Histamine-induced inositol phospholipid breakdown in the longitudinal smooth muscle of guinea-pig ileum

Food Intolerance: The Role of Histamine

Histamine is a natural amine derived from L-histidine. Although it seems that our knowledge about this molecule is wide and diverse, the importance of histamine in many regulatory processes is still enigmatic. The interplay between different types of histamine receptors and the compound may cause ample effects, including histamine intoxication and so-called histamine intolerance or non-allergic food intolerance, leading to disturbances in immune regulation, manifestation of gastroenterological symptoms, and neurological diseases. Most cases of clinical manifestations of histamine intolerance are non-specific due to tissue-specific distribution of different histamine receptors and the lack of reproducible and reliable diagnostic markers. The diagnosis of histamine intolerance is fraught with difficulties, in addition to challenges related to the selection of a proper treatment strategy, the regular course of recovery, and reduced amelioration of chronic symptoms due to inappropriate treatment prescription. Here, we reviewed a history of histamine uptake starting from the current knowledge about its degradation and the prevalence of histamine precursors in daily food, and continuing with the receptor interactions after entering and the impacts on the immune, central nervous, and gastrointestinal systems. The purpose of this review is to build an extraordinarily specific method of histamine cycle assessment in regard to non-allergic intolerance and its possible dire consequences that can be suffered.

Increasing the throughput of label-free cell assays to study the activation of G-protein-coupled receptors by using a serial agonist exposure protocol

Label-free, holistic assays, monitoring, for example, the impedance of cells on electrodes, are gaining increasing popularity in the evaluation of G-protein-coupled receptor (GPCR) ligands. It is the strength of these approaches to provide the integrated cellular response non-invasively, highly automated and with a device-dependent time resolution down to several milliseconds. With an increasing number of samples to be studied in parallel, the available time resolution is, however, reduced and the cost for the disposable sensor arrays may become limiting. Inspired by protocols from organ pharmacology, we investigated a simple serial agonist addition assay that circumvents these limitations in impedance-based cellular assays. Using a serial addition of increasing concentrations of a GPCR agonist while continuously monitoring the sample's impedance, we were able to establish a full concentration-response curve for the endogenous agonist histamine on a single layer of U-373 MG cells endogenously expressing the histamine 1 receptor (H1R). This approach is validated with respect to conventional, parallel agonist addition protocols and studies using H1R antagonists such as mepyramine. Applicability of the serial agonist addition assay was shown for other GPCRs known for their signaling via one of the canonical G-protein pathways, Gq, Gi/0 or Gs as well. The serial agonist addition protocol has the potential to further strengthen the output of label-free analysis of GPCR activation.

Augmentation of antigen receptor-mediated responses by histamine H1 receptor signaling

Histamine is considered one of the important mediators of immediate hypersensitivity and inflammation, and acts via G protein-coupled receptors. Here, we report that histamine may affect antigen receptor-mediated immune responses of T and B cells via a signal(s) from histamine H1 receptors (H1Rs). Histamine exhibited enhancing effects on the in vitro proliferative responses of anti-CD3epsilon- or anti-IgM-stimulated spleen T and B cells, respectively, at the culture condition that the fetal calf serum was dialyzed before culture and c-kit-positive cells were depleted from the spleen cells. In studies of histamine H1R knockout mice, H1R-deficient T cells had low proliferative responses to anti-CD3epsilon cross-linking or antigen stimulation in vitro. B cells from H1R-deficient mice were also affected, demonstrating low proliferative responses to B cell receptor cross-linking. Antibody production against trinitrophenyl-Ficoll was reduced in H1R-deficient mice. Other aspects of T and B cell function were normal in the H1R knockout mice. H1R-deficient T and B cells showed normal responses upon stimulation with interleukin (IL)-2, IL-4, CD40 ligand, CD40 ligand plus IL-4, and lipopolysaccharide. Collectively, these results imply that the signal generated by histamine through H1R augments antigen receptor-mediated immune responses, suggesting cross-talk between G protein-coupled receptors and antigen receptor-mediated signaling.

The effect of L-type calcium channel modulators on the mobilization of intracellular calcium stores in guinea-pig intestinal smooth muscle

1. The action of Ca2+ channel modulators has been examined on the intracellular Ca2+ signal in the longitudinal smooth muscle cells of the guinea-pig intestine after exposure to histamine and to agents known to affect intracellular Ca2+ stores. Isometric contraction has been measured simultaneously with front-surface fluorometry of fura 2-loaded preparations. 2. Histamine (10 microM) evoked a phasic and tonic increase in [Ca2+]i and contraction which were both sensitive to the Ca2+ channel blockers, nimodipine and D600. 3. Caffeine (10 mM) evoked in rapid increase in [Ca2+]i which was sustained as long as the preparation was exposed to the drug, whereas the contractile response was only phasic. In the presence of nimodipine 1 microM, the phasic contraction was absent although the fura 2-Ca2+ signal amounted to 32% of the control. 4. Ryanodine (10 microM) evoked a slow increase in [Ca2+]i and a contraction, both of which were reversed after exposure to nimodipine (1 microM) or D600 (10 microM). In the presence of diazoxide (500 microM), a hyperpolarizing agent, the ryanodine-evoked increase in [Ca2+]i and in muscle tone were inhibited. 5. Thapsigargin (1 microM) also produced an increase in [Ca2+]i and a contraction both of which were blocked by nimodipine (1 microM). 6. In Ca2+-free solution, histamine 10 microM evoked non-reproducible phasic Ca2+ signal and contraction. This response was recovered after refilling in Ca2+ containing solution. The recovery was blocked by nimodipine, D600 or diazoxide and was facilitated by the Ca2+ channel activator, Bay K 8644. When the refilling medium was supplemented with thapsigargin, the recovered response was significantly reduced, but Bay K 8644 still had some action. 7. The present results show that blockage of L-type Ca2+ channels inhibited changes in [Ca2+]i evoked by histamine, caffeine and ryanodine which are generally attributed to Ca2+ mobilization from intracellular stores. They also show that when the tissue was exposed to nimodipine, D600 and diazoxide during the procedure of refilling after depletion of intracellular stores, the action of histamine on [Ca2+]i and contraction was blocked. Bay K 8644 had an opposite effect even when the Ca2+ pumping activity of the sarcoplasmic reticulum was reduced by thapsigargin. This indicates that refilling of intracellular Ca2+ stores depleted by histamine in guinea-pig intestine mainly occurred through L-type Ca2+ channels.

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.

