Modulation of carbachol-induced inositol phosphate formation in bovine tracheal smooth muscle by cyclic AMP phosphodiesterase inhibitors

Relaxant effect of YM976, a novel phosphodiesterase 4 inhibitor, on bovine tracheal smooth muscle

Effects of 4-(3-chlorophenyl)-1,7-diethylpyrido[2,3-d]pyrimidin-2(1H)-one (YM976), a novel and selective phosphodiesterase type 4 inhibitor, on tension and adenosine 3',5'-cyclic monophosphate (cAMP) content of bovine tracheal smooth muscle were compared with those of rolipram and theophylline. YM976, rolipram and theophylline relaxed the tracheal preparations contracted with histamine in a concentration-dependent manner. The relaxant effects of YM976 and rolipram were more potent than those of theophylline. These phosphodiesterase inhibitors-induced relaxations were dramatically diminished when tracheal smooth muscle was contracted with methacholine instead of histamine. Pretreatment of the tracheal preparations with YM976 (10 microM) or rolipram (10 microM), but not with theophylline (1 mM), shifted the concentration-response curves for contractile responses to histamine; however, the same procedure failed to affect concentration-response relationships for methacholine-induced contractions. At 1 and 10 microM, both YM976 and rolipram increased the tissues cAMP content. These results suggest that YM976 relaxes tracheal smooth muscle, probably through the cAMP-dependent mechanism.

The properties of ryanodine-sensitive Ca(2+) release in mouse gastric smooth muscle cells

1. Under voltage-clamped conditions, gastric smooth muscle cells of BALB/c mice developed spontaneous (STOCs) and caffeine- (I(CAF)) and carbachol-induced (I(CCh)) transient outward currents. 2. In fura-2 microscopic measurements of intracellular Ca(2+) concentration ([Ca(2+)](i)), caffeine and carbachol (CCh) provoked similar transient [Ca(2+)](i) elevations. 3. Both I(CCh) and CCh-induced [Ca(2+)](i) elevation of single smooth muscle cells occurred in an 'all-or-nothing' fashion in contrast to the reproducible caffeine responses. 4. On the basis of the suppression of STOCs and I(CAF) by nicardipine, tetraethylammonium and iberiotoxin, but not by charybdotoxin nor apamin, it was suggested that both currents were generated by large conductance type Ca(2+)-activated K(+) channels. 5. In measurements of isometric tension, caffeine produced relaxation of gastric smooth muscle strips in a concentration-dependent manner (0.1 -- 3 mM). The concentration-dependent relaxation with caffeine was mimicked by dibutyryl cyclic AMP which produced potentiation of contraction triggered by 50 mM KCL. 6. At caffeine concentrations >3 mM, a transient contraction followed by relaxation was provoked as the quasi maximal response to caffeine. In the quasi maximal response, caffeine acted as a potent relaxant in smooth muscle strips precontracted with 50 mM KCl or 3 microM CCh. 7. The relaxation with caffeine was significantly accelerated in those strips precontracted with KCl or CCh. All these results suggest that ryanodine-sensitive Ca(2+) release, which is triggered by caffeine, is an important modifier of Ca(2+) homeostasis in the cytoplasm and the contractility of gastric smooth muscle cells of mice.

Modulation of spasmogen-stimulated Ins(1,4,5)P3 generation and functional responses by selective inhibitors of types 3 and 4 phosphodiesterase in airways smooth muscle

1. The effects of isoenzyme-selective inhibitors of phosphodiesterases PDE3 and PDE4 on cyclic AMP concentration, two indices of phosphoinositide hydrolysis, and contractile responses to spasmogens have been investigated in bovine tracheal smooth muscle (BTSM). 2. Neither the PDE3-selective inhibitor ORG 9935, nor the PDE4-selective inhibitor rolipram increased cyclic AMP levels in BTSM. However, rolipram addition in the presence of PDE3 inhibition (ORG 9935; 1 microM) concentration-dependently (-log EC50 (M), 6.55+/-0.15; n = 3) increased cyclic AMP levels to about 70% of the maximal response to the beta-adrenoceptor agonist isoprenaline. 3. Rolipram per se inhibited histamine-stimulated [3H]-inositol (poly)phosphate ([3H]-InsP(X)) accumulation by > 80% (-log EC50 (M), 6.92+/-0.11; n = 3). Although ORG 9935 (1 microM) had little effect on histamine-stimulated [3H]-InsP(X) accumulation alone it greatly facilitated the inhibitory action of rolipram (-log EC50 (M), 8.82+/-0.39; n = 3). The effects of PDE3 and/or PDE4 inhibition on [3H]-InsP(X) accumulation stimulated by muscarinic acetylcholine (mACh) receptor activation were less marked. However, combined PDE3/4 inhibition significantly decreased this response at a submaximal concentration of mACh receptor agonist (carbachol; 1 microM). 4. The greater-than-additive effect of combined PDE3/4 inhibition was also observed at the level of contractile responses to histamine and carbachol. In experiments designed to investigate the effects of PDE3 and/or 4 inhibitors on the carbachol-mediated phasic contraction, additions of rolipram (10 microM) or ORG 9935 (1 microM) were without effect, whereas added together the inhibitors caused a significant (P < 0.01) 40% reduction in the peak phasic contractile response. 5. The effect on contraction correlated with a substantial inhibitory effect of PDE3/4 inhibition on the initial increase in inositol 1,4,5-trisphosphate (InsP3) accumulation stimulated by spasmogen. Thus, in the presence of ORG 9935 (1 microM) rolipram concentration-dependently inhibited carbachol-stimulated InsP3 accumulation by > or = 50% (-log EC50 (M), 6.77+/-0.21; n = 4). 6. Carbachol (100 microM) addition caused a rapid decrease (by 67% at 10 s) in BTSM cyclic AMP level in the presence of PDE3/4 inhibition. However, omission of Ca2+ from the incubation medium prevented the carbachol-evoked decrease in cyclic AMP and this coincided with a greater inhibition (> or = 80%) of the carbachol-stimulated InsP3 response. 7. These data indicate that combined PDE3 and PDE4 inhibition has greater-than-additive effects on second messenger and functional responses to spasmogens in BTSM. Furthermore, the ability of PDE3/4 inhibition significantly to attenuate mACh receptor-mediated contractile responses, may be, at least in part, attributed to an effect exerted at the level of InsP3 generation.

