Ribonucleic acid synthesis in relation to precursor pools in regenerating rat liver

Hepatotrophic factors: implications for diabetes mellitus

In view of the importance of insulin in hepatic cell proliferation and regeneration, disturbances might be expected in these processes in diabetics. The relative importnace of insulin replacement given intraportally rather than subcutaneously is discussed. Results are presented showing that even when normoglycaemia is achieved with peripheral insulin infusion using the 'artificial pancreas' there are still abnormalities in intermediary metabolism. The incidence of cirrhosis in diabetes is reviewed and it is concluded that the evidence is poor for an increase in diabetics. Finally it is shown that in the normal diabetic rat changes are observed after partial hepatectomy consistent with an increase in redox potential within the regenerating liver. Insulin treatment improves redox status but does not completely reverse the changes shown.

Tissue-specific changes in RNA synthesis in vivo during anoxia in crucian carp

The overall energy budget for protein synthesis (i.e., transcription plus translation) is thought to consist of fixed and variable components, with RNA synthesis accounting for the former and protein synthesis the latter. During anoxia, the downregulation of protein synthesis (i.e., the variable component), to reduce energetic demand, is an important aspect of survival in crucian carp. The present study examines RNA synthesis during anoxia by labeling with [(3)H]uridine. A novel synthesis rate calculation is presented, which allows for the tissue-specific salvage of uridine, with synthesis rates finally expressed relative to DNA. After 48 h anoxia, the decline (29%) in brain RNA synthesis and increases in the heart and liver (132 and 871%, respectively) support known RNA functions during hypoxic/anoxic survival. This study provides evidence that, in an anoxia-tolerant species, survival mechanisms involving RNA are able to operate because tissue-specific restructuring of the RNA synthesis process enables fixed synthesis costs to be maintained; this may be as vital to survival as exploiting the variable energetic demand of protein synthesis.

Possible role of histone acetylation and histone H1(0) replacement for the initiation of replication in regenerating rat liver

The role of histone acetylation and DNA synthesis has been investigated extensively in the regenerating rat liver system in the presence and absence of the cyclophosphamide derivative mafosfamide. We demonstrate a mafosfamide-induced inhibition of maximum histone acetyltransferase activity followed by a second elevation of enzyme activity and an accompanying total suppression of DNA synthesis for 7-8 h. The maximum of histone acetyltransferase activity, in parallel with an elevated acetylation in vivo, the consecutive replacement of histone H1(0) amd initiation of replication occur sequentially in the presence and absence of mafosfamide, but with a temporary delay of 7-8 h. Our data indicate that modifications of histone acetyltransferase (EC 2.3.1.48) activity do not significantly influence the acetylation patterns of histones H3 and H4. The mafosfamide-induced change of histone acetyltransferase activity and acetylation in vivo, the shift of histone H1(0) exchange and the consecutive transition of initiation of replication suggest that these three events might be functionally related.

Rates of rat liver RNA degradation in vivo as determined from cytidine release during brief cyclic perfusion in situ

The breakdown of RNA and of long-lived proteins in rat liver is believed to occur largely within the lysosomal-vacuolar system. Both processes are induced by amino acid lack and suppressed by insulin, and in all circumstances a consistent lag of 15-20 min was observed between the introduction of a physiological regulator and onset of the degradative response. This lag has allowed us to determine rates of liver RNA degradation in vivo during brief cyclic perfusions, as was done previously for long-lived-protein breakdown [Hutson & Mortimore (1982) J. Biol. Chem. 257, 9548-9554]. Degradation was measured from the release of [14C]cytidine in livers of rats previously labelled in vivo with [6-14C]orotic acid. Release was linear and unaffected by physiological regulators between 2 and 12 min of perfusion. In contrast with protein breakdown, no short-lived component was observed. In animals trained to feed between 16:00 and 20:00 h, the content of liver RNA fell at an average rate of 0.26 mg/h per 100 g initial body wt. between 07:00 and 16:00 h, a loss that was within 9% of that predicted from the net release (total release minus reutilization) of cytidine in vivo. In addition, the total rate of RNA degradation determined at the end of the meal was only 12% of that at the start of the post-absorptive period 14 h later (2.1 versus 17.1%/day). This finding is fully consistent with a lysosomal mechanism for RNA degradation, since autophagy is strongly suppressed by food intake. This approach provides a comparatively simple means of approximating moment-to-moment rates of RNA degradation in the rat liver in vivo.