Effects of semotiadil fumarate, a novel Ca2+ antagonist, on cytosolic Ca2+ level and force of contraction in porcine coronary arteries

1. The mechanisms of action of semotiadil fumarate, a novel Ca2+ antagonist, were examined by measuring the cytosolic Ca2+ level ([Ca2+]i) and force of contraction in porcine coronary arteries, and by determining [3H]-pyrilamine binding to bovine cerebellar membranes. 2. Semotiadil or verapamil (0.1 and 1 microM) inhibited both the high KCl-induced increases in [Ca2+]i and force in a concentration-dependent manner. 3. Histamine (30 microM) produced transient increases followed by sustained increases in [Ca2+]i and force, which were inhibited by semotiadil and verapamil (1 and 10 microM). The agents were different in that semotiadil reduced the maximum [Ca2+]i and force responses to histamine, but not pD2 values, whereas verapamil did reduce the pD2 values for histamine, but not the maximum responses. 4. Verapamil (10 microM), but not semotiadil, inhibited histamine-induced increases in [Ca2+]i and force in Ca(2+)-free solution. Neither semotiadil nor verapamil affected the increases in [Ca2+]i and force induced by caffeine. Semotiadil even at the higher concentration (10 microM) did not displace specific binding of [3H]-pyrilamine to bovine cerebellar membranes. 5. These results suggest that semotiadil inhibits both KCl- and histamine-induced contractions mainly by blocking voltage-dependent L-type Ca2+ channels.

Blockers for excitatory effects of achatin-I, a tetrapeptide having a D-phenylalanine residue, on a snail neurone

Some histamine H1 receptor antagonists suppressed the inward current (Iin) of an Achatina identifiable neurone type, PON (periodically oscillating neurone), caused by an Achatina endogenous tetrapeptide having a D-phenylalanine residue, achatin-I (Gly-D-Phe-Ala-Asp), under voltage clamp. Achatin-I was applied locally to the neurone by brief pneumatic pressure ejection and antagonists were administered by perfusion. The dose-response curves of the effective histamine H1 antagonists indicated their potency order to suppress the Iin as follows: chlorcyclizine, promethazine, triprolidine and homochlorcyclizine > trimeprazine and clemastine > diphenylpyraline. The potent drugs were mostly piperazine and phenothiazine types. The effects of chlorcyclizine, promethazine and triprolidine on the dose (the duration of the pressure ejection)-response curve of achatin-I indicated that these drugs affected the Iin caused by achatin-I in a non-competitive manner. The antagonists for the receptors of the small-molecule neurotransmitters other than histamine H1, such as histamine H2, acetylcholine, gamma-aminobutyric acid (GABA), L-glutamic acid, dopamine, alpha- and beta-adrenalin and 5-hydroxytryptamine, had no effect on the Iin caused by achatin-I.

Agonist-induced desensitization of histamine H1 receptor-mediated inositol phospholipid hydrolysis in human umbilical vein endothelial cells

1. The regulation of histamine-induced [3H]-inositol phosphate formation was studied in human cultured umbilical vein endothelial cells (HUVEC). 2. Histamine (EC50 4.8 microM) produced a 12.7 fold increase in [3H]-inositol phosphate formation over basal levels. Prior exposure to 0.1 mM histamine (2 h) produced a 78% reduction in the response to subsequent histamine (0.1 mM) challenge. The IC50 for this histamine-induced desensitization was 0.9 microM. 3. The inositol phosphate response to histamine (0.1 mM) was inhibited by phorbol dibutyrate (IC50 40 nM; maximal reduction 64%). This effect was antagonized by both staurosporine (100 nM) and Ro 31-8220 (10 microM). However, the histamine-induced desensitization of the H1-receptor-mediated inositol phosphate response was insensitive to the protein kinase inhibitors, staurosporine, Ro 31-8220, K252a and KN62. 4. Prior exposure to sodium nitroprusside (100 microM), forskolin (10 microM) or dibutyryl cyclic AMP (1 mM) had no effect upon histamine-induced [3H]-inositol phosphate formation. 5. NaF (20 mM) and thrombin (EC50 0.4 u ml-1) also induced inositol phosphate formation in HUVEC. Histamine pretreatment (0.1 mM, 10-120 min) failed to modify the inositol phosphate response to a subsequent NaF or thrombin challenge. 6. We conclude that the desensitization of histamine H1-receptor-mediated [3H]-inositol phosphate formation occurs at the level of the receptor and involves a mechanism independent of activation of protein kinase A, G, or C, or calcium calmodulin-dependent protein kinase II.

Contractile responses to histamine and GTP gamma S in beta-escin-treated skinned smooth muscle of guinea pig ileum

To characterize the calcium (Ca2+)-releasing effects of histamine and GTP gamma S, the drug-induced tension developments were measured in beta-escin-treated skinned longitudinal smooth muscle of guinea pig ileum. Intracellular Ca2+ stores were loaded with Ca2+ by incubating the muscle for 10 min in a Ca(2+)-containing solution. Histamine (10-100 microM), applied after Ca(2+)-loading, produced a transient rise in tension. The effect of histamine was not preserved after treatment with 20 mM caffeine, a Ca(2+)-store releaser. The effect of histamine was potentiated by GTP; inhibited by GDP beta S, an antagonist of GTP for binding to G-proteins; or heparin, an antagonist of inositol 1,4,5-trisphosphate (IP3) for binding to its receptor; and mimicked by IP3. When GTP gamma S (20 microM) was applied and continued to be present for 15 min, a transient rise in tension followed by a small, sustained rise in tension was elicited. The effect of GTP gamma S was completely inhibited by GDP beta S. The initial, transient component of the biphasic GTP gamma S response was abolished or markedly inhibited after treatment with caffeine, heparin or the calcium ionophore A23187. The present results suggest that histamine and GTP gamma S cause a release of Ca2+ from caffeine-sensitive stores which is mediated by IP3 formed through a G-protein-coupled mechanism. The GTP gamma S-induced Ca2+ release is not considered to involve such an IP3-independent process as described in chemically-skinned arterial muscle.

Modulation of K+ and Ca2+ channels by histamine H1-receptor stimulation in rabbit coronary artery cells

1. The modulation of whole-cell K+ and Ca2+ currents by stimulation of histamine H1-receptors in freshly isolated single smooth muscle cells from the rabbit coronary artery was characterized using the patch-clamp technique at 35 degrees C. Single-channel K+ currents were also analysed using the cell-attached patch configuration. 2. The histamine H1-receptor agonist, 2-(2-aminoethyl)pyridine (AEP) (0.1 mM), increased the amplitude of voltage-activated inward Ba2+ currents, recorded using the perforated-patch recording technique, which could be completely blocked by the dihydropyridine antagonist, nicardipine (1 microM). 3. Whole-cell outward K+ currents in rabbit coronary artery cells could be classified into at least two components: (a) a slowly inactivating, 4-aminopyridine (4-AP)-sensitive low-noise current, and (b) a non-inactivating, tetraethylammonium (TEA)-sensitive high-noise current. 4. AEP (0.1 mM) caused changes in whole-cell outward K+ currents which depended upon membrane voltage. Specifically: (a) AEP enhanced the amplitude of outward currents at voltages between -30 and 0 mV, and (b) AEP decreased the outward currents at more positive potentials. 5. The removal of extracellular Ca2+ caused little inhibition of the effects of AEP on K+ currents, whereas the depletion of intracellular Ca2+ stores by pretreatment with ryanodine and caffeine prevented the effects of AEP on K+ channels. Moreover, acute exposure to ryanodine (10 microM) or thapsigargin (1 microM), a Ca(2+)-ATPase inhibitor, caused voltage-dependent changes in the outward currents similar to those observed with AEP. These results suggest that the voltage-dependent effects of AEP on K+ currents are mainly mediated by release of Ca2+ from intracellular stores. 6. The dual stimulatory and inhibitory effect of AEP on whole-cell K+ currents was shown to be due to a differential effect on two distinct types of K+ channels. The stimulatory effect observed over the voltage range -30 to 0 mV was prevented by pretreatment of cells with low concentrations of TEA (1 mM), whereas the inhibitory effect observed at positive potentials was prevented by pretreatment of cells with 4-AP (3 mM). 7. Single-channel recordings revealed two types of unitary K+ currents with conductances of 225 and 70 pS in the cell-attached configuration with symmetrical K+ solutions (150 mM K+ in pipette-150 mM K+ in bath). Bath application of AEP (0.1 mM) caused a marked increase in the open probability of the large conductance channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of Ca2+ modulators on acetylcholine-induced phasic and tonic contractions and A23187-induced contractions in ileal longitudinal muscle and IP3 production

