Ribonucleic acid labelling and nucleotide pools during compensatory renal hypertrophy

Dietary nucleotides and gut mucosal defence

The informational aspects of nucleic acid synthesis have attracted much more attention than the quantitative significance of DNA, rRNA, tRNA, and nucleotide synthesis. Animal and human studies suggest that in energetic terms, 5-10% of the energy used in synthesising tissue protein is expended in manufacturing an appropriate amount of synthetic machinery, that is the ribosome and tRNA. The two sources for synthesis of nucleotides are salvage of nucleotides released by intracellular degradation or derived from the diet, and nucleotides synthesised de novo from amino acids (for example, glutamine) and sugars (glucose). The comparative importance of these two processes is not well defined, but rRNA production requires a high de novo input in cell types with the capacity for rapid division (for example, lymphocytes). The gut is unusual in requiring a ready arterial supply of nucleotides synthesised by hepatic de novo pathways. Animal studies show that an exogenous supply of nucleotides (salvage) can improve liver regrowth, immune responsiveness to a microbial challenge, and gut morphology in diarrhoea models. Humans adapt to dietary nucleotide intake by downregulating de novo pathways. All total parental nutrition regimens, and most enteral regimens lack nucleotides, which may predispose to an inadequate supply of preformed nucleotides to gut and immune cells in the critically ill, artificially fed patient. Unfortunately, there are no clinical studies that answer this point at present.

Uridine and cytidine nucleotide synthesis in renal hypertrophy: biochemical differences in response to the growth stimulus of diabetes and unilateral nephrectomy

The effects of unilateral nephrectomy (UN) and streptozotocin (STZ) diabetes on the activities of enzymes involved in uridine and cytidine synthesis in early renal growth (3-14 days after stimulus to growth) have been compared. Measurements were also made of glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) and of glucose 6-phosphate (G6P), UDP-glucose, and glycogen, in relation to phosphoribosyl pyrophosphate, ribonucleotide, and complex carbohydrate formation. There were striking differences in the activities of CTP synthetase, G6PDH, and 6PGDH in the two conditions, with a three-fold increase in all three enzymes at 3 and 5 days and a two-fold increase above basal values at 14 days of STZ diabetes. The UN group showed no significant change in CTP synthetase at any stage and the activity of G6PDH and 6PGDH only kept pace with renal growth. Changes in routes of uridine synthesis were less marked, with a more rapid rise in carbamoyl-phosphate synthetase (glutamine) and a lesser response of dihydroorotate dehydrogenase in the UN relative to the STZ-diabetic groups. The enzymes of complex II and of uracil phosphoribosyltransferase showed essentially similar patterns during renal hypertrophy in UN and STZ diabetes. The parallel increase in CTP synthetase, G6PDH, and 6PGDH in the kidney in diabetes, also known to increase in growth situations in hepatomas and in renal tumors, is discussed in relation to hormone signals involved in renal growth. The importance of the concentration of CTP, and thus of CTP synthetase, in the CTP-cytidyltransferase reaction, an enzyme with a high Km for CTP, makes the present observation of the striking increase in CTP synthetase in STZ diabetes of particular interest in relation to phosphatidylcholine formation and hormone signal transduction.

Inhibition of cellular autophagy in kidney tubular cells stimulated to grow by unilateral nephrectomy

Cytoplasmic growth (hypertrophy) presupposes a positive metabolic balance brought about by increased anabolic and/or decreased catabolic processes. Degradation of cytoplasmic components takes place in autophagic vacuoles (AVs) whose volume fraction may be taken as a measure of the relative rate of degradation of cytoplasmic components. Male adult Sprague-Dawley rats (n = 80) were unilaterally nephrectomized (n = 40) or sham-operated (n = 40) and were killed 3.5-57.5 h p.o. The volume density of AVs in parenchymal cells of renal cortical convoluted tubules was determined morphometrically by systematic evaluation of large test fields in the electron microscope. During compensatory renal growth, the volume densities of autophagic vacuoles were reduced at day 0 (3.5-8 h p.o.), day 1 (20.5-33.5 h p.o.) and day 2 (44.5-57.5 h p.o.) by 49% (p less than 0.01), 43% (p less than 0.05), and 19% (n.s.), respectively, when compared with sham-operated controls. No decrease, and even an increase, in the AV-volume fraction was found in liver parenchymal cells of the unilaterally nephrectomized animals. This indicates that inhibition of autophagy is not a general response after unilateral nephrectomy, but is confined to the growing kidney, where it may represent a significant factor in the increase of cytoplasmic mass.