Estimation of rRNA synthesis and degradation rates in senescing wheat leaves

Changes in cytoplasmic and chloroplast rRNA content and rates of rRNA synthesis and degradation of detached wheat leaves were determined. It was found that rRNA loss is proportionally higher in chloroplasts than in cytoplasm. Rates of synthesis were measured by incorporation of large amounts of [3H]orotic acid into rRNA. This approach overcame size differences between pyrimidine pools of cells under different physiological status. Furthermore, these pools reached nearly the same specific radioactivity as that of the administered solution. Rates of degradation were estimated either as the difference between synthesis and net variation of rRNA or by disappearance of radioactivity from 32P-labeled rRNA. Results indicated a decrease in the net rRNA synthesis capacity of leaves after 48 h of detachment. However, the fractional rates of rRNA synthesis were maintained in both cytoplasm and chloroplasts. Ribosomal RNA degradation rates were 2.5-fold higher in chloroplast than in cytoplasm. The observed chloroplast rRNA loss is due to an increased degradation rate which is 15-fold higher than the synthesis rate 48 h after detachment.

Increased formation and keeping of ribosomes during dietary recovery of mouse kidney

In the kidney of 5-day protein-depleted mice there is a decrease of 23% in the rRNA mass. When these animals are fed with a complete diet, rRNA content is restored to its normal value after 24 h of refeeding. The mechanisms that underlie this phenomenon were studied. It was found that the activity of rRNA polymerase I in the nuclei of kidneys from refed mice showed an increase of about twofold compared with the activity in normal and protein-depleted nuclei. The in vivo incorporation of a large dose (nontrace) of [14C]orotic acid into rRNA was also twofold enhanced in kidneys from refed mice. Ribosome degradation (measured by the disappearance of radioactivity from either ribosomal proteins or rRNA previously labeled by the injection of NaH14CO3 and [14C]orotic acid to the mice, respectively) stopped during the 1st day after refeeding. The estimation of the difference between the rRNA synthesis rate and the net rRNA increase also demonstrated a decrease in the rRNA degradation rate in refed mice.

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.

Effects of partial hepatectomy on RNA polymerase activities in mouse liver

Chromatin-bound and poly[d(A-T)]dependent RNA polymerase I plus III and II activities of mouse liver were analysed 24 and 48 hr after partial hepatectomy. Chromatin-bound RNA polymerase I plus III activity showed an increase of 57% at 24 hr and 51% at 48 hr after partial hepatectomy. There was a decrease in chromatin-bound RNA polymerase II activity of 15% at 24 hr and 34% at 48 hr after partial hepatectomy. There was no significant changes in poly[d(A-T)]dependent RNA polymerase activities. Heparin caused an approximately 10-fold increase in chromatin-bound RNA polymerase II activity. The stimulation by heparin was significantly increased 48 h after partial hepatectomy. Anaesthesia and/or surgery had great influence on RNA polymerase activities. At 24 hr after operation, chromatin-bound RNA polymerase I plus III and II activities were depressed, and the liver cell chromatin was more susceptible to stimulation by heparin.

