Mechanisms of nitric oxide-independent relaxations induced by carbachol and acetylcholine in rat isolated renal arteries

1 In rat isolated renal artery segments contracted with 0.1 mu M phenylephrine and in the presence of the NO synthase inhibitor N-omega-nitro-r-arginine methyl ester (L-NAME), carbachol and acetylcholine produced endothelium-dependent relaxations. The mechanisms underlying these relaxations were studied. 2 These relaxations were not affected by ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one) or indomethacin. In arteries contracted with 20-30 mM K+, L-NAME-resistant relaxations induced by carbachol and acetylcholine were virtually absent. 3 The Na+-K+ ATPase inhibitor ouabain reduced these relaxations in a concentration-dependent manner. 4 In K+-free media, addition of K+ (5mM) produced 90.5+/-3.9% (n=3) relaxation of phenylephrine-induced tone. This relaxation was endothelium-independent and ouabain-sensitive. 5 Tetraethylammonium (TEA), charybdotoxin (ChTX) and iberiotoxin (IbTX) reduced the sensitivity of carbachol-induced relaxations, but did not change the maximal response. These relaxations were not altered by 4-aminopyridine (4-AP), glibenclamide or apamin. Acetylcholine (1 mu M)-induced relaxation was reduced by ChTX, but not by TEA or IbTX. 6 The cytochrome P450 inhibitor miconazole, but not 17-octadecynoic acid, reduced the sensitivity of carbachol-induced relaxations, without changing the maximal response. 7 In conclusion, in rat isolated renal arteries, acetylcholine and carbachol produced a non-NO/non-PGI(2) relaxation which is mediated by an endothelium-derived hyperpolarizing factor (EDHF). This factor does not appear to be a cytochrome P450 metabolite. The inhibition by ouabain of these relaxations suggests the possible involvement of Na+-K+ ATPase activation in EDHF responses, although other mechanisms cannot be totally ruled out.