RENAL-ARTERY STENOSIS AND ISCHEMIA - EFFECT ON RENAL BLOOD-FLOW AND EXTRACTION FRACTION

The clearances of [I-131]orthoiodohippurate and Tc-99m-mercaptoacetyltriglycine provide a measure of effective renal plasma flow, yet these clearances are proportional to renal plasma flow only if the extraction fraction remains constant. To determine the effect of unilateral renal artery stenosis, captopril, renal ischemia, and partial renal vein occlusion on renal blood how and the extraction fraction of [I-131]orthoiodohippurate, Tc-99m-mercaptoacetyltriglycine, and [I-125]iothalamate, we conducted a series of constant infusion studies in Sprague-Dawley rats. Renal artery flow reduction of approximately 70% decreased the extraction fraction of all three agents (P less than or equal to.05). Captopril had no effect on extraction fraction in controls, but it produced a further decrease in extraction fraction of Tc-99m-mercaptoacetyltriglycine and [I-131]orthoiodohippurate in rats with renal artery stenosis (P less than or equal to.05). Ischemia resulted in a 16% decrease in flow (P<.01) but a much larger (47% to 65%) decrease in extraction fraction of all three agents (P<.002). Partial renal vein occlusion also decreased the extraction fraction of all three agents (P less than or equal to.05). The changes in extraction fraction imply that the clearances of [I-131]orthoiodohippurate and Tc-99m-mercaptoacetyltriglycine in disease states may not be proportional to renal plasma flow. Furthermore, in rats with renal artery stenosis it appears that renal blood flow must fall below a critical threshold of approximately 58% before extraction fraction decreases; as renal blood flow is further reduced below this threshold, there is a corresponding reduction in extraction fraction (P<.01).