The influence of different sources of Ca2+ on the Emax and ED50 values of acetylcholine (ACh)-induced phasic and tonic contractions and on A23187-induced contractions was studied using different extracellular Ca2+ concentrations ([Ca2+]o) and the Ca2+ modulators TMB-8 and D600. IP3 production induced by both stimulants was also studied. The results are compatible with: (a) the mobilization of Ca2+ from an intracellular source as a primary event in the phasic response. (b) The primary involvement of a D600-sensitive inward Ca2+ current in the ACh-tonic response. (c) An inward D600-sensitive Ca2+ current associated with the ionophore transported ion. (d) The involvement of an IP3 independent, TMB-8 sensitive mechanism of Ca2+ mobilization involved in the A23187-induced responses.

GTP-binding protein involvement in membrane currents evoked by carbachol and histamine in guinea-pig ileal muscle

1. Single smooth muscle cells obtained by enzymic dispersion of the longitudinal muscle layer of guinea-pig ileum were used for recording membrane currents under whole-cell voltage clamp in response to carbachol (100 microM, unless otherwise stated) or histamine (100 microM) applied extracellularly. 2. At a holding potential of 0 mV, a transient outward current was evoked by carbachol and histamine. Responses to the two agonists were very similar in size and time course to the current response to caffeine (10 mM). The response to carbachol was virtually absent in the presence of histamine, and vice versa. Caffeine was without effect in the presence of either of these agonists. Inclusion of EGTA (10 or 20 mM) in the pipette abolished the responses to carbachol, histamine and caffeine. Thus, the outward current responses were considered to represent opening of Ca(2+)-activated K+ channels in response to a massive release of Ca2+ from the same stores by these three agents. 3. An inward current was evoked by carbachol and histamine, but not by caffeine at a holding potential of -40 mV, which was considered to represent opening of cationic channels. The carbachol-induced inward current was much longer in duration and larger in size than the histamine-induced inward current. 4. Inclusion of GDP beta S (2 mM) in the pipette abolished the inward and outward current responses to histamine, but inhibited only part of those to carbachol. 5. When the holding potential was held at 0 mV with inclusion of GTP gamma S (0.1-1 mM) in the pipette, spontaneous transient outward currents appeared immediately after break-through but disappeared a few minutes later. Under these conditions, caffeine (10 mM) was almost without effect, suggesting that GTP gamma S had released Ca2+ stores. When the holding potential was held at -40 mV and GTP gamma S (0.1 or 0.2 mM) was present in the pipette, an inward current developed a few minutes after break-through. During the GTP gamma S-induced inward current, application of carbachol or histamine produced no further inward current. However, when 0.01 mM-GTP gamma S was included in the pipette solution, carbachol- and histamine-induced inward currents were potentiated. 6. Pretreated with 2-5 micrograms/ml pertussis toxin (PTX) did not change noticeably the outward current responses to carbachol and histamine, but abolished or markedly reduced the inward current responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Subtypes and excitation-contraction coupling mechanisms for neurokinin receptors in smooth muscle of the guinea-pig Taenia caeci

This study investigated the subtype and coupling mechanisms mediating the direct contractile response to tachykinins in the guinea-pig Taenia caeci preparation in vitro. Coupling of neurokinin receptors was compared throughout with coupling of muscarinic receptors. The smooth muscle neurokinin receptors seem to be predominantly of the NK-1 subtype. Thus, the relative activities of the common naturally-occurring tachykinins fell within one order of magnitude, and the selective NK-1 receptor agonist substance P methyl ester was high in activity (0.38 relative to substance P). Some contribution from NK-3 receptors is, however, possible in view of the appreciable activity of the selective NK-3 agonist succ-[Asp6, N-MePhe8]-SP(6-11) (senktide; activity 0.004 relative to substance P), and NK-2 or NK-3 receptors in view of the higher activity of the D-isomer of [Glp6, *Pro9]-SP(6-11) as compared to its NK-1 selective L-isomer (D/L-activity ratio 1.53). Contractile actions of tachykinins were compared with carbachol for reliance on membrane-potential dependent (electromechanical) and membrane-potential independent (pharmacomechanical) coupling mechanisms. Log concentration-response curves to carbachol and substance P in normal Krebs' medium were compared with curves obtained in a high-K+ solution where processes dependent on changes in membrane potential could play no part in excitation. In the high-K+ depolarizing solution, a concentration-related relationship was maintained, though with some diminution in the maximal additional tension generated: the maximum tension with carbachol was under both conditions greater than that with substance P. The relative effects of several tachykinins and carbachol in producing receptor-mediated changes in membrane permeability through presumed receptor-operated ion channel opening, was estimated in terms of the ability to increase 86Rb-efflux, as a marker for K+, in a high-K+ depolarizing solution. Carbachol (10 microM) consistently increased 86Rb-efflux. In contrast, no permeability increase could be detected with any tachykinin tested (substance P, eledoisin, substance P methyl ester, neurokinin A, neurokinin B, 1 or 10 microM). Tachykinins and carbachol were compared in terms of ability to increase phosphatidylinositol hydrolysis. Both substance P and carbachol showed a concentration-related increase in accumulation of total inositol phosphates; though the maximal response to carbachol was considerably greater than that to any tachykinin (substance P, eledoisin, substance P methyl ester, senktide, neurokinin A, neurokinin B), or combination of two tachykinins (substance P and eledoisin, senktide and substance P methyl ester).(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

Inositol metabolism in mononuclear leucocytes from patients with atopic dermatitis

Elevated activity of cyclic adenosine monophosphate-specific phosphodiesterase (PDE) has been previously documented in the peripheral blood mononuclear leucocytes (MNLs) of patients with atopic dermatitis (AD). Because of the potential interactions of the cyclic nucleotide and inositol secondary messenger systems we sought to determine whether differences exist in the inositol uptake and metabolism between atopic and normal MNLs. We found no difference in the uptake of tritiated inositol by MNL between 19 atopic patients and 16 non-atopic control subjects. However, after stimulation of MNLs by concanavalin A, there was a significantly greater metabolism of inositol to inositol-1-phosphate (IP1) in the controls compared to the atopic MNLs; the mean percentage rise in the IP1 fraction over baseline level was 40% in the atopics, but almost 200% in controls after 2 h. Diminished inositol metabolism in atopic MNLs may explain the reduced cell-mediated immunity that characterizes the atopic state.

