Different profiles of desensitization dynamics in guinea-pig jejunal longitudinal smooth muscle after stimulation with histamine and methacholine

Monensin inhibits mast cell mediated airway contractions in human and guinea pig asthma models

Asthma is a common respiratory disease associated with airway hyperresponsiveness (AHR), airway inflammation and mast cell (MC) accumulation in the lung. Monensin, an ionophoric antibiotic, has been shown to induce apoptosis of human MCs. The aim of this study was to define the effect of monensin on MC responses, e.g., antigen induced bronchoconstriction, and on asthmatic features in models of allergic asthma. Tracheal segments from house dust mite (HDM) extract sensitized guinea pigs were isolated and exposed to monensin, followed by histological staining to quantify MCs. Both guinea pig tracheal and human bronchi were used for pharmacological studies in tissue bath systems to investigate the monensin effect on tissue viability and antigen induced bronchoconstriction. Further, an HDM-induced guinea pig asthma model was utilized to investigate the effect of monensin on AHR and airway inflammation. Monensin decreased MC number, caused MC death, and blocked the HDM or anti-IgE induced bronchoconstriction in guinea pig and human airways. In the guinea pig asthma model, HDM-induced AHR, airway inflammation and MC hyperplasia could be inhibited by repeated administration of monensin. This study indicates that monensin is an effective tool to reduce MC number and MCs are crucial for the development of asthma-like features.

Histamine activates an intracellular Ca2+ signal in normal human lung fibroblast WI-38 cells

Histamine is an inflammatory mediator that can be released from mast cells to induce airway remodeling and cause persistent airflow limitation in asthma. In addition to stimulating airway smooth muscle cell constriction and hyperplasia, histamine promotes pulmonary remodeling by inducing fibroblast proliferation, contraction, and migration. It has long been known that histamine receptor 1 (H1R) mediates the effects of histamine on human pulmonary fibroblasts through an increase in intracellular Ca2+ concentration ([Ca2+]i), but the underlying signaling mechanisms are still unknown. Herein, we exploited single-cell Ca2+ imaging to assess the signal transduction pathways whereby histamine generates intracellular Ca2+ signals in the human fetal lung fibroblast cell line, WI-38. WI-38 fibroblasts were loaded with the Ca2+-sensitive fluorophore, FURA-2/AM, and challenged with histamine in the absence and presence of specific pharmacological inhibitors to dissect the Ca2+ release/entry pathways responsible for the onset of the Ca2+ response. Histamine elicited complex intracellular Ca2+ signatures in WI-38 fibroblasts throughout a concentration range spanning between 1 µM and 1 mM. In accord, the Ca2+ response to histamine adopted four main temporal patterns, which were, respectively, termed peak, peak-oscillations, peak-plateau-oscillations, and peak-plateau. Histamine-evoked intracellular Ca2+ signals were abolished by pyrilamine, which selectively blocks H1R, and significantly reduced by ranitidine, which selectively inhibits H2R. Conversely, the pharmacological blockade of H3R and H4R did not affect the complex increase in [Ca2+]i evoked by histamine in WI-38 fibroblasts. In agreement with these findings, histamine-induced intracellular Ca2+ signals were initiated by intracellular Ca2+ release from the endoplasmic reticulum through inositol-1,4,5-trisphosphate (InsP3) receptors (InsP3R) and sustained by store-operated Ca2+ channels (SOCs). Conversely, L-type voltage-operated Ca2+ channels did not support histamine-induced extracellular Ca2+ entry. A preliminary transcriptomic analysis confirmed that WI-38 human lung fibroblasts express all the three InsP3R isoforms as well as STIM2 and Orai3, which represent the molecular components of SOCs. The pharmacological blockade of InsP3 and SOC, therefore, could represent an alternative strategy to prevent the pernicious effects of histamine on lung fibroblasts in asthmatic patients.

