Effects of SK&F 94120, an inhibitor of cyclic nucleotide phosphodiesterase type III, on human platelets

Clinical effects of phosphodiesterase 3A mutations in inherited hypertension with brachydactyly

Autosomal-dominant hypertension with brachydactyly is a salt-independent Mendelian syndrome caused by activating mutations in the gene encoding phosphodiesterase 3A. These mutations increase the protein kinase A-mediated phosphorylation of phosphodiesterase 3A resulting in enhanced cAMP-hydrolytic affinity and accelerated cell proliferation. The phosphorylated vasodilator-stimulated phosphoprotein is diminished, and parathyroid hormone-related peptide is dysregulated, potentially accounting for all phenotypic features. Untreated patients die prematurely of stroke; however, hypertension-induced target-organ damage is otherwise hardly apparent. We conducted clinical studies of vascular function, cardiac functional imaging, platelet function in affected and nonaffected persons, and cell-based assays. Large-vessel and cardiac functions indeed seem to be preserved. The platelet studies showed normal platelet function. Cell-based studies demonstrated that available phosphodiesterase 3A inhibitors suppress the mutant isoforms. However, increasing cGMP to indirectly inhibit the enzyme seemed to have particular use. Our results shed more light on phosphodiesterase 3A activation and could be relevant to the treatment of severe hypertension in the general population.

Differential effects of phosphodiesterase inhibitors on platelet activating factor (PAF)- and adenosine diphosphate (ADP)-induced equine platelet aggregation

Compounds that activate adenylate cyclase, increasing intracellular cyclic adenosine monophosphate (cAMP), inhibit equine platelet aggregation. Cyclic AMP is broken down by phosphodiesterase (PDE) and, in the present study, the effects of theophylline, a nonselective PDE inhibitor, and selective inhibitors of PDE isoenzymes PDE3, PDE4 and PDE5, on equine platelet aggregation in response to platelet activating factor (PAF) and adenosine diphosphate (ADP) have been examined. Theophylline and the PDE3 inhibitors, trequinsin and quazinone, inhibited both PAF and ADP-induced aggregation in a concentration dependent manner. The inhibition of PAF-induced aggregation was, however, significantly greater than that of the response to ADP. The inhibitory effects of theophylline and the PDE3 inhibitors on ADP- but not PAF-, induced aggregation were reversed by addition of the calcium ionophore, A23187. Rolipram and zaprinast, inhibitors of PDE4 and PDE5, respectively, had no effect on either PAF- or ADP-induced aggregation. These results demonstrate that inhibition of aggregation caused by PAF or ADP can be achieved by selective inhibition of PDE3 but suggest that there may be agonist-specific differences in the intracellular signalling pathways that regulate equine platelet aggregation.

KF31327, a new potent and selective inhibitor of cyclic nucleotide phosphodiesterase 5

The effects of KF31327 (3-ethyl-8-[2-(4-hydroxymethylpiperidino)benzylamino]-2,3-dihydro-1H-imidazo[4,5-g]quinazoline-2-thione dihydrochloride) on phosphodiesterase 5 (cyclic GMP-specific phosphodiesterase) activity and platelet aggregation were investigated and compared with those of sildenafil, a well-known phosphodiesterase 5 inhibitor. KF31327 inhibited phosphodiesterase 5 from canine trachea (K(i)=0.16 nM) more potently than sildenafil (K(i)=7.2 nM). The kinetic analysis revealed that KF31327 was a non-competitive inhibitor. In the presence of nitroglycerin (nitric oxide generator), both compounds inhibited the collagen-induced aggregation of rabbit platelets at less than 0.1 microM, augmenting intracellular cyclic GMP level without affecting cyclic AMP. In contrast, in the absence of nitroglycerin, a higher concentration (10 microM) of KF31327 was required to inhibit platelet aggregation and increased both cyclic nucleotide levels. However, 10 microM sildenafil did not affect aggregation despite elevation of cyclic GMP comparable to that in the presence of nitroglycerin. These results indicate that in the presence of nitroglycerin, the inhibition of platelet aggregation by KF31327 is due to the elevation of cyclic GMP, whereas the mechanism underlying the inhibition without nitroglycerin might be related to a rise in intracellular cyclic AMP.

A sensitive assay of human blood platelet cyclic nucleotide phosphodiesterase activity by HPLC using fluorescence derivatization and its application to assessment of cyclic nucleotide phosphodiesterase inhibitors

A selective and sensitive HPLC measurement of 3',5'-cyclic nucleotide phosphodiesterase (PDE) activity in human platelets using (3,4-dimethoxyphenyl)glyoxal (DMPG) as a fluorogenic reagent for guanine and its nucleosides and nucleotides is described. cGMP, a substrate for PDE, and GMP, which was produced by the enzyme reaction, are selectively converted by the reaction with DMPG to the fluorescent derivatives. The derivatives were separated by reversed-phase HPLC. Human platelet PDE activity was measured and the inhibitory effects of several compounds were investigated.

Synthesis and structure-activity relationships of guanine analogues as phosphodiesterase 7 (PDE7) inhibitors

The synthesis of a novel series of guanine analogues is reported. The compounds have been assessed in vitro and some analogues have been found to be inhibitors of phosphodiesterase type 7 (PDE7).

Cilostazol, a selective type III phosphodiesterase inhibitor, decreases triglyceride and increases HDL cholesterol levels by increasing lipoprotein lipase activity in rats

Cilostazol, a selective type III phosphodiesterase inhibitor, has antiplatelet and vasodilating effects. In this study, the effects of cilostazol on lipid metabolism and lipoprotein lipase (LPL) activity were studied in rats. Cilostazol was administered orally at doses of 30 or 100 mg/kg twice a day for 1-2 weeks to rats. Cilostazol decreased the serum triglyceride level in normolipidemic rats. The serum triglyceride level was reduced and HDL cholesterol level was increased by cilostazol in streptozotocin (STZ)-induced diabetic rats. The disappearance of exogenous triglyceride was accelerated by cilostazol in normolipidemic rats. Cilostazol increased post-heparin plasma LPL activity but had no effect on hepatic triglyceride lipase activity in STZ-induced diabetic rats. Cilostazol also increased LPL activity in the heart in STZ-induced diabetic rats. These findings suggest that an increase in LPL activity is responsible for the serum triglyceride lowering and HDL cholesterol elevating effects of cilostazol in rats.

