either diphenhydramine (a selective H1-antagonist) or cimetidine (a selective H2-antagonist). 2-PEA-induced dilations were significantly inhibited by diphenhydramine but not by cimetidine. Dimaprit-induced dilations were significantly blocked by cimetidine but not by diphenhydramine. ACh-, histamine-, 2-PEA- and dimaprit-induced dilations were significantly suppressed by removal of the endothelium. From these results, it is concluded that (1) isolated rat common carotid arteries have both H1- and H2-receptors, (2) there are few vasoconstrictory H1-receptors, (3) both H1- and H2-receptors mediate only vasodilation but not vasoconstriction, and (4) EDRF from the endothelium might participate in histamine-induced vasodilation via not only H1- but also H2-receptors.

Histamine-induced increases in cyclic AMP levels in bovine adrenal medullary cells

1. The effect of histamine on cellular cyclic AMP levels in cultured bovine adrenal medullary cells has been studied. 2. Histamine (0.3-30 microM) increased cyclic AMP levels transiently, with a maximal response after 5 min, a smaller response after 20 min, and no increase seen after 80 or 180 min. The EC50 at 5 min was approximately 2 microM. Histamine had no effect on cyclic AMP release from the cells over 5 min, but increased it after 90 min. 3. The cyclic AMP response to 5 microM histamine was reduced by 45% by 1 microM mepyramine and by almost 30% by 1 microM cimetidine, and was abolished by the combination of both antagonists. Cimetidine at 100 microM did not inhibit the response to histamine more than 1 microM cimetidine. The H3-receptor antagonist, thioperamide (1 microM), had no effect on the response to histamine. 4. The H1-receptor agonist, 2-thiazolyethylamine (5-100 microM) and the H2-receptor agonist, dimaprit (5-100 microM), each induced a cyclic AMP response, and gave more-than-additive responses when combined. The H3 agonist (R) alpha-methylhistamine (100 microM) had no effect either on its own or in combination with either the H1 or the H2 agonist. The response to 100 microM 2-thiazolylethylamine was unaffected by cimetidine (100 microM). 5. The cyclic AMP responses to 5 microM histamine, 100 microM thiazolylethylamine and 100 microM dimaprit were each weakly enhanced in the presence of 1 mM 3-isobutyl-1-methylxanthine. The response to dimaprit was enhanced more than 10 fold in the presence of 0.3 microM forskolin, while the responses to histamine and thiazolylethylamine were weakly enhanced.6. The cyclic AMP response to 5 microM histamine was partially reduced in the absence of extracellular Ca2 and the residual response was fully antagonized by 1 microM cimetidine and was unaffected by 1 microM mepyramine.In the absence of Ca2 , the cyclic AMP response to 100 microM thiazolylethylamine was abolished, while that to 100 microM dimaprit was unaffected.7. Reincubation of 5 microM histamine solutions with a second set of chromaffin cells, following prior incubation with another set of cells, induced a cyclic AMP response in the fresh cells. This response was reduced by a combination of mepyramine and cimetidine to the same degree as the response to fresh 5 microm histamine solutions.8. The results indicate that histamine increases cellular cyclic AMP levels in bovine chromaffin cells by three mechanisms: by acting on H1 receptors, by acting on H2 receptors, and by an interaction between H, and H2 receptors. The H1 response does not require concomitant activation of H2 receptors, is fully dependent on extracellular Ca2 +, does not depend on secreted chromaffin cell products, and is not due to reduced cyclic AMP degradation or export. The H2 cyclic AMP response is the first functional response reported for H2 receptors on chromaffin cells, is independent of Ca2 , is not due to reduced cyclic AMP export or degradation, and is likely to be mediated via a direct action through Gs. The role of these different mechanisms in the regulation of cyclic AMP-dependent processes in chromaffin cells by histamine is under investigation.