DIFFERENTIAL DEVELOPMENT OF CARBACHOL-INDUCED DESENSITIZATION IN RECEPTOR-MEDIATED Ca2+INFLUX AND Ca2+RELEASE PATHWAYS IN SMOOTH MUSCLE OF GUINEA-PIG TAENIA CAECI

1. We have found that development of carbachol (CCh)-induced desensitization to receptor agonists, but not to receptor by-passed stimulation, is transiently interrupted by a Ca2+-dependent resensitization during the early stage in the smooth muscle of guinea-pig taenia caeci. To further characterize the receptor-mediated signal transduction pathways involved in this peculiar desensitization process, we examined the desensitization processes during Ca2+ influx- and Ca2+ release-mediated contractions in response to activation of muscarinic receptors or histamine H1 receptors. 2. Desensitization treatment with 10(-4) mol/L CCh for 30 min in the presence of extracellular Ca2+ resulted in desensitization to the muscarinic agonists McN-A-343 or AHR-602, which are known to induce contraction only in the presence of extracellular Ca2+ in taenia caeci. The development of desensitization to these agonists was interrupted by a transient resensitization at 1 min. In contrast, the transient resensitization phase was lost following removal of extracellular Ca2+ during the desensitization treatment with CCh; under these conditions, the desensitization developed gradually without an apparent resensitization phase. 3. Contractions to 10(-4) mol/L CCh and 10(-4) mol/L histamine in the absence of extracellular Ca2+ were gradually desensitized without a resensitization phase following the CCh desensitization treatment, irrespective of the presence or absence of extracellular Ca2+ during CCh treatment, although the onset of the desensitization was delayed under Ca2+-free conditions. 4. These results suggest that the receptor-mediated Ca2+ influx and Ca2+ release pathways are differentially desensitized to CCh and that the transient resensitization appears to regulate the desensitization process in response to Ca2+ influx-mediated contraction. Such differential processes of desensitization in receptor-mediated bifurcated signalling pathways may determine cellular responsiveness to certain types of stimuli, depending on the different Ca2+ sources required for contraction.

Effect of histamine on phasic and tonic contractions of isolated uterine tissue from pregnant women

OBJECTIVE

To enhance our understanding of the uterotonic effect of histamine, we compared the effects of histamine on spontaneous phasic and tonic contractile activity of uterine strips from term pregnant nonlaboring women.

STUDY DESIGN

Longitudinal uterine strips were used from the lower uterine segment of term pregnant nonlaboring women undergoing elective cesarean section. The concentration-response relationship to histamine (10(-8) to 10(-4) mol/L) was determined in spontaneously contracting strips or in strips contracted tonically with a protein kinase C activator (-)-indolactam V in the presence of H(1) receptor antagonist (S[+]-chlorpheniramine maleate), H(2) receptor antagonist (cimetidine), or solvent.

RESULTS

Histamine increased spontaneous phasic myometrial contractions in a concentration-dependent manner. H(1), but not H(2), receptor antagonist significantly attenuated the response to histamine. Histamine significantly reduced tonic contractions of uterine strips induced by indolactam V. H(1) histamine receptor antagonist abolished relaxation, whereas H(2) histamine receptor antagonist had no effect.

CONCLUSION

Histamine increases spontaneous, but inhibits tonic, contractions of uterine strips from term pregnant nonlaboring women. Both effects are mediated through activation of H(1) receptors.

The fate of released histamine: reception, response and termination

Histamine released from ECL cells elicits responses from a variety of cellular targets in the vicinity. Three sets of receptors are involved (H1, H2 and H3). Receptor occupation is promptly transduced into cellular responses. The responses, in turn, are terminated by diverse mechanisms: enzymatic inactivation, cellular uptake and desensitization at the receptor level. Under specific pathological conditions, histamine effects could be exaggerated by the presence of derivatives that may be of marginal relevance under physiological conditions.

High capacity for tissue-type plasminogen activator release from vascular endothelium in vivo

BACKGROUND

Tissue-type plasminogen activator (t-PA) is the key mediator of intravascular fibrinolysis. We showed recently that local administrations both of methacholine (MCH) and of desmopressin (1-desamino-8-D-arginine vasopressin, DDAVP) induced a massive local release of t-PA in the human forearm vasculature.

OBJECTIVE

To determine whether the human vascular endothelium could respond to repeated stimulation with the same agonist, and, if so, to further evaluate the releasable endothelial pool of t-PA.

