DIFFERENTIAL SENSITIVITY TO ATP-SENSITIVE POTASSIUM CHANNEL OPENERS OF NOREPINEPHRINE-INDUCED CONTRACTION OF GUINEA-PIG AND RAT AORTA

Vasorelaxant effects of ATP-sensitive potassium (K-ATP) channel opener s were examined on the tonic phase of vascular contraction induced by norepinephrine (NE) in guinea pig and rat aorta. K-ATP channel openers , NIP-121 and cromakalim, produced glibenclamide-sensitive and concent ration-dependent relaxations in guinea pig and rat aorta preconstricte d with NE. However, the vascular relaxations induced by both K-ATP cha nnel openers were less pronounced in guinea pig aorta than in rat aort a. D-cis-Diltiazem, at the concentration up to 10(-5) M, did not appre ciably inhibit the NE-induced contraction of guinea pig aorta, whereas the compound almost completely inhibited the NE-induced contraction o f rat aorta at the same concentration. By contrast, sodium nitroprussi de relaxed the NE-induced contractions in both guinea pig and rat aort a with similar potencies. These findings suggest that vasorelaxant eff ects of K-ATP channel openers on the NE-induced sustained contraction in guinea pig aorta is not attributable to the subsequent inhibition o f Ca2+ influx through L-type voltage-gated Ca2+ channels. Lower sensit ivity of guinea pig aortic smooth muscle to K-ATP channel openers is m ost likely due to the low dependence of NE-induced contraction on the Ca2+ influx in this vascular smooth muscle.