APAMIN-SENSITIVE K-DEPENDENT HYPERPOLARIZATION IN RABBIT MESENTERIC-ARTERIES( CHANNELS MEDIATE AN ENDOTHELIUM)

1. Vascular endothelial cells release a variety of substances which af fect the membrane potential and tone of underlying vascular smooth mus cle. In the presence of N-omega-nitro-L-arginine to inhibit nitric oxi de synthase and indomethacin to inhibit cyclo-oxygenase, acetylcholine (ACh; EC(50) approximate to 1 mu M) elicited the release of an endoth elium-derived hyperpolarizing factor (EDHF) in rabbit mesenteric arter ies. 2. The hyperpolarization due to EDHF was blocked by apamin (IC50 approximate to 0.3 nM), and by other inhibitors of the apamin-sensitiv e K+ channel (10 nM scyllatoxin, 100 mu M d-tubocurarine, 300 mu M gal lamine) in the presence of indomethacin and N-omega-nitro-L-arginine. The hyperpolarization was not blocked by glibenclamide (5 mu M), iberi otoxin (10 nM), tetraetylammonium (1 mM), barium (500 mu M), 4-aminopy ridine (500 mu M), ouabain (10 mu M), bumetanide (10 mu M), or nimodip ine (100 nM). 3. In the presence of apamin and N-omega-nitro-L-arginin e, but the absence of indomethacin, ACh triggered a hyperpolarization that was blocked by glibenclamide, an inhibitor of ATP-sensitive K+ (K -ATP) channels. A similar glibenclamide-sensitive hyperpolarization wa s caused by Iloprost, a stable analogue of prostacyclin. 4. In experim ents which distinguished the effects of EDHF, prostanoids and nitric o xide, hyperpolarizations and/or relaxations triggered by ACh were anta gonized by muscarinic antagonists, the relative potencies (atropine ap proximate to 4-DAMP > pirenzepine) of which indicated that the release of all three endothelium-derived factors was mediated by M(3) recepto rs. 5. Our results suggest that ACh stimulates M(3) receptors on endot helial cells, triggering the release of nitric oxide and prostanoids, which hyperpolarize underlying smooth muscle by activation of K-ATP ch annels, and the release of an EDHF, which hyperpolarizes smooth muscle through the activation of apamin-sensitive K+ (K-AS) channels.