METHOD

Seven young, healthy men participated. MCH and DDAVP were administered as local infusions into the brachial artery. In protocol 1 (n = 3) 2 microg/min MCH was infused for 2 (30 min with a free interval of 20 min). In protocol 2 (n = 4) 70 ng/min DDAVP was infused for 2 (20 min with an interval of 75 min). Dosages and time intervals were based on the different release profiles of the two drugs observed in previous studies. Brachial arterial and venous blood samples were obtained at baseline and throughout infusions. Net release of t-PA was calculated as the product of the arteriovenous concentration gradient and forearm plasma flow.

RESULTS

Forearm blood flow was increased 3-4-fold by MCH and 2-3-fold by DDAVP infusions. During the first and second infusions of MCH, the average amounts of t-PA released were 1600 and 1000 ng/l forearm tissue, respectively. In contrast, DDAVP induced similar t-PA responses during both infusions; the average total releases of t-PA were 2300 and 2400 ng/l tissue, respectively.

CONCLUSION

The results show that the vascular endothelium is responsive to repeated stimulation both with MCH and with DDAVP. The diminished t-PA response to the second MCH infusion can not be explained in terms of depletion of intracellular pools, in view of the large amount of t-PA released by a single infusion of DDAVP. The dynamic pool of t-PA available for release is very large, but further studies are required in order to quantify the releasable endothelial stores.

Bradykinin regulates the histamine-induced Ca2+ mobilization via protein kinase C activation in human gingival fibroblasts

We previously demonstrated that histamine and bradykinin evoke an increase in intracellular Ca2+ ([Ca2+]i) in human gingival fibroblasts by using a fluorescent Ca2+ indicator Fura-2. In this paper, we further demonstrate the regulation of the histamine-induced Ca2+ mobilization by bradykinin. In fibroblasts stimulated with bradykinin (1 microM), subsequent stimulation with histamine (100 microM) failed to mobilize Ca2+, whereas bradykinin induced an increase in [Ca2+]i in the cells pre-stimulated with histamine. The attenuation of the histamine response was dependent on the concentration of bradykinin for the first stimulation. Histamine also failed to induce the formation of inositol 1,4,5-trisphosphate in fibroblasts pretreated with bradykinin. In fibroblasts pretreated with bradykinin (1 microM) for 3 min and then washed with fresh medium, the effect of histamine on [Ca2+]i quickly returned to the control level. The activation of protein kinase C by phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (PMA) elicited a marked decrease in histamine-induced Ca2+ mobilization. When the protein kinase C activity was inhibited with H7, a protein kinase C inhibitor, or was down-regulated by pretreatment with PMA for 20 h, the inhibitory effect of PMA on the histamine response was relieved. In the fibroblasts pretreated with H7 or PMA for 20 h, histamine evoked Ca2+ mobilization even after bradykinin stimulation. These results suggest that the histamine response is regulated by bradykinin receptor activation via the activation of protein kinase C in human gingival fibroblasts.

Increased desensitization by picomolar phorbol ester of the endothelium-mediated effect of histamine in the perfused rat mesenteric bed

The vasodilatatory, endothelium-mediated, effect of histamine (H), through H1 receptor, in the isolated and perfused mesenteric bed of the rat, undergoes strong desensitization during perfusion or repetitive injections of noradrenaline (NA) and H. The mesenteric bed completely desensitized to H is responsive to carbachol (C) and this latter compound does not affect the H desensitization. The homologous desensitization to C effect is very small, attaining less than 10% after 30 min of continuous perfusion. In this work the effect of inhibitors or activators of protein-kinase(s)-C (PKC) are studied during continuous perfusion of H or C in preparations preconstricted by NA. Staurosporine antagonizes the onset of the H desensitization, while the rate of desensitization in increased by phorbol-12-myristate-13-acetate (PMA). PMA, at a concentration from 10(-12) to 10(-10)M, selectively enhances the homologous desensitization of H, while at 10(-8)M it also produces a desensitization to C. At least two different PKC isoenzymes might be involved in the desensitization of the vasodilatatory effect of H and C in the isolated and perfused rat mesenteric bed.