Specific gene expression during compensatory renal hypertrophy in the rat

The compensatory growth of the kidney which is induced by unilateral nephrectomy is a highly regulated process resulting principally in hypertrophy of the remaining kidney. The events which regulate this process are unknown. We have examined the levels of transcripts for the proto-oncogenes, myc, H-ras, K-ras, and fos, and the cellular genes, H4 histone, ornithine aminotransferase, and gamma-glutamyl transpeptidase, following unilateral nephrectomy in the rat. The pattern of expression of c-myc, c-H-ras, and c-K-ras during compensatory growth of the kidney differs from the pattern of expression of these proto-oncogenes during liver regeneration, in which, unlike the kidney, hyperplasia rather than hypertrophy predominates. The lack of change in the abundance of these proto-oncogene transcripts following unilateral nephrectomy suggests a primary relationship between the expression of these proto-oncogenes and DNA synthesis and indicates there may be separate signals for cell growth, one to double cell size and one to replicate DNA. Increased mRNA transcripts for the enzymes ornithine aminotransferase and gamma-glutamyl transpeptidase were induced in the contralateral kidney after nephrectomy. The time course of expression for these two enzymes differs. The early expression of the gamma-glutamyl transpeptidase gene may indicate an involvement of this glutathione-metabolizing enzyme during renal compensatory growth, while the function of the delayed increase in ornithine aminotransferase transcripts in the remaining kidney is not apparent.

Concentration of phosphoribosyl pyrophosphate in renal hypertrophy. Contrasting effects of early diabetes and unilateral nephrectomy

Studies were made of the renal phosphoribosyl pyrophosphate (PPRibP) content and PPRibP synthetase (EC 2.7.6.1) activity in rats diabetic for 5, 14 or 20 days, or unilaterally nephrectomized (UN) for 5 days, and in doubly lesioned animals. Approximately equal degrees of renal enlargement were found after 5 days diabetes or 5 days UN. In the doubly lesioned animals the increment of growth was additive. Unilateral nephrectomy of 5 days duration, in contrast with diabetes, had no effect on the PPRibP content of the contralateral kidney, nor did it modify the renal PPRibP content when performed on animals diabetic for 5, 14 or 20 days. The activity of PPRibP synthetase was unaffected by diabetes, UN or diabetes +UN. The results pinpoint a stage of nucleotide synthesis which is differentially affected by the two stimuli, in line with evidence for differences in regulation of nucleic acid turnover in the two conditions.

Concentration of phosphoribosyl pyrophosphate in the kidney during development and in experimental diabetic hypertrophy

The effect of developmental growth on the kidney content of phosphoribosyl pyrophosphate PPRibP was studied in rats at ages between the foetal animal and up to 100 days of age. In addition, the effect of short-term diabetes (up to 14 days) on the renal content of PPRibP was studied in immature rats and in adults aged approx. 60 days. The developmental pattern of PPRibP is such that the PPRibP content is lowest in the young rat and increases as the rate of kidney growth slows. In the adult rat, the early kidney hypertrophy of diabetes is accompanied by a fall in PPRibP content and, again, the PPRibP content returns to normal as the rate of kidney hypertrophy diminishes. Induction of diabetes in the immature rat causes a lesser degree of kidney hypertrophy and also a smaller depression of renal PPRibP content. The activity of PPRibP synthetase (EC 2.7.6.1) is not significantly affected by age or diabetes. The changes in PPRibP content are discussed in relation to the generation of ribose 5-phosphate by the pentose phosphate pathway and the utilization of PPRibP for nucleotide synthesis via the 'de novo' and salvage pathways.

