Acute cardiovascular effects of the alpha2-adrenoceptor antagonist, idazoxan, in rats: influence of the basal sympathetic tone.

Intravenous administration of the alpha2-adrenoceptor antagonist, idazoxan, elicits variable cardiovascular effects, depending on experimental conditions. In this study, the effects of idazoxan were investigated in rats with high, low, or no basal sympathetic tone. In a group of conscious Sprague-Dawley rats (n = 9), mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nervous activity (RSNA) were recorded. Idazoxan (250 microg/kg, i.v.) induced a transient decrease in MAP (-12+/-3 mm Hg) that was accompanied by increases in HR (49+/-14 beats/min) and RSNA (53+/-14%). In six of nine rats, a light pentobarbitone anesthesia was given. Basal RSNA was decreased (6.0+/-1.3 microV from 12.8+/-4.1 microV; p<0.05), and the depressor effect of idazoxan was reversed to a pressor effect (21+/-6 mm Hg) associated with bradycardia (-16+/-8 beats/min) and sympathoinhibition (-56+/-15%). In eight conscious intact rats, idazoxan (250 microg/kg, i.v.) attenuated by approximately 40% the pressor response to the selective alpha1-adrenoceptor agonist, cirazoline (0.5 microg/kg, i.v.). In three groups of six to seven ganglion-blocked (chlorisondamine, 2.5 mg/kg, i.v.) conscious rats, idazoxan dose-dependently increased mean arterial pressure (MAP: 39+/-2, 55+/-3, and 69+/-4 mm Hg at 125, 250, and 500 microg/kg, i.v., respectively) with minimal changes in HR. In contrast, the noradrenaline-releasing agent, tyramine (62.5, 125, and 250 microg/kg, i.v.), dose-dependently increased both MAP and HR. The alpha1-adrenoceptor antagonist, prazosin (1 mg/kg, i.v.; n = 8) blunted by approximately 70% (p<0.01) the pressor effect of 250 microg/kg idazoxan. It is concluded that in rats with high sympathetic tone, idazoxan has depressor effects, most likely related to its peripheral alpha-adrenoceptor antagonist properties. In rats with low or no sympathetic tone, idazoxan induced pressor responses mainly secondary to its partial agonist activity at vascular postjunctional alpha1-adrenoceptors.