Ibudilast modulates platelet-endothelium interaction mainly through cyclic GMP-dependent mechanism

3-Isobutyryl-2-isopropylpyrazolo[1,5-a]pyridine (ibudilast) has been widely used in Japanese clinics for its antiasthmatic and antithrombotic effects. We investigated the mechanisms involved in the antiplatelet effects of the agent, specifically focusing on platelet-endothelium interaction. Ibudilast inhibits both phosphodiesterase (PDE) 3 and 5, the two major PDE isoforms of human platelets, with an IC50 of 31 and 2.2 microM, respectively. Cyclic guanosine monophosphate (GMP) accumulation in washed human platelets exposed to ibudilast alone increased significantly only at high concentrations of the agent (100 microM), whereas > or = 1 microM ibudilast enhanced cyclic GMP levels in the platelets cocultured with bovine aorta endothelial cells (ECs). In contrast, ibudilast enhanced cyclic AMP accumulation only at 100 microM, either with or without ECs. The synergistic effect of ibudilast and EC on cyclic nucleotide accumulation also was demonstrated by the inhibitory capability of the drug and the cells on platelet aggregation. The synergism between ibudilast and aspirin-pretreated ECs was more pronounced than that between ibudilast and N(omega)-nitro-L-arginine (L-NNA)-pretreated ECs. Ibudilast affected neither ATP diphosphohydrolase activity nor NO release from EC up to a concentration of 10 microM. We conclude that ibudilast exhibits antiplatelet properties mainly by inhibiting PDE5 to potentiate antiplatelet function of endothelium-derived NO.

Antithrombotic activity of NSP-513, a novel selective phosphodiesterase 3 inhibitor, on femoral arterial thrombosis induced by physical stenosis and electrical current: comparison of antithrombotic and hemodynamic effects

NSP-513, a novel potent and selective phosphodiesterase 3 (PDE 3) inhibitor, and cilostazol, a previously developed PDE 3 inhibitor, were compared with respect to antiplatelet, antithrombotic, and hemodynamic effects. In the in vitro antiplatelet aggregation studies, NSP-513 and cilostazol inhibited collagen-induced canine platelet aggregation with median inhibitory concentration (IC50) values of 0.093 and 3.1 miccroM, respectively, and inhibited adenosine diphosphate (ADP)-induced canine platelet aggregation with IC50 values of 0.15 and 12 microM, respectively. For ADP-induced platelet aggregation, the presence of prostaglandin E1 (PGE1; 3 and 10 nM) further decreased the IC50 values for NSP-513 to 0.11 and 0.032 microM, respectively. In ex vivo antiplatelet aggregation studies, orally administered NSP-513 (0.03-1 mg/kg) and cilostazol (50 mg/kg) inhibited collagen-induced canine platelet aggregation. In an in vivo canine femoral arterial thrombosis model, intraduodenally administered NSP-513 (0.01-0.03 mg/ kg) dose-dependently prevented thrombus formation without any changes in blood pressure, heart rate, or bleeding time. In conscious dogs, NSP-513 at oral doses of > or =0.3 mg/kg produced hemodynamic changes such as decreased blood pressure and increased heart rate and LVdP/dt(max). Thus the minimal hemodynamically effective dose of NSP-513 was 0.3 mg/kg, and the hemodynamic effects of this dose were comparable to those of 50 mg/kg of cilostazol. In conclusion, these data suggest that NSP-513 has in vivo selectivity for antiplatelet and antithrombotic activities over hemodynamic activity, and that the selectivity of NSP-513 is higher than that of cilostazol in dogs.

Antiplatelet and antithrombotic effects of a novel selective phosphodiesterase 3 inhibitor, NSP-513, in mice and rats

We investigated the effects of NSP-513, (R)-4,5-dihydro-5-methyl-6-[4-(2-propyl-3-oxo-1-cyclohexenyl)amino] phenyl-3(2H)-pyridazinone, on phosphodiesterase (PDE) isozyme activities, in vitro platelet aggregation and in vivo thrombus formation. NSP-513 selectively inhibited human platelet PDE 3 isozyme with an IC50 value of 0.039 microM. In an in vitro human platelet aggregation assay, the IC50 values (microM) of NSP-513 for platelet aggregation induced by collagen, U-46619, arachidonic acid, adenosine diphosphate (ADP), epinephrine and thrombin were 0.31, 0.25, 0.082, 0.66, 0.23 and 0.73, respectively. In a mouse pulmonary thromboembolism model, orally administered NSP-513 showed in vivo antithrombotic effects that were 320 to 470 times more potent than those of cilostazol. In a rat carotid arterial thrombosis model, intraduodenally administered NSP-513 (0.1 mg/kg), cilostazol (30 mg/kg) and aspirin (30 mg/kg) reduced thrombus formation by 75%, 66% and 48%, respectively. However, intravenously administered dipyridamole (10 mg/kg) did not significantly prevent thrombus formation. These results demonstrate that NSP-513 has the potential to prevent not only in vitro platelet aggregation but also in vivo thrombus formation and indicate that the highly selective PDE 3 inhibitory effect of NSP-513 may make this compound useful for assessing the physiological role of PDE 3.

Characterization of the isoenzymes of cyclic nucleotide phosphodiesterase in human platelets and the effects of E4021

In extracts of human platelets, three isoenzymes of cyclic nucleotide phosphodiesterase (PDE), namely, PDE2, PDE3, and PDE5, were identified; activities of PDE1 and PDE4 were not detected. In human platelets, the cGMP-hydrolytic activity was about six times higher than the cAMP-hydrolytic activity, and PDE5 and PDE3 are the major phosphodiesterase isoenzymes that hydrolyze cGMP and cAMP, respectively. PDE5 exhibited organ-specific expression in humans, and platelets were among the tissues richest in PDE5. A novel inhibitor of PDE5, sodium 1-[6-chloro-4-(3,4-methylenedioxybenzyl)aminoquinazolin-2-yl ] piperidine-4-carboxylate sesquihydrate (E4021), was a potent and highly selective inhibitor of human platelet PDE5. However, E4021 (up to 10 microM) did not inhibit 9,11-epithio-11,12-methano-thromboxane A2-induced platelet aggregation, in vitro. E4021 plus SIN-1 (3-morpholino-sydnonimine), at concentrations that had little effect individually, inhibited aggregation. These results suggest the unique distribution of phosphodiesterase isoenzymes in human platelets and the PDE5 inhibitors might be useful as a new class of antiplatelet drugs.

The effect of selective phosphodiesterase 3 and 4 isoenzyme inhibitors and established anti-asthma drugs on inflammatory cell activation

1. This study aimed to evaluate the effects of phosphodiesterase (PDE) inhibitors and currently prescribed anti-asthma drugs for their ability to inhibit inflammatory cell activation in vitro. 2. Alveolar macrophages and eosinophils were isolated from the bronchoalveolar lavage (BAL) fluid of ovalbumin (Ovalb)-sensitized guinea-pigs. Opsonized zymosan (OZ) and PAF stimulated leukotriene B4 (LTB4) release from eosinophils was measured by radioimmunoassay. Ovalb-induced superoxide generation was measured by reduction of cytochrome C. 3. Monocytes were separated from human peripheral venous blood and mast cells were dispersed from human lung fragments. Lipopolysaccharide (LPS)-induced tumour necrosis factor-alpha (TNF-alpha) release from monocytes was measured by ELISA and anti-IgE stimulated histamine release from mast cells was measured by a radioenzymatic method. 4. The beta 2 agonist, salbutamol inhibited TNF-alpha release from monocytes and histamine release from mast cells whilst having no effect on eosinophil-derived LTB4 release or macrophage superoxide generation. 5. The PDE 3 inhibitor, milrinone produced a concentration-related inhibition of TNF-alpha release from monocytes which achieved statistical significance at 10(-5) M but inhibited LTB4 release from eosinophils and superoxide generation from macrophages only at the highest concentration (10(-3) M) examined. Milrinone had no effect on histamine release from mast cells. 6. The selective PDE 4 inhibitors, denbufylline and rolipram and the corticosteroid, beclomethasone produced a concentration-related inhibition of LTB4 release from eosinophils, TNF-alpha release from monocytes and superoxide generation from alveolar macrophages whilst having no effect on histamine release from mast cells. 7. The mixed PDE 3/4 inhibitor, benzafentrine produced a concentration-related inhibition of LTB4 release from eosinophils, TNF-alpha release from monocytes, superoxide generation from alveolar macrophages and histamine release from mast cells. 8. In conclusion these data clearly show that both established anti-asthma medication as well as PDE inhibitors have the potential to inhibit inflammatory cell activation in vitro but that the anti-secretory actions of beta 2 agonists, corticosteroids and PDE inhibitors are distinct.

