The effect of cyclic AMP elevating agents on bradykinin- and carbachol-induced signal transduction in canine cultured tracheal smooth muscle cells

The pharmacological rationale for combining muscarinic receptor antagonists and β-adrenoceptor agonists in the treatment of airway and bladder disease

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Muscarinic receptors increase smooth muscle tone in airways and urinary bladder.

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β-Adrenoceptors relax smooth muscle tone and oppose muscarinic contraction.

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Opposition involves transmitter release, signal transduction and receptor expression.

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This supports the combined use of muscarinic antagonists and β-adrenoceptor agonists.

Muscarinic receptor antagonists and β-adrenoceptor agonists are used in the treatment of obstructive airway disease and overactive bladder syndrome. Here we review the pharmacological rationale for their combination. Muscarinic receptors and β-adrenoceptors are physiological antagonists for smooth muscle tone in airways and bladder. Muscarinic agonism may attenuate β-adrenoceptor-mediated relaxation more than other contractile stimuli. Chronic treatment with one drug class may regulate expression of the target receptor but also that of the opposing receptor. Prejunctional β₂-adrenoceptors can enhance neuronal acetylcholine release. Moreover, at least in the airways, muscarinic receptors and β-adrenoceptors are expressed in different locations, indicating that only a combined modulation of both systems may cause dilatation along the entire bronchial tree. While all of these factors contribute to a rationale for a combination of muscarinic receptor antagonists and β-adrenoceptor agonists, the full value of such combination as compared to monotherapy can only be determined in clinical studies.

Increased expression of G alpha q protein in bronchial smooth muscle of mice with allergic bronchial asthma

In the present study, the change in Gq protein level in bronchial smooth muscle of mice with antigen-induced airway hyperresponsiveness (AHR) was determined. BALB/c mice were actively sensitized and repeatedly challenged with ovalbumin antigen to induce bronchial smooth muscle hyperresponsiveness. The contraction induced by 10 microM AlF4(-) (generated by 10 microM AlCl3 plus 10 mM NaF) of bronchial smooth muscles isolated from the antigen-challenged mice was significantly augmented as compared with that from the control animals. The G alpha q protein level determined by immunoblotting was also significantly increased in bronchial smooth muscles of the antigen-challenged group. Thus, an upregulation of G alpha q protein may be involved in the pathogenesis of bronchial smooth muscle hyperresponsiveness, one of the causes of AHR in asthmatics.

Induction of cyclooxygenase-2 by lipopolysaccharide in canine tracheal smooth muscle cells: involvement of p42/p44 and p38 mitogen-activated protein kinases and nuclear factor-kappaB pathways

Lipopolysaccharide (LPS) was found to induce inflammatory responses in the airways and exerted as a potent stimulus for PG synthesis. This study was to determine the mechanisms of LPS-enhanced cyclooxygenase (COX)-2 expression associated with PGE(2) synthesis in tracheal smooth muscle cells (TSMCs). LPS markedly increased the expression of COX-2 and release of PGE(2) in a time- and concentration-dependent manner, whereas COX-1 remained unaltered. Both the expression of COX-2 and the generation of PGE(2) in response to LPS were attenuated by a tyrosine kinase inhibitor genistein, a phosphatidylcholine-phospholipase C inhibitor D609, a phosphatidylinositol-phospholipase C inhibitor U73122, protein kinase C inhibitors, GF109203X and staurosporine, removal of Ca(2+) by addition of BAPTA/AM plus EGTA, and phosphatidylinositol 3-kinase (PI3-K) inhibitors, LY294002 and wortmannin. Furthermore, LPS-induced NF-kappaB activation correlated with the degradation of IkappaB-alpha, COX-2 expression, and PGE(2) synthesis, was inhibited by transfection with dominant negative mutants of NIK and IKK-alpha, but not by IKK-beta. LPS-induced COX-2 expression and PGE(2) synthesis were completely inhibited by PD98059 (an inhibitor of MEK1/2) and SB203580 (an inhibitor of p38 MAPK inhibitor), but these two inhibitors had no effect on LPS-induced NF-kappaB activation, indicating that NF-kappaB is activated by LPS independently of activation of p42/p44 MAPK and p38 MAPK pathways in TSMCs. Taken together, these findings suggest that the increased expression of COX-2 correlates with the release of PGE(2) from LPS-challenged TSMCs, at least in part, independently mediated through MAPKs and NF-kappaB signalling pathways. LPS-mediated responses were modulated by PLC, Ca(2+), PKC, tyrosine kinase, and PI3-K in these cells.

