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

Activation of mGluR1 negatively modulates glutamate-induced phase shifts of the circadian pacemaker in the mouse suprachiasmatic nucleus

In mammals, photic information delivered to the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract (RHT) plays a crucial role in synchronizing the master circadian clock located in the SCN to the solar cycle. It is well known that glutamate released from the RHT terminals initiates the synchronizing process by activating ionotropic glutamate receptors (iGluRs) on retinorecipient SCN neurons. The potential role of metabotropic glutamate receptors (mGluRs) in modulating this signaling pathway has received less attention. In this study, using extracellular single-unit recordings in mouse SCN slices, we investigated the possible roles of the G_q/11 protein-coupled mGluRs, mGluR1 and mGluR5, in photic resetting. We found that mGluR1 activation in the early night produced phase advances in neural activity rhythms in the SCN, while activation in the late night produced phase delays. In contrast, mGluR5 activation had no significant effect on the phase of these rhythms. Interestingly, mGluR1 activation antagonized phase shifts induced by glutamate through a mechanism that was dependent upon Ca_V1.3 L-type voltage-gated Ca²⁺ channels (VGCCs). While both mGluR1-evoked phase delays and advances were inhibited by knockout (KO) of Ca_V1.3 L-type VGCCs, different signaling pathways appeared to be involved in mediating these effects, with mGluR1 working via protein kinase G in the early night and via protein kinase A signaling in the late night. We conclude that, in the mouse SCN, mGluR1s function to negatively modulate glutamate-evoked phase shifts.

Molecular Signaling and Transcriptional Regulation of Histamine H1 Receptor Gene

Histamine-activated histamine H₁ receptor (H1R) signaling regulates many gene expressions, mainly through the protein kinase C (PKC)/extracellular signal-regulated kinases (ERK) signaling. Involvement of other signaling, including NF-κB, Wnt, RUNX-2, and Rho A signaling was also demonstrated. In addition, cAMP production through the activation of H1R signaling was reported. H1R gene itself is also up-regulated by the activation of H1R signaling with histamine. Here, we review our recent findings in the molecular signaling and transcriptional regulation of the H1R gene. Stimulation with histamine up-regulates H1R gene expression through the activation of H1R in HeLa cells. The PKCδ/ERK/poly(ADP)ribosyl transferase-1 (PARP-1) signaling was involved in this up-regulation. Heat shock protein 90 also plays an important role in regulating PKCδ translocation. Promoter analyses revealed the existence of two promoters in the human H1R gene in HeLa cells. H1R-activated H1R gene up-regulation in response to histamine was also observed in U373 astroglioma cells. However, this up-regulation was mediated not through the PKCδ signaling but possibly through the PKCα signaling. In addition, the promoter region responsible for histamine-induced H1R gene transcription in U373 cells was different from that of HeLa cells. These findings suggest that the molecular signaling and transcriptional regulation of the H1R gene are different between neuronal cells and non-neuronal cells.

Current Knowledge and Perspectives on Histamine H1 and H2 Receptor Pharmacology: Functional Selectivity, Receptor Crosstalk, and Repositioning of Classic Histaminergic Ligands

H₁ and H₂ histamine receptor antagonists, although developed many decades ago, are still effective for the treatment of allergic and gastric acid-related conditions. This article focuses on novel aspects of the pharmacology and molecular mechanisms of histamine receptors that should be contemplated for optimizing current therapies, repositioning histaminergic ligands for new therapeutic uses, or even including agonists of the histaminergic system in the treatment of different pathologies such as leukemia or neurodegenerative disorders. In recent years, new signaling phenomena related to H₁ and H₂ receptors have been described that make them suitable for novel therapeutic approaches. Crosstalk between histamine receptors and other membrane or nuclear receptors can be envisaged as a way to modulate other signaling pathways and to potentiate the efficacy of drugs acting on different receptors. Likewise, biased signaling at histamine receptors seems to be a pharmacological feature that can be exploited to investigate nontraditional therapeutic uses for H₁ and H₂ biased agonists in malignancies such as acute myeloid leukemia and to avoid undesired side effects when used in standard treatments. It is hoped that the molecular mechanisms discussed in this review contribute to a better understanding of the different aspects involved in histamine receptor pharmacology, which in turn will contribute to increased drug efficacy, avoidance of adverse effects, or repositioning of histaminergic ligands.

Histamine 1 receptor-Gβγ-cAMP/PKA-CFTR pathway mediates the histamine-induced resetting of the suprachiasmatic circadian clock

Background

Recent evidence indicates that histamine, acting on histamine 1 receptor (H1R), resets the circadian clock in the mouse suprachiasmatic nucleus (SCN) by increasing intracellular Ca²⁺ concentration ([Ca²⁺]_i) through the activation of Ca_V1.3 L-type Ca²⁺ channels and Ca²⁺-induced Ca²⁺ release from ryanodine receptor-mediated internal stores.

Results

In the current study, we explored the underlying mechanisms with various techniques including Ca²⁺- and Cl⁻-imaging and extracellular single-unit recording. Our hypothesis was that histamine causes Cl⁻ efflux through cystic fibrosis transmembrane conductance regulator (CFTR) to elicit membrane depolarization needed for the activation of Ca_V1.3 Ca²⁺ channels in SCN neurons. We found that histamine elicited Cl⁻ efflux and increased [Ca²⁺]_i in dissociated mouse SCN cells. Both of these events were suppressed by bumetanide [Na⁺-K⁺-2Cl⁻ cotransporter isotype 1 (NKCC1) blocker], CFTR_inh-172 (CFTR inhibitor), gallein (G_βγ protein inhibitor) and H89 [protein kinase A (PKA) inhibitor]. By itself, H1R activation with 2-pyridylethylamine increased the level of cAMP in the SCN and this regulation was prevented by gallein. Finally, histamine-evoked phase shifts of the circadian neural activity rhythm in the mouse SCN slice were blocked by bumetanide, CFTR_inh-172, gallein or H89 and were not observed in NKCC1 or CFTR KO mice.

Conclusions

Taken together, these results indicate that histamine recruits the H1R-G_βγ-cAMP/PKA pathway in the SCN neurons to activate Ca_V1.3 channels through CFTR-mediated Cl⁻ efflux and ultimately to phase-shift the circadian clock. This pathway and NKCC1 may well be potential targets for agents designed to treat problems resulting from the disturbance of the circadian system.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Selective coupling of type 6 adenylyl cyclase with type 2 IP3 receptors mediates direct sensitization of IP3 receptors by cAMP

Interactions between cyclic adenosine monophosphate (cAMP) and Ca²⁺ are widespread, and for both intracellular messengers, their spatial organization is important. Parathyroid hormone (PTH) stimulates formation of cAMP and sensitizes inositol 1,4,5-trisphosphate receptors (IP₃R) to IP₃. We show that PTH communicates with IP₃R via “cAMP junctions” that allow local delivery of a supramaximal concentration of cAMP to IP₃R, directly increasing their sensitivity to IP₃. These junctions are robust binary switches that are digitally recruited by increasing concentrations of PTH. Human embryonic kidney cells express several isoforms of adenylyl cyclase (AC) and IP₃R, but IP₃R2 and AC6 are specifically associated, and inhibition of AC6 or IP₃R2 expression by small interfering RNA selectively attenuates potentiation of Ca²⁺ signals by PTH. We define two modes of cAMP signaling: binary, where cAMP passes directly from AC6 to IP₃R2; and analogue, where local gradients of cAMP concentration regulate cAMP effectors more remote from AC. Binary signaling requires localized delivery of cAMP, whereas analogue signaling is more dependent on localized cAMP degradation.

