Utilization of labelled uridine, cytidine and orotic acid for determination of ribonucleic acid synthesis in mouse liver

[3H]uridine and [3H]orotic acid were equally utilized for labelling of RNA in mouse liver. Incorporation of [3H]cytidine was 2-3 times as high as that of [3H]-labelled uridine or orotic acid. These results differ from findings in rat liver, where both cytidine and orotic acid are better utilized for RNA labelling than is uridine. The ratio between liver RNA [3H]-activity and volatile [3H]-activity was 2, 3 and 13, respectively, at 300 min after injection of labelled uridine, orotic acid and cytidine, indicating an efficient chanelling of cytidine into liver anabolic pathways.     

Metabolism of uridine and determination of liver ribonucleic acid synthesis in developing and adult mice

The metabolism of [5-3H]uridine and the incorporation of the precursor into liver RNA was studied in developing (13-day-old) and adult (45-day-old) mice. Different time-courses of labelling and increased amounts of labelled catabolic products of uridine were found in liver and blood of developing mice compared with adult animals. This is suggested to be a consequence of enlarged metabolite pools resulting from a lower total amount of uracil-degrading enzymes in the developing mice. The labelling of the uracil nucleotides was decreased in the developing liver. However, in spite of a lower specific radioactivity of UTP, the RNA-specific radioactivity of developing liver was increased compared with adult liver. Also the labelling of liver RNA with [6-14C]orotic acid was found to be increased in developing mice, thus indicating a higher rate of RNA synthesis in these animals. A more pronounced difference in liver RNA labelling between the developing and the adult mice obtained with the use of [14C]orotic acid than with [3H]uridine may suggest that the de novo pathway, relative to the salvage pathways, is more important in developing than in adult liver.     

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.     

Radioautographic visualization of differences in the pattern of [3H]uridine and [3H]orotic acid incorporation into the RNA of migrating columnar cells in the rat small intestine

The epithelium of rat small intestine was radioautographed to examine whether RNA is synthesized by the salvage pathway as shown after [3H]uridine injection or by the de novo pathway as shown after [3H]orotic acid injection. The two modes of RNA synthesis were thus investigated during the migration of columnar cells from crypt base to villus top, and the rate of synthesis was assessed by counting silver grains over the nucleolus and nucleoplasm at six levels along the duodenal epithelium--that is, in the base, mid, and top regions of the crypts and in the base, mid, and top regions of the villi. Concomitant biochemical analyses established that, after injection of either [5-3H]uridine or [5-3H]orotic acid: (a) buffered glutaraldehyde fixative was as effective as perchloric acid or trichloracetic acid in insolubilizing the nucleic acids of rat small intestine; (b) a major fraction of the nucleic acid label was in RNA, that is, 91% after [3H]uridine and 72% after [3H]orotic acid, with the rest in DNA; and (c) a substantial fraction of the RNA label was in poly A+ RNA (presumed to be messenger RNA). In radioautographs of duodenum prepared after [3H] uridine injection, the count of silver grains was high over nucleolus and nucleoplasm in crypt base cells and gradually decreased at the upper levels up to the villus base. In the rest of the villus, the grain count over the nucleolus was negligible, while over the nucleoplasm it was low but significant. After [3H]-orotic acid injection, the number of silver grains over the nucleolus was negligible at all levels, whereas over the nucleoplasm the number was low in crypt cells, but high in villus cells with a peak in mid villus. The interpretation is that, except for a small amount of label incorporated into DNA from either precursor by crypt cells, the bulk of the label is incorporated into RNA as follows. In the crypts, cells make almost exclusive use of uridine, that is, of the salvage pathway, for the synthesis of ribosomal RNA in the nucleolus and of messenger and transfer RNA in the nucleoplasm. However, when cells pass from crypt to villus, they mainly utilize orotic acid--i.e., the de novo pathway--for the synthesis of messenger and transfer RNA within the nucleoplasm.     

