Further investigations into the endothelium-dependent hyperpolarizing effects of bradykinin and substance P in porcine coronary artery

1 In porcine coronary arteries, smooth muscle hyperpolarizations produced b y the nitric oxide donor, NOR-1, and the prostacyclin analogue, iloprost, w ere compared with those induced by substance P and bradykinin and attribute d to the endothelium-derived hyperpolarizing factor (EDHF). 2 In the presence of 300 muM L-nitroarginine and 10 muM indomethacin, ilopr ost-induced hyperpolarizations were partially inhibited by 10 muM glibencla mide whereas those to NOR-1, substance P and bradykinin were unaffected. 3 Hyperpolarizations produced by maximally-effective concentrations of NOR- 1 and NS1619 were identical (to -65 mV). They were significantly less than those generated by either substance P or bradykinin (to approximately -80 m V) and were abolished by iberiotoxin 100 nM, a concentration which had esse ntially no effect on responses to substance P or bradykinin. 4 Incubation of segments of intact arteries for 16 - 22 h in bicarbonate-bu ffered Krebs solution had little effect on EDHF responses to substance P or bradykinin. In contrast, after incubation for this period of time in HEPES -buffered Tyrode solution or Krebs containing 10 mm HEPES the EDHF response to substance P was abolished and that to bradykinin was markedly reduced. The residual bradykinin-induced hyperpolarization following incubation in T yrode solution was inhibited by iberiotoxin and by 10 muM 17-octadecynoic a cid. 5 We conclude that substance P activates only the EDHF pathway in the prese nce of nitric oxide synthase and cyclo-oxygenase inhibitors. Incubation in HEPES-buffered Tyrode solution abolishes the EDHF responses to substance P and bradykinin to reveal an additional hyperpolarizing mechanism, associate d with the opening of K+ channels, activated only by bradykinin.