Renal medullary blood flow studied with the 86-Rb extraction method. Methodological considerations.

The 86-Rb extraction method was applied for a study of regional renal blood flow. In the cortex, a sampling time of 30 s led to an underestimation by about 15% as compared with the microsphere method. This was due to incomplete cortical cellular extraction of rubidium with subsequent rapid wash-out the tracer. In the renal medulla, a sampling of 30-60 s gave valid data with almost complete extraction. A sampling time of only 10 s, i.e. a time similar to the intravascular transit time, gave rise to a 50% underestimation of the inner medullary blood flow. Errors due to transport of rubidium by the tubular fluid were investigated in detail. A theoretical analysis based on equilibrium data revealed a maximal error of about 5%. Studies with micropuncture of distal tubules and studies of the urinary transport showed no or negligible contamination from tubular urine. Under control antidiuretic conditions the blood flow in the cortex was 5.2 +/- 0.2 ml . min-1 . g-1 (mean +/- SE, n = 16), in the outer stripe of the outer zone 2.2 +/- 0.1, in the inner stripe 1.5 +/- 0.1 and in the inner zone 0.69 +/- 0.06.