Role of PKA and PKC in histamine H1 receptor-mediated activation of catecholamine neurotransmitter synthesis

Activation of the histamine H1 receptor stimulates tyrosine hydroxylase (TH) to increase catecholamine neurotransmitter synthesis in mammalian brain and adrenal tissues. Histamine non-selectively activates both H1-linked phospholipase (PL) C/inositol phosphates (IP)/diacylglycerol (DAG) signaling and adenylyl cyclase (AC)/adenosine 3',5'-cyclic monophosphate (cAMP) signaling, confounding determination of signaling events involved in H(1)-mediated TH activation. This research uses two new functionally-selective H1 agonists, cis-PAB and trans-PAT, that selectively activate H1/PLC/IP/DAG and H1/AC/cAMP signaling, respectively, to characterize H(1)-mediated activation of TH in rat striatum and bovine adrenal chromaffin (BAC) cells. Histamine, cis-PAB, and trans-PAT produced a two-fold maximal TH activation by an H1 receptor mechanism in rat striatum and BAC cells. Histamine is more potent and efficacious in BAC cells (EC50 approximately 0.2 microM, Emax approximately 200% basal) versus rat striatum (EC50 approximately 0.4 microM; Emax approximately 150%). Cis-PAB and trans-PAT are more potent in rat striatum (EC50 approximately 0.1 microM for both agonists) versus BAC cells (EC50 approximately 1.0 microM for both), with similar efficacy in both preparations (Emax approximately 160% for both agonists). Signaling studies in BAC cells revealed that protein kinase (PK) A but not PKC is involved in H1 -mediated TH activation by trans-PAT and histamine, while, both PKA and PKC are involved for cis-PAB. Results for cis-PAB suggest H1/PLC/IP/DAG/PKC signaling activates PKA, downstream of cAMP formation, indicating apparent direct activation of PKA by PKC.

Reduction in voltage-gated K+ currents in primary cultured rat pancreatic beta-cells by linoleic acids

Free fatty acids (FFAs), in addition to glucose, have been shown to stimulate insulin release through the G protein-coupled receptor (GPCR)40 receptor in pancreatic beta-cells. Intracellular free calcium concentration ([Ca(2+)](i)) in beta-cells is elevated by FFAs, although the mechanism underlying the [Ca(2+)](i) increase is still unknown. In this study, we investigated the action of linoleic acid on voltage-gated K(+) currents. Nystatin-perforated recordings were performed on identified rat beta-cells. In the presence of nifedipine, tetrodotoxin, and tolbutamide, voltage-gated K(+) currents were observed. The transient current represents less than 5%, whereas the delayed rectifier current comprises more than 95%, of the total K(+) currents. A long-chain unsaturated FFA, linoleic acid (10 microm), reversibly decreased the amplitude of K(+) currents (to less than 10%). This reduction was abolished by the cAMP/protein kinase A system inhibitors H89 (1 microm) and Rp-cAMP (10 microm) but was not affected by protein kinase C inhibitor. In addition, forskolin and 8'-bromo-cAMP induced a similar reduction in the K(+) current as that evoked by linoleic acid. Insulin secretion and cAMP accumulation in beta-cells were also increased by linoleic acid. Methyl linoleate, which has a similar structure to linoleic acid but no binding affinity to GPR40, did not change K(+) currents. Treatment of cultured cells with GPR40-specific small interfering RNA significantly reduced the decrease in K(+) current induced by linoleic acid, whereas the cAMP-induced reduction of K(+) current was not affected. We conclude that linoleic acid reduces the voltage-gated K(+) current in rat beta-cells through GPR40 and the cAMP-protein kinase A system, leading to an increase in [Ca(2+)](i) and insulin secretion.

Involvement of the βγ subunits of G proteins in the cAMP response induced by stimulation of the histamine H1 receptor

Stimulation of the histamine H1 receptor has been shown to enhance adenosine 3', 5'-cyclic monophosphate (cAMP) accumulation in various cell types but, to date, the mechanism by which this occurs is still unclear. In the present study, we examined the possibility that the betagamma subunits of G proteins (G betagamma) are involved in this process in cultured Chinese hamster ovary cells transfected with the human histamine H1 receptor (CHO-H1). Histamine increased intracellular cAMP levels in a concentration-dependent manner in CHO-H1 cells, and this histamine action was abolished by pyrilamine (1 microM). Inhibition of histamine H1 receptor-G(q) protein coupling by stable expression of the C-terminal peptide of G alpha(q) protein significantly attenuated the cAMP accumulation induced by histamine. By comparison, neither BAPTA/AM (50 microM), an intracellular Ca2+ chelator, nor GF 109203X (1 microM), an inhibitor of protein kinase C, influenced the cAMP response. Histamine H1 receptor-mediated cAMP accumulation was significantly inhibited by transient transfection of CHO-H1 cells with the C-terminal peptide of beta-adrenoceptor kinase I (residues 542-685), a scavenger of G betagamma. Stable expression of the C-terminal peptide of the G alpha(s) protein, but not treatment with pertussis toxin (200 ng/ml for 24 h), attenuated the histamine H1 receptor-mediated cAMP accumulation. These results suggest that stimulation of histamine H1 receptors activates adenylyl cyclase through the release of G betagamma subunits from G proteins, thereby elevating intracellular cAMP levels.

Ligand-directed functional heterogeneity of histamine H1 receptors: novel dual-function ligands selectively activate and block H1-mediated phospholipase C and adenylyl cyclase signaling

The autacoid and neurotransmitter histamine activates the H(1) G protein-coupled receptor (GPCR) to stimulate predominantly phospholipase C (PLC)/inositol phosphate (IP) signaling and, to a lesser extent, adenylyl cyclase (AC)/cAMP signaling in a variety of mammalian cells and tissues, as well as H(1)-transfected clonal cell lines. This study reports that two novel H(1) receptor ligands developed in our laboratory, (-)-trans-1-phenyl-3-dimethylamino-1,2,3,4-tetrahydronaphthalene (trans-PAT) and (+/-)-cis-5-phenyl-7-dimethylamino-5,6,7,8-tetrahydro-9H-benzocycloheptane (cis-PAB), activate H(1) receptors to selectively stimulate AC/cAMP formation and PLC/IP formation, respectively, in Chinese hamster ovary cells transfected with guinea pig H(1) receptor cDNA. trans-PAT and cis-PAB also are shown to be functionally selective antagonists of H(1)-linked PLC/IP and AC/cAMP signaling, respectively. Whereas cis-PAB H(1) receptor activity is shown to be typically competitive, trans-PAT displays a complex interaction with the H(1) receptor that is not competitive regarding antagonism of saturation binding by the standard H(1) antagonist radioligand [(3)H]mepyramine or H(1)/PLC/IP functional activation by histamine. trans-PAT, however, does competitively block H(1)/PLC/IP functional activation by cis-PAB. Molecular determinants for trans-PAT versus cis-PAB differential binding to H(1) receptors, which presumably leads to differential activation of AC/cAMP versus PLC/IP signaling, likely involves stereochemical factors as well as more subtle steric influences. Results suggest the trans-PAT and cis-PAB probes will be useful to study molecular mechanisms of ligand-directed GPCR multifunctional signaling. Moreover, because most untoward cardiovascular-, respiratory-, and gastrointestinal H(1) receptor-mediated effects proceed via the PLC/IP pathway, PAT-type agonists that selectively enhance H(1)-mediated AC/cAMP signaling provide a mechanistic basis for exploiting H(1) receptor activation for drug design purposes.

H1-receptors: localization and role in airway physiology and in immune functions

Histamine H(1)-receptors are involved in the pathologic processes of allergy. Clinical trials of H(1)-receptor antagonists have demonstrated the efficacy of these agents in reducing the sneezing, pruritus, and rhinorrhea associated with allergic rhinitis. In the lung, H(1)-receptors mediate the bronchoconstrictive effects of histamine and increase vascular permeability, which lead to plasma exudation. H(1)-receptors are present on T cells, B cells, monocytes, and lymphocytes, and stimulation of these receptors induces pro-inflammatory effects. It has been suggested that a signal from the H(1)-receptor contributes to the antigen receptor-mediated signaling pathways that induce proliferative responses and lead to the production of cytokines and antibodies by T cells and B cells, respectively. It would appear, therefore, that the H(1)-receptor has a wider role in inflammatory processes than simply mediating the actions of histamine.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

A novel phenylaminotetralin radioligand reveals a subpopulation of histamine H(1) receptors