Transcriptional control of ribosome production in regenerating rat liver

Kinetic experiments of labelling in vivo with [14C]orotate of cellular free UMP and/or UTP, nucleolar, nucleoplasmic and cytoplasmic rRNA in normal and 12 h-regenerating rat liver were performed. The specific-radioactivity curves obtained were analysed by computer and the rates of synthesis of precursor rRNA (45S pre-rRNA) and cytoplasmic 28S and 18S rRNA calculated. (a) The rates of synthesis of 45S pre-rRNA in normal and regenerating rat liver are 1400 and 3700 molecules/min per nucleus respectively; (b) the average rates of formation of mature 28S and 18S rRNA are identical with the rates of synthesis of 45S pre-rRNA in both normal and regenerating rat liver. Thus the synthesis of rRNA in 12h-regenerating rat liver is activated 2.7-fold. The analysis of rRNA synthesis in isolated nucleoli also shows a 2.7-fold stimulation of transcription in regenerating liver. It is concluded that all the 45S pre-rRNA molecules synthesized are processed and transferred as 28S and 18S rRNA in the cytoplasm, i.e. degradation (wastage) of newly synthesized ribosomes in the nucleus does not occur in both normal and regenerating rat liver. Thus the enhanced production of ribosomes in regenerating rat liver is regulated only at the transcriptional level.

Regulation of the prereplicative changes in the synthesis and transport of messenger and ribosomal RNA in regenerating livers of normal and hypocalcemic rats

Partial hepatectomy induces increases in the synthesis of both messenger (poly(A)+) and ribosomal (poly(A)-) RNA, which precede the initiation of DNA synthesis. The increase in poly(A)+RNA, which commences soon after surgery and reaches a peak 1-3 hours later is particularly striking. Disruption of this early increase in poly(A)+RNA synthesis by colchicine (and other microtubule disrupters) or indomethacin results in a failure to initiate DNA synthesis. This suggests that prostaglandins and the microtubules are involved in the mechanism of proliferative activation. Hypocalcemia, which also prevents the initiation of DNA synthesis, has no effect on the prereplicative changes in either messenger or ribosomal RNA synthesis. These results help to define the critical stages of prereplicative development and give some insight into their regulation.

Metabolism of sialic acid in regenerating rat liver

In regenerating rat liver slices 24 h after partial hepatectomy, the incorporation of [1-14C]glucosamine into 'free sialic acid' (N-acetylneuraminic acid + CMP-N-acetylneuraminic acid) decreased to below 50% of the control values and the incorporation into protein-bound sialic acid decreased to the same extent. The incorporation of [14C]glucosamine into 'free sialic acid' decreased during the period from 6 to 47 h after hepatectomy, showing a minimum at 12 h, and recovered to the control value by 96 h. At 12 h, the activities of UDP-N-acetylglucosamine 2-epimerase (UDP-2-acetoamido-2-deoxy-D-glucose 2-epimerase, EC 5.1.3.14) and N-acyl-D-mannosamine kinase (ATP: 2-acylamino-2-deoxy-D-mannose 6-phosphotransferase, EC 2.7.1.60) in the liver were significantly decreased. The amount of protein-bound sialic acid in the liver was not changed after partial hepatectomy, but the amount in plasma was changed, with a similar pattern to that of the incorporation of [14C]glucosamine into slice 'free sialic acid'. These results indicate that the synthesis of sialic acid in the liver much decreases in the early stage of regeneration and that this may be correlated with the decreased synthesis of plasma sialoglycoproteins.

Uridine triphosphate and RNA synthesis during diabetes-induced renal growth

UTP, CTP, and RNA synthesis were studied in the renal cortex of diabetic and control rats in vivo. The incorporation of UTP into RNA (nmol/h DNA) was used as estimate of RNA synthesis rate. Two to three days after streptozotocin injection, UTP and CTP ppol size and orotate incorporation into UTP and RNA were greater in diabetic animals than in controls. In addition, RNA content and RNA synthesis rate were increased. These changes were corrected by insulin infusion. In diabetic animals, additional increases in UTP pool, RNA content, and RNA synthesis rate followed contralateral nephrectomy. This increase in RNA content was greater than in uninephrectomized controls. The changes in the diabetic renal cortex were not accompanied by increased plasma concentrations of growth hormone. The increase in RNA content in the diabetic renal cortex is probably due to increased RNA synthesis. Increased synthesis of pyrimidines and expansion of the UTP pool may make this substrate more readily available for the synthesis of UDP sugars and may facilitate the synthesis of basement membrane in diabetes.