Salbutamol potentiates the relaxant effects of selective phosphodiesterase inhibitors on guinea pig isolated trachea

The ability of low concentrations of salbutamol to potentiate the relaxant effects of the phosphodiesterase (PDE) inhibitors, rolipram, Ro 20-1724 (PDE type IV inhibitor), siguazodan and milrinone (PDE type III inhibitor) was studied on guinea pig isolated trachea. These PDE inhibitors were strong relaxants of guinea pig trachealis under basal tone, but had only a weak activity on tissues precontracted with histamine (10(-5) M). In both cases, PDE type IV inhibitors showed a relaxant effect composed of two phases. The first phase represented 20 and 40% and the second, 90 and 140%, respectively, of relaxation of basal tone and histamine-induced tone. A second characteristic of PDE type IV inhibitors was the very fast and partially reversible relaxation observed at concentrations greater than 3 x 10(-8) M (for histamine-induced tone) at the first addition of inhibitor, followed by a residual relaxant activity. The latter relaxant effect was stable at concentrations of 3 x 10(-8)-10(-5) M and was equivalent to a 20% relaxation (for histamine-induced tone). In the presence of low concentrations (10(-9) and 10(-8) M) of salbutamol, there was a significant concentration-dependent potentiation of the effects of PDE inhibitors on trachea precontracted with histamine. Salbutamol, at a concentration of 10(-9) M, potentiated the effects of PDE inhibitors between 1.4- and 3.6-fold. In the presence of salbutamol 10(-8) M, the potentiation was more marked for siguazodan (37.9-fold), milrinone (11.0-fold) and Ro 20-1724 (14.5-fold) than for rolipram (4.3-fold). These results suggest that low concentrations of salbutamol can potentiate the relaxant effects of both PDE type III and PDE type IV inhibitors. Thus, PDE type IV inhibitors, which have antiinflammatory properties, could also provide adequate bronchodilation when used in combination with lower than usual doses of beta 2-agonists.

Effects of IZP-94005 (contignasterol) on antigen-induced bronchial responsiveness in ovalbumin-sensitized guinea-pigs

1. We have investigated the novel naturally occurring marine compound, IZP-94005 (contignasterol), as a potential anti-asthma agent, using both in vivo and in vitro models of allergen-induced bronchoconstriction and airway smooth muscle contraction. 2. Tracheal rings from ovalbumin (OA)-sensitized guinea-pigs were treated with various concentrations of IZP-94005 for 20 min prior to challenge with ovalbumin. IZP-94005 (3-30 microM) inhibited responses of sensitized tracheal rings stimulated with OA in a concentration-dependent manner, with an IC50 of 10 microM. 3. IZP-94005 (10 microM) had no effect on carbachol-induced contractions of sensitized guinea-pig tracheal rings, although it did inhibit histamine-induced responses of OA sensitized guinea-pig tracheal rings. 4. The effects of IZP-94005 in vivo were examined using OA-sensitized guinea-pigs which were tracheotomized under anaesthesia and placed in a body plethysmograph. Measurements of lung resistance and compliance were performed by isovolumetric analysis of volume and trans-pulmonary pressure. 5. IZP-94005 (50 and 200 micrograms kg-1), by inhalation 20 min prior to OA challenge caused significant inhibition of the increase in lung resistance induced by OA in sensitized guinea-pigs, compared to vehicle-treated animals. Nedocromil sodium (20 mg kg-1), with a similar protocol, also inhibited OA-induced responses in this model. 6. We therefore suggest that IZP-94005 is a good candidate for further investigation as a possible antiasthma agent.

Effect of carbachol on phospholipase C-mediated phosphatidylinositol 4,5-bisphosphate hydrolysis, and its modulation by isoproterenol in rabbit corneal epithelial cells