Previously, (-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalene ([-]-trans-H(2)-PAT) was shown to activate stereospecifically histamine H(1) receptors coupled to modulation of tyrosine hydroxylase activity in guinea pig and rat forebrain in vitro and in vivo. Furthermore, the novel radioligand [(3)H](-)-trans-H(2)-PAT was shown to label selectively H(1) receptors in guinea pig and rat brain with high affinity (K(D), ~0.1 and 0.5 nM, respectively) and a B(max) about 50 and 15%, respectively, of that observed for the H(1) antagonist radioligand [(3)H]mepyramine. In the current study, [(3)H](-)-trans-H(2)-PAT-labeled cloned guinea pig and human H(1) receptors in Chinese hamster ovary (CHO) cell membranes with high affinity (K(D), ~0.08 and 0.23 nM, respectively) and a B(max) about 15% of that observed for [(3)H]mepyramine. The binding of H(2)-PAT to H(1) receptors in both CHO-H(1) cell lines was stereoselective with the (-)-trans-isomer having affinity (K(i), ~1.5 nM) about 4-, 20-, and 50-times higher than the (-)-cis-, (+)-trans-, and (+)-cis-isomers, respectively; the affinity of (-)-trans-H(2)-PAT was unaffected by excess GTP. In functional assays, (-)-trans-H(2)-PAT was a full antagonist of histamine H(1)-mediated stimulation of phospholipase C (PLC) and [(3)H]inositol phosphates (IP) formation in CHO-H(1) cells, a full inverse agonist of constitutively active H(1) receptors in COS-7-H(1) cells, and a full competitive antagonist (pA(2) = 9.2) of histamine H(1)-mediated contraction of guinea pig ileum. It is concluded that (-)-trans-H(2)-PAT is an antagonist at H(1) receptors coupled to PLC/IP formation and smooth muscle contraction. Meanwhile, the observation that [(3)H](-)-trans-H(2)-PAT labels only a subpopulation of H(1) receptors and that (-)-trans-H(2)-PAT activates H(1) receptors coupled to modulation of tyrosine hydroxylase suggests that there may be post-translational H(1) receptor heterogeneity.

Dopamine induces a biphasic modulation of hypothalamic ANF neurons: a ligand concentration-dependent effect involving D5 and D2 receptor interaction

Increasing evidence now suggests that more than one subtype of dopamine receptors is co-expressed in some of the central neurons. The neurobiological effects on the host cells when these receptors are concurrently activated by their common physiological ligand, dopamine, however, remains elusive. Among the members of the family of dopamine receptors, coupling of D1-like dopamine receptors to Gs and D2-like receptors to Gi proteins are known to augment or suppress cellular functions respectively, through modulation of adenylyl cyclase activity and consequently cAMP generation. Simultaneous activation of D1 and D2 receptors in transfected cell lines expressing the two cloned receptors, however, produced antagonistic effects. This is in contrast to in vivo studies, in which concurrent activation of D1-like and D2-like receptors by their respective agonists may induce synergistic or antagonistic effects or both. We report here that in long-term rat hypothalamic cell cultures, activation of both D1-like (D1 and D5) and D2 receptors on atrial natriuretic factor-producing neurons by dopamine yields a biphasic response. The response is ligand concentration-dependent and involves type II adenylyl cyclases. This process is mediated primarily through antagonistic and synergistic interactions of D5 and D2 receptors as the event is mimicked by the concurrent activation of these two receptors co-transfected in CHO cells. Our present findings suggest a novel action of dopamine, and the biochemical processes involved may underlie some of the pharmacological actions of atypical anti-psychotic drugs. Molecular Psychiatry (2000) 5, 39-48.

Activation of tyrosine hydroxylase by histamine in bovine chromaffin cells

Acute activation of tyrosine hydroxylase by histamine has been studied in cultured bovine chromaffin cells. Tyrosine hydroxylase activity was determined in situ by measuring 14CO2 release following the hydroxylation and rapid decarboxylation of 14C-tyrosine offered to the cells. Histamine increased tyrosine hydroxylase activity 2-fold over 10 min with an EC50 of 0.3 microM and maximal response at 10 microM. Tyrosine hydroxylase activation was detectable within 1-2 min and maintained for at least 10 min. The effect of histamine was fully blocked by the H1 antagonist mepyramine, but unaffected by H2 (cimetidine) and H3 (thioperamide) antagonists. It was mimicked by Nalpha-methylhistamine and the H1 agonist 2-thiazolylethylamine, but not by H2 (dimaprit) or H3 (R)alpha-methylhistamine) agonists. The response to histamine was reduced by 70% by removing extracellular Ca2+ and abolished by removing extracellular Ca2+ and chelating intracellular Ca2+ with BAPTA. Tyrosine hydroxylase activation by histamine was unaffected by the protein kinase C inhibitor Ro 31-8220 but was completely blocked by the protein kinase A inhibitor H89. The results indicate that histamine activates tyrosine hydroxylase and that this effect is mediated through H1 receptors by a mechanism that depends on both extracellular and intracellular Ca2+ and that requires protein kinase A.

Cyclic AMP accumulation in rat soleus muscle: stimulation by beta2- but not beta3-adrenoceptors

The beta-adrenoceptor subtypes involved in cyclic AMP accumulation in rat soleus muscle were studied using beta1- beta2- and beta3-adrenoceptor agonists and antagonists. Responses to (-)-isoprenaline were antagonised by (-)-propranolol (p KB = 8.32 at 0.1 microM) and by erythro-DL-1(7-methylindian-4-yloxy)-3-isopropylaminobuta n-2-ol (+/-)-ICI 118551) (pKB = 9.38 at 10 nM and 9.65 at 100 nM) but not by 2-hydroxy-5(2-((2-hydroxy-3-(4-((1-methyl-4-trifluoromethyl)1H-imidazole -2-yl)-phenoxy)propyl)amino)ethoxy)-benzamide monomethane sulfonate ((+/-)-CGP 20712A at 10 nM or 100 nM). The beta3-adrenoceptor agonist sodium-4-[-2[-2-hydroxy-2-(-3-chlorophenyl)ethylamino]propyl]phenoxya cetate (BRL 37344 at 10 pM or 10 microM) caused no significant change in basal cyclic AMP levels and had no effect on the level of cyclic AMP accumulation stimulated by (-)-isoprenaline, zinterol or forskolin. (-)-Isoprenaline pretreatment (400 microg kg(-1) h(-1), 14 days) abolished responses to (-)-isoprenaline (10 microM) and zinterol (1 microM) while BRL 37344 had no effect in either isoprenaline or vehicle-treated groups. These results show that beta3-adrenoceptor agonists do not stimulate cyclic AMP accumulation in rat soleus muscle and that (-)-isoprenaline induced increases in cyclic AMP levels are mediated predominantly by beta2-adrenoceptors. This suggests that the previously reported increase in glucose uptake by beta3-adrenoceptor agonists in skeletal muscle does not involve direct stimulation of adenylate cyclase.

Inhibition of proximal tubular fluid absorption by nitric oxide and atrial natriuretic peptide in rat kidney

Atrial natriuretic factor (ANF) and nitric oxide (NO) stimulate production of guanosine 3',5'-cyclic monophosphate (cGMP) and are natriuretic. Split-drop micropuncture was performed on anesthetized rats to determine the effects of ANF and the NO donor sodium nitroprusside (SNP) on proximal tubular fluid absorption rate (Jva). Compared with control solutions, SNP (10(-4) M) decreased Jva by 23% when administered luminally and by 35% when added to the peritubular perfusate. Stimulation of fluid uptake by luminal angiotensin II (ANG II; 10(-9) M) was abolished by SNP (10(-4) and 10(-6) M). In proximal tubule suspensions, ANF (10(-6) M) increased cGMP concentration to 143%, whereas SNP (10(-6), 10(-5), 10(-4), 10(-3) M) raised cGMP to 231, 594, 687, and 880%, respectively. S-nitroso-N-acetylpenicillamine (SNAP) also raised cGMP concentrations with similar dose-response relations. These studies demonstrate inhibition by luminal and peritubular NO of basal and ANG II-stimulated proximal fluid absorption in vivo. The ability of SNP to inhibit basal fluid uptake whereas ANF only affected ANG II-stimulated transport may be because of production of higher concentrations of cGMP by SNP.