Comparison of transcription stimulating, phenol-soluble non-histone chromosomal proteins in normal rat liver and transplantable hepatocellular carcinomas

The non-histone chromosomal proteins (NHCP) of a rapidly and slowly proliferating transplantable hepatocellular carcinoma (THC) were compared to those of normal and regenerating rat liver. The total quantity of NHCP is approximately threefold higher in the THCs than in either normal rat liver at 4 h and 44 h regenerating rat liver. Only those NHCP that can be extracted from chromatin by 0.35 M NaCl were further examined and it was observed that the proteins of this highly complex fraction could be further fractionated by their differential phenol-solubility. The phenol-soluble 0.35 NHCP contained protein(s) capable of stimulating the level of DNA-directed RNA synthesis in vitro. The total amount of this stimulatory activity was 5 times higher in the rapidly growing THC and 1.6 times higher in the slowly growing THC than in normal rat liver. In order to assess the contribution of cell-cycle dependent alterations on the increase in the amount of stimulatory activity in the THCs, 44 h regenerating rat livers were examined. This tissue represents a mix of cells in various stages of the cell cycle which is similar to that found in the THCs. It was found that the total quantity of NHCP in the 44 h regenerating rat liver was the same as in normal rat liver. The total amount of the stimulatory activity also was similar in both the normal and 44 h regenerating rat liver. The amount of the stimulatory activity was found to double in 4 h regenerating rat liver, however. These data suggest that the alterations observed in the NHCP of the THCs are not due solely to cell cycle dependent changes, but may represent malignancy dependent alterations.

Base composition studies on transfer RNA from normal and regenerating rat liver

The base composition of bulk tRNA isolated from regenerating rat liver, 12, 18, 24 and 30 h after partial hepatectomy, was determined by a 3H derivative method. Only a few minor statistically significant changes (2--11%), as compared to sham-operated liver, were found at 18, 24 and 30 h after hepatectomy. These included a reduction in the amounts of adenosine and 3-(3-amino-3-carboxypropyl)-uridine, and an increase in the amounts of 1-methyl-adenosine, 1-methylguanosine, 3-methylcytidine and pseudouridine. Similarly, when the base composition of tRNA fractions from control and 24-h regenerating rat liver, partially purified by one-dimensional polyacrylamide gel electrophoresis, was determined, no gross differences were observed. These results suggest that the process of liver regeneration is not accompanied by a gross alteration of the modification pattern of tRNA.

Liver growth, biosynthesis of cytidine nucleotides and level of cytochrome P-450 in rat liver after administration of alpha-hexachlorocyclohexane

The biosynthesis of cytidine nucleotides and the level of microsomal cytochrome P-450 in intact and regenerating rat liver after repeated administration of alpha-hexachlorocyclohexane (alpha-HCH) were compared. In alpha-HCH treated animals the utilization of [2-14C] orotic acid for the synthesis of cytidine nucleotides is suppressed. In 24-h regenerating liver the incorporation of labelled orotic acid into cytidine nucleotides is markedly activated; the degree of activation is lower in regenerating livers of alpha-HCH treated animals. The changes in the level of cytochrome P-450 vary inversely with the changes in the utilization of [2-14C] orotic acid for the synthesis of cytidine nucleotides. The activity of cytidine triphosphate synthetase of liver cytosol increases shortly after the administration of alpha-HCH; uridine-cytidine kinase is enhanced in the later stages of the drug action. Within 15-45 min after the administration of alpha-HCH the uptake of [U-14 C] cytidine into the liver and its incorporation into RNA cytosine are increased. After the administration of the drug the uptake of [2-14 C] uridine and its incorporation into RNA uracil is also enhanced whereas its utilization for the synthesis of cytidine nucleotides of the acid-soluble extract as well as for the RNA cytosine are suppressed.