Interplay between milrinone and adenosine in the inhibition of human platelet response

1. In this study, we investigated the influence of the inotropic agent and coronary vasodilator milrinone on platelet aggregation and intracellular levels of 3',5' cyclic adenosine monophosphate (cAMP) in human platelet-rich plasma (PRP) and whole blood (WB). Furthermore, we evaluated the influence of milrinone on the effects of adenosine, which reduces the platelet aggregation through an elevation of intraplatelet cAMP levels. 2. Milrinone decreased the platelet aggregation in response to agonists in both PRP and WB. A dose-dependent increase of intraplatelet cAMP levels was demonstrated: this result is in accordance with an effect on platelet phosphodiesterases. 3. Milrinone at low concentration and adenosine exerted additive effects on platelet aggregation and intraplatelet cAMP levels. 4. An interplay between milrinone and adenosine was shown in WB. Furthermore, dipyridamole, which prevents the uptake of endogenous adenosine, markedly enhanced the milrinone antiaggregating effect, whereas the adenosine receptor blocker, theophylline, decreased it. 5. The present data provide evidence that milrinone modulates the platelet function through an influence on intraplatelet levels of cAMP and it is able to interplay with substances stimulating adenylyl cyclase. 6. The interplay between milrinone and adenosine in the inhibition of the human platelet function could be effective during milrinone administration in the treatment of heart failure, when blood adenosine levels are significantly increased. These milrinone effects could be advantageous from a therapeutic point of view, since patients with heart failure are at risk of thrombosis and ischemic heart disease.

Differential cellular pattern of gene expression for two distinct cGMP-inhibited cyclic nucleotide phosphodiesterases in developing and mature rat brain

Cyclic GMP-inhibited phosphodiesterases are characterized by sensitivity of cAMP hydrolysis to inhibition by cGMP. This phosphodiesterase family contains at least two different isoforms (PDE3A and PDE3B) encoded by distinct genes and serving tissue-specific roles in regulation of lipolysis, glycogenolysis, myocardial contractility, and smooth muscle relaxation. Our previous work indicated an abundance of these two phosphodiesterase messenger RNAs in the embryonic rat brain, and therefore, to elucidate the potential functions of these enzymes in brain development as well as in mature brain function, the present study mapped cellular patterns of gene expression for these two enzymes from embryonic day 15 to adulthood using in situ hybridization histochemistry. Phosphodiesterase 3B isoform messenger RNA is uniformly expressed in germinal neuroepithelium and mature neurons, with distribution generally reflecting cell density. Phosphodiesterase isoform 3A messenger RNA, in contrast, demonstrates striking spatiotemporal specificity of expression, with three distinct patterns being evident. Firstly, this mRNA is highly abundant in both primary and secondary neuroepithelial germinal zones. Secondly, during early postnatal development PDE3A mRNA is transiently but highly expressed in neurons localized in basal forebrain, deep cerebellar, pontine, interpeduncular and a variety of thalamic, midbrain and brainstem nuclei. Thirdly, PDE3A mRNA is focally expressed in isolated large striatal and hippocampal neurons from the perinatal period without attenuation into adulthood. In summary, two cGMP-inhibited phosphodiesterase isoforms show distinctive patterns of gene expression in brain: PDE3B gene expression is uniform without evidence of system specificity or developmental stage specificity, suggesting that this isoform has a constitutive role in neuroepithelial metabolism, while PDE3B gene expression demonstrates a high level of spatiotemporal heterogeneity, suggesting that this isoform subserves a variety of developmental stage-specific and system-specific functions.

Cyclic nucleotide phosphodiesterases from purified human CD4+ and CD8+ T lymphocytes

BACKGROUND

CD4+ and CD8+ T-lymphocytes are suggested to differentially affect airway inflammation in asthma. Agents which increase intracellular cAMP levels, such as PDE inhibitors, have been shown to diminish lymphocyte growth and differentiation, and to affect cytokine expression. Differences in the PDE isoenzyme profile between CD4+ and CD8+ cells might form a basis to differentially modify their functions by PDE inhibitors.

OBJECTIVE

The study investigates and compares the PDE isoenzyme activity profiles of human peripheral blood CD4+ and CD8+ T-lymphocytes.

METHODS

CD4+ and CD8+ T-lymphocytes were purified (> 98%) from peripheral blood mononuclear cells by negative selection. PDE isoenzyme activity profiles were investigated using PDE isoenzyme selective inhibitors and activators.

RESULTS

In CD4+ and CD8+ T-lymphocyte homogenates, PDE IV and PDE III activities were the predominant PDE isoenzyme activities at 0.5 microM cyclic nucleotide substrate concentrations. PDE IV was localized in the soluble fraction whereas PDE III was membrane bound. Low PDE I, II and V activities were detected. About 20% of total cAMP hydrolysing capacity at 0.5 microM cAMP was insensitive to PDE isoenzyme selective inhibitors and activators and therefore could not be assigned to PDE I-IV. The PDE isoenzyme pattern was not different between CD4+ and CD8+ T-lymphocytes. Moreover, representative inhibitors of PDE III and IV activity inhibited cAMP hydrolysis in soluble fractions of both T-lymphocyte subsets with similar potency. Enzyme kinetic analysis similarly did not reveal differences between CD4+ and CD8+ T-lymphocytes.

CONCLUSION

Normal CD4+ and CD8+ T-lymphocytes are likely to be equally sensitive targets for the effects of PDE inhibitors.

