Synaptic and extra-synaptic distribution of histamine H1-receptors in rat and guinea pig brains

Localization of histamine H1-receptors in subcellular fractions from rat and guinea pig brains was examined in a [3H]mepyramine binding study. Major [3H]mepyramine binding sites with increased specific activities [( 3H]mepyramine binding vs. protein amount) were recovered from P2 fractions from both rat and guinea pig brains by differential centrifugation. Further subfractionation of both rat and guinea pig P2 fractions by a discontinuous sucrose density gradient centrifugation showed the highest recovery of [3H]mepyramine binding with further increased specific activities found in synaptic plasma membrane (SPM) fractions. Minor [3H]mepyramine binding sites with increased specific activities were detected in both rat and guinea pig P3 fractions. [3H]Mepyramine binding sites in SPM and P3 fractions showed identical Kd values in each species. These results indicate that histamine H1-receptors are located not only in synaptic but also in extra-synaptic membranes of both rat and guinea pig brains.     

Binding of 4(5)-[2-(4-azido-2-nitroanilino)ethyl]imidazole to histamine receptors in guinea pig cerebral cortical membranes

The interaction of 4(5)-[2-(4-azido-2-nitroanilino)ethyl]imidazole (AAH), a photolabile histamine receptor antagonist, with the binding of histamine, mepyramine, and tiotidine to guinea pig cerebral cortical membranes was examined to evaluate the specificity of AAH for histamine H1 and H2 receptors. Saturable, specific binding of [3H]histamine, [3H]mepyramine, and [3H]tiotidine to the membranes was observed. Competition assays were used to assess the relative affinity of AAH for H1- and H2-receptors. The rank order of IC50 values obtained was (most to least potent) (i) for competing with [3H]histamine binding: histamine greater than AAH much greater than mepyramine approximately equal to tiotidine; (ii) for competing with [3H]mepyramine binding: mepyramine much greater than AAH greater than histamine greater than tiotidine; and (III) for competing with [3H]tiotidine binding: tiotidine much greater than mepyramine greater than histamine approximately equal to AAH. The affinity of AAH for H1 receptors was ca. 14-fold greater than for H2 receptors. These findings support previous evidence obtained in isolated smooth muscle preparations that AAH shows H1-receptor selectivity as an antagonist.     

SKF 525-A and cytochrome P-450 ligands inhibit with high affinity the binding of [3H]dextromethorphan and sigma ligands to guinea pig brain.

The DM1/sigma 1 site binds dextromethorphan (DM) and sigma receptor ligands. The broad binding specificity of this site and its peculiar subcellular distribution prompted us to explore the possibility that this site is a member of the cytochrome P-450 superfamily of enzymes. We tested the effects of the liver microsomal monooxygenase inhibitor SKF 525-A (Proadifen), and other P-450 substrates on the binding of [3H]dextromethorphan, [3H]3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine and (+)-[3H]1,3-Di-o-tolyl-guanidine ([3H]DTG) to the guinea pig brain. SKF 525-A, l-lobeline and GBR-12909 inhibited the binding of the three labeled ligands with nM affinity. Each drug has identical nM Ki values for the high-affinity site labeled by the three ligands. This indicated that they displaced the labeled ligands from the common DM1/sigma 1 site. Debrisoquine and sparteine, prototypical substrates for liver debrisoquine 4-hydroxylase, displayed Ki values of 9-13 and 3-4 microM respectively against the three labeled ligands. These results, the broad specificity of the DM1/sigma 1 binding site, and its peculiar subcellular distribution, raises the possibility that this binding site is a member of the cytochrome P-450 superfamily of isozymes, rather than a neurotransmitter receptor. These findings may have important implications for the understanding of the therapeutic, side effects and toxicity of several neurotropic drugs.     