Synthesis of ribonucleic acid in normal bone in vitro

1. The incorporation of [2-(14)C]uridine into nucleic acids of bone cells was studied in rat and pig trabecular-bone fragments surviving in vitro. 2. The rapid uptake of uridine into trichloroacetic acid-soluble material, and its subsequent incorporation into a crude nucleic acid fraction of bone or purified RNA extracted from isolated bone cells, was proportional to uridine concentration in the incubation medium over a range 0.5-20.0mum. 3. During continued exposure to radioactive uridine, bulk RNA became labelled in a curvilinear fashion. Radioactivity rapidly entered nuclear RNA, which approached its maximum specific activity by 2hr. of incubation; cytoplasmic RNA, and particularly microsomal RNA, was more slowly labelled. The kinetics of labelling and rapid decline of the nuclear/microsomal specific activity ratio were consistent with a precursor-product relationship. 4. Bulk RNA preparations were resolved by zonal centrifugation in sucrose density gradients into components with approximate sedimentation coefficients 28s, 18s and 4s. 5. Rapidly labelled RNA, predominantly nuclear in location, demonstrated a polydisperse sedimentation pattern that did not conform to the major types of stable cellular RNA. Material of highest specific activity, sedimenting in the 4-18s region and insoluble in 10% (w/v) sodium chloride, rapidly achieved its maximum activity during continued exposure to radioactive precursor and decayed equally rapidly during ;chase' incubation, exhibiting an average half-life of 4.3hr. 6. Ribosomal 28s and 18s RNA were of lower specific activity, which increased linearly for at least 6hr. in the continued presence of radioactive uridine. There was persistent but variable incorporation into ribosomal RNA during ;chase' incubation despite rapid decline in total radioactivity of the acid-soluble pool containing RNA precursors.     

A direct effect of growth hormone on the incorporation of precursors into proteins and nucleic acids of perfused rat liver

1. The livers of rats were perfused in situ. When the amino acid concentration in the perfusing medium was that present in rat plasma, the addition of growth hormone to the medium stimulated the incorporation of labelled amino acids into liver protein only marginally and not to a statistically significant extent. When, however, the amino acid concentration was raised to three times that present in rat plasma, growth hormone significantly and substantially stimulated amino acid incorporation into protein within 30min. of perfusion of normal rat liver. 2. A significant effect of growth hormone on labelling of normal rat-liver protein was seen with concentrations not much greater than those reported to be present in rat plasma. 3. The labelling of nucleic acids of normal and hypophysectomized rat liver by [(3)H]orotic acid was enhanced by addition of growth hormone to the perfusing medium when normal concentrations of amino acids were used. 4. At elevated concentrations of amino acids, growth hormone stimulated labelling of nucleic acids of hypophysectomized rat liver at 30 and 60min. of perfusion. Under these conditions, nucleic acids of normal rats were labelled to about the same extent in control and hormone-treated livers at 30min. and, because of a fall in the radioactivity of the control livers, there was more labelled nucleic acids in growth-hormone-treated livers at 60min. than in the control livers. 5. Growth hormone, unlike insulin, had no inhibitory effect on the release of glucose by the perfused liver. 6. It is concluded that growth hormone can stimulate the incorporation of precursor into proteins and nucleic acids of liver directly and without the mediation of other organs or of insulin.     

Stimulation of uridine phosphorylation in primary cultures of Xenopus laevis hepatocytes by estradiol-17 beta

The effects of estrogen on the uridine uptake into cells were examined in primary cultures of liver parenchymal cells from Xenopus laevis. The total uptake of [3H]uridine into the estrogen-treated cells and its incorporation into RNA were about 1.5 times higher than the values for control cells. The uptake of [3H]adenosine and its incorporation into RNA were not affected by estrogen. An experiment in which liver parenchymal cells were double labeled with [3H]uridine and [3H]adenosine showed that estrogen elevated the specific radioactivity of the UTP pool 1.4-fold the value found for the control cells, but that of the ATP pool was not altered by estrogen. Short term labeling revealed that estrogen did not significantly alter the rate of the initial uptake of [3H]uridine into the cells, but it did stimulate [3H]uridine phosphorylation about 1.7-fold. Uridine kinase activity measured in cell-free extracts of hepatocytes treated with estrogen had a value 1.6 times that of the control cells. These data indicate that the stimulation of [3H]uridine uptake and phosphorylation in Xenopus laevis hepatocytes in the presence of estrogen is caused by the enhancement of uridine kinase activity.     

