Cardiac histamine receptors.

Current evidence pertinent to the identification of cardiac histamine receptors in the guinea pig is reviewed. Pharmacological characterization has been aided by the use of selective agonists and antagonists for both types of histamine receptors. It appears that both H1 and H2 receptors mediate the cardiac effects of histamine. Histamine H2 receptors mediate the positive chronotropic and ventricular inotropic effects. H1 receptors mediate the negative dromotropic effect of histamine and possibly the atrial inotropic effect. Histamine-induced arrhythmias involve H1 receptors (arrhythmias of conduction) or H2 receptors (arrhythmias of automaticity), or both. The receptors mediating the histamine-induced increase in coronary flow are not as clearly defined: both H1 and H2 receptors might be implicated.     

Minireview: histamine receptors in brain.

The recent availability of selective pharmacological tools has allowed several classes of histamine receptors to be characterized in brain by assessment of biochemical, neurophysiological and behavioral responses as well as by radioligand binding studies. H₁-receptors might be coupled to translocation of calcium ions and H₂-receptors to an adenylate cyclase. Histamine receptors, distinct from the H₁- and H₂-receptors, might also be present as suggested by electrophysiological and binding studies. The possible involvement of cerebral histamine receptors in the sedative activity of H₁-antihistamines, in the hypotensive activity of clonidine and in the antidepressant activity of tricyclic compounds is discussed.     

Roles of histamine in regulation of arousal and cognition: functional neuroimaging of histamine H1 receptors in human brain

Brain histamine is involved in a wide range of physiological functions such as regulation of the sleep-wake cycle, arousal, cognition, and memory mainly through interactions with histamine H1 receptors (H1Rs). Neurons producing histamine, histaminergic neurons, are exclusively located in the posterior hypothalamus and transmit histamine to almost all regions of the brain. Histamine H1 antagonists, or antihistamines, often prescribed for treatment of allergic disorders, sometimes induce sleepiness and cognitive deficits. It is understood that the mechanism of such CNS side effects is that antihistamine blocks H1Rs in the brain. The purpose of the present study was to compare the CNS side effects of different antihistamines.Subjective sleepiness was measured using the Stanford Sleepiness Scale (SSS) and psychomotor performance was examined by a tachistoscope testing system in healthy, young, Japanese volunteers (16 males, 20-28 yrs.) before and after oral administration of antihistamines such as fexofenadine (FEX) and cetirizine (CET). Additionally, H1R occupancy by antihistamines was examined by PET with 11C-doxepin in 8 volunteers.The results of SSS and psychomotor tests demonstrated that FEX tended to be less sedative than CET though the difference was not statistically significant. PET measurements revealed that no H1Rs in the cerebral cortex were occupied by FEX while about 30% of H1Rs were occupied by CET. In summary, it was confirmed that histamine and H1Rs are involved in maintaining arousal and cognition in humans, and that the severity of clinical symptoms is correlated to the amount of antihistamine that penetrated into the brain.     

Oligomerization of mu- and delta-opioid receptors. Generation of novel functional properties

The existence of dimers and oligomers for many G protein-coupled receptors has been described by us and others. Since many G protein-coupled receptor subtypes are highly homologous to each other, we examined whether closely related receptors may interact with each other directly and thus have the potential to create novel signaling units. Using mu- and delta-opioid receptors, we show that each receptor expressed individually was pharmacologically distinct and could be visualized following electrophoresis as monomers, homodimers, homotetramers, and higher molecular mass oligomers. When mu- and delta-opioid receptors were coexpressed, the highly selective synthetic agonists for each had reduced potency and altered rank order, whereas endomorphin-1 and Leu-enkephalin had enhanced affinity, suggesting the formation of a novel binding pocket. No heterodimers were visualized in the membranes coexpressing mu- and delta-receptors by the methods available. However, hetero-oligomers were identified by the ability to co-immunoprecipitate mu-receptors with delta-receptors and vice versa using differentially epitope-tagged receptors. In contrast to the individually expressed mu- and delta-receptors, the coexpressed receptors showed insensitivity to pertussis toxin and continued signal transduction, likely due to interaction with a different subtype of G protein. In this study, we provide, for the first time, evidence for the direct interaction of mu- and delta-opioid receptors to form oligomers, with the generation of novel pharmacology and G protein coupling properties.     

Somatostatin receptors in human cancer: incidence, characteristics, functional correlates and clinical implications.

