Regulation of NO-dependent acetylcholine relaxation by K + channels and the Na +–K + ATPase pump in porcine internal mammary artery

This study was designed to determine whether K + channels play a role in nitric oxide (NO)-dependent acetylcholine relaxation in porcine internal mammary artery (IMA). IMA segments were isolated and mounted in organ baths to record isometric tension. Acetylcholine-elicited vasodilation was abolished by muscarinic receptor blockade with atropine (10 -6 M). Incubation with indomethacin (3 × 10 − 6 M), superoxide dismutase (150 U/ml) and bosentan (10 − 5 M) did not modify the acetylcholine response ruling out the participation of cyclooxygenase-derivates, reactive oxygen species or endothelin. The relaxation response to acetylcholine was strongly diminished by NO synthase- or soluble guanylyl cyclase-inhibition using l-NOArg (10 − 4 M) or ODQ (3 × 10 − 6 M), respectively. The vasodilation induced by acetylcholine and a NO donor (NaNO 2) was reduced when rings were contracted with an enriched K + solution (30 mM), by voltage-dependent K + (K v) channel blockade with 4-amynopiridine (4-AP; 10 − 4 M), by Ca 2+-activated K + (K Ca) channel blockade with tetraethylammonium (TEA; 10 − 3 M), and by apamin (5 × 10 − 7 M) plus charybdotoxin (ChTx; 10 − 7 M) but not when these were added alone. In contrast, large conductance K Ca (BK Ca) , ATP-sensitive K + (K ATP) and inwardly rectifying K + (K ir) channel blockade with iberiotoxin (IbTx; 10 − 7 M), glibenclamide (10 − 6 M) and BaCl 2 (3 × 10 − 5 M), respectively, did not alter the concentration–response curves to acetylcholine and NaNO 2. Na +−K + ATPase pump inhibition with ouabain (10 − 5 M) practically abolished acetylcholine and NaNO 2 relaxations. Our findings suggest that acetylcholine-induced relaxation is largely mediated through the NO-cGMP pathway, involving apamin plus ChTx-sensitive K + and K v channels, and Na +−K +-ATPase pump activation.