The effects of carbachol (CCh) on phospholipase C(PLC)-mediated phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and its modulation by isoproterenol were investigated in SV40-adenovirus transformed rabbit corneal epithelial cells (RCEC). When examined under light microscope, these cells exhibited a cobblestone-like appearance typical of the corneal epithelial cells grown in primary culture. Addition of CCh (0.1 mM) for 30 min to RCEC, prelabeled with 32Pi, decreased the radioactivity in phosphatidylinositol 4-phosphate and PIP2 by 15 and 27%, respectively, and concomitantly increased the radioactivity in phosphatidylinositol and phosphatidic acid by 14 and 38%, respectively. When the concentration of CCh was increased to 1 mM, the changes in radioactivity were even more pronounced. Addition of CCh (0.1 mM) to the cells, prelabeled with myo[3H]inositol, increased the accumulation of [3H]inositol 1,4,5-trisphosphate ([3H]InsP3) by 115%, indicating stimulation of PLC-mediated PIP2 hydrolysis. Similar increases were also observed in [3H]InsP1 and [3H]InsP2. The effects of CCh on inositol phosphate accumulation were time- and dose-dependent, and were inhibited by atropine (10 microM), suggesting that the observed effects of CCh were mediated by activation of muscarinic cholinergic receptors. The effects of CCh were antagonized more potently by 4-diphenylacetoxy N-methyl-piperidine than by pirenzepine, indicating that the muscarinic receptors involved in PLC activation are probably of M3 type. By Western immunoblotting analysis with various anti-PLC antibodies, the RCEC were shown to contain PLC gamma 1 and PLC delta 1 in the soluble fraction and PLC beta 1 in the microsomal fraction. Addition of isoproterenol to RCEC, increased cAMP both in a time- and dose-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of muscarinic receptor-induced inositol phospholipid hydrolysis by caffeine, beta-adrenoceptors and protein kinase C in intestinal smooth muscle

1. The effects of caffeine, isoprenaline, dibutyryl cyclic AMP, isobutylmethylxanthine (IBMX), 12-O-tetradecanoylphorbol-13-acetate (TPA) or 1-oleoyl-2-acetylglycerol (OAG), (protein kinase C (PKC) activators), 2-methoxy verapamil (D600), thapsigargin and ryanodine on muscarinic acetylcholine receptor (AChR)-stimulated inositol phospholipid hydrolysis were studied in smooth muscle fragments from the longitudinal layer of the small intestine of the guinea-pig. 2. Incubation of the fragments with the muscarinic agonist, carbachol (CCh) (100 microM) resulted in rapid increases in the levels of all the inositol phosphate isomers with maximal increases in the [3H]-inositol (1,4,5) trisphosphate ([3H]-Ins(1,4,5)P3) isomer occurring 10 s following incubation. 3. The beta-adrenoceptor agonist, isoprenaline (10 microM) and dibutyryl cyclic AMP (10 microM), a membrane permeant analogue of cyclic AMP both reduced the CCh stimulation, but not the basal levels of [3H]-inositol phosphates. This inhibition by dibutyryl cyclic AMP was enhanced in the presence of the phosphodiesterase inhibitor, IBMX. CCh inhibited the isoprenaline-induced increases in the levels of cyclic AMP and this was via a pertussi toxin (PTX)-sensitive G-protein mechanism. 4. TPA (1 microM) and OAG (100 microM) a 1,2-diacylglycerol (DAG) analogue both reduced the CCh-induced increases in [3H]-inositol phosphates levels but neither affected basal values nor the basal levels of cyclic AMP. 5. D600 (10 microM), which blocks voltage-dependent Ca2+ channels, also reduced the CCh-stimulated levels of [3H]-inositol phosphates suggesting that some of the agonist-induced increases are due to a potentiating effect of Ca2+ entering the cell. 6. Caffeine (0.5-30 mM) significantly inhibited both the basal and CCh-induced increases in all the [3H]-inositol phosphate isomers. Its inhibitory action was not due to increases in cyclic AMP since caffeine had no effect on the levels of cyclic AMP at concentrations up to 30 mM. 7. Incubation with thapsigargin (1 microM) and ryanodine (10 microM) had no effect on either basal or CCh-induced inositol phospholipid hydrolysis or cyclic AMP levels. 8. The results indicate a reciprocal inhibition by beta-adrenoceptors and muscarinic AChRs of their effects on cyclic AMP and inositol phosphate levels respectively. Ca2+ entering the cell (but not the action of ryanodine or thapsigargin) potentiates while caffeine inhibits muscarinic AChR-induced rises in inositol phosphate levels. Diacylglycerols may exert a negative feedback inhibition on inositol phosphate production.

Inhibitory effect of the nonpeptide angiotensin II receptor antagonist losartan and its active metabolite, E-3174, on cAMP phosphodiesterase: additional action of the antagonists

The inhibitory effects of the nonpeptide angiotensin II (AII) receptor antagonist losartan and its active metabolite, E-3174, on bovine brain calcium/calmodulin-dependent 3':5'-cyclic nucleotide phosphodiesterase (cAMP PDE) were investigated. Losartan and E-3174 inhibited cAMP PDE activity competitively with an apparent Ki of 18.7 +/- 2 microM (N = 3) and 70.4 +/- 8 microM (N = 3) with respect to cAMP, respectively. With 1.2 mM cAMP as a substrate, cAMP PDE activities were inhibited by losartan and E-3174 in a concentration-dependent manner. The concentrations of losartan and E-3174 required to obtain 50% inhibition of the enzyme activity (IC50) were estimated to be 38.9 +/- 7 microM (N = 3) and 139.3 +/- 39 microM (N = 3), respectively. These results show that losartan is about four times more potent than E-3174 in inhibiting the enzyme. The Hill coefficient of -1.0 +/- -0.04 (N = 3) for losartan and -1.1 +/- -0.14 (N = 3) for E-3174 was obtained, indicating that one inhibitor binding site is available on cAMP PDE. This study demonstrated that losartan and E-3174 exert additional inhibitory action on cAMP PDE besides AII receptor antagonism.

