Evidence for operation of the magnification phenomenon in patients with chronic renal insufficiency

It has been proposed that the functional adaptations of surviving nephrons in advancing chronic renal failure (CRF) are not random but characterized by an excretory response which varies inversely with the number of surviving nephrons ('magnification phenomenon'). Because validation of this hypothesis in man is incomplete, we undertook to characterize the excretory response to acute volume expansion in patients with CRF. In normals, water immersion to the neck (NI) results in a redistribution of blood volume with preferential central hypervolemia (CV) in the absence of plasma compositional change. NI was utilized, therefore, to assess the response to acute CV. 13 patients with CRF (GFR = 3-65 ml/min) were studied twice while ingesting a constant diet (60-150 mEq Na/day): during a seated control study (C) and during 4 h of NI. FENa was constant during C. In contrast, during NI, there was a prompt and marked increase in FENa which markedly exceeded that of 29 normal subjects undergoing an identical study. In the CRF group the extent of the augmentation of FENa during immersion varied inversely with GFR (r = -0.54; p less than 0.05). These results provide evidence that the 'magnification phenomenon' subtends renal sodium handling in patients with CRF.