Type III cGMP-inhibited cyclic nucleotide phosphodiesterases (PDE3 gene family)

Seven different but related cyclic nucleotide phosphodiesterase (PDE) gene families have been identified. Type III cGMP-inhibited (cGI) PDEs, the PDE3 gene family, are found in many tissues. cGI PDEs exhibit a high affinity for both cAMP and cGMP, and are selectively and relatively specifically inhibited by certain agents which augment myocardial contractility, promote smooth muscle relaxation and inhibit platelet aggregation. Adipocyte, platelet, and hepatocyte cGI PDE activities are regulated by cAMP-dependent phosphorylation. Insulin-induced phosphorylation/activation of adipocyte and hepatocyte cGI PDEs is thought to be important in acute regulation of triglyceride and glycogen metabolism by insulin. Two distinct cGI PDE subfamilies, products of distinct but related genes, have been identified. They exhibit the domain structure common to PDEs with a carboxyterminal region, conserved catalytic domain and divergent regulatory domain. In their catalytic domains cGI PDEs contain a 44 amino acid insertion not found in other PDE families. The expression of cGIP1 and cGIP2 mRNAs differs in different rat tissues, suggesting distinct functions for the two cGI PDE subfamilies, i.e., cGIP1 in adipose tissue, liver, testis and cGIP2 in myocardium, platelets and smooth muscle.

Distinctive anatomical patterns of gene expression for cGMP-inhibited cyclic nucleotide phosphodiesterases

Type III cGMP-inhibited phosphodiesterases (PDE3s) play important roles in hormonal regulation of lipolysis, platelet aggregation, myocardial contractility, and smooth muscle relaxation. We have recently characterized two PDE3 subtypes (PDE3A and PDE3B) as products of distinct but related genes. To elucidate their biological roles, in this study we compare cellular patterns of gene expression for these two enzymes during rat embryonic and postnatal development using in situ hybridization. PDE3B [corrected] mRNA is abundant in adipose tissue and is also expressed in hepatocytes throughout development. This mRNA is also highly abundant in embryonic neuroepithelium including the neural retina, but expression is greatly reduced in the mature nervous system. Finally, PDE3B [corrected] mRNA is localized in spermatocytes and renal collecting duct epithelium in adult rats. PDE3B mRNA is highly expressed in the cardiovascular system, including myocardium and arterial and venous smooth muscle, throughout development. It is also abundant in bronchial, genitourinary and gastrointestinal smooth muscle and epithelium, megakaryocytes, and oocytes. PDE3A [corrected] mRNA demonstrates a complex, developmentally regulated pattern of gene expression in the central nervous system. In summary, the two different PDE3s show distinctive tissue-specific patterns of gene expression suggesting that PDE3B [corrected] is involved in hormonal regulation of lipolysis and glycogenolysis, while regulation of myocardial and smooth muscle contractility appears to be a function of PDE3A [corrected]. In addition, the present findings suggest previously unsuspected roles for these enzymes in gametogenesis and neural development.

cGMP and nitric oxide modulate thrombin-induced endothelial permeability. Regulation via different pathways in human aortic and umbilical vein endothelial cells

Previous studies have demonstrated that cGMP and cAMP reduce the endothelial permeability for fluids and macromolecules when the endothelial permeability is increased by thrombin. In this study, we have investigated the mechanism by which cGMP improves the endothelial barrier function and examined whether nitric oxide (NO) can serve as an endogenous modulator of endothelial barrier function. Thrombin increased the passage of macromolecules through human umbilical vein and human aortic endothelial cell monolayers and concomitantly increased [Ca]2+ in vitro. Inhibition of these increases by the intracellular Ca2+ chelator BAPTA indicated that cytoplasmic Ca2+ elevation contributes to the thrombin-induced increase in endothelial permeability. The cGMP-dependent protein kinase activators 8-bromo-cGMP (8-Br-cGMP) and 8-(4-chlorophenylthio)cGMP (8-PCPT-cGMP) decreased the thrombin-induced passage of macromolecules. Two pathways accounted for this observation. Activation of cGMP-dependent protein kinase by 8-PCPT-cGMP decreased the accumulation of cytoplasmic Ca2+ in aortic endothelial cells and hence reduced the thrombin-induced increase in permeability. On the other hand, in umbilical vein endothelial cells, cGMP-inhibited phosphodiesterase (PDE III) activity was mainly responsible for the cGMP-dependent reduction of endothelial permeability. The PDE III inhibitors Indolidan (LY195115) and SKF94120 decreased the thrombin-induced increase in permeability by 50% in these cells. Thrombin treatment increased cGMP formation in the majority of, but not all, cell cultures. Inhibition of NO production by NG-nitro-L-arginine methyl ester (L-NAME) enhanced the thrombin-induced increase in permeability, which was restricted to those cell cultures that displayed an increased cGMP formation after addition of thrombin. Simultaneous elevation of the endothelial cGMP concentration by atrial natriuretic factor, sodium nitroprusside, or 8-Br-cGMP prevented the additional increase in permeability induced by L-NAME. These data indicate that cGMP reduces thrombin-induced endothelial permeability by inhibition of the thrombin-induced Ca2+ accumulation and/or by inhibition of cAMP degradation by PDE III. The relative contribution of these mechanisms differs in aortic and umbilical vein endothelial cells. NO can act in vitro as an endogenous permeability-counteracting agent by raising cGMP in endothelial cells of large vessels.

Possible role of cyclic AMP phosphodiesterases in the actions of ibudilast on eosinophil thromboxane generation and airways smooth muscle tone

1. The possible role of cyclic AMP phosphodiesterase (PDE) in the inhibitory actions of ibudilast on tracheal smooth muscle contractility and eosinophil thromboxane generation was investigated. 2. Ibudilast was a non-selective inhibitor of partially purified cyclic nucleotide PDE isoenzymes from pig aorta and bovine tracheal smooth muscle, exhibiting only moderate potency against bovine tracheal PDE IV (IC50 = 12 +/- 4 microM, n = 3). Similar or slightly lower potencies were displayed against PDEs I, II, III and V. In contrast, rolipram exhibited selectivity for PDE IV (3 +/- 0.5 microM, n = 3). 3. Ibudilast (IC50 = 0.87 +/- 0.37 microM, n = 3), like rolipram (IC50 = 0.20 +/- 0.04 microM, n = 3), was a more potent inhibitor of membrane-bound PDE IV from guinea-pig eosinophils than of partially purified PDE IV from bovine tracheal smooth muscle. The potency of ibudilast increased when the eosinophil enzyme was solubilised with deoxycholate and NaCl (IC50 = 0.11 +/- 0.05 microM, n = 3) or exposed to vanadate/glutathione complex (V/GSH) (IC50 = 0.11 +/- 0.02 microM, n = 3). The potency of rolipram was also increased by solubilization (IC50 = 0.012 +/- 0.003, n = 3) or V/GSH (IC50 = 0.012 +/- 0.003, n = 3). 4. In intact eosinophils, ibudilast (0.032 microM-20 microM) potentiated isoprenaline-induced cyclic AMP accumulation in a concentration-dependent manner, being approximately 20 fold less potent than rolipram. Little or no effect on basal cyclic AMP levels was observed with either compound. The cyclicAMP-dependent protein kinase activity ratio was significantly increased following incubation of eosinophils with either ibudilast (20 MicroM) or rolipram (20 MicroM) in the absence or presence of isoprenaline.5. Leukotriene B4 (300 nM)-induced thromboxane generation from guinea-pig eosinophils was inhibited by ibudilast (IC50 = 11.3 +/- 3.7 MicroM, n = 5) and rolipram (IC50 = 0.280 +/- 0.067 MicroM, n = 5) in a concentration-dependent manner.6. Ibudilast (10 nM-1 MicroM), whilst generally less potent than rolipram (1 nM- 1 MicroM), produced concentration-dependent relaxation of spasmogen (methacholine, histamine, LTD4)-induced tone in the guinea pig isolated tracheal strip. Ibudilast was less potent in reversing the methacholine (IC50 = 1.95 +/- 0.40 JM,n =6)-induced contraction than those of histamine (IC50 = 0.18 +/- 0.70 MicroM, n =6) or leukotriene D4(LTD4, IC50 = 0.12 +/- 0.05 MicroM, n = 6). Rolipram also exhibited a similar pattern of activity, although the difference in potency against methacholine (IC50 = 0.1 +/- 0.01 MicroM, n = 6) compared with the other two spasmogens, histamine (IC50 = 0.034 +/- 0.017 MicroM, n = 7) and LTD4 (IC50 = 0.026 +/- 0.008 MicroM, n = 7), was not as great.7. These results demonstrate that ibudilast, like rolipram, has several biological actions on the eosinophil and airways smooth muscle which may be attributed to inhibition of cyclic AMP PDE. These actions may account, at least in part, for the recently reported anti-asthma effects of ibudilast.