Interleukin-1beta-induced cyclooxygenase-2 expression is mediated through activation of p42/44 and p38 MAPKS, and NF-kappaB pathways in canine tracheal smooth muscle cells

Interleukin-beta (IL-1beta) was found to induce inflammatory responses in the airways, which exerted a potent stimulus for PG synthesis. This study was to determine the mechanisms of IL-1beta-enhanced cyclooxygenase (COX)-2 expression associated with PGE(2) synthesis in tracheal smooth muscle cells (TSMCs). IL-1beta markedly increased COX-2 expression and PGE(2) formation in a time- and concentration-dependent manner in TSMCs. Both COX-2 expression and PGE(2) formation in response to IL-1beta were attenuated by a tyrosine kinase inhibitor, genistein, a phosphatidylcholine-phospholipase C inhibitor, D609, a phosphatidylinositol-phospholipase C inhibitor, U73122, protein kinase C inhibitors, GF109203X and staurosporine, removal of Ca(2+) by addition of BAPTA/AM plus EGTA, and phosphatidylinositol 3-kinase (PI3-K) inhibitors, LY294002 and wortmannin. IL-1beta-induced activation of NF-kappaB correlated with the degradation of IkappaB-alpha in TSMCs. IL-1beta-induced NF-kappaB activation, COX-2 expression, and PGE(2) synthesis were inhibited by the dominant negative mutants of NIK and IKK-alpha, but not by IKK-beta. IL-1beta-induced COX-2 expression and PGE(2) synthesis were completely inhibited by PD98059 (an inhibitor of MEK1/2) and SB203580 (an inhibitor of p38 inhibitor), but these two inhibitors had no effect on IL-1beta-induced NF-kappaB activation, indicating that activation of p42/44 and p38 MAPK and NF-kappaB signalling pathways were independently required for these responses. These findings suggest that the increased expression of COX-2 correlates with the release of PGE(2) from IL-1beta-challenged TSMCs, at least in part, independently mediated through MAPKs and NF-kappaB signalling pathways in canine TSMCs. IL-1beta-mediated responses were modulated by PLC, Ca(2+), PKC, tyrosine kinase, and PI3-K in these cells.

Potentiation of [Ca2+]i response to angiotensin III by cAMP in cortical thick ascending limb

BACKGROUND

In the rat cortical thick ascending limb (CTAL), intracellular Ca2+ ([Ca2+]i) responses to angiotensin II (Ang II) and angiotensin III (Ang III) were mediated by the Ang II subtype 1A receptor (AT1A-R), whereas the arginine vasopressin (AVP)-dependent cAMP accumulation involved the vasopressin receptor type 2 (V2-R). This work was performed in CTAL to investigate the crosstalk between these two receptors by studying their transduction pathways.

METHODS

The cAMP-dependent pathway was activated by 10 minutes of prestimulation with either forskolin, CTP-cAMP or AVP, and Ang II/Ang III-induced [Ca2+]i responses were assessed.