Two pathways of pyrimidine nucleotide biosynthesis in birds

Dillerent chicken tissues are shown to display a clearly pronounced specificity relative to [2-14C] orotic acid and [5-3H]uridine as precursors of synthesis of the pool and RNA pyrimidine nucleotides. The fraction of pyrimidine nucleotides synthetized relative to the reserve pathway (uridine utilization) decreases in the series: kidneys greater than duodenum mucosa greater than lungs greater than liver greater than pancreas greater than bone marrow greater than brain greater than spleen. The results of [2-14C]orotic acid and [53H]uridine incorporation into UMP and CMP of the liver and spleen tissues RNA are interpreted in terms of the concept on existence of separate pools of pyrimidine phosphates--RNA precursors.     

Separate pyrimidine-nucleotide pools for messenger-RNA and ribosomal-RNA synthesis in HeLa S3 cells.

Kinetic analyses of mRNA and 28-S RNA labeling [3H]uridine revealed distinctly different steady-state specific radioactivities finally reached for uridine in mRNA and 28-S RNA when exogenous [3H]uridine was kept constant for several cell doubling times. While the steady-state label of (total) UTP and of uridine in mRNA responded to the same extent to a suppression of pyrimidine synthesis de novo by high uridine concentrations in the culture medium, uridine in 28-S RNA was scarcely influenced. Similar findings were obtained with respect to labeling of cytidine in the various RNA species due to an equilibration of UTP with CTP [5-3H]Uridine is also incorporated into deoxycytidine of DNA, presumably via dCTP. The specific radioactivity of this nucleosidase attained the same steady-state value as UTP, uridine in mRNA and cytidine in mRNA. The data indicate the existence of two pyrimidine nucleotide pools. One is a large, general UTP pool comprising the bulk of the cellular UTP and serving nucleoplasmic nucleic acid formation (uridine and cytidine in mRNA, deoxycytidine in DNA). Its replenishment by de novo synthesis can be suppressed completely by exogenous uridine above 100 muM concentrations. A second, very small UTP (and CTP) pool with a high turnover provides most of the precursors for nucleolar RNA formation (rRNA). This pool is not subject to feedback inhibition by extracellular uridine to an appreciable extent. Determinations of (total) UTP turnover also show that the bulk of cellular RNA (rRNA) cannot be derived from the large UTP pool.     

Polyamines and nucleic acid metabolism in chick embryo. Incorporation of labelled precursors into nucleic acids of subcellular fractions and polyribosomal patterns

1. An increase in polyamine concentration, caused by inhibiting the amine oxidase activities with iproniazid, increased the incorporation of [³H]orotic acid into chick-embryo RNA and DNA. On the other hand, a decrease in polyamine concentration, obtained by causing an increase in amine oxidase activities, decreased [³H]orotic acid incorporation into nucleic acids. This was particularly evident for nuclear DNA and ribosomal RNA. 2. Polyribosomal patterns obtained by sucrose-density-gradient centrifugation showed highest radioactivity in the regions of 259s and 280s aggregates in those embryos in which the polyamine contents were enhanced, whereas a decrease in the radioactivity was observed when the polyamine concentrations were decreased. 3. The activity of DNA-dependent RNA polymerase, assayed in the same experimental conditions, also varied in the same fashion with changes in polyamine concentration.     

Effects of adenosine 3':5'-monophosphate and related agents on ribonucleic acid synthesis and morphological differentiation in mouse neuroblastoma cells in culture.

A variety of compounds were assessed for their ability to induce morphological differentiation and to affect the synthesis of RNA in uncloned mouse neuroblastoma cells in culture. The stimulation of morphological differentiation in uncloned cells after exposure for 48 hours to concentrations of 3 times 10-7 to 3 times 10-4 M papavarine or 10-9 to 10-3 M dibutyryl adenosine 3':5'-monophosphate (dibutyryl-cAMP) was associated, in part, with a concentration-dependent decrease in incorporation of [5-3H]uridine into ribosomal RNA (rRNA) and heterogeneous RNA (HnRNA). The latter effect on cellular RNA produced by papavarine occurred within 1 hour after its addition to the medium and was associated with impaired uptake of radioactive precursor into uridine nucleotides and reduction in the intracellular concentration of uridine 5'-triphosphate (UTP). Dibutytyl-cAMP produced a decreased in the specific radioactivity of UTP without affecting the concentration of UTP in the tumor cells. The effects of papavarine and dibutyryl-cAMP could be distinguished further by the 50% reduction of acetylcholinesterase activity produced by papavarine, but not by dibutyryl-cAMP. Papavarine did not, however, reduce the cellular level of the soluble enzyme, adenine phosphoribosyltransferase. Sodium butyrate, while producing morphological effects similar to those of papavarine and dibutyryl-cAMP at equimolar concentrations, caused no significant changes in the incorporation of [5-3H]uridine into rRNA and HnRNA; however, acetylcholinesterase activity was stimulated 6- to 7-fold above control levels. In contrast to the other differentiating agents examined, addition of 10-9 to 3 times 10-4 M concentrations of cAMP to the tissue culture medium enhanced morphological differentiation of nueroblastoma cells, and caused a 10- to 20-fold stimulation of the incorporation of [5-3H]uridine into rRNA and HnRNA at concentrations of 10-4 M and higher. This effect observed only at high concentrations of cyclic nucleotide was accompanied by an elevation in the specific acitivty of UTP, These studies suggest that the morphological response of neuroblastoma cells is not necessarily associated with concomitant alterations in the synthesis of RNA with agents other than cAMP. Observed changes in incorporation of [5-3H]uridine into RNA appear in most instances to be due to alterations in the uptake of uridine, and in the pool size and specific radioactivity of UTP.     

