Clonidine after renal ischemia to lessen acute renal failure and microvascular damage

Clonidine, an antihypertensive drug that inhibits renin release and causes a water diuresis in normal animals, was tested for its ability to reduce the severity of post-ischemic acute renal failure produced in rabbits by clamping the left renal pedicle for 1 hour and removing the opposite kidney. Clonidine significantly lessened renal failure when given during, or 1 hours after, the ischemic insult in dehydrated rabbits. It was also effective when given during the ischemic insult in vasopressin-treated water-drinking rabbits but not in control water-drinking rabbits. In vasopressin-treated rabbits, clonidine lessened renal failure observed 2 days after the ischemic insult despite the fact that in the immediate postischemic period it lowered total renal blood flow, produced hypotension, and did not bring about lower plasma renin levels. Clonidine treatment resulted in less outer medullary microvascular damage (demonstrated by colloidal carbon staining), higher outer medullary blood flow 1 to 2 hours after unclamping, fewer casts, and higher creatinine clearance and free water clearance/creatinine clearance 4 to 6 hours after unclamping compared with controls. The effect of clonidine was unrelated to plasma renin activity. Clonidine did not alter plasma vasopressin concentration. Demeclocycline and lithium, two agents that blunt renal responsiveness to vasopressin, had a beneficial effect in dehydrated animals similar to that of clonidine, but the angiotensin II antagonist saralasin and the angiotensin converting enzyme inhibitor SQ20881 did not. Normal rabbits given a large dose of vasopressin in oil plus clonidine had significantly greater urine output and free water clearance and lower urine osmolality than did rabbits given vasopressin in oil alone. These results suggest that clonidine may be beneficial because it prevents ischemic microvascular injury in the renal outer medulla, an effect that may decrease tubular obstruction by lessening desquamation of damaged tubular cells or cell constituents into the tubular lumen. Clonidine may also decrease formation of obstructive hyaline casts in collecting ducts by blunting the kidney's response to vasopressin and increasing tubular fluid flow rate.