5-hydroxytryptamine-stimulated accumulation of 1,2-diacylglycerol in the rabbit basilar artery: a role for protein kinase C in smooth muscle contraction

1. 5-Hydroxytryptamine (5-HT) produced a concentration-dependent increase in the membrane concentration of 1,2-diacylglycerol (DG) in the rabbit isolated basilar artery, but did not stimulate the hydrolysis of membrane phosphoinositide. 2. The 5-HT-induced accumulation of DG could be blocked with the putative phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC; 70 microM), but not with the protein kinase C inhibitor, 1-(5-isoquinolinesulphonyl)-2-methyl piperazine (H7; 50 microM). 3. Direct stimulation of protein kinase C with phorbol 12,13-dibutyrate (PDBu) produced sustained smooth muscle contraction which was fairly rapid in onset and could be reversed by H7 but not by NCDC. The inactive phorbol, 4 alpha phorbol 12,13-dideceonate, did not produce contraction in the basilar artery. 4. 5-HT-induced contractions (1 nM-100 microM) were blocked or greatly reduced in the presence of the protein kinase inhibitor H7 or polymyxin B, and with the phospholipase C inhibitor, NCDC. The concentrations of these inhibitors which abolished contraction to 5-HT, did not alter smooth muscle contraction produced in response to 30 mM K(+)-physiological salt solution (PSS). 5. These data suggest that DG production and the subsequent activation of PKC forms an important component of the cerebrovascular contractile response to 5-HT. As the DG does not appear to arise from membrane phosphatidylinositol, it appears that 5-HT can stimulate the production of this second messenger in cerebral arteries by a mechanism which is different from peripheral arteries.

Molecular and cellular analysis of histamine H1 receptors on cultured smooth muscle cells

Histamine is an important mediator of immediate hypersensitivity for both animals and humans. The action of histamine on target tissues is believed to be mediated by specific cell surface receptors, especially H1 and H2 receptors for hypersensitivity and inflammatory reactions, which involve stimulation of smooth muscle contractility, alterations in vascular permeability, and modifications in the activities of macrophages and lymphocytes. Although the nature of histamine receptors in the brain and peripheral tissues has been studied extensively by many laboratories, the molecular mechanism of histamine receptor-mediated reactions is not fully understood, mainly because histamine receptors are incompletely characterized from the biochemical point of view. In previous studies, we have found that the cultured smooth muscle cell line DDT1MF-2, derived from hamster vas deferens, expresses low-affinity histamine H1 receptors and responds biochemically and functionally to H1-specific stimulation (Mitsuhashi and Payan, J Cell Physiol 134:367, 1988). This cell line provides a model for analyzing the biochemical responses of H1 receptor-mediated reactions in peripheral tissues. In this review, we summarized our recent progress in the study of low-affinity H1 receptors on DDT1MF-2 cells.

The diacylglycerol kinase inhibitor, R 59022, suppresses contractility of intestinal smooth muscle

R 59022, a compound which causes the accumulation of diacylglycerol by inhibition of the enzyme diacylglycerol kinase, was tested for its effect on stimulated motor activity of the longitudinal muscle from the guinea-pig ileum. Motor responses to histamine were very potently inhibited by R 59022 (greater than or equal to 0.03 microM), which confirms its known activity as a histamine receptor antagonist. Contractions elicited by acetylcholine, substance P, or K+ depolarization were also concentration dependently depressed by R 59022 (1-30 microM); further analysis showed that substance P was antagonized in a non-competitive manner. R 59022 was significantly more active to block the tonic than the phasic component of the contractile response to K+ depolarization. The Ca2+-induced activation of the contractile apparatus in chemically skinned muscle strips was depressed by similar concentrations of R 59022 (3-30 microM). These data indicate that R 59022 suppresses the contractile activity of intestinal smooth muscle at an intracellular site of action but it is not yet clear whether this action can be accounted for by the accumulation of diacylglycerol and subsequent stimulation of protein kinase C.

Beta-adrenoceptor stimulation inhibits histamine-stimulated inositol phospholipid hydrolysis in bovine tracheal smooth muscle

1. Histamine and carbachol produced concentration-related increases in the accumulation of 3H-inositol phosphates in slices of bovine tracheal smooth muscle. 2. Noradrenaline alone produced a small stimulation of 3H-inositol phosphate accumulation which was inhibited by the alpha-adrenoceptor antagonist phentolamine. In contrast, when noradrenaline (0.1 mM) was added simultaneously with histamine it significantly reduced the inositol phosphate response to high (greater than or equal to 0.1 mM) concentrations of histamine. However, noradrenaline had no inhibitory effect on the carbachol-induced inositol phosphate response. 3. The non-selective beta-agonist isoprenaline (IC50 = 0.08 microM) and the beta 2-selective agonist salbutamol (IC50 = 0.29 microM) both produced a dose-related inhibition of the inositol phosphate response to 0.1 mM histamine. The inhibitory effect of salbutamol was antagonized by propranolol (KA = 2.4 x 10(9) M-1) and the beta 2-selective adrenoceptor antagonist ICI 118551 (KA = 1.7 x 10(9) M-1). 4. The accumulation of 3H-inositol phosphates induced by histamine increased steadily over a 40 min period after an initial lag period of 3-4 min. Following the simultaneous addition of histamine and salbutamol there was a further delay of 3-4 min before the appearance of the inhibitory effect of salbutamol. 5. The effect of histamine on inositol phosphate accumulation was accompanied by a stimulation of [3H]-inositol incorporation into membrane phospholipids which was reduced by the presence of salbutamol. However, when histamine was used to stimulate maximally [3H]-inositol incorporation during the prelabelling period, salbutamol produced a marked inhibition of histamine-stimulated 3H-inositol phosphate accumulation under conditions in which there was no change in the level of incorporation.

Short-term desensitization of guinea-pig taenia caecum induced by carbachol occurs at intracellular Ca stores and that by histamine at H1-receptors

1. In Ca-free solution, the contractile response of guinea-pig taenia caecum to 10(-4) M carbachol was mediated through muscarinic receptors and was reduced time-dependently by desensitization with 10(-4) M carbachol, but not 10(-4) M histamine. On the other hand, the response to 10(-4) M histamine was shown to be mediated through H1-receptors and to be reduced time-dependently by desensitization with either 10(-4) M histamine or 10(-4) M carbachol. 2. The maximal K+ contraction was not changed by desensitization with carbachol or histamine. Thus, contractile proteins and voltage-dependent Ca channels maintain their normal functions. 3. To study the coupling of Ca channel activity in cell surface membrane to receptor activation, the contractile responses elicited by carbachol and histamine added simultaneously with Ca to Ca-free solution were measured. The response elicited by carbachol plus Ca was not changed by desensitization with carbachol, while that elicited by histamine plus Ca was reduced by desensitization with histamine. These results show that desensitization by carbachol occurs at a post-receptor site, whereas that induced by histamine occurs at H1-receptors. 4. After desensitization with carbachol, but not histamine, the contractile response to 5 x 10(-2) M caffeine in Ca-free solution was significantly reduced. 5. These results show that short-term desensitization of guinea-pig taenia caecum by carbachol is heterologous and occurs at intracellular Ca stores, while that induced by histamine is homologous and occurs at histamine H1-receptors.