Incorporation of orotic acid into nucleotides and RNA in mouse organs during 60 minutes.

Mouse kidney and liver were found to increase their levels of radioactivity above that of serum from 2 to 60 min after administration of [6-14C]orotic acid. In spleen, thymus and brain, the radioactivity level reached a maximum soon after the injection and then decreased, as did that in serum. Sixty minutes after the injection, 44% of the administered isotope dose was found in the kidneys, 22% in the liver and 0.75% in the spleen. The 14C activity in liver UTP increased rapidly and then remained constant for 60 min. The ratio between the activities in uridine phosphates and UDP-sugars was 3:4 from 10- 60 min after injection. In the liver and kidneys, the RNA 14C activities at 60 min after injection were 15% of the activity in their acid-soluble fractions. Intraperitoneal administration was found to be preferable to intravenous administration for studies on nucleotides and RNA in mouse liver, due to the delayed incorporation of the [14C]orotic acid activity into the nucleotide pool.     

Studies in rapidly labelled ribonucleic acid in cell fractions and in tumour tissues

1. Twenty minutes after injection of [(3)H]orotic acid into rats the rapidly labelled RNA from the liver is mainly associated with the nuclear fraction and little with the ribosomal cytoplasmic fraction. 2. The thermal denaturation of RNA from the fractions was not as reversible as that of the RNA extracted from whole liver. 3. Rapidly labelled RNA is synthesized by cells from a transplantable hepatoma when incubated in the presence of [(3)H]uridine and, after extraction and centrifugation, the label is present in three main fractions: one which sediments to the bottom of a gradient and is associated with DNA, a second which sediments to the heavy side of the 28s RNA, and a third which has a peak of activity between 28s RNA and 18s RNA and is associated with DNA. 4. After labelling and extraction of the RNA from Ehrlich ascites cells the distribution of radioactive components is similar to that of the material from the hepatoma cells. 5. The difference between the tumour cells and liver is due to some extent to the method of homogenizing the tissues and the nature of the components is discussed.     

Uridine uptake and RNA synthesis in the brain of torpid and awakened ground squirrels.

1. Uptake of [3H]uridine into the nucleotide precursor pool after intraventricular injection occurs with the same intensity in the brain of torpid and normothermic awakened ground squirrels. This indicates that the membrane uridine transporters and uridine kinases operate in the hibernator's brain in a hypothermia-tolerant way. 2. Utilization of the [3H]uridine pool for synthesis of the rapidly labelled RNA in the brain of torpid ground squirrels falls more than eight times against RNA labelling in the brain of the active animals between bouts of hibernation. 3. Two hours from the beginning of the artificially provoked awakening, RNA uridine incorporation in the brain of ground squirrels has risen 6.5 times. 4. Drastic changes in [3H]uridine RNA labelling under the stable uridine uptake exclude the precursors and energy supply as the main factors determining changes in intensity of the brain RNA synthesis in the different stages of hibernation.     

Wheat embryo ribonucleates. VI. Comparison of the 3'-hydroxyl termini in 'rapidly labelled' RNA from metabolizing wheat embryos with the corresponding termini in ribosomal RNA from differentiating embryos of wheat, barley, corn and pea.