New 4-amino-7,8-dimethoxy-5H-pyrimido[5,4-b]indole derivatives: synthesis and studies as inhibitors of phosphodiesterases

A series of 4-amino-7,8-dimethoxy-5H-pyrimido[5,4-b]indole derivatives has been synthesized. These compounds resemble carbazeram and other pyridazino compounds with activity in the cardiovascular system. Some of these new compounds possess inotropic activity (Table 2), with a complementary effect on the inhibition of different CGI-PDE (Table 3). The most active compounds 5, 6d, and 7 also possess activity as vasodilators (Table 4). Some of these new compounds inhibit blood platelet aggregation induced by ADP and AA and are active as inhibitors of human platelet PDEs (Tables 5 and 6).

Existence of rolipram-sensitive phosphodiesterase in rat megakaryocyte

1. The effect of rolipram (ME3176) on ADP- and IP3-induced repetitive IK(Ca) in rat megakaryocyte was investigated by use of the nystatin perforated patch and conventional whole-cell patch-clamp techniques. 2. The ADP-induced IK(Ca) was depressed by treatment with rolipram in a concentration-dependent manner. The inhibition by rolipram disappeared after treatment with a cyclic nucleotide-dependent protein kinase inhibitor, H-8. The inhibition of IK(Ca) was also observed in the presence of cyclic AMP accumulating agents such as forskolin and isobutylmethylxanthine (IBMX). 3. Rolipram enhanced the inhibitory action of forskolin, suggesting that rolipram facilitates the accumulation of cyclic AMP by blocking its breakdown. Similar results was obtained with adenosine, an endogenous adenylate cyclase activator. 4. Intracellular application of inositol trisphosphate (IP3) induced repetitive IK(Ca) in megakaryocytes. The induced IK(Ca) was also inhibited by rolipram and by other cyclic AMP accumulating agents. 5. It was concluded that megakaryocytes possess rolipram-sensitive phosphodiesterase (PDE), which was not detected in platelets, but plays a distinct modulatory role in megakaryocytes for generating ADP-induced IK(Ca).

Effect of milrinone in human platelet shape change, aggregation and thromboxane A2 synthesis: an in vitro study

Milrinone (MIL; a cAMP-specific phosphodiesterase type-III inhibitor), added in vitro to achieve concentrations below the therapeutic levels, inhibited agonist-induced platelet shape change (PSC). Arachidonic acid (AA)-induced PSC was significantly more inhibited by a combination of MIL and indomethacin (INDO; a cyclooxygenase inhibitor) than by either alone. PSC induced by 5-hydroxytryptamine was inhibited by MIL but not by INDO; and this effect of MIL was not augmented by INDO. Whole blood-platelet aggregation (WB-PA) and platelet-rich plasma aggregation induced by potent stimulators of thromboxane A2 (TXA2) synthesis such as AA and calcium ionophore and by less potent agonists (e.g. ADP and U46619) were inhibited by MIL at or near therapeutic concentrations. WB-PA induced by collagen was significantly more inhibited by the MIL and INDO combination than by either of these agents alone whereas with ADP-induced WB-PA no additional effect could be shown when both MIL and INDO were co-incubated. MIL and similar types of drugs may be of benefit in conditions associated with platelet hyperactivity and some of these effects may be enhanced by cyclooxygenase inhibitors.

A novel class of cardiotonic agents: synthesis and biological evaluation of pyridazino[4,5-b]indoles with cyclic AMP phosphodiesterases inhibiting properties

Some fused pyridazino[4,5-b]indoles (7) were synthesized. These new compounds present a planar topography and some resemblance to carbazeram, imadozan, and other pyridazino agents with cardiotonic activity. These compounds also possess a complementary effect as inhibitors of platelet aggregation. 6-(3,5-Dimethylpyrazolyl)-1,2,4-triazolo[4,3-b]pyridazino[4, 5-b]indole (7a) has a good profile as an inodilatador with antiaggregate activity due to the inhibition of phosphodiesterase.

Effect of milrinone on thromboxane A2 synthesis, cAMP phosphodiesterase and 45Ca2+ uptake by human platelets

The phosphodiesterase inhibitors milrinone and isobutylmethylxanthine (IBMX) inhibited the conversion of [3H]cAMP to [3H]AMP by washed human platelets in concentration-dependent manners (IC50: milrinone, 2.6 x 10(-6) M; IBMX, 4.6 x 10(-6) M). Milrinone and IBMX increased cAMP levels when stimulated by a single concentration (0.3 microM) of iloprost. EC50:milrinone, 5.6 x 10(-5) M; IBMX, 3 x 10(-5) M. Milrinone was a potent inhibitor of platelet thromboxane A2 (TXA2) synthesis when stimulated by median stimulatory doses of collagen (IC50: 3 x 10(-7) M), sodium fluoride (NaF) (a non-specific G protein activator; IC50: 3 x 10(-7) M) and phorbol ester myristate acetate (PMA) (a protein kinase C activator; IC50: 2.2 x 10(-7) M). In contrast, at median stimulatory doses of A23187 and arachidonate there was a marked decrease in the potency of milrinone in inhibiting TXA2 synthesis. Milrinone had a weak inhibitory effect on TXA2 synthesis when elicited by freeze fracturing. In all experiments IBMX was a weaker inhibitor of TXA2 synthesis, although the general pattern of effects was similar to milrinone. Milrinone inhibited both collagen- and adrenaline-stimulated 45Ca2+ uptake by human platelets in dose-dependent manners. Since platelet TXA2 synthesis is dependent on Ca2+, and milrinone inhibited 45Ca2+ uptake, it is concluded that milrinone exerts its inhibitory effect on platelet activity, principally through an action on Ca2+ mobilisation/binding to effector proteins (protein kinase C and/or phospholipase A2).