Regulation of cerebral microvascular permeability by histamine in the anaesthetized rat

1. The permeability response of slightly leaky pial venular capillaries to histamine was investigated using the single microvessel occlusion technique. 2. Histamine dose-response curves showed that concentrations between 5 nm and 5 microM increased permeability, while concentrations from 50 microM to 5 mM reduced it. 3. The H2 receptor antagonist cimetidine (2 microM) blocked the effects of lower concentrations of histamine, while the H1 receptor antagonist mepyramine (3 nM) blocked those of higher concentrations of histamine. 4. The effects of lower doses of histamine were mimicked by the H2 receptor agonist dimaprit, and the effects of higher doses of histamine were mimicked by the H1 receptor agonist alpha-2-(2-aminoethyl)pyridine (AEP). 5. Low concentrations of histamine, which normally increase the permeability of Lucifer Yellow (PLY), reduced it when co-applied with the phosphodiesterase 4 (PDE4) inhibitor rolipram. Rolipram also potentiated the response to AEP, but had no effect on that to dimaprit. 6. The effects of dimaprit were blocked by reducing extracellular Ca2+ from 2.5 mM to nominally Ca2+ free, or by applying the calcium entry blocker SKF 96365.

Adrenomedullin stimulates cAMP accumulation and inhibits atrial natriuretic peptide gene expression in cardiomyocytes

Adrenomedullin (ADM) is a novel vasodilating and natriuretic peptide which may play an important role in cardiovascular regulation. In neonatal cardiomyocyte cultures we have shown that ADM leads to dose-dependent inceases in cAMP accumulation and subsequent inhibition of atrial natriuretic peptide (ANP) gene expression and secretion. Forskolin-mediated elevation of intracellular cAMP levels led to a qualitatively similar inhibitory effect on both ANP gene expression and secretion. These data show that ADM has direct effects on expression of ANP in the cardiomyocyte by a mechanism that may involve the activation of adenylate cyclase, lending further support to the hypothesis that ADM may act in vivo as an important endocrine or paracrine modulator of cardiovascular function.

Potentiation of histamine-induced catecholamine secretion by ouabain in cultured bovine adrenal chromaffin cells is dependent on calcium and sodium influx

The effects of histamine on catecholamine secretion from cultured bovine adrenal chromaffin cells were studied in the presence of ouabain, an inhibitor of Na+-K+ ATPase. The purpose of this study was to determine whether Na+, as well as Ca2+, was involved in histamine receptor-mediated catecholamine secretion. Histamine (10(-8)-10(-5) M)-induced catecholamine secretion was markedly potentiated by addition of ouabain (10(-5) M) and was inhibited by a histamine-H1 receptor antagonist or incubation in a Ca2+-free medium. Histamine-induced 45Ca2+ influx was also potentiated by addition of ouabain. Ouabain alone or in the presence of histamine increased 22Na+ influx into the cells. In an additional set of experiments, cells were preincubated in the presence or absence of Na+ for 30 min (+/- histamine and ouabain), washed and then catecholamine secretion was measured following exposure to 2.2 mM Ca2+ for 15 min. Preincubation with histamine alone with or without Na+ had no effect of Ca2+-induced secretion of catecholamine. Preincubation with ouabain alone or with ouabain plus histamine produced a slight stimulation of catecholamine secretion in Na+-free medium and a large stimulation in Na+-containing medium. These results suggested that stimulation of the histamine-H1 receptor and inhibition of the Na+ pump both increase intracellular Na+ levels, resulting in increases in Ca2+ influx and catecholamine secretion.

Inhibition of nicotinic responses of bovine adrenal chromaffin cells by the protein kinase C inhibitor, Ro 31-8220

1. The effects of the protein kinase C inhibitor, Ro 31-8220, on the responses of cultured bovine adrenal chromaffin cells to nicotine, phorbol 12, 13-dibutyrate (PDBu) and K+ have been investigated. 2. Tyrosine hydroxylase activity was measured in situ in intact cells by measuring 14CO2 evolved following the hydroxylation and rapid decarboxylation of [14C]-tyrosine offered to the cells. Secretion of endogenous adrenaline and noradrenaline was measured by use of h.p.l.c. with electrochemical detection. Cyclic AMP levels were measured in cell extracts by RIA. 3. Ro 31-8220 produced a concentration-dependent inhibition of 300 nM PDBu-stimulated tyrosine hydroxylase activity with an IC50 of < 2 microM and complete inhibition at 10 microM. It had no effect on the responses to forskolin. 4. Ro 31-8220 produced a concentration-dependent inhibition of 5 microM nicotine-stimulated tyrosine hydroxylase activity, adrenaline and noradrenaline secretion and cellular cyclic AMP levels, with an IC50 of about 3 microM and complete inhibition by 10 microM. At concentrations up to 10 microM, Ro 31-8220 had little or no effect on the corresponding responses to 50 mm K+. 5. A structural analogue of Ro 31-8220, bisindolylmaleimide V, that lacks activity as a protein kinase C inhibitor, had no effect up to 10 microM on PDBu-stimulated tyrosine hydroxylase activity or on nicotine-stimulated cyclic AMP levels or noradrenaline secretion and only marginal inhibitory effects on nicotine-stimulated tyrosine hydroxylase activity and adrenaline secretion. 6. A structurally related protein kinase C inhibitor, bisindolylmaleimide I, inhibited PDBu-stimulated tyrosine hydroxylase activity with an IC50 of < 1 microM and complete inhibition by 3 microM, but had essentially no effect on nicotine stimulated tyrosine hydroxylase activity or catecholamine secretion. 7. The results suggest that Ro 31-8220 is not only a protein kinase C inhibitor but is also a potent inhibitor of nicotinic receptor responses in adrenal chromaffin cells by a mechanism unrelated to protein kinase C inhibition. The results are consistent with Ro 31-8220 being a nicotinic receptor antagonist.

Further investigation into the signal transduction mechanism of the 5-HT4-like receptor in the circular smooth muscle of human colon

1. The nature of the receptor coupling mechanism of the 5-hydroxytryptamine4 (5-HT4) receptor in the circular smooth muscle of the human colon has been further investigated. 2. 5-HT stimulated cyclic AMP generation and caused a relaxation in a concentration-dependent fashion, with EC50 values of 175.5 and 274.9 nM respectively. DAU 6236 increased cyclic AMP formation and caused a relaxant effect but was a partial agonist relative to 5-HT. 3. The 5-HT4 receptor antagonist, GR 113808, inhibited cyclic AMP formation and relaxation induced by 5-HT with -log Ki values of 9.1 (cyclic AMP) and 8.9 (relaxation) and apparent pA2 values of 9.2 (cyclic AMP) and 9.5 (relaxation). 4. Ondansetron and methysergide failed to inhibit cyclic AMP formation or the relaxation induced by 5-HT. 5. The phosphodiesterase inhibitor, IBMX, produced a concentration-dependent relaxation (EC50 = 30 microM) and at 1 microM it enhanced the 5-HT-induced relaxation producing a leftward shift of the 5-HT concentration-effect curve with a concentration-ratio of 4.1. Rolipram caused a concentration-dependent relaxation (EC50 = 564.8 nM) and at 200 nm caused a leftward shift of the concentration-effect curve to 5-HT with a concentration-ratio of 5.5. 6. Application of the adenylyl cyclase inhibitor, SQ 22536 (0.1 mM), and the protein kinase inhibitors, H7 (100 nM) and H89 (200 nM), inhibited the relaxant effect of 5-HT inducing a rightward shift of the concentration-effect curve with concentration-ratios of 10.1, 2.7 and 4.2 respectively. 7. Forskolin stimulated cyclic AMP production and caused a relaxation. The maximum relaxant effect of forskolin (6 microM, 13.8 +/- 1.9 cm.s) was not significantly different from the maximum relaxant effect of 5-HT (10 microM, 12.7 +/- 4.9 cm.s). However, the cyclic AMP levels stimulated by forskolin (6 microM, 49.3 +/- 6.6 pmol mg-1) were markedly greater than those stimulated by 5-HT (10 microM, 7.6 +/- 2.0 pmol mg-1). 8. In conclusion, these results indicate that the 5-HT4 receptors of the circular smooth muscle of human colon mediate relaxation and inhibition of spontaneous contractions via activation of adenylyl cyclase, formation of cyclic AMP and activation of protein kinase A.