The NaCl-insoluble (2.5 M, 0 degrees C) fraction of wheat embryo RNA (iRNA) can be labelled when wheat embryos are subjected to either short-term (0.5 h) or long-term (24 h) imbibition in a medium that contains tritium-labelled adenosine, guanosine, cytidine and uridine. Electrophoretic analyses reveal that, after short-term labelling, there is a broadly heterodisperse distribution of radioactivity in 'rapidly labelled' i[3H]RNA, but after long-term labelling, there is an essentially trimodal distribution of radioactivity in i[3H]RNA. End-group analyses reveal that, after short-term labelling, adenosine is the principal 3'-hydroxyl terminus in all centrifugal subfractions of 'rapidly labelled' i[3H]RNA, whereas cytidine (in 5.8S rRNA), guanosine (in 18S rRNA) and uridine (in 26S rRNA) are the principal 3'-hydroxyl termini in centrifugal subfractions of wheat embryo i[3H]RNA. Guanosine is also the principal 3'-hydroxyl terminus in the 18S rRNA of differentiating embryos excized from both monocotyledonous (wheat, barley, corn) and dicotyledonous (pea) seedlings. The implications that the end-group measurements may have for current views about the possible biochemical involvements of 3'-hydroxyl terminal sequences in both mRNA and 18SrRNA are subjects of discussion. Incidental to the principal investigation, an existing technique for analyzing the RNA contents of cellular materials has been appropriately modified to circumvent interference from uv-absorbing pigments, which, when present, prevent application of the method to plant materials.     

In vivo studies on the metabolism of pyrimidine nucleosides (author's transl)]

3H-labelled metabolites were determined in the perchloric acid-soluble fraction of blood plasma and liver of adult male Wistar rats, following the application of [5 - 3H]uridine. Ten minutes after the injection of uridine, only 20% of the total 3H activity of the plasma could be attributed to [3H]uridine. The remaining radioactivity was found chiefly in [3H]uracil (40%) and 3H2O (20%). In the liver, at 10 min, [3H]-uridine and [3H]uracil together accounted for less than 0.5% of the total radioactivity; about 70% of the radioactivity was due to [3H]beta-alanine, and 15% to 3H2O. 45 min after the injection, 70% of the radioactivity in the plasma was due to 3H2O, whereas uridine and uracil represented about 4% and 6%, respectively. At this time, about 55% of the radioactivity in the liver was due to [3H]beta-alanine, about 40% to 3H2O, and about 5% to unidentified metabolites; [3H]uridine and [3H]uracil were not observed. A comparison of the rate of catabolism of [5-3H]-uridine, [5-3H]cytidine and [6-3H]thymidine showed that cytidine is degraded in the organism 25 times more slowly than uridine or thymidine. The biological half lives for the total degradation of the [3H]nucleosides to 3H2O, based on the values in the plasma, were: uridine 1.1 h; thymidine 1.3 h; cytidine 25 h. Furthermore, the turnover time of exogenous uridine in the plasma was found to be 9 min, which gives a half life of 6 min for the metabolism of exogenous uridine to uracil.     

Biochemical changes in mouse liver after palmitoyl-ethanolamide (PEA) administration

PEA (palmitoylethanolamide; N-(2-hydroxyethyl)palmitamide) daily and orally administered to male mice caused: (1) increased incorporation of labelled orotic acid into DNA and RNA, (2) an increase in the activity of uridine kinase and decrease of tryptophan pyrrolase, (3) decreased ribonuclease activity of isolated liver ribosomes, (4) raising of specific radioactivity after injection of labelled amino acids in both mitochondrial and microsomal fractions of liver homogenate, (5) increased incorporation of [¹⁴C]-palmitic acid and ³²P into liver phospholipids.     

Purine and pyrimidine metabolism in cultured white spruce (Picea glauca) cells: Metabolic fate of 14C-labeled precursors and activity of key enzymes

