Microsphere size and determination of intrarenal blood flow distribution in the rat

The influence of microsphere size upon the estimation of cardiac output (CO), renal blood flow (RBF) and its cortical distribution (ICBFD) was evaluated by simultaneous injection of 8.5 +/- 0.8 micrometer (SD) and 12.7 +/- 1.7 micrometer (SD) spheres in control conditions and after hemorrhagic hypotension (HH rats). The values of CO and RBF were unaffected whilst the ratio of flow to outer and inner halves of cortex (OCF/ICF) was 32% higher with 12.7 micrometer than with 8.5 micrometer spheres in both groups. Microscopic analysis of cleared kidneys slices confirmed that large spheres were more concentrated in outermost and less concentrated in innermost glomeruli than small spheres. In addition, the ratio of sphere number per outermost to that per innermost glomerulus (fs/fjm), and approximation of glomerular blood flow distribution was 1.74 and 1.76 with large spheres and 0.98 and 1.06 with small spheres in control and HH rats respectively. It is concluded that the artifact due to sphere size was not minimized in low flow conditions (HH rats) and that 8.5 micrometer spheres may be a more realistic marker of glomerular blood flow distribution in the rat than 12.7 micrometer spheres.