Polyamine metabolism in regenerating livers from normal and hypocalcemic rats

There is a marked increase in the concentration of putrescine during the first ten hours following partial hepatectomy in rats. The concentration of spermidine also increases but to a smaller degree. Putrescine levels return to normal between 10 and 24 hours after the operation, whereas the increased spermidine level is maintained. The production of putrescine and spermidine appears to be initiated by the induction of ornithine decarboxylase which shows a single peak of activity at four hours after hepatectomy. The activity of S-adenosylmethionine decarboxylase shows little change following hepatectomy. The changes in polyamine levels and the activities of the enzymes of polyamine metabolism are not affected by thyroparathyroidectomy 72 hours prior to hepatectomy. Thus although these hypocalcemic conditions considerably reduce and delay DNA synthesis and mitosis, the prereplicative changes in polyamine metabolism still occur. These data suggest that the hepatocytes in hypocalcemic animals have become activated and moved to an advanced stage of prereplicative development before being blocked.

An autoradiographic study of the 3H-uridine and 3H-thymidine incorporation in the regenerating mouse liver

An autoradiographic study was made of the 3H-uridine incorporation into RNA and DNA in nucleus and cytoplasm of parenchymal cells in the regenerating liver of the mouse after a pulse time of 2 hr. After a decreased uptake of precursor into the parenchymal nucleus during the first 6 hr compared with the normal value, incorporation increased and was maximal at 36 hr; normal values were restored at 72 hr. The cytoplasmic labelling, after an initial small decrease, reached a maximum at 12 hr; this changed to normal 48 hr after hepatectomy. RNase-digestion of the liver sections left a small incorporation in both nucleus and cytoplasm: presumably DNA. This incorporation is maximal at 12 hr over the nucleus and at 24 hr over the cytoplasm. After a 2 hr pulse of 3H-thymidine, there was a marked uptake of the precursor into DNA about 24 hr after hepatectomy. This was maximal at 48 hr and reached normal values at 72 hr. A small amount of incorporation of 3H-thymidine into DNA was seen immediately after the operation, and this population of weakly labelled nuclei was still rather large 72 hr later.

Nucleolar DNA-dependent RNA polymerase from rat liver. 2. Two forms and their physiological significance

RNA polymerase I (or A) was extracted from nuclear, nucleolar and nucleoplasmic fractions, and resolved into IA and IB forms on a phosphocellulose column. During the course of cycloheximide treatment, the activity of RNA polymerase IB decreased in the nucleoli with concomitant increase in the nucleoplasmic fraction, suggesting strongly that cycloheximide induced specific leakage of IB enzyme from the nucleolus. The activity of IA enzyme did not change in the nucleoli. When nucleoli were incubated in the presence of actinomycin D, all the IA enzyme activity and approximately 30% of the total IB enzyme activity were released in the incubation medium, whereaa 70% of IB activity remained associated with the nucleolar pellet where no IA activity was detected. The enzyme which was released into the incubation medium was tentatively designated as free or unbound RNA polymerase I and that which was associated with the nucleolar pellet as template-bound enzyme. During the treatment with cycloheximide, the activity of bound enzyme, which contained exclusively IB form, decreased rapidly, with kinetics almost identical to that of nucleolar RNA synthesis in vivo. The activity of free enzyme did not change appreciably. At 2 h after partial hepatectomy, IB enzyme activity in the free RNA polymerase fraction increased to almost twice the control, while the bound enzyme activity did not increase appreciably until 4h of regeneration. Enhancement of nucleolar RNA synthesis in vivo was not apparent at 2 h but became significant by 4 h after partial hepatectomy. These results strongly suggest that (a) the above-mentioned procedure is actually fractionating RNA polymerase I into free and bound forms, (b) RNA polymerase IB is the transcriptionally active form in vivo, (c) RNA polymerase IB exists in excess in the nucleoli, but the amount of bound IB molecules, which are engaged in transcription in vivo, must be determined by some other factor(s) than the mere concentration of IB enzyme in the nucleolus, and (d) IA form is not an artifact of isolation but is always present in vivo at a certain amount, although the exact nature of this molecule is not known at present.

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.