RESULTS

Pretreatment with 5 micromol/L forskolin significantly enhanced the [Ca2+]i response induced by 10-7 mol/L either Ang II or Ang III. Analysis of dose-response curves to Ang III in forskolin-treated CTAL demonstrated that the maximal [Ca2+]i response was significantly increased without altering the EC50. In Ca2+-free medium, the forskolin-induced potentiation of the [Ca2+]i response to Ang III was weaker but always present, suggesting that this effect was not only due to intracellular Ca2+ release but also to extracellular Ca2+ influx. Furthermore, the fact that the forskolin-induced potentiation of the [Ca2+]i response to Ang III was blocked by 10 micromol/L H-89, a specific protein kinase A (PKA) inhibitor, indicated that this effect occurred via activation of PKA. Finally, the potentiation of the [Ca2+]i response to Ang III also was observed following pretreatment with 100 micromol/L CTP-cAMP or 10-7 mol/L AVP.

CONCLUSIONS

In CTAL, there is a positive crosstalk between the adenylyl cyclase and phosphoinositide pathways mediated by V2- and AT1A-R, respectively, through activation of PKA.

Gq protein level increases concurrently with antigen-induced airway hyperresponsiveness in rats

In the present study, bronchial Gq protein level of the airway hyperresponsive rats was determined by using immunoblot analysis. In the airway hyperresponsive rats that were sensitized and repeatedly antigen challenged, the in vitro bronchial responsiveness to acetylcholine was significantly enhanced as compared with that in the sensitized control group. Moreover, the bronchial contraction induced by 10 microM AlF(4)(-) (generated by 10 microM AlCl(3) plus 10 mM NaF) was significantly elevated after repeated antigen challenge (0.44+/-0.13 and 1.09+/-0.09 g tension in the control and airway hyperresponsive groups, respectively; P<0.01). In both groups, immunoblotting with the antibody against G alpha q gave a single 42 kD band. The G alpha q protein levels in the airway hyperresponsive group (0.58+/-0.12) estimated by G alpha q/beta-actin ratio was significantly greater than those in the control group (0.30+/-0.10; P<0.05). These findings suggest that the increase in G alpha q protein level may be involved in the pathogenesis of antigen-induced airway hyperresponsiveness in rats.

Forskolin inhibits 5-hydroxytryptamine-induced phosphoinositide hydrolysis and Ca+2 Mobilisation in canine cultured aorta smooth muscle cells

The effect of forskolin on 5-hydroxytryptamine (5-HT)-induced inositol phosphate (IP) and Ca2+ mobilisation was investigated in canine cultured aorta smooth muscle cells (ASMCs). Pretreatment of ASMCs with forskolin attenuated 5-HT-induced IP accumulation and Ca2+ mobilisation in a time- and concentration-dependent manner. The half-maximal effects (pEC50) of forskolin to attenuate IP and Ca2+ responses to 5-HT occurred at concentrations of 6.28 and 6.64, respectively. Pretreatment of ASMCs with cholera toxin caused a similar inhibition on 5-HT-induced responses. Even after treatment with forskolin for 24 h, the 5-HT-induced responses were still inhibited. The inhibitory effect of forskolin resulted from both a depression of the maximal response and a shift to the right of the concentration-effect curves of 5-HT in these responses. The water-soluble forskolin analogue L-858051 [7-deacetyl-7beta-(gamma-N-methylpiperazino)-butyryl forskolin] significantly inhibited the 5-HT-stimulated IP accumulation. In contrast, the addition of 1,9-dideoxy forskolin, an inactive forskolin analogue, had little effect on IP response. Moreover, SQ-22536 [9-(tetrahydro-2-furanyl)-9-H-purin-6-amine], an inhibitor of adenylate cyclase, and both H-89 [N-(2-aminoethyl)-5-iosquinolinesulphonamide] and HA-1004 [N-(2-guanidinoethyl)-5-iosquinolinesulphonamide], inhibitors of cAMP-dependent protein kinase (PKA), attenuated the ability of forskolin to inhibit the 5-HT-stimulated accumulation of IP in ASMCs. These results indicate that activation of cAMP/PKA might inhibit the 5-HT-stimulated IP accumulation and consequently reduce Ca2+ mobilisation, or inhibit both responses independently.