Relaxation of guinea-pig trachea by cyclic AMP phosphodiesterase inhibitors and their enhancement by sodium nitroprusside

1. The effects of agents that elevate either cyclic AMP (the phosphodiesterase (PDE) III inhibitor siguazodan, salbutamol) or cyclic GMP (sodium nitroprusside (SNP)) on the relaxant activity of the PDE IV inhibitor, rolipram, were investigated in carbachol (0.1 microM) precontracted guinea-pig tracheal sheets. 2. Rolipram, siguazodan and SNP caused concentration-related reductions in tone of tissues precontracted with 0.1 microM carbachol (EC50 values 12.5; 2.73 and 0.35 microM respectively). Whilst the concentration-response relationship for the PDE III inhibitor, siguazodan, was monophasic that of the PDE IV inhibitor, rolipram, was biphasic. 3. The relaxant activity of rolipram was markedly enhanced in the presence of 10 microM siguazodan (EC50 < 0.01 microM), 0.1 microM salbutamol (EC50 0.03 microM) and 0.3 microM SNP (EC50 0.03 microM). In contrast, the relaxant activity of siguazodan was unaffected by SNP and only modestly enhanced by rolipram (10 microM) and salbutamol (0.1 microM). 4. The relaxant activity of SNP was enhanced by the PDE V inhibitor SK&F 96231 (30 microM: EC50 0.06 microM) and rolipram (30 microM, EC50 0.08 microM) but was unaffected by 30 microM siguazodan. 5. At concentrations up to 10 microM, neither siguazodan nor rolipram elevated tracheal cyclic AMP levels. However, the combination of 10 microM rolipram and siguazodan caused a two fold increase in the cyclic AMP content (from 2.19 to 4.36 pmol cyclic AMP mg-1 protein). SNP (0.1-10 microM) failed to produce a significant increase in tracheal cyclic AMP levels. At 0.1 microM the effect of SNP on tracheal cyclic AMP levels was significantly (P < 0.05) increased in the presence of rolipram but not siguadozan. 6. The results indicate that the relaxant effects of rolipram are markedly enhanced by agents that inhibit PDE III activity or elevate cyclic GMP. They support the hypothesis that SNP potentiates the effects of rolipram via the inhibitory action of cyclic GMP on hydrolysis of cyclic AMP by PDE III. The findings also suggest that whilst PDE III may be more significant in regulating basal smooth muscle tone in the absence of any exogenous stimulus to cyclic AMP accumulation, PDE IV activity may be more tightly coupled to the pool of adenylyl cyclase stimulated by beta2-adrenoceptor agonists.

Phosphoinositide metabolism in airway smooth muscle

Agonist-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, which generates inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol, is thought to be one of the major mechanisms underlying pharmacomechanical coupling in airway smooth muscle. This article is a review of the currently available information on phosphoinositide and inositol 1,4,5-trisphosphate metabolism in this tissue and includes data on inositol 1,4,5-trisphosphate-induced Ca2+ release and the receptor mediating this effect. The final section outlines the potential mechanisms underlying physiological regulation of phosphoinositide metabolism by other second-messenger pathways operative in this tissue.

Pulmonary effects of type V cyclic GMP specific phosphodiesterase inhibition in the anaesthetized guinea-pig

1. We have investigated the bronchodilator potential of type V phosphodiesterase (PDE V) inhibitors in anaesthetized ventilated guinea-pigs using the potent and selective PDE V inhibitor, SK&F 96231. We have compared its activity to that of salbutamol, the PDE III inhibitors, siguazodan and SK&F 95654 and to the PDE IV inhibitor rolipram. 2. Administered as an i.v. infusion SK&F 96231 (0.6 and 1 mg kg-1 min-1, i.v.) caused a slowly developing inhibition of histamine (100 nmol kg-1, i.v.)-induced bronchoconstriction and elevated tracheal cyclic GMP levels in the anaesthetized guinea-pig. SK&F 96231 (0.1 and 0.3 mg kg-1 min-1, i.v.) was without effect on histamine-induced bronchoconstriction. In the presence of a sub-threshold infusion of SNP (0.1 mumol kg-1 min-1, i.v.) there was a marked enhancement of SK&F 96231-induced inhibition of histamine responses such that at infusion rates that were ineffective alone, SK&F 96231 caused a > 50% inhibition of histamine responses. The stimulation of tracheal cyclic GMP accumulation by SK&F 96231 was also potentiated. 3. Administered directly into the airway, SK&F 96231 (300 micrograms in 5 mg lactose carrier) was largely without effect on histamine-induced bronchoconstriction (4.9 +/- 1.9% inhibition). In the presence of SNP (0.1 mumol kg-1 min-1, i.v.) or isosorbide dinitrate (200 micrograms administered by insufflation into the trachea) there was a marked potentiation of the inhibitory activity of SK&F 96231 (40 +/- 4% and 62 +/- 1.8% respectively). 4. Salbutamol and rolipram (3-300 microg by insufflation) caused a dose-related inhibition of histamine responses with a maximum of 91 +/- 2% and 59 +/- 10% respectively. The PDE III inhibitor, siguazodan,was without effect on histamine responses but they were reduced (27.7 +/- 4.8% at 300 microg) by SK&F95654. There was a marked enhancement of the inhibitory activity of rolipram in the presence of SK&F 95654.5. We conclude that SK&F 96231 has weak anti-spasmogenic activity in the guinea-pig in vivo, we suggest that this is primarily a consequence of a low endogenous guanylate cyclase activity in the airway. The potentiation of the anti-spasmogenic activity of SK&F 96231 by SNP suggests that a combination of PDE V inhibitor and guanylate cyclase agonist might provide significant bronchodilator activity.6. We have established that PDE IV inhibitors are bronchodilators when administered directly into the airway of anaesthetized guinea-pigs but that PDE III inhibitors are only weakly active. The marked enhancement of the inhibitory activity of rolipram by the PDE III inhibitor, SK&F 95654, indicates that inhibitors of both PDE III and PDE IV might offer greater potential as bronchodilators than inhibitors of either isoenzyme alone.