Indirect evidence for a role of phosphatidylinositol turnover in the cardiac response to H1-receptor stimulation

The influence of lithium on the positive inotropic effect of the H1-agonist 2-pyridyl-ethylamine (PEA) and of the H2-receptor agonist 4-methylhistamine was studied in isolated guinea-pig ventricular strips electrically stimulated at 1 Hz. Lithium (1-10 mM) was devoid of any effect on cardiac contraction; the positive inotropic effect of 4-methylhistamine was unaffected in the presence of 10 mM lithium. On the other hand, lithium (1-10 mM) dose-dependently shifted the dose-inotropic effect curve for PEA to the right; an antagonistic effect, qualitatively similar to that of lithium, was induced by the myoinositol antagonist 2-2'-anhydro-2-C-hydroxymethyl-myoinositol, at a concentration of 100 microM. Moreover the antagonistic effect of the higher lithium concentration (10 mM) was almost completely prevented in preparations superfused with 10 mM myoinositol. Since it is known that lithium is able to reduce the cellular availability of myoinositol by an interference with the phosphatidylinositol (PI) cycle, these results suggest that the H1-receptor-mediated increase in contractility may be linked to an increased turnover of PI, while the H2-receptor-mediated one is not.

Does the hydrolysis of inositol phospholipids lead to the opening of voltage operated Ca2+ channels in guinea-pig ileum? Studies with fluoride ions and caffeine

Fluoride ions (1-30 mM) stimulate phosphoinositide hydrolysis in guinea-pig ileum longitudinal smooth muscle slices, and this is not inhibited in the presence of indomethacin or nifedipine. This action is associated with a slow contractile response which peaks after approximately five minutes and then declines towards baseline; at this time the contractile response to a maximally effective concentration of carbachol is also inhibited. Fluoride-induced contractions are inhibited completely in the presence of nifedipine. Similarly, contractions induced by caffeine, which releases Ca2+ from intracellular stores, are also inhibited by nifedipine. These data are consistent with a model in which the activation of a G-protein by F- ions leads to the following sequential events: activation of phospholipase C, release of intracellular Ca2+, opening of voltage operated (i.e. dihydropyridine sensitive) Ca2+ channels and contraction. The transient nature of the fluoride contraction and the inhibition of the carbachol contraction may be due to a slow elevation of cAMP levels induced by F-.

Histamine H1-receptors mediate phosphoinositide hydrolysis in astrocyte-enriched primary cultures

Astrocyte-enriched primary cultures of newborn rat brain hemispheres, prelabeled with [3H]inositol, accumulated [3H]inositol phosphate but not [3H]inositol bis- and tris-phosphate, after exposure to histamine for 60 min in the presence of 10 mM LiCl. The response to histamine was not a function of contaminating meningeal fibroblasts since no accumulation of [3H]inositol phosphate was elicited by histamine in meningeal cultures. The stimulation of phosphoinositide hydrolysis by histamine in astrocytes was dose-dependent (EC50 = 1.7 microM, maximal effect = 345% over basal levels) and was mimicked by several H1-receptor agonists. The use of selective receptor antagonists confirmed that the histamine response was the result of activation of H1-receptors. The histamine-induced [3H]inositol phosphate accumulation was completely abolished by omission of Ca2+ from the incubation medium. Astrocyte membranes specifically bound the radiolabeled H1-antagonist, [3H]mepyramine with an affinity (Kd = 5.9 nM) and a density of binding sites (Bmax = 113 fmol/mg protein) similar to rat brain. These results demonstrate the presence of functional histamine H1-receptors in rat brain astrocytes and suggest a role for histamine as a neuromodulator of astrocyte function.

Comparison of the effects of some muscarinic agonists on smooth muscle function and phosphatidylinositol turnover in the guinea-pig taenia caeci

1. The effects of the muscarinic agonists acetylcholine (ACh), carbachol (CCh), AHR-602, and McN-A-343 on contractility and on inositol phosphate accumulation in the presence of lithium were compared in the taenia of the guinea-pig caecum. 2. Compared to CCh, ACh was a full agonist for contraction but AHR-602 and McN-A-343 were partial agonists producing 80-85% of the maximal response to CCh. Similar to previous findings with CCh, tonic contractions produced by AHR-602 and McN-A-343 were less sensitive to inhibition by nifedipine or verapamil than tonic contractions to ACh. 3. CCh and ACh produced similar increases in inositol phosphate accumulation and the effect of CCh (0.1 mM) was inhibited by atropine (IC50 8.5 nM) and pirenzepine (IC50 450 nM). The accumulation of inositol phosphates in the presence of AHR-602 or McN-A-343 was not significantly different (P greater than 0.05) from basal levels. 4. A concentration of 0.2 mM AHR-602 produced a parallel shift of the concentration-response curve to CCh on inositol phosphate accumulation. The IC50 value for inhibition of CCh (0.1 mM) was greater than 50 fold higher than the EC50 value for contraction produced by the partial agonist. McN-A-343 (20 microM) produced a flattening of the concentration-response curve to CCh for inositol phosphate accumulation. 5. The results suggest that the increase in phosphatidylinositol turnover produced by muscarinic agonists, like the contractile response, involves an M2-muscarinic receptor. AHR-602 and McN-A-343 are partial agonists for the contractile response and while producing no significant increase in phosphatidylinositol turnover inhibit the response to CCh.

Neurokinin3-receptors are linked to inositol phospholipid hydrolysis in the guinea-pig ileum longitudinal muscle-myenteric plexus preparation

1. Tachykinin-stimulated inositol phospholipid hydrolysis was examined in slices of longitudinal muscle from guinea-pig ileum. 2. Substance P, neurokinin A and neurokinin B induced a concentration-dependent accumulation of total [3H]-inositol phosphates in the presence of 12 mM lithium with similar maximal responses and EC50 values. 3. The selective NK1-receptor agonist, substance P methyl ester, and the selective NK3-receptor agonist succ-[Asp6, MePhe8]-SP(6-11) (senktide) also stimulated [3H]-inositol phosphate formation with maximum responses of 50.69 +/- 0.96 and 45.64 +/- 1.17% relative to 10 microM substance P, respectively. Substance P methyl ester was approximately equipotent with substance P, whereas senktide was approximately 100 times more potent. 4. When added together, maximally effective concentrations of substance P methyl ester and senktide gave responses that were fully additive. In contrast, responses to substance P and neurokinin B were not additive. 5. The stimulation of [3H]-inositol phosphate formation by substance P, neurokinin B and senktide was not affected by atropine (2 microM) or tetrodotoxin (TTX, 0.3 microM). 6. The contractile effect of senktide was inhibited completely by TTX and partially blocked by atropine. Contractions induced by substance P methyl ester were not changed in the presence of TTX or atropine. 7. [D-Pro4, D-Trp7,9,10]-SP(4-11) competitively antagonized the action of substance P methyl ester on inositol phospholipid hydrolysis and contraction, but had no significant effect on senktide-induced inositol phospholipid breakdown or contraction. 8. These results suggest that NK3-receptors in the guinea-pig ileum are coupled to inositol phospholipid hydrolysis.