RNA synthesis in regenerating mouse liver evaluated by incorporation of [methyl-14C]methionine and by determination of RNA polymerase activity

The synthesis of RNA during mouse liver regeneration was studied by two different methods at 24 and 48 h after partial hepatectomy. Total chromatin-bound RNA polymerase activity showed an increase of 32% at 24 h after partial hepatectomy. At 48 h a slight increase in total activity was also observed in regenerating liver, but the difference was not significant. The increase in total RNA polymerase activity was due to a rise in RNA polymerase I plus III activity. This enzyme activity was increased at both 24 and 48 h. The increase was 57% at 24 h and 51% at 48 h. When [methyl-14C]methionine was used for labelling of methyl groups in rRNA, there was an increased specific radioactivity of this class of RNA at both 24 h and 48 h. The increases were 263 and 103% at 24 and 48 h respectively. Thus both methods revealed an increased synthesis of rRNA during mouse liver regeneration. The results are discussed in relation to previous results from this laboratory [Yngner, Carlberg, Lewan & Engelbrecht (1979) Hoppe-Seyler's Z. Physiol. Chem. 360, 1069-1074; Yngner, Engelbrecht, Lewan & Annerfeldt (1979) Biochem. J. 178, 1-8; Yngner, Bengtsson, Carlberg, Engelbrecht & Wieslander (1980) Exp. Cell. Biol. 48, 393-403], which have shown that the incorporation of orotic acid or uridine into RNA is not increased in mouse liver regenerating after partial hepatectomy.

Renal enlargement: comparative autoradiographic studies of 3H-thymidine uptake in diabetic and uninephrectomized rats

Incorporation of 3H-thymidine into renal cortical tissue has been studied by light microscopic autoradiography in streptozotocin-diabetic rats, uninephrectomized rats, uninephrectomized diabetic rats, insulin-treated diabetic rats and control rats. The percentage of labelled cortical nuclei (the labelling index) was determined separately in glomeruli, proximal tubules and distal tubules after 2, 4 and 6 days on autoradiographs from 1 micron thick plastic embedded sections. The incorporation of thymidine in glomerular nuclei was consistently low (less than 1%) and no differences were found between the control and experimental groups. In both proximal and distal tubules an increase in thymidine incorporation was seen on day 2 followed by a decline on days 4 and 6. The maximal labelling on day 2 in proximal tubules was 9.1% in the uninephrectomized diabetic group, 3.7% in the diabetic group and 1.4% in the uninephrectomized group. In distal tubules the corresponding values were 5.2, 3.5 and 1.1%. The increase in kidney weight after 6 days was 83, 62 and 37%, respectively. Estimates of the net increase in the number of cortical tubular cells in the different experimental groups showed that the kidney enlargement followed different patterns with respect to the extent of cellular hyperplasia and hypertrophy. The kidney growth in uninephrectomized diabetic rats was dominated by tubular cellular hyperplasia, in the diabetic group hyperplasia and hypertrophy participated to approximately the same extent, whereas cellular hypertrophy was most pronounced in the uninephrectomized animals. Nuclear labelling in the insulin-treated diabetic rats was not different from that of control rats and consequently a hyperplastic effect of streptozotocin can be ruled out.(ABSTRACT TRUNCATED AT 250 WORDS)

Humoral factors in regulation of compensatory renal hypertrophy

Compensatory renal hypertrophy in transplanted kidneys shows that the major regulators are humoral. Crosscirculation experiments are confirmatory indicating further that the regulators are short-lived and must be consistently present during the early phases of hypertrophy. Controls are difficult to achieve in other systems; variables relate to nutrition, means of assay, pharmacokinetics, and abnormalities produced by the experimental model itself. A simple hypothesis to account for the events precipitating renal hypertrophy might integrate the onsets of renal hyperemia soon after contralateral nephrectomy with the activation of pre-existing stimulators specific for the kidney.

Synthesis and conservation of ribosomal proteins during compensatory renal hypertrophy

The rate of synthesis of ribosomal proteins was investigated as an index of the rate of production of ribosomes in mouse kidney during the first few days after contralateral nephrectomy. Compensatory renal hypertrophy was not associated with a major increase in the synthetic rate of ribosomal proteins and rRNA. Instead, the ratio of the rate of ribosomal-protein synthesis to that of total protein synthesis remained nearly constant. The conformation of glutaraldehyde-fixed ribosomes and ribosomal subunits was unchanged. During the early stages of compensatory renal hypertrophy the accretion of rRNA is due largely to conservation of ribosomes that would otherwise have been degraded.