Circulatory depression and ventricular arrhythmias induced by compound 48/80 in anaesthetized rats

The effects of graded doses of compound 48/80 on various cardiovascular and respiratory parameters were studied in pentobarbitone-anaesthetized rats. Following intravenous injections, this compound significantly depressed the mean blood pressure (MBP), left ventricular pressure (LVP) and dLVP/dtmax, and caused ventricular tachycardia (VT) or fibrillation (VF) and death. Heart rate (HR) response were variable, and there were no marked changes in airway resistance or blood gases. Pretreatment of the animals with either cimetidine or diphenhydramine significantly prolonged the time of onset of VT/VF but failed to alter the changes in other circulatory variables. A combination of cimetidine and diphenhydramine significantly alleviated the decreases in MBP and LVP and prevented the occurrence of VT/VF. It is suggested that the circulatory depression and the occurrence of ventricular arrhythmias following the administration of compound 48/80 result from activation of H1- and H2-receptors by elevated blood histamine levels due to release of the amine from tissues.

Histamine-stimulated increases in intracellular calcium in the smooth muscle cell line, DDT1MF-2

Suspensions of undifferentiated cultured vas deferens smooth muscle cells (DDT1MF-2) were loaded with the calcium-sensitive fluorescent dye fura-2. Exposure to histamine elicited a rapid and maintained increase in intracellular free calcium ([Ca2+] i) with an EC50 of 1.3 +/- 0.7 x 10(-5) M. The initial rise is a consequence of calcium release from intracellular stores, whereas the maintained or plateau phase, which is dependent upon the presence of extracellular calcium, is associated with calcium influx. Experiments in nominally Ca(2+)-free buffer attenuated the initial rise in [Ca2+]i (i.e. peak height) and virtually abolished the plateau phase. Re-addition of 2 mM Ca2+ (during experiments performed in nominally Ca(2+)-free buffer) resulted in a return of the plateau phase. Pretreatment with the H1-antagonist mepyramine (100 nM; Kd = 1.0 +/- 0.4 nM, N = 3) completely blocks the response to histamine, whereas tiotidine (2 microM; H2-antagonist) had no effect. In conclusion, the present data would suggest that functional H1-receptors found in hamster vas deferens smooth muscle cells are typical of the "classical" H1-receptor in both its control of intracellular Ca2+ and sensitivity to antagonism by mepyramine.

Histamine H1-receptor-mediated keratan sulfate production in rabbit chondrocytes: involvement of protein kinase C

We investigated the characteristics of the histamine H1-receptor in cultured rabbit chondrocytes. Scatchard analysis of [3H]pyrilamine, an H1-antagonist, binding to the chondrocytes revealed a single class of binding sites with KD and Bmax values of 90 +/- 12 nM and 56 +/- 11 fmol/10(4) cells, respectively. H1-agonists stimulated the production of keratan sulfate in a dose-dependent manner. Stimulation of keratan sulfate production was inhibited by pyrilamine. Protein kinase C inhibitors (sphingosine and H-7) also had inhibitory effects. Phorbol 12,13-dibutyrate, a direct activator of protein kinase C, activated the production. When protein kinase C in the chondrocytes was down-regulated by preincubation with phorbol ester, the effect of the H1-agonist on keratan sulfate production was abolished. These results indicate that the histamine H1-receptor on chondrocytes mediates the accumulation of keratan sulfate production and that protein kinase C is involved in these events.

Influence of histaminergic receptors on denervated canine gracilis muscle vascular tone during endotoxemia

The purpose of this study was to determine if endogenously released histamine and its non-neural interaction with the H1- and H2-histaminergic receptors in the peripheral vasculature can account for the decompensatory loss of peripheral vascular tone associated with the hypotension occurring during endotoxemia. A denervated in situ constant flow double canine gracilis muscle preparation that permitted one muscle to serve as a control (GMc) for the contralateral experimental muscle (GMe) was used. Endotoxemia was induced by intravenous infusion of 2 mg.kg-1.30 min-1 endotoxin. The specific H1 and H2 antagonists diphen-hydramine and cimetidine were infused either together or separately in both high and low dosages into the GMe. Blockades were validated by intra-arterial injection of histamine or the specific agonists betahistine for H1 and dimaprit for H2 receptors. The results suggest that the high-dose diphenhydramine produced a nonspecific dilation not seen with the lower dose. Because both the blocked and unblocked vascular beds exhibited the same degree of vasodilation after endotoxin, these studies do not support the hypothesis that endogenously released histamine is responsible for the loss of vascular tone. These studies do verify, however, that a nonneurally mediated loss of skeletal muscle vascular tone is an important factor to consider in the overall cardiovascular hypotension occurring during endotoxin shock.