Mechanism of action of cholera toxin on the opossum internal anal sphincter smooth muscle

Cholera toxin (CTX), an activator of G(s) protein, is an important pharmacological tool in G protein research. The effect and the mechanism of action of CTX in the gastrointestinal smooth muscle, including the internal anal sphincter (IAS), are not known. The present investigation was carried out to examine the effects of CTX on the signal transduction associated with the adenylate cyclase (AC) pathway on the basal tone of the IAS smooth muscle. CTX caused a prompt and dose-dependent fall in the basal tone of the IAS that was not affected by the neurotoxins TTX and omega-conotoxin or the nitric oxide synthase inhibitor N(G)-nitro-L-arginine. The cyclooxygenase inhibitor indomethacin, cAMP-dependent protein kinase inhibitor Rp-8-bromoadenosine 3',5' cyclic monophosphorothioate inhibited CTX-induced IAS smooth muscle relaxation. Furthermore, CTX caused a concentration-dependent relaxation of the isolated smooth muscle cells (SMC) of the IAS, which was blocked by G(s)alpha antibody (G(s)alpha-Ab). The IAS smooth muscle relaxation was accompanied with an increase in the GTPase activity that was also specifically blocked by G(s)alpha-Ab. We conclude that a major part of the inhibitory action of CTX in the IAS is via the direct response of the SMC that is linked with G(s) protein to the AC pathway. A part of the inhibitory action of CTX on the smooth muscle occurs via the activation of cyclooxygenase pathway. The relative contribution of such actions of CTX in the smooth muscle in the gastrointestinal motility disturbances following cholera infection remains to be determined.

Dissociation of intracellular Ca2+ release and Ca2+ entry response to 5-hydroxytryptamine in cultured canine tracheal smooth muscle cells

The relationship between the agonist-sensitive Ca2+ pool and those discharged by the Ca2+ -ATPase inhibitor thapsigargin (TG) were investigated in canine tracheal smooth muscle cells (TSMCs). In fura-2-loaded TSMCs, 5-hydroxytryptamine (5-HT) stimulated a rapid increase in intracellular Ca2+ ([Ca2+]i), followed by a sustained plateau phase that was dependent on extracellular Ca2+. In such cells, TG produced a concentration-dependent increase in [Ca2+]i, which remained elevated over basal level for several minutes and was substantially attenuated in the absence of extracellular Ca2+. Application of 5-HT after TG demonstrated that the TG-sensitive compartment partly overlapped the 5-HT-sensitive stores. Pre-treatment of TSMCs with TG significantly inhibited the increase in [Ca2+]i induced by 5-HT in a time-dependent manner. Similar results were obtained with two other Ca2+ -ATPase inhibitors, cyclopiazonic acid and 2,5-di-t-butylhydroquinone. Although these inhibitors had no effect on phosphoinositide hydrolysis, Ca2+ -influx was stimulated by these agents. These results suggest that depletion of the agonist-sensitive Ca2+ stores is sufficient for activation of Ca2+ influx. Some characteristics of the Ca2+ -influx activated by depletion of internal Ca2+ stores were compared with those of the agonist-activated pathway. 5-HT-stimulated Ca2+ influx was inhibited by La3+, membrane depolarisation, and the novel Ca2+ -influx blocker 1-¿beta-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl¿-1H-imidazole hydrochloride (SKF96365). Likewise, activation of Ca2+ influx by TG also was blocked by La3+, membrane depolarisation, and SKF96365. These results suggest that (1) in the absence of PI hydrolysis, depletion of the agonist-sensitive internal Ca2+ stores in TSMCs is sufficient for activation of Ca2+ influx, and (2) the agonist-activated Ca2+ influx pathway and the influx pathway activated by depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool are indistinguishable.

