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 1 mM aminopyridine (inhibitor of Kv channels), but not by 1 μM glibenclamide (inhibitor of KATP channels) or 50 nM iberiotoxin (inhibitor of KCa 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 μM- 10 mM) induced concentration-dependent contraction in Ca2+-free, depolarizing (50 mM KCl) medium. Sildenafil (0.1–100 μM) produced reversible concentration-dependent inhibition of the response to CaCl2, which was completely abolished by the highest sildenafil concentration. By contrast, only 100 μM zaprinast inhibited the response to CaCl2. The L-type Ca2+ channel activator Bay K 8644 (0.1 nM–1 μM) induced concentration-dependent potentiation of the response to CaCl2 inhibited by 100 μM sildenafil. Moreover, Bay K 8644 (0.1 nM–1 μM) 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 μM) and zaprinast (1 or 100 μM). These results show that sildenafil relaxes the rabbit basilar artery by increasing K+ efflux through Kv 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.