Effects of hyperosmotic: Mannitol infusion on hemodynamics of dog kidney

This study evaluated the effect of systemic infusion of hypertonic mannitol on renal hemodynamics (aortic pressure [P]-renal blood flow [RBF] relationship, glomerular filtration rate [GFR], and effective renal plasma flow [ERPF]) during 50% reduction of left kidney blood flow. Conditioned mongrel dogs anesthetized with halothane were hydrated by continuous infusion of lactated Ringer's solution containing creatinine to measure GFR and p-aminohippurate (PAH), to measure ERPF. The left kidney was exposed and two hydraulic occluders were placed, one around the aorta just above the renal arteries and the other around the left renal artery. Experimental design consisted of measuring P near the left renal artery, RBF by electromagnetic flowmeter, and ERPF and GFR by clearance methods in both kidneys in response to stepwise reduction in the aortic pressure by aortic occlusion before and after 50% reduction in the left kidney blood flow. The P-RBF relationship, GFR, and ERPF thus obtained were compared with those obtained during systemic intravenous infusion of 20% mannitol for a period of 1 h. We found that 1) a transient increase occurred in RBF with step reduction of P from 80 to 60 mm Hg under control conditions; 2) reducing the RBF by 50% changed the shape of the P-RBF relationship from a convex to the P axis to a linear form with a marked shift toward the P axis; 3) infusion of mannitol, during reduced RBF, caused a significant shift of the P-RBF curve toward the RBF axis and returned the linear P-RBF relationship toward normal, but had no effect on altered yield pressure; and 4) infusion of hypertonic mannitol had slightly increased GFR and ERPF in the right (unconstricted) kidney. However, hypertonic mannitol significantly increased GFR and ERPF values in the left (constricted) kidney suggesting a beneficial effect of mannitol on ischemic kidney. The results are consistent with the hypothesis that infusion of hypertonic mannitol to ischemic kidney increases RBF, presumably by decreasing the intrarenal vascular resistance. We speculate that this compensatory response may be mediated either 1) by stimulating the release of a vasodilator substance (e.g., prostaglandins), or 2) by washing out interstitial sodium, thereby reducing the sensitivity of the renal vasculature to ischemia-induced stimulation of renin-angiotensin system.