The effect of Ca2+ on the translocation of protein kinase C in bovine tracheal smooth muscle

The role of calcium in protein kinase C redistribution was studied in bovine tracheal smooth muscle preparations contracted by methacholine. Previous results have shown that, in the presence of normal extracellular Ca2+, 10 microM methacholine produced a sustained contraction and a sustained translocation of protein kinase C from the cytosol to the membrane. In the present study, when tissues were preincubated in Ca(2+)-free buffer containing 1 mM EGTA, methacholine produced a rapid but transient elevation in membrane-associated protein kinase C activity which was detected at 30 s and had returned to basal within 20 min. The redistribution of protein kinase C from the cytosol to the membrane induced by 1 microM methacholine in normal Ca2+ was reversed by removal of the extracellular Ca2+ and addition of 2 mM EGTA during agonist stimulation. Removal of the Ca2+ caused approximately 50% relaxation after 10 min. Verapamil (30 microM) partially reversed the methacholine-induced protein kinase C redistribution and caused approximately 40% relaxation after 15 min. Sodium nitroprusside (10 microM) caused a rapid relaxation and complete reversal of the protein kinase C redistribution induced by methacholine. High K+ (60 mM) also induced a sustained contraction and redistribution of protein kinase C from the cytosol to the membrane. Suitable antagonists were added to the bathing medium to block the effects of endogenous mediators which could be released by KCl-induced depolarization. Thus, translocation of protein kinase C is obtained in the absence of receptor activation.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of elevated cyclic AMP levels on histamine-H1-receptor-stimulated inositol phospholipid hydrolysis and calcium mobilization in the smooth-muscle cell line DDT1MF-2

The effects of raising cyclic AMP levels, by forskolin stimulation, beta-adrenoceptor activation or cyclic AMP phosphodiesterase inhibition, on inositol phospholipid hydrolysis and increases in intracellular free [Ca2+] ([Ca2+]i) elicited by a range of agonists have been investigated in the hamster vas deferens smooth-muscle cell line DDT1MF-2. Isoprenaline (log [EC50 (M)] = -7.7 +/- 0.2), forskolin and the type IV cyclic AMP phosphodiesterase inhibitor rolipram elicited significant increases in the accumulation of cyclic [3H]AMP. Pretreatment with forskolin (10 microM) attenuated histamine (100 microM)- and N6-cyclopentyladenosine (CPA; 300 nM)-induced release of intracellular Ca2+, observed when cells are stimulated in Ca(2+)-free buffer containing 0.1 mM EGTA. Forskolin had no effect on ATP (100 microM)- or bradykinin (1 microM)-stimulated release of intracellular Ca2+. Histamine-induced intracellular Ca2+ release was also inhibited by pretreatment with rolipram (100 microM) or the membrane-permeant cyclic AMP analogue (Sp)-adenosine 3',5'-monophosphothioate (100 microM). Isoprenaline (1 microM) pretreatment (in the presence of 10 microM rolipram, a concentration which on its own did not decrease the histamine response) attenuated histamine-induced intracellular Ca2+ release. Forskolin inhibited histamine (100 microM)- and CPA (100 nM) stimulated accumulation of [3H]-inositol phosphates, but was without effect on ATP or bradykinin responses. Addition of forskolin (in the presence of 100 microM rolipram) after the cells had been stimulated with histamine (in experiments initiated in Ca(2+)-free buffer) inhibited the rise in [Ca2+]i observed when extracellular Ca2+ (2 mM) was re-applied (owing to receptor-mediated Ca2+ influx). Finally, the refilling of intracellular Ca2+ stores (after receptor-mediated Ca2+ influx is blocked by mepyramine) can be demonstrated in the presence of raised cyclic AMP levels.

Theophylline produces over-additive relaxation of canine tracheal smooth muscle when combined with beta-agonists: The dose-response relationship

The interaction between theophylline (T) and the beta-agonists albuterol (A) and isoproterenol (I) was examined using canine cervical tracheal smooth muscle devoid of epithelium contracted with 0.1 or 0.3 microM methacholine. Greater functional antagonism with beta-agonists vs. T was confirmed and an ability of T to potentiate beta-agonist relaxation was demonstrated. The EC50 for T increased from 0.13 +/- 0.02 to 0.37 +/- 0.07 mM (mean +/- SEM) in preparations contracted with 0.1 or 0.3 microM methacholine, respectively, while that for I increased from 0.036 +/- 0.008 to 0.17 +/- 0.03 microM, a significantly larger change (P < 0.025). In tissues contracted with 0.3 microM methacholine and pretreated with 10 micrograms/ml of T IC50 values from composite concentration-response curves for I and A were displaced to the left and Emax was increased (56.6 to 71.5% for I, 44 to 61% for A, P < 0.0002). Addition of 10 micrograms/ml T resulted in relaxations which exceeded that calculated by the fractional product method for additive, independent action (P < 0.0001 for I, P < 0.0002 for A at 0.3 microM methacholine), suggesting that at least part of T's action was over-additive. Five, 10 and 20 micrograms/ml T enhanced the effectiveness of single concentrations of I by factors of 1.47 +/- 0.14 (P < 0.05), 2.72 +/- 0.26 (P < 0.01) and 5.34 +/- 0.55 (P < 0.01), respectively, in preparations contracted with 0.1 microM methacholine: I enhanced the effectiveness to a lesser degree. Using two approaches, positive interaction or over-additivity between T and beta-agonists has been demonstrated.