Presence of renotrophic factor in plasma of unilaterally nephrectomized rats

1. Renal cortical slices cut in two halves were incubated separately in culture medium. One half slice was used as control while the other half was treated experimentally. After incubation, their dry weights and/or their protein content were estimated. 2. Addition to half cortical slices of 30 micrograms mg-1 of freeze-dried plasma from rats killed 24 hr after unilateral nephrectomy resulted in an increase of dry weight or protein content of the order of some 10% after 4-8 hr incubation. 3. The effect of freeze-dried plasma from rats killed 24 hr after unilateral nephrectomy was specific to the renal cortex. When added to half slices from either the renal medulla or the liver there were no changes in their dry weight. 4. Addition of 30 micrograms mg-1 of freeze-dried plasma from rats killed 24 hr after bilateral nephrectomy had no significant effect on the dry weight of half cortical slices incubated for various periods of time, from 4 to 48 hr. 5. The renotrophic activity of plasma from unilaterally nephrectomized rats, killed 24 hr after the operation, appears to be metabolized or destroyed by incubation with cortical slices in 4 hr. 6. Freeze-dried plasma from normal unilaterally nephrectomized rats produced an increase dry weight when incubated for 4 hr with half cortical slices from early (3 days) hypophysectomized rats. When incubated with half cortical slices from late (23 days) hypophysectomized rats, the increase in dry weight was about 2 times as great as that observed with cortical slices from normal animals. 7. The possible reason for the production of renotrophic activity in plasma of unilaterally nephrectomized rats, and for its absence in plasma of anephric animals, is discussed.

The measurement of colonic mucosal-submucosal blood flow in man

1. A method is described for the measurement of colonic mucosal-submucosal blood flow in man, by studying surgically created colostomies. 2. A local radioisotopic clearance technique utilizing a stable preparation of [125I]4-iodoantipyrine is employed. The indicator is injected directly into the colostomy under study and its gamma emission is recorded by a scintillation detector. 3. A radioautographic study was carried out at laparotomy in humans to facilitate the interpretation of the recorded washout curves. This demonstrated that the tracer was cleared from both the mucosa and submucosa throughout the period of study. 4. Mucosal-submucosal blood flow was calculated according to Kety (1949) from the monoexponential clearance curves obtained, and amounted to 31.7 +/- 11 ml./min. 100 g (S.D. of an observation, n = 30). 5. The results from two consecutive measurements in seventeen patients showed that the mean change between first and second readings was not significant (t test). In addition the between-patient variation (12.2) was significantly greater than the within-patient variation (2.6) for consecutive recordings (F test; P less than 0.01). 6. The within-patient between-days variation (12.9; n = 10) was found to be similar to the between-patient variation. 7. It is concluded that the technique permits measurement of local colonic blood flow in man and by consecutive measurements, it may be used to evaluate local changes in blood flow following reflex or pharmacological stimulation.

Anabolism versus catabolism of [5-3H]uridine and its relationship to ribonucleic acid labelling in mouse liver after partial hepatectomy

The balance between anabolism and catabolism of [5-(3)H]uridine was studied in the mouse after partial hepatectomy. Labelling of RNA and UDP-glucose was determined and evaluated in relation to changes in the specific radioactivity of UTP. The amounts of labelled catabolic products of uridine were increased several-fold in liver and blood after partial hepatectomy. The specific radioactivity of RNA decreased to about 60% of the control value at 6h and was in the same range as that of control liver at 24h after operation. Decreased labelling of RNA and UDP-glucose was attributable to decreased specific radioactivity of UTP. No changes in the size of the UTP pool or in the balance between uridine anabolism and catabolism were found that could explain the decreased specific radioactivity of UTP. Rather, the alterations in the labelling of this metabolite induced by the partial hepatectomy may be related to decreased phosphorylating capacity in the liver cells and/or dilution of the labelled precursor in an expanded uridine pool. The enhanced amounts of uridine catabolic products in liver and blood were probably a consequence of accumulation and altered incorporation of the metabolites from the blood into the liver cells. Despite the increased amounts of labelled catabolic products and the decreased labelling of RNA, the results reported here actually suggest decreased uridine catabolism and slightly increased RNA synthesis in mouse liver after partial hepatectomy. The results stress the importance of proper controls in determination of nucleic acid synthesis and in metabolic studies by use of labelled precursors.