Regulation of the human histamine H1 receptor stably expressed in Chinese hamster ovary cells

1. The human H1 receptor gene expressed in Chinese hamster ovary cells (CHOhumH1) encodes a classical histamine H1 receptor with a pharmacology similar to that of the H1 receptor found in guinea-pig cerebellum and the endogenously expressed human H1 receptor in 1321N1 astrocytoma cells as determined by [3H]-mepyramine binding studies. 2. In CHOhumH1 cells, histamine induced a concentration-dependent rise in inositol phosphates (EC50 2.23 +/- 0.97 microM) and a rapid increase of [Ca2+]i, followed by a sustained increase of [Ca2+]i upon addition of 100 microM histamine. 3. Short-term exposure of CHOhumH1 cells to histamine (100 microM) resulted in a decrease of subsequent histamine-induced Ca2+ responses. The histamine-induced desensitization appeared to be heterologous as the ATP-induced Ca2+ response was also found to be affected. 4. The process of heterologous histamine-induced desensitization of the Ca2+ response in CHOhumH1 cells can be ascribed to an alteration at the level of the intracellular Ca2+ pool, as the Ca2+ response of caffeine (10 mM), which releases Ca2+ from intracellular Ca2+ stores was also attenuated upon short-term histamine exposure. 5. In CHOhumH1 cells the PKC activator, PMA, was found to inhibit the histamine (100 microM)-induced Ca2+ response concentration-dependently (IC50 0.2 +/- 0.03 microM) as well as the ATP (100 microM)-induced Ca2+ response. However, this inhibition was only partial and less effective than histamine-pretreatment. Moreover, in CHOhumH1 cells PKC downregulation induced by long-term exposure to PMA (1 microM) did not affect the histamine-induced desensitization nor did pretreatment with the specific PKC inhibitor Ro-31-8220 (10 microM), indicating that in CHOhumH1 cells PKC is probably not involved in the heterologous desensitization. 6. Long-term treatment of CHOhumH1 cells with histamine or other H1 agonists resulted in a time- and concentration-dependent decrease in the number of H1 receptor binding sites (maximal reduction: 47 +/- 5%). 7. Long-term exposure of CHOhumH1 cells to ATP or PMA did not affect H1 receptor density. 8. Both histamine (100 microM)- and ATP (100 microM)-induced Ca2+ responses were affected upon long-term exposure of cells to histamine (100 microM), which might be explained by an alteration at a level distant from the receptor. 9. These results show that in CHOhumH1 cells the human histamine H1 receptor is susceptible to short-term and long-term receptor regulation in which PKC does not seem to play a role. The CHOhumH1 cells therefore provide an excellent model system for studying the mechanism(s) of PKC-independent H1 receptor regulation.

Characteristics of the binding of [3H]-mepyramine to intact human U373 MG astrocytoma cells: evidence for histamine-induced H1-receptor internalisation