Ascorbic acid augments the adenylyl cyclase-cAMP system mediated POMC mRNA expression and beta-endorphin secretion from hypothalamic neurons in culture

Besides acting as an important cofactor in the biosynthesis of catecholamine, ascorbic acid (AA) also modulates the activity of peptidylglycine-alpha-amidating monooxygenase for the post-translational modification of neuropeptides such as alpha-MSH and TRH. We report here a novel action of AA in modulating the secretion of immunoreactive beta-endorphin (ir-beta EP) and mRNA expression of proopiomelanocortin (POMC) following the activation of cAMP-dependent protein kinase A pathway in rat hypothalamic neurons. Primary cultures of hypothalamic neurons from neonatal rats as previously described were employed in the present studies. Six days after plating, cultures were replenished with serum-free media and incubated with vehicle or various doses of AA in the presence or absence of forskolin, 3-isobutyl-1-methylxanthine (IBMX), N6,2'-O-dibutyryladenosine 3'5'-(cyclic)monophosphate [(Bu)2cAMP]. Whereas the basal ir-beta EP release was 22.0 +/- 0.4 pg/well (mean +/- S.E.; n = 3), 10 microM of forskolin treatment increased ir-beta EP release approximately 4.2-fold. Co-incubation with AA enhanced forskolin induced ir-beta EP release and that this enhancing effect of AA was both time related and dose-dependent, with an ED50 of approximately 10 microM and an Emax of 100 microM. At the concentration of 10 microM, AA augmented ir-beta EP release approximately 6.1-fold that of cultures treated with forskolin alone. A similar potentiating effect of AA was also seen in cultures co-treated with IBMX or with (Bu)2cAMP. These enhancing effects of AA were similarly found in the abundance of total cAMP and of POMC mRNA of cultures which received identical treatments. However, it is important to point out that AA alone did not modulate ir-beta EP release or the abundance of POMC mRNA or total cAMP levels of the hypothalamic cultures when protein kinase A pathway was not activated. We thus conclude that AA augments cAMP-dependent protein kinase A pathway-induced production and release of beta EP from rat hypothalamic neurons in culture. Furthermore, this biological effect of AA is, at least in part, mediated through enhancing the responsiveness of the adenylyl cyclase-cAMP system.

Stimulation of cyclic AMP formation in the circular smooth muscle of human colon by activation of 5-HT4-like receptors

5-HT stimulated cyclic AMP generation in human colonic circular smooth muscle in a concentration-dependent fashion (EC50 = 229.1 nM). DAU 6236 also increased cyclic AMP formation and was a partial agonist relative to 5-HT. GR 113808 inhibited the cyclic AMP formation induced by 5-HT with a -log Ki value of 9.1 and an apparent pA2 value of 9.2. Ondansetron and methysergide failed to inhibit cyclic AMP formation induced by 5-HT. These results indicate that the 5-HT4 receptors of human colonic circular muscle mediate relaxation and inhibition of spontaneous contractions via formation of cyclic AMP.

Increases in cyclic AMP levels couple to H1 receptors in atria from autoimmune myocarditis mice

We have previously shown that myocardium from experimental autoimmune myocarditis expresses H1 receptors not present in normal mice heart. ThEA acting via H1 receptors, augments cyclic AMP production in atria from autoimmune myocarditis mice without any effect on atria from control mice. Addition of mepyramine before ThEA caused cyclic AMP levels to fall to a level similar to basal, confirming the H1 receptor participation. Histamine at low concentrations mimicked the ThEA action on H1 receptor-stimulation of cyclic AMP production by autoimmune myocardium. The fact that the inhibition of phospholipase C blocked the cyclic AMP stimulation by ThEA, supports the assumption that this action is secondary to receptor-mediated hydrolysis of phosphoinositides, generating some oxidative metabolites (IP3-DAG), which in turn may be responsible for the cyclic AMP effect. So, the inhibition of protein kinase C and calcium/calmodulin partially prevented the stimulatory action of ThEA on cyclic AMP levels in autoimmune myocardium, suggesting that both pathways are implicated in this effect. Data shows that the stimulation of H1 receptors by specific agonist in atria from autoimmune myocarditis mice, augments the cyclic AMP, requiring the hydrolysis of phosphoinositide cycle. The role of this cyclic AMP augmentation in myocardium from autoimmune myocarditis mice, will provide a basis to assess the role of this second messenger as an important factor in the regulation and/or modulation of the physiological behaviour of the heart in the course of autoimmune myocarditis.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Protein kinase A and nicotinic activation of bovine adrenal tyrosine hydroxylase

1. Stimulation of nicotinic cholinoceptors on bovine chromaffin cells increases phosphorylation of three serine residues in tyrosine hydroxylase (TOH) and activates TOH. One of the serines is a target for protein kinase A phosphorylation, and phosphorylation of this serine is adequate alone to cause TOH activation. The role of protein kinase A in nicotinic activation of TOH was therefore investigated. 2. TOH activity was studied in situ in intact, cultured, bovine adrenal chromaffin cells, by measuring 14CO2 evolved following the hydroxylation and rapid decarboxylation of [14C]-tyrosine offered to the cells. 3. Nicotine (5 microM), forskolin (1 microM) and 8-bromo-cyclic AMP (8-Br-cyclic AMP, 1 mM) each increased TOH activity by up to 200% over 10 min. The effect of nicotine was completely abolished by removal of extracellular Ca2+. 4. TOH activation by all three drugs was blocked by H89 (3-20 microM), which inhibits protein kinase A by competing for the ATP binding site on the kinase. Adenosine 3':5'-cyclic monophosphorothioate Rp-diastereomer (Rp-cAMPS) (1 mM), an inhibitor of protein kinase A that competes with cyclic AMP for the regulatory subunit of the kinase, abolished the activation of TOH by nicotine, and reduced that by forskolin and 8-Br-cyclic AMP. Both H89 and Rp-cAMPS inhibited basal TOH activity by 50-80%. 5. A structural analogue of H89, H85 (3-20 microM), which lacks activity as a protein kinase A inhibitor, did not inhibit either the activation of TOH by nicotine (5 microM) or basal TOH activity. Neither sodium nitroprusside (0.3-1O microM) nor 8-Br-cyclic GMP (1 mM) increased TOH activity.6. In digitonin-permeabilized chromaffin cells, forskolin (3 microM), cyclic AMP (10 microM) and Ca2+ (approx.2 micro M free Ca2+) each increased TOH activity. The response to all three drugs was blocked by H89(10 microM), which also reduced basal TOH activity in the permeabilized cells.7. Maximal activation of TOH by forskolin was achieved with 10 micro M forskolin. This concentration was less than the EC50 for forskolin-induced cyclic AMP accumulation in these cells. The activations of TOH by forskolin (1O microM) and nicotine (5 microM) were additive.8. The results indicate that both basal TOH activity and nicotinic activation of TOH in bovine chromaffin cells require protein kinase A activity. However, it is unlikely that nicotinic activation of TOH is directly mediated by an activation of protein kinase A in response to elevated cyclic AMP levels.It is possible that protein kinase A plays a permissive role in allowing nicotinic cholinoceptors to activate TOH by another signalling pathway.

Histochemical, pharmacological, biochemical and chromatographic evidence that pituitary adenylyl cyclase activating peptide is involved in inhibitory neurotransmission in the taenia of the guinea-pig caecum

The possibility that pituitary adenylyl cyclase-activating peptide (PACAP) is an inhibitory neurotransmitter has been investigated in the taenia of the guinea-pig caecum. The action of PACAP on muscle contractility and its ability to alter levels of adenosine-3':5'-cyclic monophosphate (cyclic AMP) and guanosine-3':5'-cyclic monophosphate (cyclic GMP) were investigated. PACAP-1-27 was an effective agonist, giving relaxations comparable in magnitude to isoproterenol; its EC50 was 3.4 x 10(-7) M. PACAP (10(-6) M) caused an almost two-fold increase in cyclic AMP levels; but the level of cyclic GMP was not affected. The relaxation caused by PACAP was slow in onset, with a latency of 5.8 +/- 0.8 s and reached a maximum at 9.1 +/- 1.1 s after onset. The relaxation was significantly reduced by apamin (10(-6) M) and suramin (10(-4) M) but was not reduced by tetrodotoxin (10(-7) M). Relaxation of the taenia coli caused by electrical stimulation of the inhibitory nerves was greatly reduced by apamin but only slightly reduced by suramin. PACAP-like immunoreactivity (-IR) was localised immunohistochemically in varicose nerve fibres within the taenia coli and in the underlying myenteric plexus and circular muscle. Approx. 50% of vasoactive intestinal peptide (VIP)-IR nerve fibres in the taenia also had immunoreactivity for PACAP; conversely, almost all PACAP-IR fibres were immunoreactive for VIP. PACAP-IR and substance P (SP)-IR were generally in separate fibres; only about 5% of SP-IR fibres were PACAP-IR. Radioimmunoassay revealed tissue concentrations of PACAP-1-27 and PACAP-1-38 of 1.0 +/- 0.1 and 2.1 +/- 0.3 (SEM) pmol/g wet weight of tissue, respectively. Material with PACAP-1-27 immunoreactivity co-eluted with authentic PACAP-1-27 on gel filtration chromatography, and PACAP-1-38 immunoreactivity also co-eluted with the authentic peptide. This study provides structural, chemical and pharmacological evidence that PACAP could be involved in inhibitory neurotransmission to the taenia coli of the guinea-pig caecum.

