Role of K+ and Ca2+ fluxes in the cerebroarterial vasoactive effects of sildenafil

The aim of this study was to assess the role of K+ and Ca2+ fluxes in the cerebroarterial vasoactive effects of the phosphodiesterase-5 inhibitor sildenafil. We used isolated rabbit basilar arteries to assess the effects of extracellular K+ raising on sildenafil-induced vasodilatation, and studied the pharmacological interaction of sildenafil with selective modulators of membrane K+ and Ca2+ channels. Expression of Kv1 subunits of K+ channels was assessed at messenger and protein levels. Parallel experiments were carried out with zaprinast for comparison. Sildenafil (10 nM-0.1 mM) induced concentration-dependent relaxation of endothelin-1 (10 nM)-precontracted arteries, which was partially inhibited by depolarization with KCl (50 mM), 3 mM tetraethylammonium (non-selective K channel blocker) or I mM aminopyridine (inhibitor of K-v channels), but not by 1 mu M glibenclamide (inhibitor of K-ATP channels) or 50 nM iberiotoxin (inhibitor of K-Ca channels). Arterial smooth muscle expressed messengers for Kv1.2, Kv1.3, Kv1.4, Kv1.5 and Kv1.6, and proteins of Kv1.1, Kv1.2 and Kv1.4. CaCl2 (10 mu M-10 mM) induced concentration-dependent contraction in Ca2+-free, depolarizing (50 mM KCl) medium. Sildenafil (0.1-100 mu M) produced reversible concentration-dependent inhibition of the response to CaCl2, which was completely abolished by the highest sildenafil concentration. By contrast, only 100 mu M zaprinast inhibited the response to CaCl2. The L-type Ca2+ channel activator Bay K 8644 (0.1 nM-1 mu M) induced concentration-dependent potentiation of the response to CaCl2 inhibited by 100 mu M sildenafil. Moreover, Bay K 8644 (0.1 nM-1 mu M) induced concentration-dependent contraction in slightly depolarizing (15 mM) medium, which was inhibited to the same extent and in a concentration-dependent way by sildenafil (0.1-100 mu M) and zaprinast (1 or 100 mu M). These results show that sildenafil relaxes the rabbit basilar artery by increasing K+ efflux through K, channels, which in turn may affect Ca2+ signalling. Expression of Kv1 subunits involved in this pharmacological effect occurs at the messenger and, in some cases, at the protein level. In addition to this phosphodiesterase-5-related effect, sildenafil and zaprinast inhibit cerebroarterial vasoconstriction at least in part by directly blocking L-type Ca2+ channels, although a decrease in the sensitivity of the contractile apparatus to Ca2+ can not be discarded. (C) 2007 Elsevier B.V. All rights reserved.