1. The kinetics of the binding of 5 nM [3H]-mepyramine to sites on intact human U373 MG astrocytoma cells, sensitive to inhibition by 2 microM pirdonium, were temperature-dependent. At 37 degrees C the half-time for association was 0.9 +/- 0.4 min and at 4 degrees C 19 +/- 3 min. Dissociation of bound [3H]-mepyramine was fast at 37 degrees C, t0.5 1.5 +/- 0.3 min, but at 6 degrees C dissociation initiated by dilution or addition of unlabelled mepyramine was negligible over 120 min. The very slow dissociation at 6 degrees C made it possible to reduce the level of pirdonium-insensitive binding from 56 +/- 5% to 39 +/- 5% by washing the cells in ice-cold medium before filtration. 2. The binding of [3H]-mepyramine sensitive to 2 microM temelastine, measured after 10 min equilibration at 37 degrees C, failed to saturate and was resolved into an hyperbola and an apparently linear component, whereas the fit to the binding of [3H]-mepyramine sensitive to 2 microM pirdonium was not significantly improved over that to an hyperbola. The mean Kd for the binding of [3H]-mepyramine to the saturable component, 2.5 +/- 0.4 nM, was in close agreement with the value of 3.5 nM for mepyramine derived from inhibition of histamine H1-receptor-mediated inositol phosphate formation in U373 MG cells. 3. Curves for the inhibition of the binding of 5 nM [3H]-mepyramine to U373 MG cells by histamine H1-receptor antagonists were biphasic and were fitted to a two site-model. Affinities calculated from the best-fit IC50 values for the high-affinity site correlated well with those expected for binding to H1-receptors. 4. The percentages of the high-affinity site in curves of the inhibition of [3H]-mepyramine binding to intact U373 MG cells by two tertiary amine antagonists, norpirdonium and 4-methyldiphenhydramine, 68 +/- 3 and 63 +/- 4%, were significantly greater than the percentages of the high-affinity site in the inhibition curves of their quaternary derivatives, 50 +/- 1 and 45 +/- 3%, respectively. Similarly, the percentage of the high-affinity site for unlabelled mepyramine, 65 +/- 7%, was greater than for the non-cell penetrant H1-antagonist temelastine, 42 +/- 5%. 5. Incubation of U373 MG cells with 100 microM histamine at 37 degrees C, followed by washing twice in ice-cold medium and then incubation with 1-15 nM [3H]-mepyramine for 120 min at 4 degrees C, resulted in a decrease in the binding of [3H]-mepyramine sensitive to 2 microM pirdonium, compared to control cells not exposed to histamine. The binding of [3H]-mepyramine in the absence of pirdonium was not altered by histamine pretreatment, whereas the level of the pirdonium-insensitive binding was significantly increased, except after 1 min exposure to histamine. The decreases in the pirdonium-sensitive binding after 5, 10 and 60 min incubation with 100 microM histamine were 41 +/- 6, 56 +/- 6 and 67 +/- 8%, respectively, but the decrease after 1 min incubation with histamine, 16 +/- 8%, was not statistically significant. 6. The results are consistent with the binding of [3H]-mepyramine to intact U373 MG cells being to both plasma membrane and intracellular histamine H1-receptors. The high-affinity binding sensitive to the non-cell penetrant quaternary compounds and to temelastine is thus to plasma membrane H1-receptors. On exposure to 100 microM histamine receptors are translocated to the intracellular pool, since the change in the high-affinity binding of [3H]-mepyramine is primarily in the level of the pirdonium-insensitive binding, rather than in the total binding.

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.

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.

Desensitization of histamine H1 receptor-mediated inositol phosphate accumulation in guinea pig cerebral cortex slices

1. Histamine stimulated the production of [3H]-inositol phosphates in untreated (control) guinea-pig cerebral cortex slices with a best-fit EC50 of 17 +/- 4 microM, and a best-fit maximum response of 385 +/- 23% over basal accumulation. 2. Histamine pretreatment desensitized guinea-pig cortex slices to a subsequent challenge with histamine, which was observed as a reduction in the best-fit maximum response to 182 +/- 32% over basal accumulation. 3. The time-course for the histamine-induced production of [3H]-inositol phosphates was approximately linear over 90 min of stimulation in both control and histamine pretreated slices. The rate of production in pretreated slices was significantly slowed compared to control, such that by 90 min of histamine stimulation the desensitized slices produced 2.8 times the basal [3H]-inositol phosphate accumulation compared to 5.3 fold the basal [3H]-inositol phosphate accumulation in the control slices. 4. Displacement of [3H]-mepyramine binding to homogenates of guinea-pig cerebral cortex by mepyramine and histamine revealed that histamine pretreatment did not alter the apparent affinity of the H1 receptor for histamine (control Kd = 6.3 +/- 0.7 microM, desensitized Kd = 7.9 +/- 1.6 microM) or mepyramine (control Kd = 3.4 +/- 0.8 nM, desensitized Kd = 3.4 +/- 1.3 nM), nor was there any reduction in the calculated maximum number of [3H]-mepyramine binding sites (control Bmax = 192 +/- 31 fmol mg-1 protein, desensitized Bmax = 220 +/- 50 fmol mg-1 protein). 5. The histamine-mediated desensitization of response in guinea-pig slices was mediated by the HI receptor subtype, since the attentuated maximum histamine-stimulated [3H]-inositol phosphate accumulation could not be prevented by inclusion of an H2- (ranitidine) and an H3- (thioperamide) receptor antagonist during the pretreatment period.6. The desensitized histamine-stimulated [3H]-inositol phosphate accumulation recovered to 90% of control levels over a period of 150 min after the removal of the conditioning dose of histamine, with a half-time of recovery of about 95 min.

