Effects of saline on continuously recorded filtration fractions in cat kidney

The radioactivity of [51Cr]EDTA and [125I]albumin in arterial and in deep and superficial renal venous blood was continuously measured in an extracorporeally recirculating system. The filtration fraction of plasma in the superficial (FFs) and in the deep (FFD) venous drainage area was computed every 100 s. Following infusion of Ringer fluid a decrease in the FFD/FFs ratio was observed--mainly due to a fall in FFD. The decrease in the FFD/FFs ratio was more pronounced with infusion of Ringer fluid directly into the renal artery than with infusion at the same rate (1.0-1.5 ml . min-1) into a jugular vein, showing that a direct renal effect is involved. The sensitivity of the FFD/FFs reaction is high, as the fluid loads given were moderate. The fall in FFD may be due to a preferential increase in medullary blood perfusion, which, by wiping out the medullary hyperosmolality, promotes diuresis.