3H]mepyramine binding to histamine H1 receptors in bovine retina

The promethazine-sensitive [3H]mepyramine binding was used to determine the presence of histamine H1 receptors in membranes from bovine retina. Specific mepyramine binding to retinal membranes was reversible, saturable and of high affinity. The apparent dissociation constant (KD = 2.2 +/- 0.4 nM) and the density of binding sites (Bmax = 60.9 +/- 5.1 fmol/mg protein), obtained in equilibrium studies, were similar to those found in bovine brain cortex. Binding was stereospecific and the inhibitory potencies of H1 and H2 antagonists indicated that [3H] mepyramine binding sites in the retina have characteristics of H1 receptors.     

The binding of [3H]mepyramine to histamine H1 receptors in monkey brain

Several laboratories have reported ligand binding studies using radioactive histamine H1 antagonists to label the H1 receptors in mammalian brain. We have extended these studies to a detailed examination of the binding of [3H]mepyramine to monkey brain and have shown that the distribution is similar to that in man, with specific binding sites being concentrated in the frontal cortex with relatively low binding to the pons and basal ganglia. The binding shows a single saturable component with a KD of about 1 nM and a Hill plot slope close to unity. These observations are the same for all structures tested. Comparison with data from other laboratories suggests that in this species, the histamine receptor is the same as that in peripheral tissues. From Ki values for various ligands and comparison of KD estimates in other species, the receptor seems to be essentially identical to the H1 receptor in central and peripheral tissues of the guinea pig and also to that in human brain. The rat and possibly the dog have minor differences from the monkey in terms of KD values for [3H]mepyramine binding.     

Interaction of hexachlorobenzene with the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin in vitro and in vivo. Evidence that hexachlorobenzene is a weak Ah receptor agonist

Hexachlorobenzene (HCB) produces hepatic porphyria and induces the hepatic cytochrome P450 isozymes P450c (P450IA1) and P450d (P450IA2) in rodents. These and other effects of HCB resemble those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which acts via its binding to the aromatic hydrocarbon (Ah) receptor. We therefore examined the ability of HCB to interact with this receptor in vitro and in vivo. HCB, at concentrations of 1 microM or higher, inhibited the specific binding of [3H]TCDD (0.3 nM) to the Ah receptor in vitro, whereas the solubility of [3H]TCDD was affected only at 100 microM HCB. The inhibition was competitive, with a KI of approximately 2.1 microM. In rats fed a diet containing 3000 ppm HCB for varying times (4 h to 7 days), the specific binding of [3H]TCDD in hepatic cytosol was reduced by up to 40%, as observed previously for known Ah receptor agonists. The decrease in [3H]TCDD specific binding in cytosol of HCB-treated rats was due principally to a decrease in the number of binding sites for [3H]TCDD rather than competition from residual HCB. As shown by immunoblotting and radioimmunoassay, HCB induced the cytochrome P450 isozymes P450c and P450d, which are regulated by the Ah receptor, as well as the phenobarbital-inducible isozymes P450b and P450e. Together these results indicate that HCB is a weak agonist for the Ah receptor, and suggest that some of its effects may be mediated by its interaction with this gene-regulatory protein.     

H1-histaminergic activation stimulates inositol-1-phosphate accumulation in chromaffin cells

Adrenal medullary chromaffin cells maintained in vitro were prelabeled with [3H]inositol and the accumulation of [3H]inositol-1-phosphate, was determined following stimulation with a variety of pharmacological agents. Carbachol, bradykinin, and histamine produced significantly greater accumulation of [3H] inositol-1-phosphate over basal levels, with histamine producing the greatest effect. H1-histamine receptor antagonists, mepyramine, pyrilamine, tripelennamine and clemastine were all able to reduce or completely block the histamine response. The two specific H2-histamine receptor antagonists, cimetidine and ranitidine, had no effect on this response. Histamine dose-response characteristics in the presence of mepyramine and clemastine suggest the H1 antagonism to be competitive in nature.     

