Renal blood flow distribution measured by microspheres during isovolemic hematocrit alteration in rats

The distribution of 15-micron microspheres was measured in rat kidneys before and after the animals had undergone either isovolemic hematocrit increase or isovolemic hematocrit decrease. Raising the systemic arterial hematocrit from 46 +/- 1% (mean +/- SEM) to 59 +/- 1% caused a significant decrease in the rate of urinary sodium and potassium excretion (UNaV, UKV), but not significant changes in total renal blood flow (RBF), filtration fraction (FF), outer cortical flow, inner cortical flow, or rate of urinary volume excretion (V). Fractional sodium excretion was also unchanged suggesting that the decreased UNAV was the result of a decreased glomerular filtration rate (GFR). The measured decrease in GFR was not statistically significant. Lowering the systemic hematocrit from 45 +/- 1% to 33 +/- 1% caused a significant fall in FF as well as significant increases in the rate of urine volume excretion (V), UNaV, UKV, and GRF. There was a significant increase in RBF but again no change in the flow distribution between inner and outer cortex. The findings show that hematocrit alterations alone do not immediately lead to a redistribution of blood flow between inner and outer cortex of the rat kidney.