Interaction of endothelium with dilation produced by inhibitors of cyclic nucleotide diesterases in mouse brain arterioles in vivo

BACKGROUND AND PURPOSE

In vitro evidence gathered from extracerebral conductance vessels suggests interaction between the endothelium-derived relaxing factor for acetylcholine (EDRFACh) and cyclic nucleotide action in vascular smooth muscle. The purpose of this study was to examine this interaction in vivo in pial arterioles. As had been done in vitro, we used phosphodiesterase inhibitors that elevate cyclic nucleotide levels in vascular smooth muscle.

METHODS

Pial vessels of mice were observed with television microscopy. Diameter of the arterioles was monitored with an image-splitting technique. The responses to topically applied phosphodiesterase inhibitors were tested before and after focal endothelial injury or before and during application of N-guanidino-L-monomethyl arginine (L-NMMA). Both treatments are known to eliminate the endothelium-dependent response to acetylcholine in this preparation.

RESULTS

Phosphodiesterase inhibitors dilated pial arterioles. This was true for phosphodiesterase inhibitors elevating levels of both adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) as well as for inhibitors thought to selectively affect either nucleotide. The dilation was inhibited by endothelial injury and by L-NMMA.

CONCLUSIONS

The data suggest that brain arterioles are dilated by both cAMP and cGMP but that this action is impaired if EDRFACh levels are reduced. Since EDRFACh elevates cGMP levels, these data support the hypothesis that the relaxing actions of cGMP and cAMP depend upon adequate basal levels of cGMP in vascular smooth muscle. This hypothesis, originally introduced in studies of extracerebral conductance arteries in vitro, can now be applied to brain resistance vessels in vivo.

The effect of SK&F 95654, a novel phosphodiesterase inhibitor, on cardiovascular, respiratory and platelet function

1. SK&F 95654 inhibited the guanosine 3':5'-cyclic monophosphate (cyclic GMP)-inhibited phosphodiesterase (cGI-PDE) with an IC50 value of 0.7 microM. The IC50 values were greater than 100 microM for the other four phosphodiesterase isoenzymes tested. The R-enantiomer of SK&F 95654 (IC50 = 0.35 microM) was a more potent inhibitor of cGI-PDE than was the S-enantiomer (IC50 = 5.3 microM). 2. In the guinea-pig working heart, SK&F 95654 produced a positive inotropic response without altering heart rate. 3. Oral administration of SK&F 95654 to conscious dogs caused dose-dependent increases in left ventricular dp/dtmax in the range 10-50 micrograms kg-1. These positive inotropic responses were maintained for 3 h without simultaneous changes in heart rate or blood pressure. The peak effects on left ventricular dp/dtmax were similar for orally and intravenously administered compound, indicating good oral bioavailability. 4. SK&F 95654 caused a potent inhibition of U46619-induced aggregation in both a human washed platelet suspension (WPS) (IC50 = 70 nM) and in human platelet-rich plasma (PRP) (IC50 = 60 nM), indicating that the compound shows negligible plasma binding. 5. The R-enantiomer of SK&F 95654 was twenty fold more potent as an inhibitor of platelet aggregation than was the S-enantiomer. The similarity of this ratio to that obtained on the cGI-PDE suggests that SK&F 95654 inhibits platelet aggregation via its effects on cGI-PDE. This was also indicated by studies which showed that SK&F 95654 increased adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels and activated cyclic AMP-dependent protein kinase in human platelets. 6. Collagen-induced aggregation of rat PRP was also inhibited by SK&F 95654 (ICso = 65 nM). The effects of SK&F 95654, administered intravenously, on ex vivo platelet aggregation were studied in the conscious rat. At 1 mg kg-', SK&F 95654 inhibited aggregation for at least 4 h post dose and was more potent than the two other cGI-PDE inhibitors studied (siguazodan and SK&F 94120).7. In contrast to its potent effects on heart and platelets, SK&F 95654 caused only a modest relaxation of histamine- or U46619-induced bronchoconstriction in the anaesthetized, ventilated guinea-pig.8. Taken together, these results indicate that SK&F 95654 may be a suitable agent for the treatment of congestive heart failure.

Properties of base-substituted and carboxyl-esterified analogues of griseolic acid, a potent cAMP phosphodiesterase inhibitor

Griseolic acid (GA) is a potent cyclic AMP (cAMP) phosphodiesterase (PDE) inhibitor that has an adenine base and two carboxyl groups in its molecule (Nakagawa F, Okazaki T, Naito A, Iijima Y and Yamazaki M, J Antibiot 38: 823-829, 1985). GA analogues were synthesized in which the adenine group was substituted with guanine (6-deamino-2-amino-6-hydroxygriseolic acid, G-GA) or hypoxanthine (6-deamino-6-hydroxygriseolic acid, H-GA). Their inhibitory activities to cyclic GMP (cGMP) PDE and cAMP PDE were compared with GA. For cGMP PDE from rod outer segments of bovine retina, the IC50 values of GA, G-GA and H-GA were 18, 0.040 and 0.12 microM, respectively, with 0.25 microM cGMP as substrate. For type IV PDE isozyme from mouse 3T3 fibroblast cells, the IC50 values of GA, G-GA and H-GA were 0.021, 15 and 11 microM, respectively, with 0.25 microM cAMP as substrate. Thus, GA and G-GA were found to be base-selective inhibitors of type IV PDE of 3T3 cells and type V PDE of bovine retinas, respectively. Esters of carboxylic acids of GA were synthesized in order to increase permeability into cells, and their efficacy was tested by measuring the accumulation of cAMP in 3T3 cells. The dipivaloyloxymethyl ester of GA was found to increase cAMP levels at 0.1 microM, while GA and 3-isobutyl-1-methylxanthine were active only above 100 microM, and the dimethyl ester of GA was inactive. The dipivaloyloxymethyl ester of GA seems to exert its activity after conversion to GA in the cell, since the pivaloyloxymethyl ester was easily hydrolysed by the enzyme action and the dipivaloyloxymethyl ester of GA itself was much less potent an inhibitor of PDE. The dipivaloyloxymethyl ester of GA inhibited thrombin-induced aggregation of platelets and stimulated lipolysis of adipocytes at low concentrations.

Characterization of the cyclic nucleotide phosphodiesterase isoenzymes present in rat epididymal fat cells

Soluble and particulate fractions from extracts of rat epididymal fat cells were shown to exhibit a number of different phosphodiesterase activities, as determined by substrate specificity and sensitivity to activators and inhibitors. These activities were then further characterized following separation by MonoQ fast protein liquid chromatography (FPLC). A cyclic AMP-specific activity, unaffected by the presence of calcium and calmodulin and inhibited by rolipram, was the major soluble phosphodiesterase. This fraction also contained distinct calcium and calmodulin- and cyclic-GMP-stimulated activities. Over 80% of the phosphodiesterase activity in the particulate fraction could be accounted for by an insulin-activated cyclic AMP and cyclic GMP-hydrolysing enzyme, which was sensitive to inhibition by cyclic GMP, SKF 94120, SKF 95654 and cilostamide, and eluted as a single peak during MonoQ chromatography. At 1 microM cyclic AMP, the phosphodiesterase activity in the soluble fraction was about eight times greater than in the particulate fraction. Specific inhibitors to the particulate phosphodiesterase (cilostamide and SKF 95654) were added to incubations of isolated fat cells, and were able to potentiate sub-maximal concentrations of isoproterenol in the stimulation of lipolysis. These inhibitors were also able to reverse the antilipolytic effect of insulin, demonstrating the importance of the particulate phosphodiesterase in insulin action, despite the fact that its activity represents only a small proportion of the total phosphodiesterase activity in fat cells. Inhibitors of the major soluble phosphodiesterase had no effect on lipolysis.

