DIRECT MEASUREMENT OF RENAL MEDULLARY BLOOD-FLOW IN THE DOG

We studied the responses of total renal blood flow (RBF) and renal med ullary blood flow (RMBF) to changes in renal perfusion pressure (RPP) within and below the range of renal autoregulation in the anesthetized dog (n = 7). To measure RMBF, we developed a technique in which the m edulla is exposed by excising a section of infarcted cortex and a mult iple optical fiber flow probe, connected to a lasser-Doppler flowmeter , is placed on the medulla. At the baseline RPP of 120 +/- 1 mmHg, RBF was 2.58 +/- 0.33 ml.min(-1).g perfused kidney wt(-1), and RMBF was 2 22 +/- 45 perfusion units. RPP was then decreased in consecutive 20-mm Hg steps to 39 +/- 1 mmHg. At 80 +/- 1 mmHg, RBF remained at 89 +/- 4% of the baseline value; however, RMBF had decreased significantly (P < 0.05) to 73 +/- 4% of its baseline value. The efficiency of autoregul ation of RBF and of RMBF within the RPP range of 120 to 80 mmHg was de termined by calculating an autoregulatory index (AI) for each paramete r using the formula Al = (%Delta blood flow)/(%Delta RPP). An AI of 0 indicates perfect autoregulation, and an index of 1 indicates a system with a fixed resistance. The AI for RBF averaged 0.33 +/- 0.12 over t his pressure range and showed a significantly greater (P < 0.05) autor egulatory ability than did the RMBF (0.82 +/- 0.13). Decreasing perfus ion pressure <80 mmHg produced significant decreases in both RBF and R MBF. The results suggest that, in the anesthetized dog, RMBF is not au toregulated with the same efficiency as RBF within the pressure range of 80 to 120 mmHg. In conclusion, we have developed a direct method fo r measurement of RMBF in the dog and have demonstrated its use in the investigation of renal medullary hemodynamics.