[A study on the relationship between histamine receptors and bronchial hyperresponsiveness in asthma]

We measured the bronchial response to histamine in 26 allergic asthmatics and 17 healthy subjects before and after the bronchial pretreatment with selective H2-receptor antagonist ranitidine. We also investigated the results of the new H1-receptor antagonist terfenadine on the prevention and treatment of asthma. Selective H1-receptor stimulation with histamine (pretreated with 18.5mg ranitidine) failed to enhance the effect of histamine in asthma group (P greater than 0.05), whereas it enhanced the histamine response obviously in healthy group (P less than 0.001). We conclude that: 1. BHR to histamine in allergic asthma is related to a functional depression of H2 receptors in airways; 2. terfenadine is useful in the prevention and treatment of asthma.

Inhibition by cations of antagonist binding to histamine H1-receptors: differential effect of sodium ions on the binding of two radioligands

1. Measurements have been made of the inhibition by mono- and divalent cations of the binding of [3H]-(+)-N-methyl-4-methyldiphenhydramine ([3H]-QMDP) to histamine H1-receptors in homogenates of guinea-pig cerebellum. 2. The binding of [3H]-QMDP was inhibited by monovalent cations with an order of potency Li+ = Na+ greater than K+ greater than Cs+ = Rb+. The IC50 for Li+ was 39 mM, but that for K+ was 132 mM. Hill coefficients for inhibition curves for Li+ and Na+ were less than 1. 3. Divalent cations also inhibited the binding of [3H]-QMDP. The most potent cations examined were Hg2+, Cd2+ and Zn2+, with IC50 values of 5, 17 and 41 microM, respectively. Ca2+ and Mg2+ were relatively weak inhibitors (IC50 12 and 34 mM, respectively). The potency of Ni2+, Co2+ and Mn2+ was intermediate between these groups. Hill coefficients for inhibition curves for Hg2+, Cd2+ and Zn2+ were greater than 1, but Hill coefficients for the other cations were less than 1. 4. Both mono- and divalent cations also inhibited the binding of [3H]-mepyramine. The divalent cations were approximately equipotent in inhibiting the binding of [3H]-QMDP and [3H]-mepyramine. The same was true for Li+. However, Na+ was markedly more effective against [3H]-QMDP binding than against the binding of [3H]-mepyramine. 5. The effect of 40 mM Li+ on the parameters of binding of [3H]-mepyramine was to increase the best-fit value of the concentration giving half-maximal binding EC50, by approximately 2 fold without having any significant effect on the maximum amount of binding. Cd2+ (15 microM) caused both an increase in EC 0 and a decrease in Bmax (32 +/- 4% inhibition). Na+, 100 mm, had no significant effect on either EC50 or Bmax for [3H]-mepyramine binding.

Mechanisms implicated in the histamine response of the sheep ureterovesical junction

The isotonic response of the sheep ureterovesical junction to histamine receptor agonists and antagonists was studied in vitro. Histamine (10(-7)-10(-4) M) produced a concentration-dependent contraction which was dependent on the extracellular calcium concentrations. Nifedipine (10(-9)-10(-6) M) partially inhibited the contractile response to histamine. 2-Methylhistamine and 2-pyridylethylamine elicited contractile effect in a concentration-related manner. The order of potency was histamine greater than 2-methylhistamine greater than 2-pyridylethylamine. Dimaprit and 4-methylhistamine produced no response in this tissue. The histamine contraction was antagonized by mepyramine, the pA2 value being 8.41, but not by cimetidine (10(-4) M). Scopolamine (10(-5) M) and indomethacin (10(-6) M) had an inhibitory effect. Phentolamine (10(-5) M), propranolol (10(-7)) and hexamethonium (10(-6) M) did not alter the histamine-induced contractions. The present results indicate that the response to histamine in sheep ureterovesical junction is mediated mainly by a direct action on the smooth muscle through excitatory histamine H1-receptors and partly by an indirect action via the stimulation of intramural cholinergic nerves.