A new cGMP phosphodiesterase isolated from bovine platelets is substrate for cAMP- and cGMP-dependent protein kinases: evidence for a key role in the process of platelet activation

The biochemical differences among cGMP phosphodiesterases in platelets have not been thoroughly examined, primarily due to the lack of sufficient purified material. This report describes a simple method developed to isolate a specific bovine platelet cGMP phosphodiesterase. This enzyme is cytosolic in its native form and was purified to an apparent homogeneity by ion-exchange chromatography, affinity chromatography, and density gradient centrifugation. Cyclic GMP binds to a "pseudo-site" when the catalytic site is deprived of Mg++. The affinity for cGMP at alkaline pH in presence of EDTA and IBMX (Kd = 60 nM) suggests that the removal of Mg++ by EDTA converts the catalytic site to a binding site. A ligand affinity chromatography was designed to take advantage of these features. The core enzyme has a molecular weight 190,000 composed of 2 subunits (MW 95,000) and has a specific activity of 2.5 mumol/min/mg. Moreover, this enzyme was phosphorylated by cAMP- and cGMP-dependent protein kinases, suggesting that its activity could be indirectly regulated by cyclic nucleotides. Agents elevating cGMP and cAMP inhibit platelet activation by inhibiting protein kinase C and thrombin induced hydrolysis of phosphatidylinositol 4,5 diphosphate. The antiaggregating properties of some of these agents might therefore be attributed to the fact that they are inhibitors of phosphodiesterases.

Effects of zaprinast and rolipram on platelet aggregation and arrhythmias following myocardial ischaemia and reperfusion in anaesthetized rabbits

1. This study was designed to compare the effects of two selective inhibitors of certain phosphodiesterase (PDE) isoenzymes on arrhythmias induced by coronary artery occlusion and reperfusion. The drugs used were zaprinast which inhibits guanosine 3':5'-cyclic monophosphate (cyclic GMP)-specific PDE (PDE V) and rolipram which inhibits cyclic GMP-insensitive, adenosine 3':5'-cyclic monophosphate (cyclic AMP)-specific PDE (PDE IV). 2. Pretreatment of anaesthetized rabbits with zaprinast (300 micrograms kg-1 plus 30 micrograms kg-1 min-1) had no significant effect on ischaemia- or reperfusion-induced ST-segment changes, or arrhythmias. In contrast, rolipram (30 micrograms kg-1 plus 3 micrograms kg-1 min-1) and (100 micrograms kg-1 plus 10 micrograms kg-1 min-1) increased the severity of arrhythmias. With the higher dose of rolipram, ST-segment changes were increased in magnitude and mortality due to ventricular fibrillation during ischaemia or reperfusion was increased to 80% compared with 30% in controls (n = 10 per group). 3. Zaprinast caused small but significant increases in heart rate and arterial blood pressure whereas rolipram decreased diastolic arterial pressure, increased left ventricular (LV) dP/dtmax and substantially increased heart rate. 4. At the end of each experiment platelet aggregation was measured ex vivo. Pretreatment of rabbits with either dose of rolipram had no significant effect on platelet aggregation induced by adenosine diphosphate (ADP), collagen, arachidonic acid or thrombin or on isoprenaline- or prostacyclin-induced inhibition of aggregation. Aggregatory responses to ADP and collagen were increased in platelets obtained from rabbits which had received zaprinast. 5. These results indicate that in the dose used here, the PDE V inhibitor zaprinast had no significant effect on arrhythmias. The effects of the PDE IV inhibitor rolipram on haemodynamics, combined with its lack of antiplatelet activity, may have contributed to the exacerbation of arrhythmias observed during myocardial ischaemia and reperfusion.

Selective inhibition of a high affinity type IV cyclic AMP phosphodiesterase in bovine trachealis by AH 21-132. Relevance to the spasmolytic and anti-spasmogenic actions of AH 21-132 in the intact tissue

The ability of a papaverine-derived bronchodilator, AH 21-132, to inhibit cyclic nucleotide hydrolysis and to increase the cAMP content and the activity of cAMP-dependent protein kinase (A-kinase) was evaluated in bovine tracheal smooth muscle (BTSM) and related to the mechanical effects elicited by this compound in vitro. AH 21-132 (100 nM-1 mM) produced a concentration-related relaxation of BTSM pre-contracted with methacholine (MCh) that was subject to marked functional antagonism. AH 21-132 (100 microM) also displayed anti-spasmogenic activity preventing the generation of tone induced by low, but not high, concentrations of MCh. Neither the spasmolytic nor anti-spasmogenic effects of AH 21-132 were antagonized by the beta 2-adrenoceptor blocking drug ICI 118551 (50 nM). Three Ca(2+)- and calmodulin-independent cyclic nucleotide phosphodiesterases (PDE) were resolved from the soluble fraction of BTSM homogenates by Q-Sepharose anion exchange chromatography. These PDEs were identified by kinetic and inhibitor sensitivity criteria as the Type II (cGMP-stimulated), Type IV (Ro 20-1724-inhibited) and Type V (cGMP-specific) isoenzymes. A small amount (approximately 5%) of a Type III PDE seemed to be present but this was not identified with certainty. AH 21-132 selectively inhibited Type IV PDE in a competitive manner with an IC50 and KI of 3.7 and 2.7 microM, respectively. AH 21-132 similarly increased the cAMP content (from 5.3 to 23.1 pmol/mg protein after 1 mM AH 21-132) and activated A-kinase (from 29.6% to 53.5% after 1 mM AH 21-132) in intact BTSM over the same concentration range at which this compound influenced tone. In addition, AH 21-132 in high concentrations (greater than 100 microM), while exerting no direct effect on A-kinase itself, markedly potentiated (ca. four-fold at 3 mM AH 21-132) the ability of cAMP to activate A-kinase without affecting the affinity of cAMP for this enzyme. It is concluded that the spasmolytic and anti-spasmogenic effects of AH 21-132 in BTSM may be related, in part, to its ability to inhibit Type IV PDE, increase the intracellular cAMP content and so activate A-kinase. A cyclic nucleotide-dependent mechanism is therefore proposed. In addition, the ability of AH 21-132 to augment cAMP-dependent phosphorylation in a cell-free system, when Type IV PDE is inhibited fully, provides the possibility that the observed relaxation elicited by high concentrations of AH 21-132, while cAMP-dependent, does not require any further increase in the intracellular cAMP concentration.

