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The changes in the medullary and cortical blood flows of isolated dog kidney were measured by means of a light and small thermocouple inserted in each region.When the perfusion pressure rose abruptly, the medulla responded increasing the flow independently of the initial perfusion pressure, while the cortical flow varied according to the initial perfusion pressure value. From 70 to 160 mm Hg of initial perfusion pressure, the cortex showed a doublehumped response, that is, an initial increase in blood flow f ollowed by a decrease that did not reach the control value and finally a late increase. The urinary flow increased concomitantly with the late increase in the cortical blood flow. When the initial perfusion pressure was under 70 mm Hg or over 160 mm Hg the cortex responded similarly to the medulla.The α adrenergic blocking agent (MA 1277) did not change any of the responses above mentioned but potassium cyanide (4 mM/L in the perf using blood) suppressed the decrease in blood flow shown by the cortex, in such a way that only an increase was observed. Under KCN action the urinary flow increased simultaneously with the increase in the perfusion pressure. The intra-arterial in jection of acetylcholine, in doses of 2.5 μg/g of kidney tissue, produced a decrease in the cortical blood flow while the medullary blood flow responded by an initial increase followed by a decrease. These responses were concomitant with a decrease in the total renal flow. When the MA 1277 was added (5 mM/L) to the perfusing blood the response to acetylcholine mentioned above, changed in such a manner that only increases in the cortical, medullary and total renal blood flows were observed. The different effects of large doses of acetylcholine on cortical and medullary resistance are discussed in terms of different amount of sympathetic transmitter released.The present findings suggest that during an abrupt increase in the perfusion pressure the diminution phase of the double-humped cortical flow response was caused by an active contraction of the vascular smooth muscles; while the increment in medullary flow could be due to the blood coming from another source different from the efferent blood vessels of the juxtamedullary glomeruli.

THE CORTICAL AND MEDULLARY BLOOD FLOWS OF THE ISOLATED DOG'S KIDNEY