B2 kinin receptor upregulation by cAMP is associated with BK-induced PGE2 production in rat mesangial cells

In the rat mesangial cell (MC), activation of the bradykinin B2 receptor (B2R) by bradykinin (BK) is associated with both phospholipase C (PLC) and A2 (PLA2) activities and with inhibition of adenosine 3',5'-cyclic monophosphate (cAMP) formation leading to cell contraction. Because cAMP plays an important role in the regulation of gene expression in general, we investigated the effect of increasing the intracellular cAMP concentration ([cAMP]i) in mesangial cells on the B2 mRNA expression, on the density of B2 receptor binding sites, on the BK-induced increase in both the free cytosolic Ca2+ concentration ([Ca2+]i), and in the prostaglandin E2 (PGE2) production. Forskolin, PGE2, and cAMP analog, 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), were used to increase [cAMP]i. Twenty-four-hour treatment with forskolin, PGE2, and 8-BrcAMP resulted in significant increases in B2 receptor binding sites, which were inhibited by cycloheximide. The maximum B2 receptor mRNA expression (160% above control) was observed in cells treated during 24 h with forskolin and was prevented by actinomycin D. In contrast, the D-myo-inositol 1,4,5-trisphosphate (IP3) formation and the BK-induced increase in [Ca2+]i, reflecting activation of PLC, were not affected by increased levels of [cAMP]i. However the BK-induced PGE2 release, reflecting PLA2 activity, was significantly enhanced. These data bring new information regarding the dual signaling pathways of B2 receptors that can be differentially regulated by cAMP.

Effect of forskolin on bradykinin-induced calcium mobilization in cultured canine tracheal smooth muscle cells

The effects of increases in intracellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) on bradykinin (BK)-induced generation of inositol phosphates (IPs) and Ca2+ mobilization were investigated in canine cultured tracheal smooth muscle cells (TSMCs). Pretreatment of TSMCs with either forskolin or dibutyryl cyclic AMP attenuated BK-stimulated responses. The inhibitory effects of these agents produced both a depression of the maximal response and a shift to the right of the concentration-response curves of BK. The water-soluble forskolin analogue L-858051, 7-deacetyl-7 beta-(r-N-methylpiperazino)-butyryl forskolin, significantly attenuated BK-stimulated IPs accumulation, while 1,9-dideoxy forskolin, an inactive forskolin, had little effect on IPs response. Moreover, SQ-22536, 9-(tetrahydro-2-furanyl)-9-H-purin-6-amine, an inhibitor of adenylate cyclase, and both H-89, N-(2-aminoethyl)-5-isoquinolinesulfonamide, and HA-1004, N-(2-guanidinoethyl)-5-isoquinolinesulfonamide, inhibitors of cyclic AMP-dependent protein kinase (PKA), reversed the ability of forskolin to attenuate BK-stimulated IPs accumulation. The KD and Bmax, values of the BK receptor for [3H]BK binding were not significantly changed by forskolin treatment for 30 min and 4 h. The AlF4(-)-induced IPs accumulation was attenuated by forskolin, indicating that G protein(s) are directly activated by AlF4- and uncoupled to phospholipase C by forskolin treatment. These results suggest that activation of cyclic AMP/PKA might inhibit the BK-stimulated PI breakdown and consequently reduce the [Ca2+]i increases or inhibit independently both responses, which is distal to the BK receptor in canine cultured TSMCs.