Up-regulation of thrombomodulin by activation of histamine H1-receptors in human umbilical-vein endothelial cells in vitro

Previous reports demonstrated that the expression of thrombomodulin (TM) in endothelial cells was modulated by various agents. Although TM was down-regulated by endotoxin or cytokines, up-regulation of TM was accomplished when endothelial cells were stimulated with unphysiologically high concentrations of cyclic AMP derivatives or tumour-promoting phorbol esters. We investigated the expression of TM in human umbilical-vein endothelial cells (HUVECs) by physiological substances that can be released into the bloodstream. Histamine (0.1-10 microM, 1-48 h) increased TM activity, TM antigen in cell lysates and TM mRNA levels, but 5-hydroxytryptamine and bradykinin had no effect. Enhancement of TM activity by histamine was completely blocked by the H1-selective antagonist pyrilamine, whereas the H2-antagonist cimetidine had no effect, showing that histamine up-regulates TM activity via H1-receptors on HUVECs. Enhanced TM activity by histamine and the resultant increase in protein C activation might play a role in a feedback regulation for prevention of vascular thrombosis.

Blockade by the local anaesthetic, tetracaine, of desensitization of Ca-induced Ca release after muscarinic stimulation in smooth muscle

1. Desensitization of contractile responses dependent on release of intracellularly stored Ca elicited by carbachol, histamine or caffeine was measured after desensitizing treatment with carbachol or histamine in the presence or absence of local anaesthetics in Ca-free solution containing 2 mM EGTA in the smooth muscle of guinea-pig taenia caecum. 2. Histamine-induced homologous desensitization was inhibited by tetracaine and procainamide. Dibucaine did not exert an inhibitory effect on the desensitization. This is consistent with our previous findings concerning the effects of local anaesthetics on the desensitization of histamine H1-receptors measured under normal physiological conditions. 3. Carbachol induced a functional change of intracellular Ca stores which resulted in heterologous desensitization. Tetracaine completely blocked carbachol-induced desensitization of the caffeine-elicited contraction, but in the case of carbachol-induced desensitization of carbachol- and histamine-elicited contractions, this blocking effect of tetracaine was very weak and absent, respectively. The other local anaesthetics used did not affect the desensitization. These results suggest that the Ca-induced and inositol trisphosphate-induced Ca release mechanisms were both desensitized by carbachol and that the desensitization of the Ca-induced Ca release mechanism was selectively blocked by tetracaine.

Identification and characterization of histamine H1- and H2-receptors in guinea-pig left atrial membranes by [3H]-mepyramine and [3H]-tiotidine binding

1. Histamine receptors in the membranes prepared from guinea-pig left atria were characterized with [3H]-mepyramine and [3H]-tiotidine binding. 2. The binding of the H1-antagonist, [3H]-mepyramine, was saturable and of high affinity with a maximum binding capacity of 307 +/- 27 fmol mg-1 protein (n = 14) and with an equilibrium dissociation constant (KD) of 1.5 +/- 0.2 nM (n = 14). The binding was rapid and readily reversible. 3. The competition curve for [3H]-mepyramine binding by histamine was biphasic and revealed high and low affinity states of binding. The addition of 5'-guanylylimidodiphosphate (GppNHp) (100 microM) converted this heterogeneous binding into homogeneous binding of low affinity. 4. The competition curves of H1-antagonists with [3H]-mepyramine had Hill coefficients not significantly different from unity, consistent with competition with [3H]-mepyramine at a single site. GppNHp did not shift the competition curves. 5. Dissociation constants for H1-antagonists determined from inhibition of [3H]-mepyramine binding correlated well with the constants derived from inhibition of the positive inotropic response of guinea-pig left atria to histamine. 6. The H2-antagonist, [3H]-tiotidine, labelled an apparently homogeneous population of recognition sites with a maximum binding capacity of 41 +/- 8 fmol mg-1 protein (n = 6) and a KD of 10.8 +/- 1.2 nM (n = 6). 7. Although histamine competed for [3H]-tiotidine binding in a concentration-dependent manner, the curve was monophasic and was not shifted by GppNHp. 8. It is concluded that both H1- and H2-receptors exist in guinea-pig left atria. H1-receptors probably couple to intracellular effector(s) through a guanine nucleotide-dependent transducing mechanism. On the other hand, H2-receptors seem unlikely to be linked to guanine nucleotide regulatory proteins in guineapig left atria, which may explain the failure of histamine to cause an increase in cyclic AMP in spite of the presence of H2-receptors.