Histamine H1-receptor in the retina: species differences

Histamine H1-receptors in membranes of the various mammalian retinas were studied by [3H]mepyramine binding assay. Specific [3H]mepyramine bindings to bovine, pig, dog and human retinas were observed with the dissociation constants (KD), 3.8 +/- 1.2 nM, 1.8 +/- 0.6 nM, 2.6 +/- 0.6 nM and 3.0 +/- 0.9 nM, respectively, which were similar to those found in brains. But there was no detectable specific binding in the guinea-pig and rabbit retinas. The number of binding sites (Bmax) ranged from negligible value to 290.7 +/- 51.7 fmole/mg protein(human retina). Some H1-antagonists acted as potent agents in competing with [3H]mepyramine binding to bovine and pig retinas. These results indicated that histamine H1-receptors exist in some mammalian retina and have similar characteristics to those in brain membranes, but they distributes in the wide difference of the binding capacities among the species, while in brain variations were smaller.     

Histamine Stimulation of Inositol 1-Phosphate Accumulation in Lithium-Treated Slices from Regions of Guinea Pig Brain

Histamine stimulated the accumulation of [3H]inositol 1-phosphate in the presence of 10 mM LiCl in [3H]inositol-loaded tissue slices from several regions of guinea pig brain. The level of [3H]inositol 1-phosphate increased approximately linearly, after an initial lag period, up to a time of 120 min. In the absence of lithium ions the accumulation of the 1-phosphate stimulated by histamine in cerebral cortical and hippocampal slices was markedly reduced. Lithium ions had much less effect on the response to histamine in cerebellar slices. The characteristics of the response to histamine were consistent with mediation by H1 receptors, and the affinity constants derived for mepyramine (2.3 X 10(9) M-1) and methapyrilene (1.8 X 10(8) M-1) were similar to those reported from measurements on other H1 responses in the guinea pig. The EC50 for histamine was similar in cerebellum, cerebral cortex, hippocampus, and hypothalamus. The position of the dose-response curve for histamine in cerebral cortical slices was similar to that of the curve for the receptor binding of histamine deduced from histamine inhibition of [3H]mepyramine binding.     

Cytochrome P450 isozymes catalyzing 4-hydroxylation of parkinsonism-related compound 1,2,3,4-tetrahydroisoquinoline in rat liver microsomes.

Microsomal 4-hydroxylase of 1,2,3,4-tetrahydroisoquinoline (TIQ), a possible candidate for causing Parkinson disease, was characterized by using rat hepatic microsomes and purified P450 isozymes. Kinetic analysis revealed that Km and Vmax values (mean +/- SE) for hepatic microsomal TIQ 4-hydroxylase of male Wistar rats were 319.6 +/- 26.8 microM and 12.13 +/- 1.43 pmol.min-1.mg-1 protein, respectively. When TIQ 4-hydroxylase activity was compared in Wistar (an animal model of extensive debrisoquine metabolizers) and Dark Agouti (an animal model of poor debrisoquine metabolizers) rats, significant strain (Wistar greater than Dark Agouti) and sex (male greater than female) differences were observed. The microsomal activity toward TIQ 4-hydroxylation was increased by pretreatment of male Wistar rats with P448 inducers (beta-naphthoflavone and sudan I), but not with phenobarbital. Pretreatment with propranolol, an inhibitor of P450 isozymes belonging to the P450 IID gene subfamily, decreased TIQ 4-hydroxylase activity. P450 BTL, a P450 isozyme belonging to the IID subfamily, showed TIQ 4-hydroxylase activity of 64.1 pmol.min-1.nmol P450(-1), which was 3.2-fold that of microsomes (20.9 pmol.min-1.nmol P450(-1)). Antibody (IgG) against this isozyme suppressed microsomal TIQ 4-hydroxylase activity concentration-dependently. A male-specific P450 ml (P450IIC11) catalyzed this reaction to a much lesser extent (10.0 pmol.min-1.nmol P450(-1)), and its antibody did not affect the microsomal activity. These results suggest that TIQ 4-hydroxylation in hepatic microsomes are catalyzed predominantly by a P450 isozyme (or isozymes) belonging to the IID gene subfamily in non-treated rats and its immunochemically related P450 isozyme (or isozymes), and that a P450 isozyme (or isozymes) belonging to the IA subfamily also participates in TIQ 4-hydroxylation in rats pretreated with P448-inducers.     