Translocation of protein kinase C in bovine tracheal smooth muscle strips: the effect of methacholine and isoprenaline

Investigations into the mechanisms involved in the contraction of smooth muscle have suggested that the generation of diacylglycerol and the activation of protein kinase C (PKC) may be important in the generation or maintenance of smooth muscle tone. The present study examined the possible role of PKC in the contraction of bovine tracheal smooth muscle. Methacholine (10 microM) induced a rapid elevation in PKC activity associated with the membrane fraction. PKC levels were significantly elevated in the membrane fraction 30 s after agonist addition, reached a maximum at 1 min and then declined to and remained at a lower level which was still elevated above basal. A concomitant decrease in cytosolic PKC activity of smaller magnitude was observed during this period of stimulation. This methacholine-induced re-distribution of PKC from the cytosol to the membrane was concentration-dependent and was blocked by atropine. Pre-treatment of tissues for 2 min with 100 microM isoprenaline prevented both the re-distribution of PKC and the contraction produced by 1 microM methacholine. Addition of 1 microM isoprenaline to tissues pre-contracted with 1 microM methacholine reversed the re-distribution of PKC produced during contraction and completely relaxed the tissues. Thus, under these conditions, translocation of PKC from the cytosol to the membrane seems to be well correlated with contractions of bovine tracheal smooth muscle. Whether the PKC translocation is responsible for the observed changes in muscle tone or whether the enzyme translocation is a result of drug-induced changes in [Ca2+]i, remains to be determined.

Control of cyclic AMP levels in primary cultures of human tracheal smooth muscle cells

1. [3H]-adenosine 3':5'-cyclic monophosphate ([3H]-cyclic AMP) responses were studied in primary cultures of human tracheal smooth muscle cells derived from explants of human trachealis muscle and in short term cultures of acutely dissociated trachealis cells. 2. Isoprenaline induced concentration-dependent [3H]-cyclic AMP formation with an EC50 of 0.2 microM. The response to 10 microM isoprenaline reached a maximum after 5-10 min stimulation and remained stable for periods of up to 1 h. After 10 min stimulation, 1 microM isoprenaline produced a 9.5 fold increase over basal [3H]-cyclic AMP levels. The response to isoprenaline was inhibited by ICI 118551 (10 nM), (apparent KA 1.9 x 10(9) M-1) indicating the probable involvement of a beta 2-adrenoceptor in this response in human cultured tracheal smooth muscle cells. However, with 50 nM ICI 118551 there was a reduction in the maximum response to isoprenaline. Prostaglandin E2 also produced concentration-dependent [3H]-cyclic AMP formation (EC50 0.7 microM, response to 1 microM PGE2 6.4 fold over basal). 3. Forskolin (1 nM - 100 microM) induced concentration-dependent [3H]-cyclic AMP formation in these cells. A 1.6 fold (over basal) response was also observed following stimulation with NaF (10 mM). 4. The nonselective phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) (0.1 mM) and the type IV, cyclic AMP selective, phosphodiesterase inhibitor rolipram (0.1 mM) both elevated basal [3H]-cyclic AMP levels by 1.8 and 1.5 fold respectively. IBMX (1-100 microM) and low concentrations of rolipram (< 10 microM), also potentiated the response to 1 microM isoprenaline. Inhibitors of the type III phosphodiesterase isoenzyme (SK&F 94120 and SK&F 94836) were without effect upon basal or isoprenaline-stimulated cyclic AMP responses in these cells.5. Carbachol (1 nM-I 00 microM) produced concentration-dependent inhibition of the [3H]-cyclic AMP response to 1 microM isoprenaline in human cultured tracheal smooth muscle cells (IC50 0.24 JM). Carbachol(1 JM) inhibited the [3H]-cyclic AMP response to 1 JM isoprenaline by 60%. This effect of carbachol was itself inhibited by atropine (50 nM) (KA 2.3 x 109 M-') indicating the involvement of a muscarinic receptor.6. These results show that primary cultures of human tracheal smooth muscle cells demonstrate cyclic AMP responses to direct receptor stimulation, adenylyl cyclase activation and inhibition with nonselective and type IV-selective cyclic AMP phosphodiesterase isoenzyme inhibitors, and that the cyclic AMP response to isoprenaline can be inhibited by muscarinic receptor stimulation.

