Long-Lasting Changes of Rat Blood Pressure to Vasoconstrictors and Vasodilators Induced by Nitric Oxide Donor Infusion: Involvement of Potassium Channels1

Abstract

We investigated the effects of the exposure of the rat vascular system to nitric oxide (NO), using infusion of either NO donor sodium nitroprusside (SNP) orS-nitroso-acetyl-dl-penicillamine (SNAP) on mean arterial pressure (MAP) responses to vasoconstrictors (phenylephrine, angiotensins I and II) and to vasodilators (bradykinin, acetylcholine, SNP, and iloprost). SNP (250 nmol/kg/ min) or SNAP (85 nmol/kg/min) infused for 30 min decreased MAP by 40 to 60 mm Hg. MAP returned to normal levels 5 to 10 min after the end of infusion. After infusion of SNP or SNAP the effects of phenylephrine, angiotensin I, and angiotensin II were reduced by 40 to 80%, whereas the responses to bradykinin or acetylcholine were enhanced by 50 to 80%. These changes in vascular responsiveness persisted for at least 24 h after the SNP infusion. Pretreatment with either tetraethylammonium (360 μmol/kg) or 4-aminopyridine (4-AP; 1 μmol/kg) did not alter the effects of phenylephrine or bradykinin in control animals, but prevented SNP-induced changes in responsiveness to phenylephrine or bradykinin. On the other hand, administration of tetraethylammonium, even 24 h after SNP infusion, reversed hyporesponsiveness to phenylephrine, whereas 4-AP was ineffective. Tetraethylammonium and 4-AP did not alter the increased responses to bradykinin. Glibenclamide was without effect in any situation. These results indicate that NO-induced changes on vascular responsiveness to vasoconstrictors and vasodilators are much more profound and long-lasting than described previously and that the effects of NO appear to be, at least in part, mediated by persistent activation of a tetraethylammonium-sensitive population of K⁺ channels.