Somatostatin receptors (SS-R) have been identified in membrane homogenates or tissue sections from several hundred tumors. SS-R were found in most neuroendocrine tumors, i.e. GH and TSH producing pituitary tumors, endocrine gastroenteropancreatic (GEP) tumors, paragangliomas, pheochromocytomas, medullary thyroid carcinomas (MTC) and small cell lung carcinomas. SS-R were also expressed in a majority of malignant lymphomas, in several brain tumors (all meningiomas, most astrocytomas) and in breast tumors. The majority of tumors expressing SS-R are rather differentiated (i.e. astrocytomas vs glioblastomas), but exceptions exist (high grade malignant lymphomas). An inverse relationship exists between SS-R and receptors for epidermal growth factor (EGF-R) incidence in lung tumors, glial tumors and most breast tumors, whereas meningiomas express simultaneously both receptors. A minority of tumors (ovarian tumors, MTC, insulinomas) express a subtype of SS-R, characterized by low affinity for the octapeptide SS analog octreotide. The function mediated by SS-R in human tumors may differ according to the tumor type. SS-R in pituitary and GEP tumor mediate hormone secretion inhibition with, in addition, possibly some antiproliferative effects. In meningiomas, however, activation of SS-R inhibits forskolin-stimulated adenylate cyclase activity, and weakly stimulates proliferation. Whereas SS-R seem to mediate antiproliferative effects in animal models and cell lines of lymphomas, breast and lung tumors, such an effect has not yet been convincingly documented in human primary tumors. The clinical implications of the presence of SS-R in tumors are manyfold: (1) as a predictive marker for efficient therapy with octreotide in pituitary and GEP tumors; (2) as a diagnostic marker: for pathobiochemical classification of tumors, using in vitro detection methods; for clinical evaluation using in vivo scanning techniques; (3) as a prognostic marker; and (4) as a potential radiotherapeutic target.     

Structural and functional characteristics of S1P receptors

The sphingosine-1-phosphate (S1P) family of G protein-coupled receptors (GPCR) regulates essential cellular processes such as proliferation, migration, cytoskeletal organization, adherens junction assembly, and morphogenesis. S1P, a product from the breakdown of sphingomyelin, binds to the five members of this receptor family, S1P(1), S1P(2), S1P(3), S1P(4), and S1P(5), previously referred to as endothelial differentiation gene (EDG)-1, -5, -3, -6, and -8. S1P receptors are widely expressed in different tissues, so it is not surprising that the S1P receptor family regulates many physiological processes, such as vascular maturation, cardiac development, lymphocyte trafficking, and vascular permeability. FTY720, a new S1P receptor agonist, is undergoing clinical trials as an immunosuppressor. Understanding the physiological role of these receptors and the basics of the ligand-receptor interaction will potentially provide new therapies to control a variety of diseases.     

Different antagonist binding properties of human and rat histamine H3 receptors

Different histamine H3-receptor antagonists have been tested in displacement studies at human and rat H3 receptors in stably transfected cells. Based on an actual rhodopsin structure, models for receptor antagonist interaction were developed for receptors of both species. Similarities and discrepancies in binding profiles can be explained, but not quantified by hydrophilic interactions with Asp114 and an important lipophilic binding pocket modified by two nearby amino acids.     

Polypeptide hormone receptors: characteristics and applications.

Major developments in the area of polypeptide hormone receptors have been reviewed. Receptors are high affinity, high specificity binding sites which appear to be located largely, if not entirely, on the plasma membrane of cells. Receptors are proteins intimately associated with and influenced by lipids. Receptor sites and degrading sites appear to be readily distinguishable entities. The binding of hormone to receptor is distinct and has been dissociated from subsequent steps leading to hormonal response. There is no direct relationship between receptor occupancy and the magnitude of target response to hormone. So called 'spare' receptors can be viewed thermodynamically as enhancing target tissue sensitivity to hormone. The binding of hormone to receptor appears to be a point at which regulation of tissue sensitivity can be influenced either through altering the affinity for hormone or the number of receptors. One factor apparently involved in the regulation of receptor levels is the hormone itself. Receptors have been used to develop assay procedures which have significantly complemented the bioassay and radioimmunoassay. Finally, the measurement of receptor levels in disease has provided new insights into pathophysiology.     

Structural and functional characteristics and properties of metzincins

In this review the main families of endopeptidases belonging to the clan of metzincins of zinc-dependent metal-loproteinases in organisms of wide evolutional range from bacteria to mammals are considered. The data on classification, physicochemical properties, substrate specificity, and structural features of this group of enzymes are given. The activation mechanisms of metzincins, the role of these proteins in organisms, and their participation in various physiological processes are discussed.     