Human bronchial cyclic nucleotide phosphodiesterase isoenzymes: biochemical and pharmacological analysis using selective inhibitors

1 The aims of the present study were to characterize the cyclic nucleotide phosphodiesterase (PDE) isoenzyme activities present in human bronchi and to examine the ability of selective isoenzyme inhibitors to relax histamine and methacholine precontracted preparations of human bronchi. 2 Three separations of pooled human bronchial tissue samples were performed. Ion-exchange chromatography showed that the soluble fraction of human bronchial preparations contains PDE I, II, III, IV and V isoenzyme activities. Multiple forms of PDE I and PDE IV were observed and PDE IV was the main cyclic AMP hydrolytic activity. 3 3-Isobutyl-l-methylxanthine (IBMX) non-selectively inhibited all separated isoenzyme activities. Zaprinast selectively inhibited PDE V, but also effectively inhibited one of the two PDE I isoforms identified. The PDE IV selective inhibitors rolipram and RO-201724, inhibited the PDE IV activities as did the dual PDE III/IV inhibitor, Org 30029. Org 9935, a PDE III selective inhibitor, potently attenuated part of the PDE IV activity peak in one of three separations performed, indicating that some PDE III activity may co-elute with PDE IV under the experimental conditions employed. 4 PDE IV-selective (rolipram), PDE III-selective (Org 9935) and dual PDE III/IV (Org 30029) inhibitors were effective relaxants of human bronchial smooth muscle. The PDE V/PDE I inhibitor, zaprinast was relatively ineffective. 5 The present study demonstrates in human bronchi, as in animal airways smooth muscle, that inhibitors of PDE III, PDEIV and dual PDE III/IV have potentially useful bronchodilator activity and are worthy of further consideration as anti-asthma drugs.

Comparative effects of BRL 38227, nitrendipine and isoprenaline on carbachol- and histamine-stimulated phosphoinositide metabolism in airway smooth muscle

1. The ability of BRL 38227 and nitrendipine to affect muscarinic agonist and histamine-stimulated [3H]-inositol phosphate accumulation in slices of bovine tracheal smooth muscle has been studied and compared with the established inhibitory effects of isoprenaline on this pathway. 2. Pre-addition of BRL 38227 (5 microM), nitrendipine (1 microM) or isoprenaline (10 microM) significantly inhibited the subsequent inositol phosphate response to histamine at all concentrations studied (10- 1000 microM). BRL 38227 and nitrendipine also significantly inhibited the [3H]-inositol phosphate response to low (1 microM), but not high (100 microM) concentrations of carbachol. Isoprenaline had no effect at any concentration of carbachol studied. 3. Nitrendipine (IC50 = 95 nM) and BRL 38227 (IC50 = 322 nM) caused concentration-related inhibitions of the inositol phosphate response to histamine (100 microM). Similar maximal inhibitions were caused by each agent (55-58%). Inhibitory effect of BRL 38227 was reduced in potency (IC50 = 5.5 microM), but not magnitude, in the presence of glibenclamide (0.5 microM). 4. Time-course studies comparing the effects of BRL 38227 addition 15 min before, and 10 min after histamine challenge showed that for pre-addition a distinct (less than 2 min) lag occurred following histamine addition before the inhibitory effect of BRL 38227 was manifest. In contrast, when BRL 38227 was added 10 min after histamine, an inhibitory effect was immediately apparent. 5. Further evidence for an initial, 'protected' phase of inositol phosphate accumulation was provided by the finding that BRL 38227 pre-addition had no effect on the early (0-300 s) time-course of inositol 1,4,5-trisphosphate mass accumulation. 6. The inhibitory effect of BRL 38227, but not that of nitrendipine or isoprenaline, on histaminestimulated [3H]-inositol phosphate accumulation was completely prevented in the presence of an elevated extracellular K+ (65 mM) concentration. 7. The results demonstrate that membrane hyperpolarization, and/or blockade of voltage-operated Ca2"-channels can regulate agonist-stimulated phosphoinositide metabolism in airway smooth muscle. The possible contribution of this regulatory mechanism to the relaxant properties of these agents is discussed.

The presence of five cyclic nucleotide phosphodiesterase isoenzyme activities in bovine tracheal smooth muscle and the functional effects of selective inhibitors

1. The profile of cyclic nucleotide phosphodiesterase (PDE) isoenzymes and the relaxant effects of isoenzyme selective inhibitors were examined in bovine tracheal smooth muscle. The compounds examined were the non-selective inhibitor 3-isobutyl-1-methylxanthine (IBMX), zaprinast (PDE V selective), milrinone and Org 9935 (4,5-dihydro-6-(5,6-dimethoxy-benzo[b]thien-2-yl)-5-methyl-1 (2H)-pyridazinone; both PDE III selective), rolipram (PDE IV selective) and Org 30029 (N-hydroxy-5,6-dimethoxy-benzo[b]-thiophene-2-carboximidamide HCl a dual PDE III/IV inhibitor). 2. Ion exchange chromatography showed three main peaks of PDE activity. The first peak was stimulated by Ca2+/calmodulin (PDE I), the adenosine 3':5'-cyclic monophosphate (cyclic AMP) hydrolytic activity of the second peak was stimulated by guanosine 3':5'-cyclic monophosphate (cyclic GMP) (PDE II) whilst that of the third peak was not significantly modified by any regulator (PDE IV). Calmodulin affinity chromatography revealed the additional presence of cyclic GMP-specific PDE (PDE V) in the first peak. A clearly distinct peak of cyclic GMP-inhibited PDE (PDE III) was not observed. However, Org 9935 inhibited the third activity peak more effectively in the presence, than in the absence, of rolipram (3 mumol l-1), indicating the presence of PDE III activity. 3. Rolipram was the most potent inhibitor of PDE IV. The mean -log50 IC50 values for rolipram, IBMX, milrinone, Org 30029, Org 9935 and zaprinast were 5.9 +/- 0.1, 4.9 +/- 0.1, 4.7 +/- 0.1, 4.6 +/- 0.1 and 4.6 +/- 0.1, respectively. 4. Rolipram was a potent relaxant of both histamine (1 pumol -') and methacholine (0.03 pmol -') precontracted preparations; (pD2 values; histamine 7.1 +/- 0.1, methacholine 6.8 /-+ 0.2 and 4.5 +/- 0.1, biphasic relaxation). IBMX also relaxed all preparations (pD2 values; histamine 5.6 +/- 0.1, methacholine 5.6 +/- 0.1) whilst zaprinast (pD2 values; histamine 5.2 +/- 0.1, methacholine 4.4 +/- 0.3), milrinone (pD2 values; histamine 5.2 + 0.1, methacholine 4.3 + 0.3) and Org 9935 (pD2 values; histamine 4.1 + 0.1, methacholine 4.1 +/- 0.2) did not completely relax preparations at concentrations up to 100 pImol I-. Org 30029 (pD2 values; histamine 6.2 +/- 0.1, methacholine 5.4 +/- 0.1) was a more effective relaxant than can be explained on the basis of PDE IV inhibition alone.5. We conclude that bovine tracheal smooth muscle contains five distinct PDE isoenzymes. PDE IV appears to be more important in the modulation of tissue function than PDE III and PDE V.

