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1 We have investigated the role of cyclic nucleotide phosphodiesterase IV (PDE IV) in the relaxation of human bronchus and guinea-pig trachea in vitro and in guinea-pigs in vivo. 2 Functional studies showed that the selective PDE IV inhibitors, rolipram and denbufylline, relaxed human and guinea-pig preparations in vitro. 3 Two clinically used xanthine non-selective PDE inhibitors, theophylline and pentoxifylline, were also effective in these preparations, but were much less potent than the selective agents used. 4 The rank order of potency for the four PDE inhibitors in both species was similar. 5 Biochemical studies indicated that PDE IV was the major PDE isoform present in the human bronchial tissue. PDEs I, II and V were also identified. 6 Theophylline and pentoxifylline were, as expected, non-selective inhibitors of the human enzymes, but there was a good correlation between PDE IV inhibitory and bronchorelaxation potencies, suggesting that PDE IV inhibition is important for the clinical bronchodilator activities of the two xanthine compounds. 7 We have confirmed the ability of selective PDE IV inhibitors to cause bronchodilatation in guineapigs in vivo. 8 We conclude that our study has provided further evidence that selective PDE IV inhibitors could act as bronchodilators in the clinic.

1 This study aimed to investigate the 5-hydroxytryptamine (5-HT) receptors mediating contraction of ring preparations isolated from human pulmonary arteries and veins. In functional studies, the responses to 5-HT, sumatriptan, ergotamine, serotonin-O-carboxymethyl-glycyl-tyrosinamide (SCMGT), a-methyl 5-HT (a-Me) and 2-methyl 5-HT (2-Me) were studied with WAY100635, GR127935, ritanserin, zacopride and SB204070 as antagonists. 2 All agonists produced concentration-dependent contractions of human pulmonary artery and vein preparations. The order of potency (7log EC 50 values) was ergotamine (6.88)45-HT (6.41)5SCMGT (6.20)=sumatriptan (6.19) 5a-Me (6.04) in the artery, and ergotamine (7.84)45-HT (6.96)4suma-triptan (6.60)=a-Me (6.56)4SCMGT (6.09) in the vein. The potency of each agonist, except for SCMGT, was greater in vein than in artery preparations. Contractile responses to 5-HT were similar in intact and endothelium-denuded preparations but responses to sumatriptan were enhanced in artery rings without endothelium. 3 GR127935 (1 nM to 0.5 mM) produced an unsurmountable antagonism of the response to 5-HT, sumatriptan, ergotamine and SCMGT. Ritanserin (1 nM to 1 mM) also reduced the maximum contractile responses to 5-HT, ergotamine and a-Me in artery and vein preparations without aecting those to sumatriptan and SCMGT. In endothelium-denuded preparations, surmountable antagonism of sumatriptan by GR127935 (in the presence of ritanserin) and of a-Me by ritanserin (in the presence of GR127935) allowed for the calculation of the apparent pK B values of GR127935 (9.17+0.11 in artery and 9.11+0.05 in vein) and ritanserin (8.82+0.09 in artery and 8.98+0.12 in vein). 4 WAY100635 (1 nM to 1 mM), zacopride (1 nM to 1 mM), or SB204070 (1 nM) did not signi®cantly alter the concentration-response curves for 5-HT, sumatriptan, ergotamine, SCMGT or 2-Me in human pulmonary artery or vein thus indicating that 5-HT 1A , 5-HT 3 and 5-HT 4 receptors are presumably not involved in the contractile response to these agonists. 5 Binding studies using selective radioligands for dierent 5-HT receptors could not detect the presence of 5-HT 1A receptor binding in human pulmonary blood vessels whereas the 5-HT 1B/1D radioligand [ 3 H]-5-CT signi®cantly labelled a population of speci®c binding sites in both vessel types. The presence of 5-HT 2A receptors could also be inferred from the level of binding of [ 3 H]-ketanserin to membranes obtained from human pulmonary vessels, although signi®cance could not be reached for arteries. 5-HT 4 speci®c receptor binding was scarce in veins and absent in the case of arteries. 6 These ®ndings indicate that the human pulmonary artery and vein have a mixed functional population of 5-HT 1B/1D and 5-HT 2A receptors mediating the contractile response to 5-HT which is consistent with results of the binding studies.

1 We have investigated the in vitro cardiac actions of flosequinan and of its major metabolite in man, BTS 53554. 2 Positive inotropic activity was seen with flosequinan in guinea-pig isolated ventricles, the threshold concentration for effect being less than 1 X 10-5 M. BTS 53554 was approximately half as potent as the parent compound.3 In guinea-pig working whole hearts flosequinan increased left ventricular dp/dtmax, indicating a positive inotropic action. This effect was accompanied by increases in heart rate, cardiac output and stroke volume. 4 The virtual complete inhibition of inotropic responses to flosequinan and BTS 53554 by carbachol suggests that these responses are adenosine 3':5'-cyclic monophosphate (cyclic AMP)-mediated. 5 Flosequinan was shown to increase calcium inward current in guinea-pig ventricle, an action consistent with a cyclic AMP involvement in the response. 6 The inotropic activity of flosequinan was not potentiated by the selective phosphodiesterase (PDE) III inhibitor SK&F 94120, a result which indicates that flosequinan does not increase cyclic AMP concentrations via stimulation of adenylate cyclase. 7 Flosequinan inotropic responses were potentiated by rolipram, a selective PDE IV inhibitor, a result consistent with flosequinan being itself a PDE III inhibitor. 8 Biochemical studies with purified enzymes confirmed that flosequinan and BTS 53554 are relatively selective inhibitors of PDE III. 9 A comparison of pharmacological and biochemical data for both flosequinan and BTS 53554 indicates that their PDE III inhibitory potency is sufficient to account for their inotropic activity.

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