Characterization of histamine-induced contraction in rat isolated aorta

High concentrations of histamine (greater than 10 microM) contract rat aortic rings and the effect is greatly enhanced when the endothelium is removed. The present study was aimed at characterizing the histamine-induced contractions of de-endothelialized rat aortic rings. These contractions were poorly inhibited by the histamine H1-receptor antagonist, mepyramine (1 and 10 microM) and insensitive to the histamine H2-receptor antagonist, cimetidine (10 microM), and to the cyclooxygenase inhibitor, indomethacin (5 microM). In contrast, the alpha-adrenoceptor antagonists, prasozin and pentholamine, antagonized these contractions in a concentration-dependent manner (respective apparent pKB values 9.7 and 7.9) and nifedipine (3 microM) reduced them by about 75%. Pretreatment of de-endothelialized rings with 8-bromo-cyclic GMP and of intact rings with methylene blue resulted in respective inhibition and enhancement of histamine-induced contractions, quite similarly to the effects in the presence and in the absence of endothelium, respectively. Histamine elicited endothelium-dependent relaxation of aortic rings precontracted by prostaglandin F2 alpha. This relaxation was abolished in the presence of mepyramine (1 microM). However, mepyramine failed to mimic the enhancing effect of endothelium removal on histamine-induced contractions of resting aortic rings. It is concluded that, in rat aorta, (1) contractions induced by high concentrations of histamine (greater than 10 microM) are probably mediated by alpha 1-adrenoceptors; and (2) spontaneous, but not histamine-stimulated, release of endothelium-derived relaxing factor is mainly involved in the modulation of histamine-induced contractions.

Atypical Schild plots with histamine H1 receptor agonists and antagonists in the rabbit aorta

Competitive antagonists of histamine H1 receptor were investigated for their effects on histamine-induced responses in the rabbit aorta. Antazoline-induced antagonism gave linear Schild plots with slope equal 1, while the other antagonists [+)-brompheniramine, (+/-)-chlorpheniramine, diphenhydramine and mepyramine) produced 'atypical' plots with slopes generally less than unity in the thoracic aorta. Schild plots obtained with these antagonists were evaluated using a two independent component model. The high affinity parameters thus estimated were compatible with those that have been reported for these antagonists. No such heterogeneity was observed in the abdominal aorta when diphenhydramine was investigated with different H1 agonists. The results suggest the presence of at least two components in H1-mediated responses in the thoracic aorta; these components are equally antagonized by antazoline, but differentially antagonized by the other antagonists used.

Cyclic AMP generating systems in vertebrate retina: effects of histamine and an established retinal modulator, dopamine

Histamine (HA), 1-1000 microM, significantly stimulated both basal and forskolin-activated cAMP generation in chicken and adult hen retina. The action of HA was reproduced by the selective H2-receptor agonists dimaprit and 4-methyl-histamine, but not by the selective H1-receptor agonist 2-thiazolylethylamine, and it was antagonized by the specific H2-receptor blockers cimetidine and tiotidine, but not by the H1-receptor blocker mepyramine. In parallel experiments, dopamine, an established retinal neuromodulator acting through the D1-type of receptor, also stimulated basal and forskolin-driven adenylate cyclase activity in homogenate of chicken retina. It is suggested that chicken retina contains HA H2-receptors which are positively coupled to the adenylate cyclase system.

