Intrarenal red cell and plasma volumes in the non-diuretic rat

Summary

The intrarenal distribution of⁵¹Cr-labelled red cells and¹²⁵I-γM-immunoglobulin (¹²⁵I-IgM) was studied in non-diuretic rats. IgM was chosen as a macroprotein which will either stay intravascularly or only very slowly permeate the renal blood vessels. Tracer equilibration times of 1, 2 and 10 min were used. In the three renal zones (cortex, outer and inner medulla) the IgM distribution volume was not found to change with time. The IgM volume was thus taken to be a measure of the plasma volume. For the whole rat kidney the plasma and the red cell volumes were found to be 9 and 6 μl per 100 mg kidney respectively. Plasma and red cell volumes were substantially higher in the inner and outer medulla than in the cortex. The intrarenal haematocrits calculated from the plasma and red cell volumes were expressed relative to the total body haematocrits. Values in the order of 0.90 were obtained for the whole kidney, cortex and outer medulla; substantially lower figures, 0.45–0.60, were found for the inner medulla. The low inner medullary haematocrit is interpreted haemodynamically as a “small vessel haematocrit” caused by axial accumulation of red cells, although pre- or postglomerular separation of red cells and plasma cannot be excluded as an explanation.