Pharmacological and functional characterization of bradykinin receptors in canine cultured tracheal epithelial cells

1. A direct [3H]-bradykinin ([3H]-BK) binding assay has been used to characterize the BK receptors in canine cultured tracheal epithelial cells (TECs). Based on receptor binding assay, TECs have specific, saturable, high-affinity binding sites for [3H]-BK. 2. The specific [3H]-BK binding was time- and temperature-dependent. Equilibrium of association of [3H]-BK with the BK receptors was attained within 30 min at room temperature and 1 h at 4 degrees C, respectively. 3. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 1.5 +/- 0.2 nM and a maximum receptor density (Bmax) of 53.2 +/- 5.2 fmol mg-1 protein. The Hill coefficient for [3H]-BK binding was 1.00 +/- 0.02. The association (K1) and dissociation (K-1) rate constants were (7.6 +/- 1.1) x 10(6) M-1 min-1 and (9.2 +/- 1.5) x 10 M-3 min-1, respectively. KD, calculated from the ratio of K-1 and K1, was 1.2 +/- 0.3 nM, a value close to that calculated from Scatchard plots of binding isotherms. 4. Neither a B1 receptor selective agonist (des-Arg9-BK, 0.1 nM - 10 microM) nor antagonist ([Leu8, des-Arg9]-BK, 0.1 nM - 10 microM) significantly inhibited [3H]-BK binding to TECs, which excludes the presence of B1 receptors in canine TECs. 5. The specific binding of [3H]-BK to canine TECs was inhibited by the B2 receptor selective antagonists ([D-Arg0, Hyp3, Thi5, D-Tic7, Oic8]-BK (Hoe 140, 0.1 nM-10 microM) and [D-Arg0, Hyp3, Thi5.8, D-Phe7]-BK, 0.1 nM - 10 microM) and agonists (BK and kallidin, 0.1 nM-10 microM) with a best fit by a one-binding site model. The order of potency for the inhibition of [3H]-BK binding was kallidin = BK = Hoe 140 > [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK. 6. BK and kallidin significantly induced concentration-dependent accumulation of IPs with a half-maximal response (EC50) at 17.6 +/- 3.5 and 26.6 +/- 5.3 nM, respectively, while the B1-selective agonist, des-Arg9-BK did not stimulate IPs accumulation and the B1-selective antagonist [Leu8, des-Arg9]-BK did not inhibit BK-induced IPs accumulation. Two B2-selective antagonists, Hoe 140 and [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK, inhibited BK-stimulated IPs accumulation with apparent pKB values of 8.8 +/- 0.3 and 7.0 +/- 0.3, respectively. 7. It is concluded that the pharmacological characteristics of the BK receptors in canine cultured TECs are primarily of the B2 receptor subtype which might regulate the function of tracheal epithelium through the activation of this receptor subtype coupling to PI hydrolysis.

Effect of cAMP elevating agents on carbachol-induced phosphoinositide hydrolysis and calcium mobilization in cultured canine tracheal smooth muscle cells

The effects of increases in intracellular adenosine 3',5'-cyclic monophosphate (cAMP) on carbachol-induced generation of inositol phosphates (IPs) and increases in intracellular Ca2+ ([Ca2+]i) were investigated in canine cultured tracheal smooth muscle cells (TSMCs). The cAMP elevating agents, cholera toxin (CTX) and forskolin, induced concentration- and time-dependent cAMP formation with half-maximal effects (-logEC50) at concentrations of 7.6 +/- 1.3 g/ml and 4.8 +/- 0.9 M, respectively. Forskolin caused a concentration-dependent inhibition of carbachol-induced increase in [Ca2+]i with half-maximal inhibition (-logEC50) at 5.2 +/- 0.7 M. Pretreatment of TSMCs with either CTX (10 micrograms/ml, 4 h), forskolin (10-100 microM, 30 min), or dibutyryl cAMP (1 mM, 30 min) inhibited carbachol-stimulated Ca2+ mobilization and IPs accumulation. The inhibitory effects of these agents produced both depression of the maximal response and a shift to the right of the concentration-response curve of carbachol without changing the EC50 values. After treatment with forskolin for 24 h, carbachol-induced IPs accumulation and Ca2+ mobilization were close to those of control group. SQ-22536 [9-(tetrahydro-2-furanyl)-9H-purin-6-amine, 10 microM], an inhibitor of adenylate cyclase, and HA-1004 [N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride, 50 microM], an inhibitor of cAMP-dependent protein kinase (PKA), attenuated the ability of forskolin to inhibit carbachol-induced IPs accumulation. Moreover, the inactive analogue of forskolin, 1,9-dideoxy forskolin, did not inhibit these responses evoked by carbachol, suggesting that activation of cAMP/PKA was involved in these inhibitory effects of forskolin. The KD and Bmax values of the muscarinic receptor (mAChR) for [3H]-N-methyl scopolamine binding were not significantly changed by forskolin treatment for 30 min and 24 h, suggesting that the inhibitory effect of forskolin is distal to the mAChR. The locus of this inhibition was further investigated by examining the effect of forskolin treatment on AIF4(-)-stimulated IPs accumulation in canine TSMCs. The AIF4(-)-induced response was inhibited by forskolin, supporting the notion that G protein(s) are directly activated by AIF4- and uncoupled to phospholipase C by forskolin treatment. We conclude that cAMP elevating agents inhibit carbachol-stimulated generation of IPs and Ca2+ mobilization in canine cultured TSMCs. Since generation of IPs and increases in [Ca2+]i are very early events in the activation of mAChRs, attenuation of these events by cAMP elevating agents might well contribute to the inhibitory effect of cAMP on tracheal smooth muscle formation.