Effect of H1-receptor blockade on late cutaneous reactions to antigen: a double-blind, controlled study

This study is of the effect of the blockade of histamine H1 receptors by a long-acting antihistamine on the immediate and late clinical response to antigen (Ag) and on the recruitment of eosinophils in the late-phase cutaneous reaction. Ten adult volunteers with late-phase reactions to the intradermal injection of either Dermatophagoides pteronyssinus or Phleum pratense (timothy) pollen performed a double-blind, crossover study. Each volunteer took astemizole, 10 mg, or identical placebo, daily for 2 weeks. Ag in the concentration that induced a late reaction in the screening visit was injected intradermally at the end of each drug period. The early reaction was measured serially for 30 minutes and the late reaction at 4 and 6 hours. Biopsies of the Ag and control sites were also performed at 6 hours. After a 6-week washout period, subjects then took the opposite medication for 2 weeks and returned for skin testing and biopsy. Skin testing demonstrated that astemizole inhibited the immediate response to both histamine and allergen but had no effect on the late response at 4 hours and at 6 hours. Biopsy specimens revealed no significant effect on eosinophil recruitment at 6 hours. We conclude that histamine H1-receptor blockade has no effect on the late cutaneous reaction to Ag.

Effect of histamine on tracheal mucosal perfusion, water content and airway smooth muscle in sheep

We examined the response of tracheal mucosal blood flow normalized for systemic arterial pressure (Qtrn), water content (VH20) and luminal dead space (Vtr) to nebulized histamine in intact, lightly anesthetized sheep. Nebulized histamine produced rapid increases in mean Qtrn (+84%) and VH2O (+85%), and a decrease in mean Vtr (-17%) (P less than 0.05) within 5 min post completion of challenge. Mean Vtr rapidly returned to baseline, while mean Qtrn and VH2O remained elevated for 60 and 90 min after challenge, respectively. Pretreatment with chlorpheniramine (H1-antagonist) blocked the changes in Vtr and VH2O, and attenuated the increase in Qtrn. Metiamide (H2-antagonist) pretreatment abolished the increase in Qtrn and blunted the increase in VH2O, but had no effect on the decrease in VTR. 2-methylhistamine (H1-agonist) decreased mean Qtrn and Vtr (P less than 0.05) and dimaprit (H2-agonist) increased mean Qtrn (P less than 0.05) without changing Vtr. Neither 2-methylhistamine nor dimaprit significantly altered VH2O. Atropine blocked histamine induced decreases in Vtr and slightly attenuated the increases in Qtrn and VH2O. Thus, histamine increased airway smooth muscle tone and mucosal water content principally via H1 receptors, and mucosal perfusion via H2 receptors. The airway smooth muscle contraction involved muscarinic pathways.

The influence of H1-, H2- and H3-receptors on the spontaneous and ConA induced histamine release from human adenoidal mast cells

The effects of the H3-agonist R-alpha-methylhistamine (R-alpha-MeHA) and the H3-antagonist thioperamide on the spontaneous and concanavalin A (ConA) induced histamine release from human mast cells were tested and compared with the effect of some H1- and H2-receptor active substances. R-alpha-MeHA (10(-9)-10(-7) M) exerted no effect on histamine release whereas thioperamide increased the spontaneous release at 10(-6)-10(-4) M but inhibited the ConA induced release in a narrow concentration range (10(-6)-10(-5) M). This enhancement might be taken as an indication of the existence of H3-receptor dependent autoregulation although presently other mechanism cannot be excluded.

Homologous histamine H1 receptor desensitization results in reduction of H1 receptor agonist efficacy

Prolonged exposure of the guinea-pig intestinal longitudinal smooth muscle to histamine caused homologous desensitization of the H1 receptor, which led to reduced H1 receptor-mediated production of [3H]inositol phosphates as well as to reduced H1 agonist-induced contractions. [3H]Mepyramine binding studies showed that desensitization affected neither the agonist affinity nor the number of H1 receptors. Combining the data from the binding studies and the contraction measurements it was found that desensitization results in a selective reduction of agonist efficacy.