Macromolecular metabolism in compensatory renal hypertrophy

1. The initial biochemical changes of compensatory hypertrophy occur well within 1 hour of unilateral nephrectomy and perhaps within the first few minutes.2. The initial increment in rRNA is from decreased metabolism rather than from increased synthesis.3. Changes in the processing of mRNA precursors are probably also important.4. Compensatory hypertrophy is regulated by a humoral stimulus or stimuli.5. The stimulus needs to be present virtually all of the time during the early phases of compensatory hypertrophy.6. The stimulus is related to loss of renal mass, not to loss of renal function.

A model for cytoplasm-governed gene regulation

A model of cytoplasm-governed transcription is presented. The nuclear membrane has a selective permeability towards nuclear pre-mRNA molecules which are provided with group-specific non-translated "passwords". RNA transcription on the chromatin proceeds under a dual control. One of them is gene regulation according to the Britten-Davidson and Georgiev models. The other is cytoplasm-governed regulation mediated through the selective transport of mRNA from nucleus to cytoplasm. Pre-mRNA molecules which are not "in immediate demand" by the cytoplasm and therefore accumulating the nucleus repress their own synthesis by end-product inhibition. The interrelationship of the two types of regulation in the course of cell development is discussed.

Regeneration of renal proximal tubules after mercuric chloride injury is accompanied by increased binding of aminoacyl-transfer ribonucleic acid

Homogenates of rat kidney cortex obtained 1,3 or 14 days after a single injection of HgCl2 were used to prepare the post-microsomal pH5 supernatant fraction. The activity of this fraction for peptide synthesis from [14C]phenylalanyl-tRNA was significantly increased at 1 and 3 days, at which time the proximal tubules are regenerating [Cuppage & Tate (1967) Am. J. Pathol. 51, 405-429]. This increased activity could not be attributed to a decreased inhibitory activity, but was due to an increased aminoacyl-tRNA binding, i.e. elongation-factor-1 activity, in the supernatant fraction.

Ribonucleic acid synthesis in the renal cortex at the initiation of compensatory growth

The mechanisms responsible for the increase in RNA per cell during the first 48h of renal compensatory growth were studied in the renal cortex. Unilaterally nephrectomized, sham-operated or non-operated rats were used. Incorporation into RNA of labelled precursors was studied in vivo and in vitro. Sham-operation produced significant changes in precursor incorporation, absolute amounts of UTP and RNA, and the rate of RNA synthesis. At 6h after surgery, the amount of RNA decreased in sham-operated controls, whereas that in growing cortex remained unchanged. Incorporation into RNA in vivo was greater in the growing cortex, although the rate of RNA synthesis was not increased. At 24h, precursor incorporation into RNA and UTP and RNA synthesis were all increased in the growing cortex. In contrast with results obtained in vivo, slices of growing cortex incorporated less labelled precursor into RNA than did cortex slices from sham-operated controls, from 3 to 48h. Maximal differences were found from 6 to 24h. An attempt was made to equalize endogenous precursor pool sizes by increasing the concentration of unlabelled uridine in the media; incorporation differences were narrowed significantly. Serum from nephrectomized animals did not increase precursor incorporation into RNA in vitro. An increase in RNA synthesis is an important factor in RNA accretion in the renal cortex beyond 12h of compensatory growth. This is accompanied by increased UTP content and preceded by expansion of other pools. The amount of labelled precursor incorporated into RNA is greatly influenced by its delivery rate to the growing kidney in vivo and by intracellular dilution of expanded precursor pools in vitro.

Conservation of ribosomal RNA during compensatory renal hypertrophy. A major mechanism in RNA accretion

After removal of one mouse kidney, compensatory hypertrophy in the remaining kidney is marked in 2 days by a 20% average increase in ribosomal RNA (rRNA) per cell. Both 28S and 18S RNA are conserved during the initial stages of compensatory renal hypertrophy to an extent sufficient to account for the rest of the observed accumulation of rRNA. Like some cultured cells, the kidney conserves rRNA during physiological growth.