Mechanism of histamine-induced calcium efflux from cultured bovine adrenal chromaffin cells: possible involvement of an Na+/Ca2+ exchange mechanism

The effect of stimulation of the histamine receptor on Ca2+ mobilization in cultured bovine adrenal chromaffin cells was examined. Histamine (10(-5) M) increased the intracellular free Ca2+ ([Ca2+]i) to a peak in the presence or absence of extracellular Ca2+, followed by decrease with time. Histamine (10(-8)-10(-5) M) also stimulated 45Ca2+ efflux from cultured bovine adrenal chromaffin cells in a concentration dependent manner. Its stimulatory effect on 45Ca2+ efflux was inhibited by the specific histamine H1 receptor antagonist mepyramine. The increase in histamine-stimulated 45Ca2+ efflux was inhibited by deprivation of extracellular Na+ and by the Na+/Ca2+ exchange inhibitor amiloride. In addition, histamine stimulated 22Na+ influx into the cells, and this action was inhibited by amiloride. These results suggest that stimulation of the histamine H1 receptor regulates Na+/Ca2+ exchange in cultured bovine adrenal chromaffin cells.

H3 receptor-mediated inhibition of noradrenaline release: an investigation into the involvement of Ca2+ and K+ ions, G protein and adenylate cyclase

The present study was aimed at the identification of mechanisms following the activation of histamine H3 receptors. Mouse brain cortex slices preincubated with 3H-noradrenaline were superfused and the (H3 receptor-mediated) effect of histamine on the electrically evoked tritium overflow was studied under a variety of conditions. The extent of inhibition produced by histamine was inversely related to the frequency of stimulation used to evoke tritium overflow and to the Ca2+ concentration in the superfusion medium. An activator (levcromakalim) and blocker (glibenclamide) of ATP-dependent K+ channels did not affect the electrically evoked tritium overflow and its inhibition by histamine. A blocker of voltage-sensitive K+ channels, tetraethylammonium (TEA), increased the evoked overflow and attenuated the inhibitory effect of histamine. TEA also reduced the inhibitory effect of noradrenaline and prostaglandin E2 on the evoked overflow. When the facilitatory effect of TEA on the evoked overflow was compensated for by reducing the Ca2+ concentration in the superfusion medium, TEA did no longer attenuate the effect of histamine. Exposure of the slices to the SH group-alkylating agent N-ethylmaleimide increased the evoked overflow and attenuated the inhibitory effect of histamine; both effects were counteracted by the SH group-protecting agent dithiothreitol, which, by itself, did not affect the evoked overflow and its inhibition by histamine. Mouse brain cortex membranes were used to study the effect of the H3 receptor agonist R-(-)-alpha-methylhistamine on the basal cAMP accumulation and on the accumulation stimulated by forskolin or noradrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoids potentiate the adenylyl cyclase-cAMP system mediated immunoreactive beta-endorphin production and secretion from hypothalamic neurons in culture

Beta-endorphin(beta EP)1-31, a potent opioid peptide of proopiomelanocortin (POMC) derivatives, is produced and released from neurons at arcuate nuclei of the rat hypothalamus. Although dexamethasone (DM) suppresses the production and secretion of POMC related peptides from rat pituitary corticotrophs, the effect of glucocorticoids on the function of hypothalamic beta EP neurons remains unclear. Employing long term monolayer cultures of neonatal rat hypothalamic cells, we report here that 4 day treatment with 10 microM of forskolin increased ir-beta EP levels in cell content and culture media by approximately 1.7 (P < 0.05) and 4.1 times (P < 0.01) above vehicle treated control cultures (mean +/- S.E.M., 47.3 +/- 2.6 pg/well and 40.4 +/- 3.0 pg/well; n = 3) respectively. Although 4 day treatment with DM alone had little effect on the release and the cell content of ir-beta EP, it significantly enhanced forskolin-induced elevation of ir-beta EP levels in cell content and in culture media. The effect of DM was dose-related and time-dependent, with an EC50 of about 1 nM; at this concentration DM enhanced ir-beta EP secretion about 2.1 times (P < 0.01) above that induced by 10 microM of forskolin alone. Furthermore, the potentiating effect of DM was specifically suppressed by 100 nM of RU38486 (P < 0.01), a glucocorticoid receptor antagonist, but not by an equivalent dose of RU28318, a mineralocorticoid receptor antagonist. In addition, Northern blot analysis showed that forskolin (10 microM) increased the abundance of POMC mRNA 1.4 fold above that of vehicle treated control cultures. Whereas by itself, DM (10 nM) had little effect on the level of POMC mRNA, it enhanced forskolin-stimulated increase of the abundance of POMC mRNA approximately 2.6 times. Moreover, DM also augmented 1.6 times (P < 0.05) forskolin-induced but not 3-isobutyl-1-methylxanthine (IBMX)-induced increase of cAMP production (5.5 +/- 0.4 pmol/well; mean +/- S.E.M., n = 3) in the cultures. Taken together, our findings suggest that in contrast to the inhibitory effect on pituitary corticotrophs, glucocorticoids enhance the production and secretion of beta EP from rat hypothalamic neurons by facilitating the stimulatory effect mediated, in part, through the adenylyl cyclase-cAMP system.

Modulation of neurotransmitter release via histamine H3 heteroreceptors

Presynaptic H3 receptors occur on histaminergic neurones of the CNS (autoreceptors) and on non-histaminergic neurones of the central and autonomic nervous system (heteroreceptors). H3 heteroreceptors, most probably located on the postganglionic sympathetic nerve fibres innervating the resistance vessels and the heart, have been identified in the model of the pithed rat. Furthermore, we could show in superfusion experiments that H3 heteroreceptors also occur on the sympathetic neurones supplying the human saphenous vein and the vasculature of the pig retina and on the serotoninergic, dopaminergic and noradrenergic neurones in the brain of various mammalian species, including man. The effects of three recently described H3 receptor ligands were studied in superfused mouse brain cortex slices. The potency of the novel H3 receptor agonist imetit exceeded that of R-(-)-alpha-methylhistamine (the reference H3 receptor agonist) by one log unit and that of histamine by almost two log units. Clobenpropit was shown to be a competitive H3 receptor antagonist, exhibiting a pA2 as high as 9.6 (exceeding the pA2 of the reference H3 receptor antagonist thioperamide by one log unit). The irreversible antagonism of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was also studied. Interactions of the H3 heteroreceptor with the dopamine autoreceptor in mouse striatal slices and the alpha 2-autoreceptor in mouse brain cortex slices could be demonstrated. Activation of alpha 2-autoreceptors decreases the H3 receptor-mediated effect. Blockade of alpha 2-autoreceptors increases the H3 receptor-mediated effect only if the alpha 2-autoreceptors are simultaneously activated by endogenous noradrenaline. The H3 receptor-mediated inhibition of noradrenaline release in mouse brain cortex slices was attenuated by the K+ channel blocker tetraethylammonium but this attenuation was abolished by reduction of the Ca2+ concentration in the medium (to compensate for the facilitatory effect of tetraethylammonium on noradrenaline release). Accordingly, we assume that the H3 receptors are not coupled to voltage-sensitive K+ channels. Pertussis toxin and N-ethylmaleimide attenuated the H3 receptor-mediated effect in the mouse brain cortex, suggesting that the H3 receptors are coupled to a G protein (eg Gi or Go). However, negative coupling to an adenylate cyclase does not appear to exist since an H3 receptor-mediated inhibition of cAMP accumulation was not obtained in mouse brain cortex membranes. H3 receptor ligands are currently undergoing clinical testing and might become new remedies for the treatment of disease of the gastrointestinal and bronchial system and the CNS.