Affinities of Histamine H1-Antagonists in Guinea Pig Brain: Similarity of Values Determined from [3H]Mepyramine Binding and from Inhibition of a Functional Response

Affinity constants for five antagonists at histamine H1-receptors in guinea pig brain have been determined from inhibition of the potentiation by histamine of the adenosine-induced accumulation of cyclic AMP in cerebral cortical slices. This action of histamine appeared to be mediated solely through H1-receptors. The affinity constants obtained were similar to those determined on peripheral H1-receptors and from inhibition of high-affinity [3H]mepyramine binding. This provides strong evidence that at least some of the [3H]mepyramine binding sites in guinea pig brain can be identified with functional H1-receptors.     

HETEROGENEITY OF HISTAMINE Hi-RECEPTORS: SPECIES VARIATIONS IN [3H]MEPYRAMINE BINDING OF BRAIN MEMBRANES

[³H]Mepyramine binds with high affinity to membranes from brain of human, rat, guinea-pig, rabbit and mouse with drug specificity indicating an association with histamine H₁receptors. Considerable species differences occur in the affinity of [³H]mepyramine, with guinea-pig and human having 34 times greater affinity than rat, mouse or rabbit. The greater affinity of [³H]mepyramine in guinea-pig than in rat is attributable both to faster association and slower dissociation rates in guinea-pig. Species differences in affinity for H₁ receptor sites occur for some antihistamines but not for others. Some tricyclic antidepressant and neuroleptic drugs are extremely potent inhibitors of [³H]mepyramine binding, exceeding in potency any H₁ antihistamines examined. The tricyclic antidepressant doxepin and the neuroleptic clozapine are the most potent of all drugs examined in competing for [³H]mepyramine binding. The regional distribution of specific [³H]mepyramine binding differs considerably in the various species examined.     

The dopamine transporter and cytochrome P45OIID1 (debrisoquine 4-hydroxylase) in brain: resolution and identification of two distinct [3H]GBR-12935 binding proteins

Two [3H]GBR-12935 binding proteins, identified as the dopamine transporter and cytochrome P45OIID1, were solubilized in digitonin from canine striatal membranes, and were resolved following wheat germ agglutinin (WGA)-lectin column chromatography. Protein adsorbed to and specifically eluted from WGA-lectin with N-acetylglucosamine displayed saturable, high affinity (KD approximately 3 nM), and sodium-dependent binding of [3H]GBR-12935, which was inhibited in a concentration-dependent and stereoselective manner by dopamine uptake blockers and substrates with a pharmacological profile indicative of the dopamine uptake site. Protein not adsorbed to WGA-lectin also bound [3H]-GBR-12935 with high affinity (approximately 7 nM), in a sodium-independent manner, and was insensitive to classical dopamine uptake blockers and substrates such as mazindol or dopamine, corresponding to the so-called "piperazine acceptor" site seen in native membranes. [3H]GBR-12935 binding to this latter protein was, however, inhibited by various compounds with a pharmacological profile indicative of a form of cytochrome P450 designated P45OIID1 (debrisoquine/sparteine monooxygenase) with the following rank order of inhibitory potency: GBR-12909 greater than budipine greater than alpha-lobeline greater than quinidine greater than alpha flupenthixol greater than SKF-525A greater than sparteine greater than quinine. Ki values obtained for inhibition of [3H]-GBR-12935 binding to neuronal WGA passthrough fractions by these drugs correlate well with their respective Ki values for liver P45OIID1 activity. Western blotting and immunoprecipitation analysis with rabbit anti-rat P45OIID1 antibody also supported the identity of the mazindol-insensitive [3H]GBR-12935 binding site (or piperazine acceptor site) as P45OIID1. Furthermore, a [3H]GBR-12935 binding protein with pharmacological and immunological characteristics similar to those of P45OIID1 was solubilized from both bovine and human liver membranes, and GBR-12909 was found to be a potent competitive inhibitor (Ki approximately 100 nM) of sparteine monooxygenase activity in human liver microsomes. These data clearly indicate that [3H]GBR-12935 and its analogs display similar affinities for both the dopamine transporter and neuronal P45OIID1, and that this radioligand may be a useful probe of P45OIID1 activity in brain and liver. The exact molecular and functional association (if any) between these two distinct binding protein populations remains to be established; however, it is tempting to speculate that P45OIID1 is involved in the catabolism and processing of neurotransmitters subsequent to their reuptake into target cells.     

