Dominant role of an endothelium-derived hyperpolarizing factor (EDHF)-like vasodilator in the ciliary vascular bed of the bovine isolated perfused eye

1 The roles of the endothelium-derived nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor (EDHF) in mediating vasodilator responses to acetylcholine and bradykinin were assessed in the ciliary vascular bed of the bovine isolated perfused eye preparation. 2 Vasodilatation to acetylcholine or bradykinin was unaffected by the nitric oxide synthase inhibitor, L-NAME (100 muM), or the cyclo-oxygenase inhibitor, flurbiprofen (30 muM), but was virtually abolished following treatment with a high concentration of KCI (30 mm), or by damaging the endothelium with the detergent, CHAPS (0.3%, 2 min). 3 Acetylcholine-induced vasodilatation was unaffected by glibenclamide (10 muM), an inhibitor of ATP-sensitive K+ channels (K-ATP(+)), but was significantly attenuated by TEA (10 mm), a nonselective inhibitor of K+ channels. 4 The small conductance calcium-sensitive K+ channel (SKCa+) inhibitor, apamin (100 nM), and the large conductance calcium-sensitive K+ channel (BKCa+) inhibitor, iberiotoxin (50 nM), had no significant effect on acetylcholine-induced vasodilatation. In contrast, the intermediate (IKCa+)/large conductance calcium-sensitive K+ channel inhibitor, charybdotoxin (50 nM), powerfully blocked these vasodilator responses, and uncovered a vasoconstrictor response. 5 The combination of apamin (100 nM) with a sub-threshold concentration of charybdotoxin (10 nM), significantly attenuated acetylcholine-induced vasodilatation, but the combination of apamin (100 nM) with iberiotoxin (50 nM) had no effect. 6 In conclusion, blockade by a high concentration of KCI, by charybdotoxin, or by the combination of apamin with a sub-threshold concentration of charybdotoxin, strongly suggests that vasodilatation in the bovine isolated perfused eye is mediated by an EDHF.