Histamine H1 receptor activation mediates the preferential release of adrenaline in the rat adrenal gland

Histamine elicited the release of catecholamines from "in vitro" perfused rat adrenals with an EC50 of 3 microM. This concentration was in the same range as those which caused a fall in the arterial blood pressure when infused intravenously in anaesthetized rats. Histamine stimulation was potently blocked by dexclorfeniramine (IC50 = 300 pM), but unaffected by ranitidine, suggesting the involvement of H1 receptors. Histamine release preferentially adrenaline. Mast cells were not detected within adrenal medulla by histochemical techniques. Compound 48/80 did not trigger catecholamine release. Catecholamine secretion evoked by splanchnic nerves stimulation was not modified by a combination of H1 and H2 antagonists. In conclusion, the histamine that elicited adrenaline release from rat adrenals comes from blood circulation not from local mast cells or splanchnic nerves. These effects are mediated through the activation of H1 receptors.

Secretogranin II: regulation of synthesis and post-translational proteolysis in bovine adrenal chromaffin cells

Secretogranin II (SgII), also called chromogranin C, is an acidic tyrosine-sulfated secretory protein found in secretory granules in a wide variety of endocrine cells and neurones. Although less abundant than chromogranin A (CGA) and chromogranin B (CGB), SgII is found in adrenal medullary chromaffin granules. In the present study we investigated the regulation of SgII biosynthesis in bovine chromaffin cells maintained in primary culture. Cellular proteins were labelled with [35S]methionine and the heat stable chromogranin enriched fraction was isolated. Following electrophoretic separation, the 86 kDa SgII band was identified by sequence analysis using the Edman degradation procedure. The radioactivity incorporated in the 86 kDa SgII band was used as an index of the SgII synthesis rate. We found that stimulation of chromaffin cells with nicotine and histamine and to a smaller extent with angiotensin II and bradykinin significantly enhanced the rate of SgII synthesis. In contrast direct depolarization with K+ may not be sufficient to induce modifications in SgII synthesis suggesting that the raise of cytosolic calcium evoked by high K+ may not be sufficient to induce modifications in SgII synthesis . The possible second messenger pathways involved in the control of SgII biosynthesis were investigated by using protein kinase C and adenylate cyclase activators. We observed that 12-O-tetradecanoylphorbol 13-acetate (TPA) and forskolin increased the basal rate of SgII synthesis. Incubation with both TPA and forskolin was required to obtain an effect comparable to that produced by nicotine or histamine suggesting that these secretagogues recruit both protein kinase C- and cyclic AMP-dependent mechanisms to stimulate SgII synthesis.

Pertussis toxin enhances proenkephalin synthesis in bovine chromaffin cells

The synthesis of the neuropeptide precursor proenkephalin was measured in bovine adrenal chromaffin cells following radiolabeling with [35S]methionine. Treatment of chromaffin cells with pertussis toxin (100 ng/ml) approximately doubled proenkephalin synthesis without altering total protein synthesis. Pertussis toxin pretreatment also increased proenkephalin synthesis in chromaffin cells exposed to vasoactive intestinal peptide (VIP) and 3-isobutyl-1-methylxanthine (IBMX). Combinations of IBMX plus nicotine, VIP, or histamine also synergistically enhanced proenkephalin synthesis, with no further elevation when the cells were also pretreated with pertussis toxin. The action of forskolin, a direct activator of adenylate cyclase, on proenkephalin synthesis was similarly potentiated by pertussis toxin or IBMX, presumably reflecting the abilities of both the toxin and this phosphodiesterase inhibitor to enhance the cyclic AMP response to forskolin. In contrast, increased synthesis of proenkephalin in response to phorbol esters was not affected by pertussis toxin treatment. These results suggest that pertussis toxin potentiates proenkephalin synthesis primarily through inactivation of guanine nucleotide-binding proteins that inhibit adenylate cyclase, although other signaling pathways may also be involved.

Salmeterol, a long-acting beta 2-adrenoceptor agonist mediating cyclic AMP accumulation in a neuronal cell line

1. The accumulation of cyclic AMP stimulated by salmeterol, a long-acting beta 2-adrenoceptor agonist and by isoprenaline, a non-selective beta-adrenoceptor agonist have been compared in the B50 neuroblastoma cell line. 2. Salmeterol produced a concentration-dependent increase in the accumulation of total [3H]-cyclic AMP in B50 cells yielding an EC50 value of 37 nM which was lower than that obtained with isoprenaline (294 nM). The maximum response to salmeterol was only 46% of that obtained with isoprenaline. 3. The beta 2-adrenoceptor antagonist, ICI 118551, inhibited the responses to both salmeterol (apparent KD 2.2 nM) and isoprenaline (apparent KD 1.6 nM). However, the beta 1-adrenoceptor antagonist, atenolol, produced no significant effect at concentrations up to 100 microM. 4. Salmeterol (1 microM) changed the concentration-response curve of isoprenaline in the manner of a partial agonist interacting with a full agonist. The KD of salmeterol obtained from the interaction was 55.6 nM. 5. Whereas salmeterol has a slow onset of action in airway smooth muscle compared to other beta 2-adrenoceptor agonists, in B50 monolayers both salmeterol and isoprenaline produced a rapid increase in cyclic AMP accumulation (t1/2 1.1 min and 0.4 min respectively). 6. Despite the existence of cyclic AMP efflux mechanisms that exist in this cell line it was possible to investigate the duration of agonist action by measuring intracellular levels of the second messenger. Replacement of drug-containing medium with fresh buffer led to a rapid reduction in intracellular levels of cyclic AMP in isoprenaline-stimulated cells whereas cyclic AMP accumulation was sustained for much longer periods in salmeterol-stimulated cells. However, the persistent action of salmeterol could be reversed by the addition of a beta2-selective antagonist.7. These results confirm that salmeterol has a high affinity, but low efficacy (relative to isoprenaline) for beta2-adrenoceptors coupled to cyclic AMP accumulation and that the drug persists at its site of action for long periods in the B50 neuronal cell line.

Ionic mechanisms involved in the secretory effects of histamine in the rat adrenal medulla

Histamine activation of H1 receptors elicited the release of adrenaline from in vitro perfused rat adrenal with an EC50 of 3 microM. Neither Na+ deprivation nor complete membrane depolarization abolished the histamine-mediated secretory response but it was partially dependent on extracellular Ca2+. Nitrendipine and BAY-K-8644 affected the release induced by histamine concentrations at over 3 microM. Delayed application of histamine pulses, after external Ca2+ removal, led to a decline in to a plateau at 50% of the initial release. Pretreatment with ionomycin abolished this Ca2+ deprivation-resistant component. These data suggest that secretion evoked by low concentrations of histamine occurs by mobilization of Ca2+ from internal stores whereas higher concentrations use Ca2+ from both intracellular and extracellular sources.

Regulation of cyclic AMP levels by phorbol esters in bovine adrenal medullary cells

Cyclic AMP responses to phorbol esters were studied in cultured bovine adrenal medullary cells. Phorbol esters that activate protein kinase C (PKC: phorbol 12,13-dibutyrate, phorbol 12,13-didecanoate) increased cellular cyclic AMP levels by up to 100% over 5 min, and this was maintained for up to 3 h. The effect was mimicked by 1,2-dioctanoyl-sn-glycerol but not by inactive phorbol esters. The effect of active phorbol esters was concentration dependent over the range 50-500 nM, and was abolished by the PKC inhibitor, Ro 31-8220 (10 microM). The response was enhanced by 3-isobutyl-1-methylxanthine (1 mM) and by forskolin (0.3 microM), was enhanced following pertussis toxin pretreatment (100 ng/ml, 7.5 h) and was unaffected by removing extracellular Ca2+. The phorbol ester cyclic AMP response was additive with that to K+ depolarisation, and synergised with those to prostaglandin E1 and dimaprit. The results indicate PKC activation increases cyclic AMP formation in bovine adrenal medullary cells, probably by a direct action on adenylate cyclase or Gs.