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.

Phorbol ester-mediated desensitization of histamine H1 receptors on a cultured smooth muscle cell line

The present study was undertaken in order to examine the effect of protein kinase C (PKC) on histamine H1 receptors (H1R) present on the smooth muscle cell line, DDT1MF-2. [3H]-pyrilamine binding revealed that specific [3H]-pyrilamine binding sites were reduced by pretreatment with 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of PKC, but not the Kd. The TPA analogue, 4 alpha phorbol 12,13-didecanoate, which does not activate PKC, failed to induce down-regulation of H1R. TPA-induced down-regulation of H1R was inhibited by pretreatment with 1-(5-Isoquinilinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), a PKC inhibitor, in a dose dependent manner. The H-7 analogue, H-8, which is a less potent inhibitor of PKC, but a potent inhibitor of cyclic nucleotide dependent protein kinase, had no effect on H1R. Moreover, treatment with TPA inhibited histamine-induced increases in [Ca2+]i in cells loaded with the fluorescent indicator, indo-1. These data suggest that H1R in DDT1MF-2 cells are functionally regulated by PKC.

Oscillatory activation of calcium-dependent potassium channels in HeLa cells induced by histamine H1 receptor stimulation: a single-channel study

We have used the patch-clamp method (O.P. Hamill et al., Pfluegers Arch., 391:85-100, 1981) in order to investigate the activation pattern of a calcium-dependent potassium channel following H1 receptor stimulation in HeLa cells. Our results essentially indicate that the stimulation of H1 receptors by exogenous histamine at concentrations greater than 1 microM induces an oscillatory activation pattern of calcium-dependent potassium channels characterized by the occurrence of channel current bursts separated by long silent periods. It was also found that the occurrence of these bursts could be directly correlated with transmembrane potential oscillations, the latter being the resulting effect of the calcium-dependent potassium channel synchronous openings. In addition, the cyclic activation of the calcium-dependent potassium channels could be initiated by the addition of histamine to a calcium-free external medium, indicating that the stimulation of the H1 receptors in HeLa cells is mainly related to the release of calcium from internal stores. Finally, the membrane-permeable cyclic AMP analog dibutyryl cyclic AMP was found to be ineffective in initiating single-channel events such as those triggered by exogenous histamine. It is proposed that the oscillatory activation of the calcium-dependent potassium channels in HeLa cells results from a repetitive transient increase in cytosolic free calcium concentration consequent to the H1 receptor stimulation.

Histamine-induced calcium release and phorbol antagonism in cultured airway smooth muscle cells

Primary cultures of airway smooth muscle cells were exposed to histamine, and intracellular free calcium transients were measured by the calcium-sensitive dye fura-2. Stimulation with 100 microM histamine resulted in a rise in intracellular calcium from an unstimulated level of 178 +/- 25 to 497 +/- 154 nM Ca2+ (SE; n = 14) and a return to base-line free calcium concentration within 1 min of stimulation. Pretreatment of cells with the H1 receptor blocker pyrilamine (2.5 microM) abolished the response; however, the calcium transient was not altered by pretreatment with the H2 blocker cimetidine (50 microM), by chelation of external calcium, or by pretreatment with 2 mM Co2+ or 5 microM nifedipine. Activation of protein kinase c by 200 nM phorbol 12-myristate 13-acetate (PMA) resulted in no detectable rise in cytosolic calcium but completely blocked the release of internal calcium by histamine. We conclude that 1) histamine causes a transient rise of cytosolic calcium in airway smooth muscle, 2) the rise in cytosolic calcium is mediated by H1 receptor coupling that triggers release of internal calcium stores, and 3) activation of protein kinase c blocks the histamine-induced release of intracellular calcium.

Breakdown of phosphoinositides in airway smooth muscle: lack of influence of anti-asthmatic drugs

Hydrolysis of membrane inositol phospholipids during agonist-induced contraction in bronchial smooth muscle leads to formation of inositol phosphates. Inositol phosphates are associated with intracellular Ca++ mobilization, which in smooth muscle leads to contraction. We have investigated the effects of inhibitors of the contraction, theophylline, isoproterenol (isoprenaline), and verapamil, on contraction due to carbachol and histamine in bovine airway smooth muscle, and on the formation of inositol phosphates in the same preparation. Since phospholipase C and A2 are involved in the formation of inositol phosphates, we have also studied the effect of inhibitors of phospholipases, dexamethasone and mepacrine, on the accumulation of inositol phosphates. Theophylline, isoproterenol and verapamil elicited a concentration-dependent relaxation of pre-contracted smooth muscle, with the following order of potency: Isoproterenol greater than verapamil greater than theophylline. The relaxant effect was more effective on histamine than on carbachol-induced contraction and depended on the initial airway tone. However, neither theophylline, isoproterenol or verapamil, nor dexamethasone or mepacrine changed the basal level of inositol phosphates or affected the rise due to agonists. We conclude that the smooth muscle effects of theophylline, isoproterenol, verapamil, dexamethasone and mepacrine are not mediated by interference with membrane phosphoinositide breakdown.

Studies on the adenosine-receptor mediating the augmentation of histamine-induced inositol phospholipid hydrolysis in guinea-pig cerebral cortex

Incubation (45 min) of slices of guinea-pig cerebral cortex with adenosine alone had no significant effect on the accumulation of [3H]-inositol phosphates but enhanced the response to histamine H1-receptor stimulation in a concentration-dependent manner. The effect of adenosine on agonist-stimulated inositol phospholipid hydrolysis appeared to be selective for histamine H1-receptor stimulation since it did not augment the phosphoinositide responses to carbachol, noradrenaline, 5-hydroxytryptamine or elevated KCl. The accumulation of [3H]-inositol phosphates induced by histamine increased linearly between 5 and 45 min incubation with agonist. However, following the simultaneous addition of histamine and adenosine, there was a marked delay in the appearance of the augmentation produced by adenosine. The augmentation of [3H]-inositol phosphate accumulation was mimicked by a number of adenosine analogues. The rank order of potency was; cyclopentyladenosine greater than R-phenyl-isopropyladenosine 5'-N-ethylcarboxamidoadenosine greater than 2-chloroadenosine. This is consistent with the order expected for an adenosine A1-receptor effect but the EC50 values were in the micro- rather than nanomolar range. The response to 2-chloroadenosine was antagonized by the xanthine adenosine-antagonists, cyclopropyltheophylline, 8-phenyltheophylline, 3-isobutyl-1-methylxanthine and theophylline, and the non-xanthine alloxazine.