THE BINDING OF [3H]MEPYRAMINE TO HISTAMINE H1 RECEPTORS IN GUINEA-PIG BRAIN

The promethazine-sensitive binding of [³H]mepyramine to a membrane fraction from guinea-pig whole brain is saturable with a dissociation constant of 1.7 × 10^-9M. The maximum amount of [³H]mepyramine binding varied widely between preparations, range 122–365 pmol/g protein, with a mean value of 227 ± 52 pmol/g protein. The inhibition of [³H]mepyramine binding by a number of drugs correlated closely with their potency as histamine H₁ antagonists. (+) Chlorpheniramine was 240-fold more potent as an inhibitor of [³H]mepyramine binding than (-)-chlorpheniramine. All antagonists inhibited the binding of [³H]mepyramine to the same extent, but the Hill coefficients characterising the inhibition curves did not all approximate to unity, the value expected for a simple antagonist-receptor equilibrium. The distribution of histamine H₁ receptors, defined by the promethazine-sensitive binding of [³H]mepyramine, in 11 different brain regions was uneven with the largest amounts in cerebellum, superior and inferior colliculus and hypothalamus and the smallest in caudate nucleus, brain stem and spinal cord.     

Characterization of thromboxane A2/prostaglandin H2 receptors and histamine H1 receptors in cultured guinea-pig tracheal smooth-muscle cells.

We characterized thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors and histamine H1 receptors in Guinea-pig cultured tracheal smooth-muscle cells (TSMC). [3H]SQ 29,548 (a TXA2 antagonist)-binding sites were saturable and a high affinity with a dissociation constant of 6.2 +/- 0.60 nM (mean +/- S.E.) and a receptor density of 46 +/- 4.6 fmol/10(6) cells. [3H]SQ 29548 binding was completely inhibited by TXA2 mimetics or antagonists. Intracellular calcium concentration ([Ca2+]i) in TSMC was increased with U46619 stimulation and the increase was attenuated by TXA2 antagonists, the potencies of which correlated with those inhibiting the activities of the [3H]SQ 29548 binding. [3H]Mepyramine (a H1 antagonist)-binding sites were also present in TSMC. [3H]Mepyramine had a single class of low-affinity-binding sites with a dissociation constant of 2.6 +/- 0.081 microM and a receptor density of 10.6 +/- 0.11 nmol/mg protein. [3H]Mepyramine binding in TSMC membrane was inhibited by H1 antagonists, but not by H2 antagonists. The inhibition constants of mepyramine in TSMC were 910-times lower than those in tracheal membranes. In contrast, the histamine-induced increase in [Ca2+]i in TSMC was inhibited in the presence of low concentrations of H1 antagonists. All these observations provide evidence that TXA2/PGH2 receptors, mepyramine-binding sites and/or H1 receptors are expressed in cultured TSMC.     

Molecular basis for the interaction of histamine with the histamine H2 receptor.

We undertook these studies to characterize the molecular basis of the interaction of histamine with the H2 receptor. Key areas of homology in the structures of the histamine H2 and beta 2 adrenergic receptor suggested specific transmembrane amino acids that might be important for binding of histamine. A third transmembrane aspartic acid of the histamine receptor (Asp98), thought to serve as a counter anion that interacts with the cationic amine moiety of histamine, was mutated to Asn98, and the mutated receptor was expressed in Hepa cells. Removal of the negatively charged amino acid abolished both binding of the H2 receptor antagonist [methyl-3H]tiotidine and histamine stimulated increases in cellular cAMP content. Mutation of a fifth transmembrane aspartic acid (Asp186) to Ala186 or Asn186 by itself or in conjunction with mutation of another fifth transmembrane amino acid (Thr190 to Ala190) resulted in a loss of [methyl-3H] tiotidine binding, although the generation of cAMP in response to histamine was maintained. The histamine receptor with only a Thr190 to Ala190 or Cys190 mutation retained the ability to bind [methyl-3H]tiotidine, but both the affinity and efficacy of binding were reduced. These data lead us to propose a model for histamine binding in which Asp98 is essential for histamine binding and action, Asp186 defines H2 selectivity, and Thr190 is important in establishing the kinetics of histamine binding, but is not essential for H2 selectivity.     

