A BIOCHEMICAL STUDY ON COMPENSATORY RENAL ENLARGEMENT AFTER UNILATERAL NEPHRECTOMY IN RATS

Abstract

Hypertrophy and hyperplasia of the remaining kidney in response to unilateral nephrectomy were studied, analyzing patterns of changes in DNA, RNA and protein contents. Sixty rats were subjected to surgical excision of right kidney, and 20 rats were subjected to detachment of right kidney from its surrounding tissue as sham-operated controls. On the 1st, 3rd, 5th and 7th day after the nephrectomy, 15 each of the rats were sacrificed. Removed kidneys were weighed, seperated into cortex and medulla, and minced. Parts of minced cortex and medulla were used for estimation of DNA, RNA and protein. From the remnants of minced cortex, tubules were isolated by collagenase treatment and sucrose density centrifugation. The minced cortex was incubated in Hanks' solution containing 0.1% collagenase (Type I, Sigma Co.) at 37°C for 25min, and filtered through two layers of nylon gauze. Large aggregates were further incubated with collagenase and filtered. The filtrates were combined, and centrifuged for 1min at 50g. The pellet was washed three times with Hanks' solution by suspension and centrifugation. The final pellet was suspended in Hanks' solution, and layered on a discontinuous sucrose gradient (75%, 57% and 8.6%w/v). After centrifugation for 60min at 1, 000g, the fragments concentrated at the interface between 57% and 8.6% layers were collected as a tubular fraction.
DNA, RNA and protein were measured by Burton's method, orcinol method and Lowry's method, respectively. In order to compensate for individual differences, the values were usually expressed as the ratio of the values of the left kidney to those of the right.
Nephrectomy induced a compensatory enlargement of the remaining kidney. An increase by 20% in weight of the remaining kidney was observed within 7 days after nephrectomy, and the extent of the increase was similar in cortex and medulla. In cortex, total RNA and protein contents increased on the 1st day after nephrectomy, and the increase continued almost linearly up to the 7th day, while total DNA content increased on the 3rd day, but slightly thereafter. Thus, a decrease in DNA concentration (mg/g wet tissue) and increase in RNA/DNA and protein/DNA ratios, except on the 3rd day, were noticed. These results suggest that hypertrophic response began on the 1st day and continued at least up to the 7th day, in addition to hyperplastic response, provoked on the 3rd day. On the other hand, behaviors of macromolecular contents in the medulla after nephrectomy were characterized by almost linear increments in total RNA and protein contents, and a decrease in DNA concentration throughout the experimental period. The increase in total DNA content was very slight. Therefore, the enlargement of the medulla induced by nephrectomy appeared to be attributed mainly to hypertrophy, and slightly to hyperplasia.
Since recovery of tubules in the isolation method was not quantitative, behaviors in total DNA, RNA and protein contents were not measurable. However, an increase in RNA/DNA ratio in tubules after nephrectomy was remarkable, while an increase in protein/DNA ratio was obscure. These results suggest that nephrectomy induces hyperplastic, more than hypertrophic response in tubules of the remaining kidney, because hypertrophy is expected to be accompanied with remarkable increment in protein/DNA ratio.
Clinical significance of this compensatory response was discussed.