Relationship between stimulated phosphatidic acid production and inositol lipid hydrolysis in intestinal longitudinal smooth muscle from guinea pig

Accumulation of [32P]phosphatidic acid (PA) and total [3H]inositol phosphates (IPs) was measured in the longitudinal smooth-muscle layer from guinea-pig small intestine. Stimulation with carbachol, histamine and substance P produced increases in accumulation of both [3H]IPs and [32P]PA over the same concentration range. The increase in [32P]PA accumulation in response to carbachol (1 microM-0.1 mM) was inhibited in the presence of atropine (0.5 microM). Buffering the external free [Ca2+] to 10 nM did not prevent the carbachol-stimulated increase in [32P]PA accumulation. Carbachol and Ca2+ appear to act synergistically to increase accumulation of [32P]PA. In contrast, although incubation with noradrenaline also increased accumulation of [3H]IPs, no increase in accumulation of [32P]PA could be detected. These results suggest that an increase in formation of IPs is not necessarily accompanied by an increase in PA formation, and imply the existence of receptor-modulated pathways regulating PA concentrations other than by phospholipase-C-catalysed inositol phospholipid hydrolysis.

The role of calcium in the cyclic AMP response to histamine in rabbit cerebral cortical slices

The effect of calcium on the H1- and H2-receptor components of the cyclic AMP response to histamine in rabbit cerebral cortical slices has been investigated. Removal of calcium ions from the incubation medium during the preparation, preincubation and final incubation of brain slices significantly reduced the cyclic AMP responses to adenosine, histamine and the H2-selective agonist, impromidine. Removal of calcium ions from the incubation medium during only the final incubation with agonists did not influence the responses to adenosine, histamine, impromidine and the H1-selective agonist, 2-thiazolylethylamine. Final incubation of rabbit cerebral cortical slices in calcium-free buffer containing EGTA (1 mM) however, selectively reduced the cyclic AMP responses to the H1-agonists histamine and 2-thiazolylethylamine without affecting the response to impromidine or adenosine. These latter incubation conditions significantly reduced the maximal extent of the augmentation of impromidine- or adenosine-stimulated cyclic AMP accumulation produced by H1-receptor stimulation, without affecting the EC50 values of the H1-agonists. Calcium-free/EGTA conditions did not, however, alter the dose-response parameters for the response to the H2-agonist, impromidine. These data provide further evidence that the two histamine receptor systems affect cyclic AMP accumulation in rabbit cerebral cortical slices by different mechanisms.

Effect of the tachykinin antagonist, [D-Pro4, D-Trp7,9,10] substance P-(4-11), on tachykinin- and histamine-induced inositol phosphate generation in intestinal smooth muscle

The effect of the tachykinin antagonist, [D-Pro4, D-Trp7,9,10] substance P-(4-11), on inositol phosphate accumulation produced by tachykinins and by histamine in strips of longitudinal muscle from the guinea-pig small intestine was investigated in the presence of 12 mM Li+. The two tachykinins substance P (SP) and kassinin (20 nM-20 microM) caused an accumulation of inositol phosphates in a concentration-dependent manner. This was seen with an agonist contact time of only 30 s. SP and kassinin were roughly equipotent in inducing inositol phosphate accumulation, which is consistent with their relative potencies in causing muscle contraction. The tachykinin antagonist (20 microM) produced a shift to the right of the dose-response curves for inositol phosphate accumulation caused by SP and kassinin. However, the effect of kassinin was inhibited much more than that of SP, which is consistent with a similar differential antagonism of the contractions induced by these agonists. The tachykinin antagonist also depressed histamine-induced accumulation of inositol phosphates whereas histamine-induced contractions had previously been found unaffected by the antagonist. These findings show that the tachykinin antagonist is not totally selective with regard to agonist-induced accumulation of inositol phosphates in intestinal smooth muscle. This may suggest that the antagonist not only acts on tachykinin receptors but also has another site of cellular action.

Changes in membrane potential and phosphoinositides during alpha 1-adrenoceptor stimulation in smooth muscle cells of guinea-pig taenia caeci

The effect of a submaximal concentration of adrenaline (3-5 microM) was studied in taenia caeci smooth muscle cells. Membrane potential hyperpolarization was observed in intact muscle preparations and this response could be separated into two phases, depending on the state of a membrane-bound calcium compartment. The effect of alpha 1-adrenergic stimulation was also measured by [3H]inositol incorporation into phospholipid and inositol phosphate fractions of taenia cell suspensions both in the absence and presence of 2.5 mM extracellular calcium. In the absence of extracellular calcium the inositol phospholipids increased within 15 s after stimulation, followed by enhanced inositol phosphates. With calcium present there was a biphasic increase in the phosphatidylinositol 4,5-bisphosphate (PIP2) fraction with a simultaneous release of inositol phosphates. Lithium ions affected the incorporation of label into the lipids but not into the inositol phosphate fractions. These findings suggest that, in taenia caeci cells, alpha 1-adrenergic-induced membrane hyperpolarization resulting in muscle relaxation is associated with changes in the PIP2 content.

Differential effect of temperature on histamine- and carbachol-stimulated inositol phospholipid breakdown in slices of guinea-pig cerebral cortex

Slices of guinea-pig cerebral cortex were incubated with [3H]-inositol at 37 degrees C before exposure to histamine or carbachol at 37 degrees C or 25 degrees C. Histamine-stimulated accumulation of [3H]-inositol 1-phosphate ([3H]-IP1) at 25 degrees C was only 5-7% of that at 37 degrees C, whereas for carbachol the response at 25 degrees C was 45-49% of that at 37 degrees C. The affinity of benzilylcholine, obtained from inhibition of carbachol-induced accumulation of [3H]-IP1 was similar at 25 degrees C and 37 degrees C, but the EC50 for carbachol was lower at 25 degrees C (20 +/- 2 microM) than at 37 degrees C (42 +/- 2 microM). The IC50 for histamine inhibition of [3H]-mepyramine binding to homogenates of guinea-pig cerebral cortex did not differ significantly at 25 degrees C and 37 degrees C. Histamine-induced accumulations of [3H]-IP2 and [3H]-IP3 at 25 degrees C, expressed as a percentage of the accumulation at 37 degrees C, were also much less than the corresponding value for carbachol. These observations imply that the locus or pathway(s) of agonist-induced formation of [3H]-IP1 are not the same for histamine and carbachol.