Purification of a histamine H3 receptor negatively coupled to phosphoinositide turnover in the human gastric cell line HGT1.

The histamine H3 receptor agonist (R)alpha-methylhistamine (MeHA) inhibited, in a nanomolar range, basal and carbachol-stimulated inositol phosphate formation in the human gastric tumoral cell line HGT1-clone 6. The inhibition was reversed by micromolar concentrations of the histamine H3 receptor antagonist thioperamide and was sensitive to cholera or pertussis toxin treatment. Using [3H]N alpha-MeHA as specific tracer, high affinity binding sites were demonstrated with a Bmax of 54 +/- 3 fmol/mg of protein and a KD of either 0.61 +/- 0.04 or 2.2 +/- 0.4 nM, in the absence or presence of 50 microM GTP[gamma]S, respectively. The binding sites were solubilized by Triton X-100 and prepurified by gel chromatography. They were separated from the histamine H2 receptor sites by filtration through Sepharose-famotidine and finally retained on Sepharose-thioperamide. The purified sites concentrated in one single silver-stained protein band of 70 kDa in SDS-polyacrylamide gel electrophoresis. They specifically bound [3H]N alpha-MeHA with a KD of 1.6 +/- 0.1 nM and a Bmax of 12,000 +/- 750 pmol/mg of protein. This corresponds to a 90,225-fold purification over cell lysate and a purity degree of 84%. Binding was competitively displaced by N alpha-MeHA (IC50 = 5.8 +/- 0.7 nM), (R) alpha-MeHA (IC50 = 9 +/- 1 nM), and thioperamide (IC50 = 85 +/- 10 nM), but not by famotidine (H2 antagonist) or by mepyramine (H1 antagonist). These findings provide the first evidence for solubilization, purification, and molecular mass characterization of the histamine H3 receptor protein and for the negative coupling of this receptor phosphatidylinositol turnover through a so far unidentified G protein.     

3H]U-69593 labels a subtype of kappa opiate receptor with characteristics different from that labeled by [3H]ethylketocyclazocine

[3H]U-69593 is an opiate agonist that has been reported to bind in vitro with high affinity and selectivity to the kappa receptor subtype. The studies reported here were designed to determine the optimal conditions for labeling kappa receptors with [3H]U-69593 and to further characterize the binding site. The effects of temperature and NaCl on [3H]U-69593 binding were of particular interest because previous studies reported that [3H]ethylketocyclazocine ([3H]EKC) and [3H]bremazocine binding to kappa receptors was optimal at 4 degrees C in the presence of NaCl. Those conditions were not found to be optimal for [3H]U-69593 binding. Although the pharmacological specificity and Bmax of [3H]U-69593 binding was similar at room temperature and at 4 degrees C, the binding affinity was approximately three times lower at 4 degrees C than at room temperature. In addition, NaCl had an effect on [3H]U-69593 binding that was opposite that on [3H]EKC binding at 4 degrees C (100 nM DAGO and 100 nM DADLE were included in all [3H]EKC assays to prevent binding to mu and delta receptors), i.e. NaCl decreased, rather than increased, [3H]U-69593 binding at 4 degrees C. These differences between [3H]U-69593 and [3H]EKC binding at 4 degrees C were accentuated by a vast difference in the density of the binding sites [Bmax approximately equal to 12 fmol/mg protein for [3H]U-69593 vs approximately equal to 375 fmol/mg protein for [3H]EKC at 4 degrees C in the presence of NaCl) and suggested that [3H]U-69593 might bind selectively to a kappa receptor subtype. This concept was supported by competition experiments. In particular, the site labeled by [3H]EKC at 4 degrees C was found to be relatively insensitive (compared to [3H]U-69593 and [3H]EKC binding at room temperature) to the kappa agonist U-50488H, a close analog to U-69593. Based on these findings, we propose that [3H]U-69593 (and U-50488H) labels a kappa receptor subtype which differs from that labeled by [3H]EKC at 4 degrees C.     

