Contribution of K + channels and ouabain‐sensitive mechanisms to the endothelium‐dependent relaxations of horse penile small arteries

Penile small arteries (effective internal lumen diameter of 300–600 μm) were isolated from the horse corpus cavernosum and mounted in microvascular myographs in order to investigate the mechanisms underlying the endothelium‐dependent relaxations to acetylcholine (ACh) and bradykinin (BK). In arteries preconstricted with the thromboxane analogue U46619 (3–30 n M ), ACh and BK elicited concentration‐dependent relaxations, pD 2 and maximal responses being 7.71±0.09 and 91±1% ( n =23), and 8.80±0.07 and 89±2% ( n =24) for ACh and BK, respectively. These relaxations were abolished by mechanical endothelial cell removal, attenuated by the nitric oxide (NO) synthase (NOS) inhibitor, N G ‐nitro‐ L ‐arginine ( L ‐NOARG, 100 μ M ) and unchanged by indomethacin (3 μ M ). However, raising extracellular K + to concentrations of 20–30 m M significantly inhibited the ACh and BK relaxant responses to 63±4% ( P <0.01, n =7) and to 59±4% ( P <0.01, n =6), respectively. ACh‐ and BK‐elicited relaxations were abolished in arteries preconstricted with K + in the presence of 100 μ M L ‐NOARG. In contrast to the inhibitor of ATP‐sensitive K + channels, the blockers of Ca 2+ ‐activated K + (K Ca ) channels, charybdotoxin (30 n M ) and apamin (0.3 μ M ), each induced slight but significant rightward shifts of the relaxations to ACh and BK without affecting the maximal responses. Combination of charybdotoxin and apamin did not cause further inhibition of the relaxations compared to either toxin alone. In the presence of L ‐NOARG (100 μ M ), combined application of the two toxins resulted in the most effective inhibition of the relaxations to both ACh and BK. Thus, pD 2 and maximal responses for ACh and BK were 7.65±0.08 and 98±1%, and 9.17±0.09 and 100±0%, respectively, in controls, and 5.87±0.09 ( P <0.05, n =6) and 38±11% ( P <0.05, n =6), and 8.09±0.14 ( P <0.01, n =6) and 98±1% ( n =6), respectively, after combined application of charybdotoxin plus apamin and L ‐NOARG. The selective inhibitor of guanylate cyclase, 1H‐[1,2,4]oxadiazolo[4,3‐a]quinoxalin‐1‐one (ODQ, 5 μ M ) did not alter the maximal responses to either ACh or BK, but slightly decreased the sensitivity to both agonists, δpD 2 being 0.25±0.07 ( P <0.05, n =6) and 0.62±0.12 ( P <0.01, n =6) for ACh and BK, respectively. Combined application of ODQ and charybdotoxin plus apamin produced further inhibition of the sensitivity to both ACh (δpD 2 =1.39±0.09, P <0.01, n =6) and BK (1.29±0.11, P <0.01, n =6), compared to either ODQ or charybdotoxin plus apamin alone. Exogenous nitric oxide (NO) present in acidified solutions of sodium nitrite (NaNO 2 ) and S‐nitroso‐cysteine (SNC) both concentration‐dependently relaxed penile resistance arteries, pD 2 and maximal responses being 4.84±0.06 and 82±3% ( n =12), and 6.72±0.07 and 85±4% ( n =19), respectively. Charybdotoxin displaced to the right the dose‐relaxation curves for both NO (δpD 2 0.38±0.06, P <0.01, n =6) and SNC (δpD 2 0.50±0.10, P <0.01, n =5), whereas apamin only reduced sensitivity (δpD 2 =0.35±0.12, P <0.05, n =5) and maximum response (65±9%, P <0.05, n =6) to SNC. ODQ shifted to the right the dose‐relaxation curves to both NO and SNC. The relaxant responses to either NO or SNC were not further inhibited by a combination of ODQ and charybdotoxin or ODQ and charybdotoxin plus apamin, respectively, compared to either blocker alone. In the presence of 3 μ M phentolamine, 5 μ M ouabain contracted penile resistance arteries by 50±6% ( n =17) of K‐PSS, but did not significantly change the relaxant responses to either ACh, BK or NO. However, in the presence of L ‐NOARG ouabain reduced the ACh‐ and BK‐elicited relaxation from 94±3% to 16±5% ( P <0.0001, n =6), and from 98±2% to 13±3% ( P <0.0001, n =5), respectively. Combined application of ODQ and ouabain inhibited the relaxations to NO from 92±2% to 26±3% ( P <0.0001, n =6). The present results demonstrate that the endothelium‐dependent relaxations of penile small arteries involve the release of NO and a non‐NO non‐prostanoid factor(s) which probably hyperpolarize(s) smooth muscle by two different mechanisms: an increased charybdotoxin and apamin‐sensitive K + conductance and an activation of the Na + ‐K + ATPase. These two mechanisms appear to be independent of guanylate cyclase stimulation, although NO itself can also activate charybdotoxin‐sensitive K + channels and the Na + ‐K + pump through both cyclic GMP‐dependent and independent mechanisms, respectively.British Journal of Pharmacology (1998) 123 , 1609–1620; doi: 10.1038/sj.bjp.0701780