Origin of the humoral factor responsible for compensatory renal hypertrophy.

1. Rats were unilaterally nephrectomized and the remaining kidney was removed 10 min after the operation.2. Cortical slices from both kidneys were incubated in culture medium in the presence of freeze-dried plasma from a control animal, for 4 hr. When renal cortical slices from a normal animal were now added to and incubated in the medium in which cortical slices from the remaining kidney (i.e. the kidney removed 10 min after unilateral nephrectomy) had been incubated, they showed an increase of protein content and dry weight. This increase did not occur when slices from normal kidneys were incubated in culture medium in which the first kidney had been incubated.3. The increase of protein content and dry weight of cortical slices from normal kidneys did not occur when incubated in culture medium in which cortical slices from the remaining kidney had been incubated when freeze-dried plasma from normal rats had been substituted by freeze-dried plasma from anephric rats.4. Since normal plasma which does not normally promote protein accretion in normal cortical slices in vitro can do so after it has been incubated with cortical slices from a kidney removed 10 min after unilateral nephrectomy, it is suggested that normal plasma contains a precursor of renal origin which is ;activated' when in the presence of a remaining kidney removed shortly after unilateral nephrectomy.