Zardaverine as a selective inhibitor of phosphodiesterase isozymes

The pyridazinone derivative zardaverine has recently been introduced as a potent bronchodilator in vivo and in vitro. In addition, zardaverine exerts a positive inotropic action on heart muscle in vitro. The actions of zardaverine are thought to be mediated via inhibition of phosphodiesterase (PDE) activity. Recent data suggest that there are multiple forms of phosphodiesterases and at least five different isozyme families are now recognized. In the present study, the effects of zardaverine on the different PDE isozymes were investigated in several tissues. PDE isozymes were separated by chromatography on Q-sepharose. Zardaverine inhibited the cyclic GMP-inhibitable PDE III from human platelets and the rolipram-inhibitable PDE IV from canine trachea and human polymorphonuclear (PMN) cells with IC50-values of 0.58, 0.79 and 0.17 microM, respectively. The pyridazinone derivative affected the calmodulin-stimulated PDE I, the cyclic GMP-stimulated PDE II and the cyclic GMP-specific PDE V only marginally at concentrations up to 100 microM. Zardaverine inhibits the ADP-induced aggregation of human platelets with an IC50 of 1.6 microM. This inhibition was synergistically increased by activators of adenylate cyclase such as PGE1 and forskolin. In human PMN cells, zardaverine inhibited the zymosan-induced superoxide anion generation with an IC50 of 0.40 microM. Again, this effect was increased by activators of adenylate cyclase. These data clearly demonstrate that zardaverine is a selective inhibitor of PDE III and PDE IV isozymes.

Differential modulation of tissue function and therapeutic potential of selective inhibitors of cyclic nucleotide phosphodiesterase isoenzymes

Since the discovery of cyclic nucleotide phosphodiesterase 30 years ago, there have been major advances in our knowledge of this group of isoenzymes. Five families, each composed of several isoforms and having differing tissue distributions, have been described. David Nicholson and colleagues compare the tissue distribution of phosphodiesterase isoenzymes and discuss the differential effects of inhibition of particular isoenzymes, with differing subcellular localization, on tissue function. They also review the potential use of isoenzyme selective phosphodiesterase inhibitors in a range of clinical disorders such as heart failure, asthma, depression and dementia.

Cyclic nucleotide phosphodiesterases: pharmacology, biochemistry and function

This article is a review of cyclic nucleotide phosphodiesterase(s) (CN PDE) from the point of view of the relationships between the newer aspects of the complex enzymology of CN PDE and recent major advances in CN PDE pharmacology. A consolidation of isozyme nomenclature to the proposed family designations is recommended. Emphasis is placed on the importance of defining the subcellular localization of isozymes expressed in a given tissue and cyclic GMP substrate and regulatory roles in CN PDE isozyme functions. CN PDE inhibitors that may be useful for experimental and clinical purposes are discussed. Examples of these inhibitors include CGS 9343B, TCV-3B, KW-6, MIMAX, Dihydroisoquinolines, Trequinsin, bipyridine and dihydropyridazinone cardiotonics, Rolipram, SQ 65442, Zaprinast and Dipyridamole.

Effects of isozyme-selective phosphodiesterase inhibitors on rat aorta and human platelets: smooth muscle tone, platelet aggregation and cAMP levels

The inhibitors of the cGMP-inhibited, low-Km cAMP phosphodiesterase--milrinone and OPC 3911--and an inhibitor of a non-cGMP-inhibited low-Km cAMP phosphodiesterase--rolipram--were used to evaluate the functional importance of the two cAMP phosphodiesterase activities in vascular smooth muscle and in platelets. Vinpocetine, an inhibitor of a calcium-calmodulin-dependent phosphodiesterase was also studied. OPC 3911 and milrinone relaxed the contracted rat aorta, inhibited ADP-induced platelet aggregation and also enhanced isoprenaline-induced relaxation as well as the antiaggregatory effects of adenosine. In platelets, OPC 3911 and milrinone increased cAMP levels, but in the rat aorta the increase was significant only for milrinone (OPC 3911 P = 0.062). In both tissues OPC 3911 and milrinone enhanced the increase in cAMP caused by activators of adenylate cyclase (isoprenaline/adenosine). Rolipram had no effects on aggregation or cAMP levels in platelets and no overadditive effects in combination with adenosine. Rolipram had little effect on relaxation and cAMP levels, did not alter isoprenaline-induced relaxation of guanfacin-contracted rat aorta, but showed synergistic effects with isoprenaline in raising cAMP levels. In PGF2 alpha-contracted aorta rolipram enhanced relaxation caused by isoprenaline. Vinpocetine had a relaxant effect without affecting cAMP levels, but had no effect on platelets. These results support the concept that the cGMP-inhibited phosphodiesterase is an important modulator of vascular smooth muscle tone and platelet function. The role of the non-cGMP-inhibited phosphodiesterase in these tissues is less obvious.

Use of a synthetic dodecapeptide (malantide) to measure the cyclic AMP-dependent protein kinase activity ratio in a variety of tissues

1. The cyclic AMP-dependent protein kinase activity-ratio assay was investigated by comparing histone and a synthetic peptide, malantide [Malencik & Anderson (1983) Anal. Biochem. 132, 32-40], as substrates. 2. In several tissues the activity ratio was higher when assayed with histone as the substrate; this result was obtained in control tissues and also in those incubated with agents known to increase cyclic AMP. The effect of these agents to increase the activity ratio was more clearly demonstrated with malantide. 3. The higher activity ratios observed with histone are due to: (a) measurement of phosphorylation not catalysed by cyclic AMP-dependent protein kinase; (b) activation of cyclic AMP-dependent protein kinase by histone during the assay. 4. When tissues were homogenized in buffers without NACl, lower activity ratios were found, owing to the catalytic subunit being artifactually removed from the supernatant. 5. We conclude that the measured activity ratio more faithfully reflects that in the tissue when NaCl is included in the homogenization buffer and malantide is used in the assay. This was confirmed in experiments where cyclic AMP-dependent protein kinase was added to the tissue before homogenization, and no dissociation of the exogenous enzyme was observed.

The effects of siguazodan, a selective phosphodiesterase inhibitor, on human platelet function

1. The effects of siguazodan (SK&F 94836) a selective phosphodiesterase (PDE) inhibitor with inotropic and vasodilator activity, were studied on human platelets. 2. Siguazodan selectively inhibited the major cyclic AMP-hydrolysing PDE in human platelet supernatants. The inhibited enzyme has been variously termed cyclic GMP-inhibited PDE or PDE-III. 3. In platelet-rich plasma (PRP), siguazodan inhibited U46619-induced aggregation more potently than that induced by ADP and collagen. Treatment of the PRP with aspirin had no effect on the potency of siguazodan. 4. In washed platelets, siguazodan increased cyclic AMP levels and reduced cytoplasmic free calcium [( Ca2+]i). ADP decreased the ability of siguazodan to raise cyclic AMP and this may explain its lower potency in inhibiting responses to ADP. 5. Siguazodan has anti-platelet actions over the same concentration range that it is an inotrope and vasodilator.