Ca 2+ -activated K + (K Ca ) channels are involved in the relaxations elicited by sildenafil in penile resistance arteries Dolores Prieto ⁎ , Luis Rivera, Sara Benedito, Paz Recio, Nuria Villalba, Medardo Hernández, Albino García-Sacristán Departamento de Fisiología, Facultad de Farmacia, Universidad Complutense de Madrid, 28040-Madrid, Spain Received 28 July 2005; received in revised form 21 November 2005; accepted 19 December 2005 Available online 27 January 2006 Abstract The aim of the present study was to evaluate the role of K + channels in the vasorelaxant effect of the phosphodiesterase 5 inhibitor, sildenafil, in isolated horse penile resistance arteries mounted in microvascular myographs. In phenylephrine-precontracted arteries, sildenafil elicited potent relaxations which were markedly reduced by raising extracellular K + , by the non-selective blocker of Ca 2+ -activated K + channels (K Ca ), tetraethylammonium and by the blocker of large- and intermediate-conductance K Ca channels, charybdotoxin. Sildenafil relaxant responses were also reduced by the selective inhibitor of large conductance K Ca (BK Ca ) channels iberiotoxin, but not by the blocker of small conductance K Ca channels apamin. The inhibitor of the cGMP-dependent protein kinase (PKG), Rp-8-Br-PET-cGMPS, reduced the relaxations elicited by sildenafil but combined treatment with iberiotoxin and Rp-8-Br-PET-cGMPS did not further inhibit these relaxations, compared to the effect of either blocker alone. Iberiotoxin also shifted to the right the relaxations elicited by both the NO donor, S-nitrosoacetyl-D,L-penicillamine (SNAP) and the adenylate cyclase activator forskolin; treatment with both iberiotoxin and Rp-8-Br-PET-cGMPS did cause an additional inhibition. The present results demonstrate that the relaxant effect of sildenafil and NO in penile resistance arteries is due in part to activation of BK Ca channels through a PKG-dependent mechanism. © 2006 Elsevier B.V. All rights reserved. Keywords: Sildenafil; K + channel; Relaxation; Nitric oxide; Protein kinase G; Penile resistance artery 1. Introduction Stimulation of K + channels is a powerful mechanism of smooth muscle relaxation and both cyclic guanosine 3′,5′- monophosphate (cGMP) and cyclic adenosine 3′,5′-monopho- sphate (cAMP) induce vasodilatation at least in part by modulation of K + channel activity (Lincoln et al., 2001; Schubert and Nelson, 2001; Schlossmann et al., 2003). Thus, large conductance Ca 2+ -activated K + (BK Ca ) channels are activated by intracellular Ca 2+ and also by membrane depolarization and are particularly abundant in vascular smooth muscle cells (Nelson and Quayle, 1995; Schubert and Nelson, 2001). Physiological activation of BK Ca is an important buffering mechanism to counteract active tension induced by vasoconstrictors and also to maintain basal levels of tone in arterial smooth muscle, since they respond to increases in intracellular Ca 2+ by attenuating transmembrane Ca 2+ influx via repolarization-induced closure of L-type voltage-dependent Ca 2+ channels (Nelson and Quayle, 1995; Schubert and Nelson, 2001). K Ca channels play a key role in the regulation of corporal and arterial smooth muscle from the penis (Christ, 2000). Under basal conditions there is enough K Ca channel activity to regulate membrane potential (Nelson and Quayle, 1995) and therefore these channels are involved in the maintenance of the resting tone of both corpus cavernosum and penile resistance arteries (Prieto et al., 1998; Simonsen et al., 2002; Spektor et al., 2002). Moreover, K Ca channels are downstream mediators of the nitric oxide (NO)/cGMP signalling cascade involved in the relaxation of penile erectile tissue; NO is released from nerves and endothelium upon sexual stimulation and relaxes tonically constricted helicine arteries and corporal smooth muscle to induce penile erection (Andersson and Wagner, 1995; Simonsen European Journal of Pharmacology 531 (2006) 232 – 237 www.elsevier.com/locate/ejphar ⁎ Corresponding author. Tel.: +34 913947193; fax: +34 913947184. E-mail address: dprieto@farm.ucm.es (D. Prieto). 0014-2999/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ejphar.2005.12.033