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 (NaNO2) was reduced when rings were contracted with an enriched K+ solution (30 mM), by voltage-dependent K+ (Kv) channel blockade with 4-amynopiridine (4-AP; 10−4 M), by Ca2+-activated K+ (KCa) 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 KCa (BKCa), ATP-sensitive K+ (KATP) and inwardly rectifying K+ (Kir) channel blockade with iberiotoxin (IbTx; 10−7 M), glibenclamide (10−6 M) and BaCl2 (3 × 10−5 M), respectively, did not alter the concentration–response curves to acetylcholine and NaNO2. Na+−K+ ATPase pump inhibition with ouabain (10−5 M) practically abolished acetylcholine and NaNO2 relaxations. Our findings suggest that acetylcholine-induced relaxation is largely mediated through the NO-cGMP pathway, involving apamin plus ChTx-sensitive K+ and Kv channels, and Na+−K+-ATPase pump activation.