The measurement of glomerular blood flow in the rat kidney: influence of microsphere size.

1. Radioactively labelled microspheres were used to determine glomerular blood flow in glomerular populations with distinct vascular characteristics. Two batches of microspheres (15 +/- 5.0 micrometer diameter and 7.0--10 micrometer diameter) were utilized. 2. The results show that the larger microspheres overestimate the superficial glomerular blood flow (414 +/- 61 nl/min, mean +/- s.e.m.) and underestimate the deep glomerular blood flow (98 +/- 10 nl/min), when compared with the data obtained with 7.0--10 micrometer diameter microspheres (317 +/- 30 nl/min and 209 +/- 23 nl/min, respectively). 3. The rheological artefact associated with the use of larger microspheres is confirmed by finding an uneven size distribution of microspheres lodged in the glomeruli. In each of three experiments, the mean diameter of 200 microspheres lodged in the superficial glomeruli (16.43 +/- 0.27 micrometer, 15.87 +/- 0.23 micrometer and 16.58 +/- 0.27 micrometer) was significantly greater than that found in the deep glomeruli (15.36 +/- 0.15 micrometer, 15.25 +/- 0.21 micrometer and 15.73 +/- 0.24 micrometer; P less than 0.01, less than 0.05 and less than 0.01, respectively). No such difference was detected when the 7.0--10 micrometer spheres were used. 4. Glomerular blood flow can be measured at all depths of the rat's cortex and the demonstrated rheological artefact associated with use of the larger spheres is circumvented with the use of 7.0--10 micrometer microspheres.