Histamine H1-receptor-mediated inositol phospholipid hydrolysis in DDT1MF-2 cells: agonist and antagonist properties

1. The effect of histamine H1-receptor stimulation on inositol phospholipid hydrolysis has been investigated in the hamster vas deferens smooth muscle cell line, DDT1MF-2. 2. Histamine (EC50 = 27 microM) stimulated the accumulation of [3H]-inositol phosphates in DDT1MF-2 cells prelabelled with [3H]-myo-inositol. 2-Thiazolylethylamine (EC50 42 microM) produced a maximal response of similar magnitude to histamine while the maximal response obtained with N alpha-methylhistamine (EC50 = 72 microM) and 2-pyridylethylamine (EC50 = 85 microM) were much lower (circa 65%, histamine = 100%). 3. The H1-selective agonists 2-(3-fluorophenyl)-histamine (2-FPH) and 2-(3-chlorophenyl)-histamine (2-CPH) both appeared to act as partial H1-agonists in this system. Both compounds produced maximal responses of only 30% (with respect to histamine = 100) and were able to antagonize the inositol phosphate response to histamine (estimated Kp = 10.4 and 18.9 microM for 2-FPH and 2-CPH respectively). 4. The response to histamine was antagonized by the H1-antagonists, mepyramine (KD 0.4 nM), (+)-chlorpheniramine (KD 1.2 nM) and promethazine (KD 0.3 nM). Furthermore, the (-)-isomer of chlorpheniramine was approx. three orders of magnitude less potent than the corresponding (+)-isomer. 5. The response to histamine (0.1 mM) was not altered by prior treatment of cells with pertussis toxin (100 ng ml-1; 24 h) whereas the inositol phosphate response to adenosine A1-receptor stimulation in this cell line was significantly attenuated under these conditions. 6. These data indicate that histamine-stimulated inositol phospholipid hydrolysis in DDT1MF-2 cells is mediated via a classical H1-receptor. Furthermore, the results also suggest that histamine HI- and adenosine A,-receptors activate phospholipase C in DDTMF-2 cells via two different G-protein-coupled pathways.

Short-term desensitization of the histamine H1 receptor in human HeLa cells: involvement of protein kinase C dependent and independent pathways

1. In this study we have investigated the effects of short-term exposure of cells to histamine on the subsequent H1 receptor responsiveness in HeLa cells, using Ca2+ fluorescence microscopy and video digital imaging. 2. In HeLa cells, histamine (100 microM) induces an immediate H1 receptor-mediated biphasic elevation of the intracellular Ca2+ concentration ([Ca2+]i) (basal [Ca2+]i: 81 +/- 30 nM, histamine-induced Ca2+ response: first phase: 1135 +/- 79 nM; second phase: 601 +/- 52 nM, n = 11). 3. The histamine H1 receptors on HeLa cells are readily susceptible to desensitization since repetitive exposure of the same group of cells to histamine (100 microM) markedly affected the release and influx component of the induced Ca2+ response (second application of histamine: first phase: 590 +/- 92 nM, second phase: 279 +/- 47 nM; third application of histamine: first phase: 454 +/- 127 nM, second phase: 240 +/- 45 nM, n = 6). Video digital imaging revealed an increase in the lag time between stimulation and monitoring of the Ca2+ response and a reduced increase in [Ca2+]i after desensitization with histamine. 4. Neither the release component of the ATP response (50 microM) nor the caffeine (3 mM)-induced Ca2+ release were found to be affected by desensitization with 100 microM histamine. However, the second phase of the ATP response was significantly reduced after desensitization with histamine (control cells: 516 +/- 33 nM; desensitized cells: 331 +/- 96 nM, n = 4, P < 0.05).5. Activation of protein kinase C (PKC) by phorbol-12-myristate-1 3-acetate was found to inhibit the histamine as well as ATP-induced Ca2" response in a dose-dependent manner.6. In PKC downregulated cells the second phase of the histamine-induced Ca2+ response was significantly elevated, indicating the involvement of PKC in the negative feedback on the Ca2+ influx(control cells: second phase: 601 +/- 52 nM (n = 11); PKC downregulated cells: second phase:890 +/- 90nM, n = I0, P<0.05).7. Homologous desensitization of H, receptor responsiveness was still observed in PKC downregulated cells, implying the rapid activation of a regulatory mechanism other than PKC.8. Based on our experimental data we suggest that short-term desensitization of the histamine H,receptor evolves from two different processes: a selective reduction of the histamine-induced Ca2+ release, mediated by a PKC-independent pathway, and a non-selective inhibition of the receptormediated Ca2+ influx activated by a PKC-dependent pathway.

