Effect of erythrocytes on the function and morphology of the isolated perfused rat kidney

We have examined the effects of erythrocytes on the function and morphology of isolated rat kidneys perfused with a physiological concentration of bovine albumin (45 g/l). (1) In kidneys perfused without red cells, renal vascular resistance (RVR) was low (4.2 +/- 0.3 mm Hg/ml/min/g), fractional sodium excretion (FeNa) was high (14.5 +/- 1.8%) and concentrating ability impaired (maximum urine osmolality 343 +/- 4 mmol/kg). The erythrocyte-free kidney also developed necrosis of the cells of the medullary thick ascending limb (mTAL). (2) Erythrocytes at a hematocrit of 4-6% did not alter RVR but prevented ischemic changes in the mTAL and reduced FeNa to 9.4 +/- 0.03%. Concentrating ability was not improved by a hematocrit of 4-6% despite the presence of a morphologically normal mTAL. (3) At a hematocrit of 40-45%, RVR was increased (to 11.2 +/- 0.4 mm Hg/ml/min/g) and FeNa was further lowered to 3.5 +/- 0.6%. Also, urinary concentrating ability was markedly improved (maximum urine osmolality 640 +/- 35 mmol/kg). (4) The isolated perfused kidney (IPK) at a hematocrit of 40-45% was able to autoregulate renal perfusate flow rate of GFR but autoregulation was incomplete. A 50% increase in perfusion pressure from 100 to 150 mm Hg increased renal perfusate flow rate and GFR 27 and 29%, respectively. Thus the IPK is not able to autoregulate as efficiently as the kidney in vivo, even in the presence of red cells at a normal hematocrit.