Utilization of uridine for RNA synthesis in the insect cell line CP-1268 derived from the codling moth, Laspeyresia pomonella

The utilization of [3H]-5-uridine by CP-1268 cells was studied. Uridine was rapidly transported into these cells by a concentration dependent, saturable process. Exogenous uridine rapidly equilibrated with cellular nucleotide pools and virtually all of the uridine transported into the cells was phosphorylated. Uridine incorporation into RNA was studied by continuous and pulse-labeling techniques in the prescence or absence of actinomycin D and cordycepin. These studies have shown that the pattern of unstable RNA precursor and relatively stable RNA product relationship known to exist in mammalian cells similarly exists in insect cells in vitro. This pattern varied markedly with pulse-labeling time and required the addition of RNA inhibitors to block reincorporation of intracellular labeled metabolites during the chase.

Galactose metabolism in regenerating rat liver

1. Rats trained on a controlled lighting and feeding schedule were subjected to partial hepatectomy or sham operation. 2. After a large period of about 6h the activity of UDP-galactose 4-epimerase increased threefold, reaching a maximum 4 days after partial hepatectomy, and returned to normal values within a fortnight. 3. The enzyme pattern of the UDP-galactose-glycoprotein galactosyltransferase was biphasic, one peak appearing at 20 h, the second at 72 h after partial hepatectomy. 4. The rise in enzyme activities could be blocked by the injection of actinomycin D, and the Km values for UDP-glucose and UDP-galactose were nearly identical in regenerating and adult liver. It is therefore concluded that the increase in enzyme activity is due to synthesis de novo of enzyme protein.

Effects of gamma-irradiation on biosynthesis of different types of ribonucleic acids in normal and regenerating rat liver

1. The effect of gamma-irradiation (4000rd) on the synthesis of ribosomal (pre-rRNA) and heterogeneous nuclear RNA (pre-mRNA) in normal and in regenerating rat liver was studied by using 40 min labelling with [6(-14)C]orotic acid. 2. Partial hepatectomy caused a sharp transient increase in the specific radioactivity of the endogenous low-molecular-weight RNA precursors in the livers of both normal and irradiated rats. Irradiation of intact animals did not affect the pool. 3. Irradiation enhanced the synthesis of pre-rRNA for at least 12h. The synthesis of pre-mRNA was also enhanced, but only in the first 3h after irradiation. 4. Partial hepatectomy strongly stimulated the synthesis of both pre-rRNA and pre-mRNA. 5. The synthesis of pre-rRNA was enhanced also in regenerating liver of animals irradiated before or after the operation. The conclusion can be drawn that the early increase in the synthesis of ribosomal RNA is a non-specific cellular response to different injuring factors. 6. The only case where irradiation caused an early inhibition of RNA synthesis was that of pre-mRNA in regenerating liver. This supports the hypothesis that ionizing radiation does not suppress the transcription per se but affects the mechanisms of activation of new genes (cellular programming).

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.

Qualitative and quantitative aspects of the biosynthesis of ribonucleic acid and of protein in the liver and the lung of the Syrian golden hamster

1. The incorporation of orotic acid and of uridine into total RNA was measured in vivo in liver and lung of the Syrian golden hamster. Specific activities of total acid-soluble UMP were measured in both organs. An estimation of the rate of RNA biosynthesis showed that hamster lung synthesizes RNA at about one-half of the rate of that of hamster liver. 2. The apparent K(m) and V(max.) values of a few enzymes involved in pyrimidine biosynthesis were measured in the 100000g supernatants of liver and lung. The apparent K(m) values were very similar in both organs. From the estimated V(max.,) it was concluded that hamster lung cells have less capacity to metabolize orotic acid than have liver cells. 3. A time-response and a dose-response study showed that actinomycin D inhibits pulmonary RNA synthesis as efficiently as hepatic RNA synthesis. 4. Protein synthesis, measured as the incorporation of leucine, was inhibited in both organs 30min after a dose of 2mg of cycloheximide/kg. The dose-response patterns were similar in both liver and lung 3h after cycloheximide. 5. It is concluded that RNA and protein synthesis in vivo in hamster lung are very similar to the corresponding reactions in liver. Alterations of RNA and protein synthesis by toxic agents can therefore be evaluated in lung with a similar approach to that used to study the pathological biochemistry of liver.