Desensitization of histamine H1 receptor-mediated cyclic GMP production in guinea-pig lung

Histamine H1 receptor-mediated production of cGMP in guinea-pig lung tissue becomes rapidly desensitized after previous exposure to histamine. This desensitization is clearly concentration dependent and appears to be homologous. Responses to histamine are also inhibited by previous treatment with phorbol 12,13-dibutyrate. Yet, the time course of the inhibition is considerably slower and the maximal inhibition is significantly less compared to receptor desensitization. Moreover, the effects of the phorbol ester are not confined to H1 receptor responses. Since the effects of receptor desensitization are also not prevented by several protein kinase C inhibitors, the development of homologous H1 receptor desensitization is not dependent upon protein kinase C activation, but is caused by a yet unidentified mechanism.

The change in the threshold for short-term desensitization in isolated smooth muscle cells showing an all-or-none response to acetylcholine

1. Isolated smooth muscle cells from guinea-pig taenia caecum were prepared by collagenase digestion. The cells showed an all-or-none response to acetylcholine (ACh) under our experimental conditions. 2. Desensitized cells showed an all-or-none response but required a higher concentration of ACh for induction of contraction, indicating that the desensitization was due to a change in the threshold concentration. 3. In [3H]-quinuclidinyl benzilate ([3H]-QNB) binding to the desensitized cells, KD and Bmax were not significantly different from those estimated in the control cells. The competitive inhibition curve for specific binding of [3H]-QNB by ACh in the desensitized cells was in agreement with that of control cells. 4. The ACh-stimulated increase of the 45Ca2+ influx was very rapid and correlated well with the contraction of the cells. The concentrations of ACh inducing the maximal 45Ca2+ influx were increased by desensitization. 5. These results indicated that although the binding of ACh to the receptor was not changed by desensitization, the threshold concentration of ACh for their contraction was raised by desensitization, and the 45Ca2+ influx accompanying the contraction was shifted to the side of high concentration of ACh. 6. These results suggest that the development of short-term desensitization is due to an uncoupling of the receptor from the mechanism for initiation of the contraction.

Histamine H1-receptor-mediated cyclic GMP production in guinea-pig lung tissue is an L-arginine-dependent process

Histamine produces a rapid and massive increase of the c-GMP level of guinea-pig lung tissue. The EC50 value for this in vitro response is found to be 27 microM and the c-GMP level is maximally 9-fold elevated by 100 microM histamine. The response is stereoselectively inhibited by the enantiomers of chlorpheniramine, indicating H1-receptor involvement. Preincubation of lung tissue with 200 microM NCDC, a phospholipase C inhibitor, reduces the histamine (100 microM) responses to 16 +/- 3% (N = 6) of the control c-GMP production. Inhibition of protein kinase C by 50 microM H-7 does not significantly attenuate the H1-receptor response, whereas omittance of extracellular Ca2+ results in almost complete inhibition of the c-GMP production. The histamine-induced c-GMP response is inhibited by hemoglobin, methylene blue and the antioxidants butylated hydroxytoluene and nordihydroguaretic acid, indicating the involvement of a nitric oxide-dependent activation of soluble guanylate cyclase. This suggestion is supported by the concentration-dependent inhibition of the c-GMP production by NG-monomethyl-L-arginine (NMA). At a concentration of 20 microM NMA the histamine (100 microM) response is inhibited to 34 +/- 8% (N = 6) of the control response. This inhibition is reversed to 127 +/- 20% (N = 6) by the exogenous addition of 1 mM L-arginine. These findings show that after an initial H1-receptor-mediated, phospholipase C-dependent, Ca(2+)-mobilization the enzymatic conversion of L-arginine to nitric oxide is stimulated. This nitric oxide production is finally responsible for the activation of soluble guanylate cyclase, leading to the production of c-GMP.

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.