MEDULLARY BLOOD-FLOW RESPONSES TO CHANGES IN ARTERIAL-PRESSURE IN CANINE KIDNEY

Although it is well recognized that whole kidney and cortical blood fl ow exhibit efficient autoregulation in response to alterations in rena l arterial pressure (RAP), the autoregulatory behavior of medullary bl ood flow (MBF) has remained uncertain. We have evaluated MBF responses to stepwise reductions in RAP for both short-term (2 min, n = 6) and longer periods (15 min, n = 7) using single-fiber laser-Doppler flowme try with needle probes inserted into the mid-medullary region in dener vated kidneys of 13 anesthetized dogs. The changes in cortical blood f low (CBF) were assessed with either a surface probe or a needle probe inserted into the cortex. Control total renal blood flow (RBF), assess ed by electromagnetic flow probe in these dogs, was 5.2 +/- 0.3 ml . m in(-1). g(-1), and glomerular filtration rate was 0.97 +/- 0.05 ml . m in(-1). g(-1)(n = 7). RBF, MBF, and CBF all exhibited efficient autore gulatory behavior during changes in RAP from 150 to 75 mmHg. The slope s of RAP vs. RBF, CBF, as well as MBF, were not significantly differen t from zero within this range of RAP. Below RAP of 75 mmHg, all indexe s of blood flow showed linear decreases with reductions in pressure. T he data indicate that blood flow in the renal medulla of dogs exhibits efficient autoregulatory behavior, similar to that in the cortex.