Ca2+‐dependent down‐regulation of human histamine H1 receptors in Chinese hamster ovary cells

G_q/11 protein‐coupled human histamine H₁ receptors in Chinese hamster ovary cells stimulated with histamine undergo clathrin‐dependent endocytosis followed by proteasome/lysosome‐mediated down‐regulation. In this study, we evaluated the effects of a sustained increase in intracellular Ca²⁺ concentrations induced by a receptor‐bypassed stimulation with ionomycin, a Ca²⁺ ionophore, on the endocytosis and down‐regulation of H₁ receptors in Chinese hamster ovary cells. All cellular and cell‐surface H₁ receptors were detected by the binding of [³H]mepyramine to intact cells sensitive to the hydrophobic and hydrophilic H₁ receptor ligands, mepyramine and pirdonium, respectively. The pretreatment of cells with ionomycin markedly reduced the mepyramine‐ and pirdonium‐sensitive binding sites of [³H]mepyramine, which were completely abrogated by the deprivation of extracellular Ca²⁺ and partially by a ubiquitin‐activating enzyme inhibitor (UBEI‐41), but were not affected by inhibitors of calmodulin (W‐7 or calmidazolium) and protein kinase C (chelerythrine or GF109203X). These ionomycin‐induced changes were also not affected by inhibitors of receptor endocytosis via clathrin (hypertonic sucrose) and caveolae/lipid rafts (filipin or nystatin) or by inhibitors of lysosomes (E‐64, leupeptin, chloroquine, or NH₄Cl), proteasomes (lactacystin or MG‐132), and a Ca²⁺‐dependent non‐lysosomal cysteine protease (calpain) (MDL28170). Since H₁ receptors were normally detected by confocal immunofluorescence microscopy with an antibody against H₁ receptors, even after the ionomycin treatment, H₁ receptors appeared to exist in a form to which [³H]mepyramine was unable to bind. These results suggest that H₁ receptors are apparently down‐regulated by a sustained increase in intracellular Ca²⁺ concentrations with no process of endocytosis and lysosomal/proteasomal degradation of receptors.

     

Solubilization, separation, and partial characterization of histamine H1 and H2 receptors from calf thymocyte membranes.

Histamine membrane receptors are defined as either H1 (blocked by diphenhydramine-like antagonists) or H2 (blocked by cimetidine-like agents). We now report the solubilization, separation, and partial characterization of specific H1 and H2 membrane receptors from calf thymocytes. Membrane fragments were incubated with [3H]histamine either alone or with unlabeled histamine, diphenhydramine, or cimetidine. Maximal specific binding occurred with incubation at 37 degrees C for 2 h at a concentration of 5 x 10(-6) M [3H]histamine. Labeled receptors were solubilized from membranes with 0.3 M KCl and 1% Nonidet 40. Chromatography of the solubilized labeled receptors on ion exchange columns revealed two classes of receptor. One class bound to DEAE-cellulose and eluted as a sharp peak at 0.15 M NaCl/Pi. The other bound to phosphocellulose and eluted as a sharp peak at 0.55 M NaCl/Pi. Initial incubation of the membranes in the presence of the H1 receptor antagonist diphenhydramine virtually abolished the DEAE-cellulose peak, while incubation with cimetidine, the H2 receptor antagonist, blocked the phosphocellulose peak. We conclude that H1 and H2 histamine receptors are physically separable and can be defined by their ability to bind to either DEAE-cellulose or phosphocellulose.