Acute and chronic sympathoinhibition on carotid artery diameter of spontaneously hypertensive rats: Effects of clonidine and flesinoxan

1. Hypertensive conduit arteries are thicker and stiffer than those of norm otensive controls. Whether they are specifically sensitive to central sympa thoinhibition has never been investigated. 2. The effects of acute (24 h infusion) and chronic (4 week infusion) treat ments with clonidine (0.01 and 0.1 mg/kg per day) and flesinoxan (1 and 3 m g/kg per day) on carotid artery diameter were investigated in spontaneously hypertensive rats. At the end of treatment, blood pressure (BP) was record ed in the rats while they were conscious, Rats were then anaesthetized for carotid artery diameter measurements using an ultrasonic echo-tracking devi ce. 3, In conscious rats, clonidine significantly decreased BP and heart rate ( HR) following acute but not chronic treatment. In contrast, flesinoxan sign ificantly decreased BP following both the acute and chronic treatment, In a naesthetized animals, the two agents have opposite effects on isobaric caro tid artery diameter, with a decrease under clonidine and an increase under flesinoxan. After 1 weeks infusion, the reactivity of aortic rings was stud ied in organ chambers. Flesinoxan, but not clonidine, caused the relaxation of potassium chloride precontracted aortic segments. 4. The results indicate that although clonidine and flesinoxan are centrall y acting antihypertensive agents, the drug-induced changes in isobaric caro tid diameter may be influenced by local factors independent of the central action of the two drugs.