In order to examine the biosynthesis, interconversion, and degradation of purine and pyrimidine nucleotides in white spruce cells, radiolabeled adenine, adenosine, inosine, uracil, uridine, and orotic acid were supplied exogenously to the cells and the overall metabolism of these compounds was monitored. [8‐¹⁴C]adenine and [8‐¹⁴C]adenosine were metabolized to adenylates and part of the adenylates were converted to guanylates and incorporated into both adenine and guanine bases of nucleic acids. A small amount of [8‐¹⁴C]inosine was converted into nucleotides and incorporated into both adenine and guanine bases of nucleic acids. High adenosine kinase and adenine phosphoribosyltransferase activities in the extract suggested that adenosine and adenine were converted to AMP by these enzymes. No adenosine nucleosidase activity was detected. Inosine was apparently converted to AMP by inosine kinase and/or a non‐specific nucleoside phosphotransferase. The radioactivity of [8‐¹⁴C]adenosine, [8‐¹⁴C]adenine, and [8‐¹⁴C]inosine was also detected in ureide, especially allantoic acid, and CO₂. Among these 3 precursors, the radioactivity from [8‐¹⁴C]inosine was predominantly incorporated into CO₂. These results suggest the operation of a conventional degradation pathway. Both [2‐¹⁴C]uracil and [2‐¹⁴C]uridine were converted to uridine nucleotides and incorporated into uracil and cytosine bases of nucleic acids. The salvage enzymes, uridine kinase and uracil phosphoribosyltransferase, were detected in white spruce extracts. [6‐¹⁴C]orotic acid, an intermediate of the de novo pyrimidine biosynthesis, was efficiently converted into uridine nucleotides and also incorporated into uracil and cytosine bases of nucleic acids. High activity of orotate phosphoribosyltransferase was observed in the extracts. A large proportion of radioactivity from [2‐¹⁴C]uracil was recovered as CO₂ and β‐ureidopropionate. Thus, a reductive pathway of uracil degradation is functional in these cells. Therefore, white spruce cells in culture demonstrate both the de novo and salvage pathways of purine and pyrimidine metabolism, as well as some degradation of the substrates into CO₂.     

Ribonucleic acid labelling and nucleotide pools during compensatory renal hypertrophy

During the first 48h of compensatory renal hypertrophy induced by unilateral nephrectomy, RNA content per cell increased by 20-40%. During this period, rates of RNA synthesis derived from the rates of labelling of UTP and RNA after a single injection of [5-(3)H]uridine showed no change in the rate of RNA synthesis (3.1nmol of UTP incorporated into RNA/min per mg of RNA). ATP and ADP pools were not changed. The rate of RNA synthesis was considerably in excess of the increment of total RNA appearing in the kidneys. With [5-(3)H]uridine as label, only continuous infusion for 24h could produce an increase (60%) in the specific radioactivity of renal rRNA in mice with contralateral nephrectomies. With a single injection of [methyl-(3)H]methionine used to identify methyl groups inserted into newly synthesized rRNA, the specific radioactivity of this rRNA was unchanged 5h after contralateral nephrectomy, increased by 60% at 9-48h, and returned to normal values at 120h. Most RNA synthesized in both nephrectomized and sham-nephrectomized mice has a short half-life. Since total cellular RNA content increases in compensatory hypertrophy despite unchanged rates of rRNA synthesis, the accretion of RNA might involve conservation of ribosomal precursor RNA or a change in rate of degradation of mature rRNA.     

KINETICS OF THE INCORPORATION OF SOME TRITIATED NUCLEIC ACID PRECURSORS AND [3H]LYSINE INTO MOUSE BRAIN CELLS FOLLOWING INTRAPERITONEAL INJECTION

Abstract— Intraperitoneal injection into white mice of the same amount of radioactivity (0.5 mCi) of [³H]uridine and [³H]lysine demonstrated by autoradiography that there was a much greater labelling of nerve cells from lysine than from uridine. For uridine, the choroid plexus cell nuclei gave maximal labelling within 1 h, with a decrease after 6 h. The plexus nuclei of lysine-injected animals gave almost the same amount of labelling during the experimental period of 48 h. In nerve cells, labelling from uridine increased in the nuclei up to 18 h after injection and there was an almost parallel increase in the labelling in the cytoplasm and neuropil. These results are compared with earlier reports on the results from intravenous injection of uridine. In lysine-injected animals the nerve cell nuclei and cytoplasm showed a fairly constant amount of label over 48 h, but the neuropil counts increased steeply. The activity of the blood was determined by scintillation counting during the 48-h period, and, as with uridine injection, was found to be almost constant over this period. A small series of animals was injected with 0.5 mCi of [³H]uracil, [³H]guanine, [³H]guanosine or [³H]cytidine for comparison. The autoradiograms from animals injected with these bases showed very slight labelling; that from guanosine was heavy in plexus nuclei, slight in nerve cells, and from cytidine it was heavy in plexus cells and moderate in nerve cells.