Multiple mechanisms of bradykinin-induced contraction in rat and guinea pig smooth muscles in vitro

Bradykinin caused graded contraction in the guinea pig ileum, trachea and urinary bladder and rat uterus and vas deferens in vitro. The order of potency (EC50, nM) was: ileum (3) > uterus (5) > trachea (15) > vas deferens (41) > urinary bladder (52) and the maximal responses (percentage to 80 mM KCl) were: 152 +/- 8 (ileum), 122 +/- 6 (uterus), 97 +/- 3 (urinary bladder), 75 +/- 5 (trachea) and 33 +/- 3 (vas deferens). Responses to bradykinin in guinea pig ileum and urinary bladder and rat vas deferens and uterus were markedly attenuated in Ca(2+)-free medium with or without EGTA or by nicardipine, whereas those in guinea pig trachea depended almost exclusively on intracellular Ca2+ sources which were sensitive to ryanodine. Treatment of the animals with pertussis toxin only inhibited bradykinin-induced contraction of the rat uterus. Furthermore, the protein kinase C inhibitors, H7 (5-isoquinolinysulfonyl-2-methyl-piperazine) and staurosporine, antagonized in a graded manner bradykinin responses in guinea pig ileum and trachea and rat vas deferens, indicating the possible dependence on activation of protein kinase C mechanisms, while responses of the rat uterus rely on coupling by a pertussis toxin-sensitive G protein. Thus, bradykinin acting at B2 receptors may induce contractions in several smooth muscles from rat and guinea pig through activation of multiple second messenger pathways.

Ca2+ increase and Ca(2+)-influx in human tracheal smooth muscle cells: role of Ca2+ pools controlled by sarco-endoplasmic reticulum Ca(2+)-ATPase 2 isoform

