Intrarenal distribution of albumin and immunoglobulin M in the non-diuretic-rat.

The intrarenal distribution of 125I-immunoglobulin M (125I-IgM) and 131I-albumin was studied in non-diuretic rats. Tracer equilibration times of 1, 10 and 60 min were used. In the 1 min series, in all renal zones (cortex, outer and inner medulla) the albumin volumes did not differ significantly from those of the much larger molecule, IgM. The 1 min values were found to be 9, 6, 17, and 24 mul per 100 mg tissue for whole kidney, cortex, outer and inner medulla, respectively. In the 10 and 60 min series the cortical albumin and IgM volumes did not differ significantly either, while in the medulla the albumin volumes slightly exceeded the IgM volumes. The 10 min values did not differ significantly from the corresponding 1 min values, except that in the cortex the albumin value was higher at 10 than at 1 min. All 60 min values were significantly higher than the corresponding 1 min values. In the medullary zones these differences can be explained for the most part as caused by accumulation of free radioiodide. In the cortex free radioiodide plays a minor role and the 60 min distribution volumes of both proteins (whether corrected for free radioiodide or not) exceed the 1 min volumes by some 50 per cent. The 1 min values can be taken to represent the intravascular compartment; the 60 min values include the extravascular compartment. In conclusion, the present study indicates that: 1. The extravascular pool of albumin constitutes only a minor fraction of the total intrarenal pool-in the cortex about one third. 2. The equilibration of tracer proteins (IgM or albumin) between plasma and the extravascular compartments of the kidney is a rather slow process; the results are compatible with a mean transit time of tracer albumin from plasma to lymph of 26.5 min, as found by others [19]. 3. Extravasation of IgM and albumin most likely takes place via large vescicles or "big leaks".