Beta-adrenoceptor induced inhibition of muscarinic receptor-stimulated phosphoinositide metabolism is agonist specific in bovine tracheal smooth muscle

The ability of the beta-adrenoceptor agonist isoprenaline to inhibit agonist-stimulated phosphoinositide metabolism was examined in bovine tracheal smooth muscle slices prelabelled with [3H]inositol. Accumulation of [3H]inositol phosphates was enhanced by the muscarinic agonists carbachol, oxotremorine and pilocarpine although the latter were only partial agonists for this response. Histamine stimulation of [3H]inositol phosphates was sensitive to mepyramine but maximal responses were only comparable to those of pilocarpine. Preincubation of tracheal slices with isoprenaline reduced the maximal phosphoinositide response to histamine and pilocarpine but the responses to carbachol and oxotremorine were unaffected. The inhibitory effect of isoprenaline (IC50 = 0.04 microM) was reversed competitively by 1 microM propranolol. The non-selective phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (1 mM) resulted in a more severe suppression of the histamine and pilocarpine responses and also produced a significant suppression of the maximal response to oxotremorine and a small shift in the carbachol dose-response curve. The different susceptibility of agonist-stimulated phosphoinositide hydrolysis to isoprenaline and IBMX are discussed in relation to the relative intrinsic activity of the agonists and/or the role of different muscarinic receptor subtypes.

Lack of effect of zaprinast on methacholine-induced contraction and inositol 1,4,5-trisphosphate accumulation in bovine tracheal smooth muscle

1. The effects of zaprinast (M&B 22948), a selective guanosine 3':5'-cyclic monophosphate (cyclic GMP) phosphodiesterase inhibitor, and sodium nitroprusside on cyclic GMP content, phosphoinositide hydrolysis and airway smooth muscle tone were examined in flurbiprofen pretreated bovine tracheal smooth muscle (BTSM). 2. Anion-exchange chromatography of the soluble fraction of BTSM homogenates resolved three peaks of Ca2+/calmodulin-independent phosphodiesterase (PDE) activity that corresponded to type Ia (cyclic GMP-specific, zaprinast-inhibitable), type II (cyclic GMP-stimulated) and type IV (Ro 20 1724-inhibitable) PDE isoenzymes. Zaprinast caused a selective inhibition of the type Ia PDE isoenzyme (IC50 0.94 microM) with respect to the type II and IV (IC50 s 93 microM and 197 microM respectively) isoenzymes. 3. Pretreatment of BTSM strips with zaprinast (10 microM) for 20 min affected neither the initial rate of force development, nor the resultant magnitude of contraction induced by methacholine (10 microM). In addition, zaprinast (10 microM; 20 min) did not affect the cumulative concentration-response relationship induced by methacholine. In contrast, sodium nitroprusside (300 microM) either alone, or in combination with zaprinast (10 microM), significantly attenuated tone induced by low, but not high concentrations of methacholine. This resulted in a non-parallel, rightwards shift of the methacholine concentration-response curves (nitroprusside: 4.0 fold; nitroprusside/zaprinast: 4.8 fold at the EC50 values), without a reduction in the maximum tone generated. 4. In BTSM slices, zaprinast (10 or 100 microM) did not influence basal or methacholine (10 microM)-stimulated cyclic GMP accumulation or inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) mass accumulation over a 60s incubation period, although it did significantly increase cyclic GMP content over longer (30 min) stimulation periods. 5. In [3H]-inositol prelabelled BTSM slices, stimulated in the presence of 5mM LiCl, methacholine (10 microM) caused a marked increase in total [3H]-inositol phosphate accumulation. This effect was not inhibited by zaprinast (10 microM), sodium nitroprusside (300 microM), or a combination of these drugs despite these agents markedly increasing tissue cyclic GMP content. 6. These findings demonstrate that despite zaprinast being a potent and selective inhibitor of the type Ia PDE isoenzyme in a cell-free system, this drug only increases cyclic GMP content in BTSM following prolonged agonist-stimulation. This may explain its lack of inhibitory effect on methacholine-induced tone. The inability of drugs which increase tissue cyclic GMP content and exhibit anti-spasmogenic activity to inhibit methacholine-stimulated Ins(1,4,5)P3 formation suggests that, unlike vascular smooth muscle, cyclic GMP-dependent mechanisms do not regulate receptor-mediated phosphoinositide hydrolysis in BTSM.