INFLUENCE OF GRADED ARTERIAL PRESSURE DECREMENT ON RENAL CLEARANCE OF CREATININE, P-AMINOHIPPURATE AND SODIUM

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Abstract
When arterial infusion pressure to the kidney is gradually reduced by gradual aortic occlusion, clearances of p-aminohippurate and creatinine are well maintained near control values through a range of about 150 -100 mm. Hg. Subsequently, clearances decrease as glomerular filtration rate and urine flow cease at about 60 mm. Hg. During this latter phase, creatinine clearances fall more rapidly than the PAH clearances, so that the filtration fraction decreases. Calculation of renal resistance changes by the method of Lamport indicates that maintenance of renal clearances is due to afferent arteriolar dilatation; efferent arteriolar resistance and post-arteriolar resistance remain essentially constant. This emphasizes the role of the afferent arterioles as a buffering mechanism to maintain blood flow and glomerular filtration in opposition to systemic arterial blood pressure changes. Renal excretion of Na decreases as glomerular filtration rate is decreased by aortic occlusion. This is because as filtration is reduced, the Na load to the tubular reabsorptive mechanism is decreased with the result that tubular reabsorption becomes more complete. The data suggest that a "threshold" for Na reabsorption is complete. The normal kidney offers a load somewhat above this threshold so that small amts. of Na are normally excreted. The significance of these findings as they bear on the problem of renal Na retention in congestive heart failure is discussed.