1,4-Dithiothreitol-induced changes in histamine H1-agonist efficacy and affinity in the longitudinal smooth muscle of guinea-pig ileum

The effect of 1,4-dithiothreitol (DTT) on histamine H1-receptor agonist affinity and efficacy has been investigated in longitudinal muscle strips of guinea-pig ileum. Exposure of ileal smooth muscle to DTT significantly increased the maximal responses to the partial agonists SKF71473 and DE-2PEA, indicative of an increase in agonist efficacy. This effect was paralleled by a small decrease in EC50 values. In contrast, DTT produced a parallel displacement of the concentration-response curve to the full agonist histamine in the same muscle strips. Studies in which phenoxybenzamine and benzilylcholine mustard were used to reduce the maximum response to histamine suggested that DTT altered both agonist affinity and efficacy. The affinity constant for histamine, calculated by the method of Furchgott & Bursztyn (1967), increased by 2.7 fold in the presence of DTT. Furthermore, agonist efficacy also appeared to increase in the presence of DTT since the maximum response to histamine following phenoxybenzamine treatment increased on application of DTT. [3H]-mepyramine binding studies confirmed that DTT increased agonist affinity. DTT produced a significant parallel shift to the left of the displacement curves for histamine, 2-methylhistamine, 2-pyridylethylamine and 2-thiazolylethylamine. The results of this study therefore suggest that DTT potentiates H1-receptor-mediated contractile activity in guinea-pig ileum by increasing both agonist efficacy and affinity.

Regional variation in the characteristics of histamine H1-agonist mediated breakdown of inositol phospholipids in guinea-pig brain

The position of dose-response curves for histamine-induced accumulation of [3H]-inositol 1-phosphate ([3H]-IP1) in lithium-treated slices of guinea-pig brain prelabelled with [3H]-inositol differed significantly between cerebellum (EC50 5.1 +/- 1.0 microM) and cerebral cortex (EC50 16.3 +/- 0.7 microM). The Hill coefficients of the curves, 1.33 +/- 0.28 and 1.24 +/- 0.03, respectively, did not differ significantly. 2-Methylhistamine, N alpha,N alpha-dimethylhistamine and betahistine were partial agonists in both cerebellum and cerebral cortex, but all produced a greater percentage of the maximum response to histamine in cerebellar slices. In hippocampal slices the response of the partial agonists was intermediate between that in cerebellum and that in cerebral cortex. The four agonists produced an appreciable accumulation of [3H]-inositol 1-phosphate in cerebellar slices even in the absence of Li+ ion. The EC50 and Hill coefficients characterizing the dose-response curves for the four agonists were the same whether 10 mM LiCl was present or not. The affinity constant for mepyramine inhibition of the histamine-induced response was similar in cerebellum, 4.2 +/- 0.6 X 10(8) M-1, and cerebral cortex, 4.6 +/- 1.0 X 10(8) M-1. Curves of mepyramine inhibition of the responses to a fixed concentration of histamine gave no indication of any second component in the response to histamine in either cerebellum or cerebral cortex. The parameters of histamine inhibition of [3H]-mepyramine binding were similar in homogenates of guinea-pig cerebellum and cerebral cortex. These results indicate that H1-agonist-induced accumulation of IP1 may not be as directly related to agonist-receptor interaction as simple reaction schemes suggest.

Enhancement of histamine H1-receptor agonist activity by 1,4-dithiothreitol in guinea-pig cerebellum and cerebral cortex

The disulphide bond-reducing agent 1,4-dithiothreitol (1 mM) produced a marked potentiation of histamine-stimulated accumulation of [3H]inositol phosphates in lithium-treated slices of guinea-pig cerebellum and cerebral cortex. This was seen as a parallel shift of the concentration-response curve for histamine to lower agonist concentrations, with no significant effect on the maximal response or Hill coefficient. Dithiothreitol similarly potentiated the augmentation of adenosine-stimulated cyclic AMP accumulation elicited by histamine in guinea-pig cerebral cortex. Studies with partial agonists suggested that this potentiating effect was associated with an increase in agonist efficacy rather than a change in agonist binding affinity. Thus, dithiothreitol increased the maximal accumulation of [3H]inositol phosphates produced by both 2-pyridylethylamine and 2-methylhistamine, which appeared to act as partial agonists in guinea-pig cerebral cortex. Dithiothreitol similarly increased the maximal extent of the augmentation of adenosine-stimulated accumulation of cyclic AMP produced by 2-methylhistamine. The site of action of dithiothreitol is not known; however, a comparison of the effect of dithiothreitol on muscarinic and histamine H1-receptor-mediated phosphoinositide responses in guinea-pig cerebral cortex suggests that it is before the stage at which the receptor-effector pathways are shared by these two receptor systems.

Histamine-induced hydrolysis of polyphosphoinositides in guinea-pig ileum and brain

The effect of histamine and the H1-selective agonist, 2-pyridylethylamine, on the accumulation of inositol monophosphate (InsP), inositol bisphosphate (InsP2) and inositol trisphosphate (InsP3) has been examined in lithium-treated slices of guinea-pig cerebellum and ileal smooth muscle. Following 45 min incubation, histamine produced a large accumulation of [3H]InsP and a smaller accumulation of [3H]InsP2 and [3H]InsP3 in both tissues. In cerebellar slices all three responses to histamine were potently and competitively inhibited by the selective H1-receptor antagonist, mepyramine. In contrast, incubation of ileal slices with mepyramine (0.1 microM) produced only a small reduction (circa 20%) in the maximal accumulation elicited by histamine of each [3H]inositol phosphate with no significant effect on the EC50 or Hill coefficient. However, when 2-pyridylethylamine, instead of histamine, was used to stimulate inositol phospholipid hydrolysis in ileal smooth muscle, the agonist-induced responses appeared to be competitively antagonised by mepyramine. The results presented indicate that there is an apparent dissociation between histamine-induced InsP3 accumulation and H1-receptor-mediated contractile activity in ileal smooth muscle and suggest that agonist-induced inositol phospholipid hydrolysis in this tissue may be involved in other cellular events separate from those involving calcium.

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.

Selective enhancement of histamine H1-receptor responses in guinea-pig ileal smooth muscle by 1,4-dithiothreitol

1,4-Dithiothreitol (DTT; 1 mM, 30 min preincubation) produced a small, non-specific potentiation of spasmogenic activity in longitudinal muscle strips of guinea-pig small intestine. A direct comparison of contractile responses elicited by histamine and a range of H1- and non-H1-receptor agonists indicated that DTT produced a significantly greater potentiation of H1-receptor responses. This apparently selective increase in tissue sensitivity to histamine H1-receptor agonists did not appear to be a consequence of the inhibition of histamine N-methyl transferase or diamine oxidase activity. Potentiation of the responses to histamine by DTT was still observed in the presence of SKF 91488 (10 microM) and aminoguanidine (1 microM). The potentiation elicited by DTT was readily reversed by the sulphydryl oxidizing agent dithiobis-(2-nitrobenzoic acid) (DTNB). This suggests that the mechanism of action of DTT involves the reduction of disulphide bonds. Exposure of ileal smooth muscle to DTT following desensitization with histamine (100 X EC50 [- DTT]) resulted in a 6.9 +/- 0.7 fold shift of the concentration-response curve to lower agonist concentrations. Conversely, following potentiation of the response to histamine with DTT, exposure of the tissue to desensitizing concentrations of histamine resulted in a dextral shift of the dose-response curve (dose ratio = 39.5 +/- 1.2) to higher agonist concentrations. The results of this study suggest that DTT may be a useful tool with which to investigate histamine H1-receptor mechanisms in ileal smooth muscle.