CARDIOVASCULAR EFFECTS OF A NOVEL, POTENT AND SELECTIVE PHOSPHODIESTERASE-5 INHIBITOR, DMPPO - IN-VITRO AND IN-VIVO CHARACTERIZATION

1 The aim of this study was to investigate the cardiovascular effects of a novel, potent and specific phosphodiesterase 5 (PDE 5) inhibitor, 1,3 dimethyl-6-(2-propoxy-5-methane ylamidophenyl)-pyrazolo[3,4-d]pyr imidin-4-(5H)-one (DMPPO) in phenylephrine-precontracted rat aortic ri ngs and different in vivo rat preparations. 2 DMPPO elicited a concent ration-dependent relaxation of rat aortic rings with functional endoth elium. DMPPO-induced relaxation was abolished by endothelium removal o r pretreatment with the soluble guanylate cyclase inhibitor, methylene blue (10 mu M). 3 In aortic rings without endothelium, the potency (p D(2)=-log(10) EC(50)) of atrial natriuretic peptide (ANP) to induce re laxation increased from 8.13+/-0.05 in the absence of DMPPO to 8.32+/- 0.05 and 8.52+/-0.08 in the presence of 30 nM and 100 nM DMPPO, respec tively. Similarly, the potency of sodium nitroprusside (SNP) in induci ng relaxation increased from 7.38+/-0.07 in the absence of the PDE 5 i nhibitor to 8.07+/-0.11 and 8.15+/-0.08 in the presence of 30 nM and 1 00 nM DMPPO, respectively. In contrast, relaxation to the adenylate cy clase activator, forskolin, was unchanged by DMPPO (100 nM). 4 In ring s without endothelium, DMPPO (100 nM) increased by 2.5 fold intracellu lar levels of guanosine 3':5'-cyclic monophosphate (cyclic GMP). Moreo ver, DMPPO (100 nM) potentiated the increases in cyclic GMP levels ind uced by ANP (30 nM) by 3 fold and SNP (30 nM) by 2.7 fold. Adenosine 3 ':5'-cyclic monophosphate (cyclic AMP) levels were not modified by DMP PO. 5 In anaesthetized normotensive or spontaneously hypertensive rats (SHR), DMPPO (2 and 5 mg kg(-1), i.v.) lowered blood pressure without affecting heart rate. Similarly, in conscious SHR, orally administere d DMPPO (5 mg kg(-1)) induced a 25 mmHg decrease in blood pressure for at least 7 h without modifying heart rate. Meanwhile, urinary cyclic GMP was increased by 50% whereas cyclic AMP remained unchanged. 6 In n ormotensive anaesthetized rats, sodium nitroprusside (SNP) (i.v. bolus ) induced a decrease in blood pressure which rapidly returned to basel ine. In DMPPO (1 mg kg(-1), i.v.)-treated rats, the hypotensive effect s of SNP (10 to 100 mu g kg(-1)) were prolonged over time whereas the peak effect was unchanged.7 In pithed rats, phenylephrine (i.v. bolus) induced dose-dependent increases in blood pressure. Pretreatment with DMPPO (5 mg kg(-1), i.v.) partially inhibited the presser response to phenylephrine (0.3 to 100 mu g kg(-1)). 8 In conclusion, the potent a nd selective PDE 5 inhibitor, DMPPO, produces relaxation in isolated v essels in the presence of a cyclic GMP drive and reduces blood pressur e of intact animals. Its high oral bioavailability and long duration o f action should make it a useful tool to study the role of cyclic GMP in various biological systems.