Regulation of cyclic AMP metabolism in bovine adrenal medullary cells

The capacity of cultured bovine adrenal medullary cells to metabolize and export cyclic AMP has been studied. Basal cellular cyclic AMP levels were increased 50% by 100 microM 3-isobutyl-1-methylxanthine (IBMX) and rolipram, a class IV (cyclic AMP-specific) phosphodiesterase (PDE) inhibitor. They were not affected by inhibition of class I (Ca2+/calmodulin-dependent), class III (cyclic GMP-inhibited) or class V PDE (cyclic GMP-specific) with vinpocetine or 3-isobutyl-8-methoxymethyl-1-methylxanthine (8-methoxymethyl-IBMX), SK&F 94120, or MB 22,948, respectively, all at 100 microM. Furthermore, only IBMX and rolipram enhanced the cyclic AMP response to 0.3 microM forskolin. Rolipram had an EC50 of < or = 1 microM and was equally effective at 100 microM and 1 mM. IBMX enhanced cyclic AMP levels significantly more at 1 mM than at 100 microM. Neither vinpocetine nor 8-methoxymethyl-IBMX (100 microM) enhanced the Ca(2+)-dependent cyclic AMP response to K+ depolarization. Elevation of cyclic GMP levels with sodium nitroprusside (10 or 100 microM), to activate any cyclic GMP-stimulated class II PDE and to inhibit any cyclic GMP-inhibited class III PDE, also had no effect on basal or forskolin-stimulated cyclic AMP levels. In the presence of IBMX (1 mM), forskolin (5 microM) caused a rapid and large increase in cellular cyclic AMP levels which was maximal after about 5 min and declined slightly over 3 hr. Over this period, extracellular cyclic AMP levels rose almost linearly reaching levels 2-3 times those in the cells. The results indicate bovine adrenal medullary cells have a high capacity for sustained cyclic AMP export. Furthermore, two PDE isozymes appear to degrade cyclic AMP in these cells, a rolipram-sensitive, cyclic AMP-specific, class IV isozyme and a rolipram-insensitive isoform.

Identification of adenylate cyclase-coupled histamine H2 receptors in guinea pig tracheal smooth muscle cells in culture and the effect of dexamethasone

Histamine acts on airway contractile elements through at least two different receptor subtypes: H1, which mediates Ca(2+)-dependent contraction, and H2, which stimulates cyclic adenosine monophosphate (cAMP) synthesis and possibly relaxation. The aim of this study was to determine the relative contribution of the different receptor subtypes to histamine-stimulated cAMP production by guinea pig tracheal smooth muscle (GPTSM) cells in primary culture. Histamine and N-alpha-methylhistamine induced concentration-dependent cAMP synthesis; these effects were entirely blocked by 10(-4) M cimetidine, an H2-receptor antagonist, whereas 10(-6) M thioperamide, a selective H3 blocker, was ineffective. The H3 agonist, R-(alpha)-methylhistamine, did not stimulate cAMP synthesis. Triprolidine, an H1 antagonist, did not modify histamine (10(-5) M)-stimulated cAMP synthesis. Histamine (10(-5) M) doubled [Ca2+]i in GPTSM. A 24-h pretreatment of GPTSM cells with 10(-6) M dexamethasone enhanced cAMP synthesized in response to 10(-5) M histamine and to 5 x 10(-6) M forskolin but did not significantly alter either the affinity or the binding capacity for [3H]-tiotidine, an H2-receptor antagonist. These results indicate that GPTSM cells in culture express H2 but not H3 receptors, which are linked to adenylate cyclase; their functional expression does not seem to be modulated by the concurrent activation of H1 receptors, whose presence in GPTSM is evidenced by a histamine-stimulated increase in [Ca2+]i. The most likely site of action of dexamethasone in enhancing histamine-stimulated cAMP synthesis is at the level of adenylate cyclase since the steroid had no effect on the H2 receptor itself.

Histamine stimulates exocytosis in a sub-population of bovine adrenal chromaffin cells

Both nicotinic stimulation and histamine are able to raise cytosolic free calcium concentration in the majority of cells in a population of bovine adrenal medullary chromaffin cells to comparable levels. Nevertheless, histamine induces much less catecholamine secretion than does nicotine. In order to test whether this is due to heterogeneity in the responses of chromaffin cells to histamine we examined exocytosis in response to nicotine and histamine using an immunofluorescence method based on staining with anti-DBH to detect inserted secretory vesicle membrane. The results show that while up to 98% of the chromaffin cells in culture undergo exocytosis in response to nicotine, histamine stimulates exocytosis in only a sub-population of cells.

Protein Phosphorylation in Bovine Adrenal Medullary Chromaffin Cells: Histamine-Stimulated Phosphorylation of Tyrosine Hydroxylase

Histamine can cause the release of catecholamines from bovine adrenal medullary chromaffin cells by a mechanism distinct from that of the depolarizing agents nicotine or high K+ buffer. It was the aim of this study to determine the protein phosphorylation responses to histamine in these cells and to compare them with those induced by depolarization. A number of proteins showed increases in phosphorylation in response to histamine especially when analyzed on two-dimensional polyacrylamide gel electrophoresis or by phosphopeptide mapping; one protein of 20,000 daltons was markedly dephosphorylated. Emphasis was given to the effects of histamine on tyrosine hydroxylase (TOH) phosphorylation, because this protein showed the most prominent changes on one-dimensional gels. Histamine acted via H1 receptors to increase TOH phosphorylation; the response was blocked by the H1 antagonist mepyramine and could be mimicked by the H1 agonist thiazolylethylamine, but not by the H2 agonist dimaprit. The H3 agonist (R) alpha-methylhistamine increased TOH phosphorylation at high concentrations, but the response was blocked entirely by mepyramine. Histamine rapidly increased the phosphorylation of TOH, with a maximum reached within 5 s and maintained for at least 30 min. This was in marked contrast to nicotine-stimulated protein phosphorylation of TOH, which was rapidly desensitized. The initial phosphorylation response to histamine was independent of extracellular Ca2+ for at least 3 min, but the sustained response required extracellular Ca2+. This was in contrast to the situation with both nicotine and high K+ buffer, which under the conditions used here caused a response which was dependent on extracellular Ca2+ at all times investigated. In the presence of histamine, the phosphopeptide profiles for TOH were essentially the same with or without Ca2+, suggesting that the same protein kinases were involved, but at longer times there was evidence of new phosphorylation sites. The mechanism or mechanisms whereby histamine modulates TOH phosphorylation are discussed with emphasis on the differences from depolarizing agents.

Cholinoceptor regulation of cyclic AMP levels in bovine adrenal medullary cells

1. The regulation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels by cholinoceptors has been studied in cultured bovine adrenal medullary cells. 2. Acetylcholine (100 microM), nicotine (10 microM) and dimethylphenylpiperazinium (20 microM) each increased cellular cyclic AMP levels 2 to 4 fold over 5 min in the absence of phosphodiesterase inhibitors. The muscarinic agonist acetyl-beta-methylcholine (100 microM) had no effect either on its own or on the response to nicotine. The responses to acetylcholine and nicotine were unaffected by atropine (1 microM) but were abolished by mecamylamine (5 microM). 3. Cellular cyclic AMP increased transiently during continuous exposure to nicotine (1-20 microM), with the largest response seen after 5 min, a smaller response after 20 min, and no change in cyclic AMP levels seen after 90 or 180 min. The maximal response after 5 min stimulation was seen with 5-10 microM nicotine and the EC50 was about 2 microM. In contrast, extracellular cyclic AMP levels did not change after 5 or 20 min stimulation with nicotine, but increased slightly after 90 min and further after 180 min. 4. The cellular cyclic AMP response to nicotine (10 microM) was unchanged or weakly enhanced in the presence of the unselective phosphodiesterase inhibitor, isobutylmethylxanthine, and was unchanged in the presence of rolipram. Nicotine did not interact synergistically with low concentrations of forskolin. The response was however completely abolished in the absence of extracellular Ca2+.