Thromboxane A2 release from the endothelium mediates transient P2 purinoceptor-induced contractions of the human corpus cavernosum

M. Faria1, J.-M. LaFuente-de-Carvalho0,2, P. Correia-de-Sá1. 1ICBAS - Universidade do Porto, Lab. Farmacologia e Neurobiologia / UMIB, 4050-313 Porto, Portugal, 2CHP - Hospital de Santo António, Serv. Urologia, 4099-001 PORTO, Portugal

Data from our group indicate that ATP and its metabolites (ADP and adenosine) are potent relaxants in the human corpus cavernosum (HCC). The nucleotide is, thus, regarded as an important mediator of initiation and maintenance of human penile erection. Nevertheless, synchronous activation of vasoconstrictor P2 purinoceptors may partially counteract ATP-mediated relaxation, a mechanism that might be implicated in the pathophysiology of human erectile dysfunction. In this study, we investigated the pharmacological profile of P2 purinoceptors mediating contraction of HCC strips isolated from organ donors and from impotent men with drug-resistant vasculogenic erectile dysfunction (VED) submitted to penile prosthesis implantation. Values of P<0.05 (unpaired Student’s t-test) were considered significant. The preferential P2X1 and P2X3 receptor agonist, α,β-MeATP (0.3-300 μM, EC50~1 µM), transiently increased the tonus of HCC strips pre-contracted with 1 µM phenylephrine (PE) in a concentration-dependent manner (max. tension, 34±3% above PE, n=21-27, P<0.05). ATP (3-3000 µM, EC50~100 µM) mimicked the contractile effect of α,β-MeATP, yet the maximal tension reached a smaller magnitude (26±3% above PE, n=41, P<0.05). ADP (10-3000 µM) and its stable analogue, ADPβS (1-300 µM) transiently increased the tension of precontracted HCC strips, but their maximal effects were not greater than 20% and 12%, respectively. ADPβS was more potent (EC50 of ~10 µM) than the native compound, ADP (EC50 of ~1 mM). In contrast to ADP and ADPβS, contractile responses to α,β-MeATP (21±3%, n=7-10, P<0.05) and ATP (9±2%, n=20, P<0.05) were partially desensitized in VED patients as compared to controls. Pre-incubation with the selective P2X1 antagonist, NF023 (10 µM), shifted to the right the concentration-response curve of α,β-MeATP (0.3-300 μM) in control individuals. Blockade of P2Y12 and P2Y13 purinoceptors with MRS 2395 (10 µM) and MRS 2211 (10 µM) receptors, respectively, shifted to the right the concentration-response curve of ADPβS (0.3-300 µM), both in control and in VED HCC strips, whereas the selective P2Y1 receptor antagonist, MRS 2179 (0.3 µM), was without effect. Coupling of ADPβS (0.3-300 µM)-induced contractions to adenylate cyclase, but not to phospholipase C, pathway was demonstrated using selective inhibitors, respectively MDL 12,330A (10 µM) and U73122 (3 µM). Preincubation of HCC strips with the cycloxygenase inhibitor, indomethacin (10 µM), reduced (>60%) the maximal tension produced by both α,β-MeATP (0.3-300 μM) and ADPβS (0.3-300 µM); the NO synthase inhibitor, L-NAME (100 µM), had no effect. GR 32191 (10 nM), a selective TP receptor antagonist, prevented almost completely (>90%) ADPβS (0.3-300 µM)-induced contractions of HCC strips. Data suggest that ADP-sensitive P2Y12 and P2Y13 purinoceptors coupled to adenylate cyclase act together with fast desensitizing ionotropic P2X1 receptors to increase smooth muscle tension in precontracted HCC strips. Results also indicate that endothelium-derived thromboxane A2, acting via TP receptor, may be an important mediator of P2 purinoceptor-induced contraction in the HCC.

Work supported by FCT (FEDER funding, UMIB-215/94 and SFRH/BD/25470/2005) and Soc. Port. Andrologia.