1. The contribution of sarco-endoplasmic reticulum Ca(2+)-ATPases (SERCA)-regulated Ca2+ stores to the increase in intracellular free calcium ([Ca2+]i) induced by bradykinin (BK) was investigated in fura-2 loaded human tracheal smooth muscle cells (TSMC). For this purpose, we used thapsigargin, a selective inhibitor of Ca(2+)-ATPases of intracellular organelles. 2. Thapsigargin (10(-9) to 10(-6) M) induced a dose-dependent increase in [Ca2+]i in the presence of external Ca2+ with an EC50 value of 7.33 +/- 1.26 nM. In Ca(2+)-free conditions, the addition of Ca2+ (1.25 mM) caused an increase in [Ca2+]i which was directly proportional to the pre-incubation time of the cells with thapsigargin. Net increases of 60 +/- 9, 150 +/- 22 and 210 +/- 27 nM were obtained after 1, 3 and 5 min, respectively. 3. In the presence of extracellular Ca2+, BK induced a typical biphasic increase in [Ca2+]i with a fast transient phase and a sustained phase. The sustained component was reversed by addition of a bradykinin B2-receptor antagonist (Hoe 140, 10(-6) M) to the buffer as well as by deprivation of Ca2+. The transient phase induced by BK, histamine and carbachol was inhibited in a time-dependent way by preincubation of the cells with thapsigargin. 4. Comparative western blotting of human TSMC membranes using anti-SERCA2 isoform-specific antibodies clearly showed the greater expression of the 100-kDa SERCA2-b isoform compared with the SERCA2-a isoform. 5. Our data show that thapsigargin-sensitive Ca2+ stores contribute significantly to the activation of human TSMC which suggests a role for these stores in the subsequent induction of Ca2+ influx. These stores appear to be controlled by the Ca2+-ATPases (SERCA2-b isoform) which could also participate in the regulation of Ca2+ influx through the plasma membrane.

Pharmacological characterization of bradykinin receptors in canine cultured tracheal smooth muscle cells

1. [3H]-bradykinin was used to characterize the bradykinin receptors associated with canine cultured tracheal smooth muscle cells (TSMCs). Receptor binding assay showed that TSMCs had specific, saturable, high-affinity binding sites for [3H]-bradykinin. 2. The specific [3H]-bradykinin binding increased linearly with increasing cell concentrations. The equilibrium for association of [3H]-bradykinin with the bradykinin receptors was attained within 2 h at 4 degrees C and 1 h at room temperature, respectively. 3. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 2.5 +/- 0.3 nM and a maximum receptor density (Bmax) of 25.1 +/- 0.3 fmol mg-1 protein. The Hill coefficient for [3H]-bradykinin binding was 1.00 +/- 0.02. The association (K1) and dissociation (K-1) rate constants were (8.67 +/- 2.60) x 10(6) M-1 min-1 and 0.024 +/- 0.005 min-1, respectively. KD, calculated from the ratio of K-1 and K1 was 2.8 +/- 0.5 nM, a value close to that of KD calculated from Scatchard plots of binding isotherms. 4. The B1 receptor selective agonist, (des-Arg9-bradykinin, 0.1 nM-10 microM) and antagonist ([Leu8, des-Arg9]-bradykinin, 0.1 nM-10 microM) did not did not inhibit the [3H]-bradykinin binding to TSMCs, which excludes the presence of B1 receptors in canine TSMCs. 5. The specific binding of [3H]-bradykinin to canine TSMCs was inhibited by B2 receptor selective antagonists ([D-Arg0, Hyp3, Thi5, D-Tic7, Oicl-bradykinin, Hoe 140, 0.1 nM-10 micro M and [D-Arg0, Hyp3,Thi5,8, D-Phe7-bradykinin, 0.1 nM-10 micro M) and agonists (bradykinin and kallidin, 0.1 nM-10 micro M) with a best fit by a one-binding site model. The order of potency for the inhibition of [3H]-bradykinin binding was kallidin = bradykinin = Hoe 140> [D-Arg0, Hyp3, Thi5,8, D-Phel-bradykinin.6. Preincubation of TSMCs with forskolin for 24 h led to an up-regulation of B2 receptors, increasing in Bmax from 25.1 +/- 0.3 to 218 +/- 24 fmol mg-1 protein without changing the KD values. [3H]-bradykinin binding to TSMCs was inhibited by the B2 receptor selective antagonists and agonists, but not by the B1 receptor selective reagents. The up-regulation of the B2 receptor by forskolin was mediated through protein synthesis, since cycloheximide blocked this response.7 It is concluded that the pharmacological characteristics of the bradykinin receptors in canine cultured TSMCs are primarily of the B2 receptor subtype.