Multiple opioid receptors and GTP-binding proteins.

We believed that GTP-binding protein (G-protein)-coupling receptor always transduces stimulatory signals to G-proteins. From our recent experiments using reconstitution techniques, however, it was revealed that some receptors transduce an inhibitory or no signal to G-proteins in specific tissues, despite some interaction between them. Here we discuss the molecular basis of mechanisms of such diverse modes of functional coupling between different subtypes of opioid receptors and G-proteins.     

AMPA和KA受体通道的分子结构及功能特性

近期的克隆研究证实,尽管α－氨基－３－羧基－５－甲基异恶唑－４－丙酸（ＡＭＰＡ）和红藻氨酸（ＫＡ）受体能被相同的激动剂激活,但它们是不同的相互独立的受体复合体。同时,已经证明非Ｎ－甲基－门冬氨酸（ＮＭＤＡ）受体的某些亚型具有Ｃａ＾２＋通透性,这种特性以前认为只有ＮＭＤＡ受体才具有。     

Characterization of functional histamine H1 receptors on a cultured smooth muscle cell line

Cultured cells of the smooth muscle line DDT1MF-2, which was derived from a hamster vas deferens tumor, expressed histamine H1-type receptors and responded biochemically and functionally to H1-specific stimulation. The H1-receptor antagonist [3H]-pyrilamine bound specifically to 9.7 x 10(6) sites/DDT1MF-2 cell with a dissociation constant (Kd) of 219 nM. The addition of histamine to suspensions of fura-2-loaded DDT1MF-2 cells elicited a rapid, transient, and stimulus concentration-dependent increase in the intracellular concentration of Ca2+ with an EC50 of 3 x 10(-5) M, which demonstrated H1 receptor specificity. Moreover, in order to evaluate in vitro contractile response of individual DDT1MF-2 cells, the degree of intracellular actin polymerization was quantified by a DNase inhibition assay. The percentage of nonpolymerized or G-actin in DDT1MF-2 cells was reduced in a histamine concentration-dependent manner with an EC50 of 1 x 10(-5) M and H1 receptor specificity. Histamine-induced actin polymerization was accompanied by changes in cell shape that were consistent with cellular contraction, as assessed by flow cytometry. The H1-type receptors of cultured DDT1MF-2 cells thus couple histamine stimulation to a variety of functional responses of smooth muscle cells.     

Fluorescent muscarinic EGFP-hM1 chimeric receptors: design, ligand binding and functional properties

We describe the construction, expression and characterization of recombinant proteins comprising the enhanced green fluorescent protein (EGFP) fused to the amino-terminal part of the muscarinic hM1 receptor together or not with an additional hexahistidine tag placed at the C-terminal end of the receptor. Expression of the fluorescent proteins reaches levels identical to those of the wt hM1 receptor, provided that fusion takes place at the very N-terminal end of the receptor. Also correct protein folding and targeting to plasma membrane is obtained upon addition of a signal peptide promoting amino-terminal domain translocation through the membrane. Ligand binding properties of--and activation of the calcium release response by--the fusion proteins are almost identical to those of the wild-type muscarinic receptor, indicating that such fluorescently-labelled receptors are valuable model systems for further functional, biochemical and structural studies.     

Solubilization of histamine H-1, GABA and benzodiazepine receptors.

Dopamine 3-0-sulfate is present in considerable amounts in mammalian plasma and peripheral tissues. Incubation of dopamine 3-0-sulfate (0.1 μmole) with purified bovine dopamine-β-hydroxylase resulted in the formation of free norepinephrine (7.3 × 10^?3 μmole). The conversion to norepinephrine was inhibited by 0.6 mM of fusaric acid, an inhibitor of dopamine-β-hydroxylase. The reaction of dopamine 3-0-sulfate with dopamine-β-hydroxylase followed Michaelis-Menten kinetics. The calculated K_m was 17 mM, different from the K_m for free dopamine (0.1 mM). The incubation medium does not contain any sulfatase activity.     

Physico-chemical characteristics and functional properties of membrane-bound hemoglobin]

It was shown that fetal form of haemoglobin (Hb) is mainly connected with membrane of peripheral red blood corpuscles. Membrane-bound haemoglobin has relatively high affinity to O2, low Hill's coefficient, much higher peroxidase activity in comparison with cytosolic Hb.