Incorporation of different labelled precursors into chloroplast RNA of Chlorella

Radioactive uridine is incorporated by Chlorella strain 211-8b/p into ribosomal subunits and their rapidly labelled RNA comigrates with chloroplast RNA on polycrylamide gels.Ribosomal particles which can be labelled by short pulses of orotic acid cosediment with the particles labelled by uridine pulses and contain the same RNA species as these when separated either on sucrose gradients or on polycrylamide gels. This incorporation is, like that of uridine, sensitive to rifampin and chloramphenicol, but insensitive to cycloheximide.A comparative study of short-time incorporation of uridine, orotic acid and guanosine into the RNA of Chlorella showed that all three precursors were incorporated mainly into RNA of chloroplastic origin. However, guanosine was also partly incorporated into cytoplasmic rRNA. Nitrogen-deficient cells always incorporated part of all three precursors into cytoplasmic rRNA, but the proportions of these were different among the different precursors.These results are consistent with the hypothesis that the described differences in the incorporation of the above mentioned precursors into RNA of different cellular compartments are largely attributable to effects of pool sizes.

Uptake of orotate and thymidine by normal and regenerating rat livers

1. At 1h after operation livers from partially hepatectomized rats showed a 60-100% increase in the capacity to concentrate (3)H radioactivity from orotate, thymidine or uridine with respect to the radioactivity in plasma. Uptake of [(3)H]cytidine into liver was unaffected, as was entry of any precursor studied into any tissue other than liver. 2. This increase in intracellular radioactivity was detectable 10min after operation with both orotate and thymidine. With orotate the augmentation had disappeared by 3 days, but with thymidine it was still evident 8 days after partial hepatectomy, when [(3)H]thymidine incorporation into DNA was no longer increased. Competition studies established that orotate was not entering the liver by the same mechanism as thymidine. 3. In the soluble fraction of the liver all the (3)H radioactivity from orotate was present as uridine nucleotides. Thymidine was not phosphorylated, and was believed to be catabolized.

The conversion of orotic acid into uridine 5'-monophosphate by isolated perfused normal and regenerating rat livers

The release of (14)CO(2) from [7-(14)C]orotic acid was measured in isolated perfused normal and regenerating rat livers. With some limitations, the release of (14)CO(2) from [7-(14)C]orotic acid can be used to estimate UMP synthesis in perfused livers. Isolated perfused livers rapidly pick up labelled orotic acid added to perfusate and convert most of it into UMP. Perfused regenerating livers produce approx. 2.5 times as much UMP/g of liver as do perfused normal livers. However, the absolute amount of orotic acid converted into UMP is higher in perfused normal livers than in perfused regenerating livers. Perfused regenerating livers do not differ in their orotic acid uptake and UMP synthesis from livers of comparable size in which regeneration is not taking place. The total amount of orotic acid taken up by the liver (rather than the rate of uptake) and the size of the liver appear to be the determining factors in UMP production. The results suggest that the decrease in liver size caused by partial hepatectomy may be in itself sufficient to account for an increase in the flow of metabolites in the pyrimidine pathway at the early stages of liver regeneration.

Uptake of amino acids and nucleic acid precursors by regenerating rat liver

1. At 0.5-1.0h after partial hepatectomy the intracellular acid-soluble fraction of rat liver took up twice as much radioactivity from [(3)H]orotic acid, [(3)H]uridine and [(3)H]thymidine as did similar fractions from sham-operated animals. This increase in penetration was not prevented by adrenalectomy or actinomycin, both of which decreased precursor uptake into nuclear RNA at this time. The increase in entry was still shown by thymidine at 22h after operation, at the height of the S period. Adenine penetration was not increased 1h after partial hepatectomy. 2. Plasma concentrations of ornithine and possibly methionine, tyrosine and lysine were raised 1.5h after partial hepatectomy. [(3)H]Lysine entry into regenerating liver at this time was increased by 60%; [(3)H]valine uptake was unaffected. Intracellular amounts of tyrosine, phenylalanine and ornithine in the liver were also increased. 3. The relation of these events to the start of liver regeneration is discussed.