Optical isomers of rocastine and close analogues: synthesis and H1 antihistaminic activity of its enantiomers and their structural relationship to the classical antihistamines

The enantiomers of 2-[2-(dimethylamino)ethyl]-3,4-dihydro-4-methylpyrido[3,2-f]-1,4- oxazapine-5(2H)-thione (rocastine) and two of its more potent analogues were prepared with an enantiomeric purity of greater than 99.9%. The antihistaminic activity of these compounds was assessed by their ability to block histamine-induced lethality in guinea pigs and to inhibit [3H]mepyramine binding to guinea pig cortex. In this series, compounds having the R configuration at the 2-position are at least 300 times more potent than the S isomers. Conformational analysis and molecular modeling suggest that rocastine can adopt a conformation in which the pyridine ring, ether oxygen, and protonated amine functions are positioned similarly to the corresponding elements of the probable binding conformers of some of the more classical antihistamines. This conformation, boatlike in the oxazepine ring with the side chain quasi-equatorial and folded back toward the ring, is the likely binding conformer at the histamine H1 receptor, and the available structure-activity relationship data is consistent with this interpretation.

The mechanism of [3H]noradrenaline release by histamine and its analogs from the rat vas deferens

In this study the mechanism by which histamine and H1 and H2 agonists evoked an overflow of radioactivity from rat vasa deferentia preloaded with [3H]noradrenaline was investigated. The overflow evoked by the various agonists was unaffected by the presence of such receptor antagonists as propranolol, phentolamine, cimetidine, or scopolamine. On the other hand, the overflow evoked by all agonists except dimaprit was inhibited by mepyramine and by two well-known neuronal uptake inhibitors, cocaine and desipramine. The inhibition by mepyramine has been attributed to its effect on the neuronal uptake process. Metabolic profile studies showed that 3,4-dihydroxyphenylglycol (DOPEG) was the major constituent in the evoked overflow caused by histamine, 2-methylhistamine, 4-methylhistamine, and dimaprit and that the overflow evoked by 2-pyridylethylamine and 2-thiazolylethylamine consisted predominantly of unchanged noradrenaline. Based on these findings, it is concluded that all of the agonists tested evoke noradrenaline release intraneuronally by entering the adrenergic nerve terminals. While dimaprit might enter by passively diffusing into the adrenergic nerves, other agonists seem to use the neuronal uptake process. Noradrenaline released intraneuronally is subsequently degraded by neuronal monoamine oxidase to form DOPEG. However, there are qualitative and quantitative differences in the metabolic profile of the overflow evoked by various agonists. It is suggested that these differences could arise from their additional properties, such as their effect on the neuronal uptake process and (or) their ability to act as substrate for neuronal monoamine oxidase.

[A comparative pharmacologic study of histamine H-1 antagonists]

In a representative group of 10 conventional antihistaminic agents, a comparison of their H1-antagonistic efficacy was carried out in a series of pharmacological tests. A very good correlation of findings obtained under in vivo conditions in guinea-pigs (tests of detoxication of histamine and histamine aerosol) and a test in rats, which examined the influence on local changes induced by histamine in the dermal region, was demonstrated. No statistically significant correlation of in vivo findings and receptor binding studies on membranes of the rat brain (displacement of (3H)-mepyramine) was found. The tests based on displacement of (3H)-mepyramine are usable primarily for the needs of screening research. The predictive value of pharmacological findings obtained under in vivo conditions is substantially higher.

Vasomotor effect of histamine on pig and cattle coronary artery in vitro

Vasomotor effects of histamine were examined in isolated coronary arteries from pigs and cattle. Histamine produced a concentration-dependent contraction in these arteries. These contractile responses were dose-dependently inhibited by diphenhydramine. The slopes of the Schild plots, however, were significantly lower than unity in both species. Cimetidine potentiated the histamine-induced contractions at relatively high doses of histamine (larger than 10(-5) M) in pig coronary arteries, but did not show a significant effect in cattle arteries. After the removal of endothelium, the Schild plot of diphenhydramine against histamine gave a straight line with a pA2 value of 7.80 and slope of 1.00 in pigs, confirming the competitive nature of the antagonism. In cattle, the slope was significantly lower than unity; however, in the presence of cimetidine, it was not significantly different from unity. Dimaprit did not contract the cattle coronary arteries with endothelium, but contracted them after the removal of endothelium. These results suggest that histamine-induced vasoconstriction in pig and cattle coronary arteries is mainly dependent on the H1-receptors in the smooth muscle cells, and that H1- and H2-receptors in the endothelial cells of pigs and H2-receptors in the smooth muscle cells of cattle modify the histamine-induced vasoconstrictions.