Hypotensive effect of clonidine during sodium depletion in the rat.

Clonidine was nonhypotensive in conscious unrestrained rats maintained on a normal sodium intake. In contradistinction, clonidine caused a dose-related hypotension in conscious unrestrained rats subjected to sodium depletion via furosemide. The plasma renin activity of normal and sodium-depleted rats was reduced after the administration of clonidine (100 mug/kg, iv) by 22.8% and 34.4%, respectively. Intravenous infusion of an angiotensin II antagonist, 1-Sar-8-Ala-angiotensin II, caused a significant reduction of arterial blood pressure in sodium-depleted rats but not in normal rats. Similarly, bilateral nephrectomy reduced arterial blood pressure and completely abolished the hypotensive effect of clonidine in sodium-depleted rats. Subcutaneous administration of chlorisondamine caused a significantly greater reduction of arterial blood pressure in sodium-depleted rats than it did in normal rats. Treatment of normal and sodium-depleted rats with 6-hydroxydopamine reduced the arterial blood pressure of both groups to approximately 85 mm Hg and completely abolished the hypotensive effect of clonidine in the sodium-depleted rats. The data presented in this paper are consistent with the conclusion that clonidine acts at some site in the sympathetic nervous system of sodium-depleted rats to inhibit renal nerve activity with a resultant suppression of renin secretion and a reduction of the angiotensin II-maintained arterial blood pressure. A similar sequence of events occurring in normal rats would not result in hypotension because their arterial blood pressure is not maintained by angiotensin II.