Thymidine kinase and deoxyribonucleic acid metabolism in growing and regenerating livers from rats on controlled feeding schedules

Rats accustomed to eating during the first 8h of a daily 12h dark period re-established about 80% of intact liver weight, protein and DNA within 4 days following partial hepatectomy; further increases were not observed. Liver thymidine kinase activity and thymidine incorporation into liver DNA exhibited marked daily oscillations during liver regeneration. Maximum values were observed near the end of the dark period both in intact growing rats and in rats partially hepatectomized 2h before the end of the dark period. The time of day of surgery affected thymidine kinase activity and thymidine incorporation into DNA at specific times following partial hepatectomy. This seriously affects the interpretation of reports of experiments where the time of day of killing has been held constant and time of surgery varied. Highly significant correlation coefficients were observed for thymidine incorporation before killing versus thymidine kinase activity at time of killing and for thymidine versus orotic acid incorporation into DNA of livers from rats partially hepatectomized 2h before the end of the dark period and killed at 12h intervals. Thymidylate phosphatase activity returned to the normal amount at a rate similar to that for liver protein. Thymidylate phosphatase did not affect the validity of the thymidine kinase assay. The relationship of [(14)C]orotic acid to [(3)H]thymidine incorporation into liver DNA varied with the time of day, with the age of the rat and during the regeneration of the liver.     

Sequential changes in ornithine decarboxylase thymidine kinase, and other enzyme activities in regenerating liver in rats on controlled feeding schedules.

The changes in activity of five enzymes including ornithine decarboxylase (ODC), tyrosine aminotransferase (TAT), thymidine kinase (TK), ornithine aminotransferase (OAT) and serine dehydratase (SDH) in the early stage of the regenerating rat liver have been studied under a controlled feeding and lighting schedule. The first three enzyme activities were stimulated sequentially by partial hepatectomy. The earliest response was observed in ODC activity. A significant increase in this enzyme activity was observed at 2 hrs and the maximal level was at 4 hrs after the operation. TAT began to increase at 4 hrs and the maximal level was at 8 hrs. The TK activity was induced at about 24 hrs and the highest value was at 48 hrs after partial hepatectomy.A significant decrease in OAT activity was observed at 24 hrs after the operation and subsequently. Although a decrease in SDH activity was also observed this decrease did not seem to correlate directly with the regeneration process, since a lowered level of the enzyme activity was also found in the sham operated group.     

Postexercise muscle and liver glycogen metabolism in male and female rats

Male and female Wistar rats were run for 5 min at 1.7 mph at a 17% grade to determine whether a sex difference exists in the rate of glycogen resynthesis during recovery in fast-twitch red muscle, fast-twitch white muscle, and liver. Rats were killed at one of three time points: immediately after the exercise bout, and at 1 or 4 h later. Males had significantly higher resting muscle glycogen levels (P less than 0.05). Exercise resulted in significant glycogen depletion in both sexes (P less than 0.01). Males utilized approximately 50% more glycogen during the exercise bout than females (P less than 0.05). During the food-restricted 4-h recovery period, muscle glycogen was repleted significantly during the 1st h (P less than 0.05). Liver glycogen was not depleted as a result of the exercise bout, but fell during the first h of recovery (P less than 0.05) and remained low during the subsequent 3 h. The greater glycogen utilization in red and white fast-twitch muscle during exercise by males could represent a true sex difference but could also be attributable in part to the males having performed more work as a result of 20% greater body mass. We conclude that no sex difference was observed in the rates of muscle glycogen repletion after exercise or in liver glycogen metabolism during and after exercise, and rapid postexercise muscle glycogen repletion occurred at a time of accelerated liver glycogen depletion.     

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.     

Effect of controlled feeding on the isoenzymes of pyruvate kinase, hexokinase and aldolase in liver during development

Controlled feeding schedules have been used to maintain rats during the first thirty days following birth to investigate the role of food in determining the course of liver development. Use of the ‘8 + 16’ feeding schedule and a new ‘16 + 8’ suckling schedule allowed rats to be reared through the neonatal period without access to solid food until the day they were weaned. Food-independent increases in Pyruvate kinase type I isoenzyme, glucokinase and aldolase B take place in the late suckling period. Thus it appears that the change in the nutritional status of the animal at weaning is not a major factor in liver development.     

Glucosamine metabolism in regenerating rat liver.

1. Glycoprotein synthesis was investigated with [1-14C]glucosamine in vivo. [14C]Glucosamine was administered intravenously 24h after hepatectomy to rats. 2. Incorporation into the acid-soluble fraction was maximum at 15 min after injection both in sham-operated and hepatectomized rats. 3. Enhancement of incorporation into UDP-N-acetylhexosamine in regenerating liver was observed. However, its specific activity was lower, because of a greater enhancement of synthesis de novo of the amino sugar. 4. In the liver acid-insoluble fraction, maximum incorporation of [14C]glucosamine was at 30 min in sham-operated rats and 2 h in hepatectomized rats respectively. 5. In sham-operated rats, incorporation into the plasma acid-insoluble fraction followed that of the liver acid-insoluble fraction, but hepatectomy resulted in a rapid enchancement of incorporation into plasma. 6. It is concluded that synthesis of liver glycoproteins is stimulated after partial hepatectomy and that glycoproteins synthesized are released rapidly into the plasma.     

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.     

Modification of the template capacity of liver chromatin for form-B ribonucleic acid polymerase by food intake in rats under controlled feeding schedules.

Nuclei from liver of rats accustomed to eating during the first 8h of a daily 12h dark period demonstrate an increased capacity to synthesize RNA 6H after the beginning of the feeding period. 2. This increase is accompanied by a higher yield of extractable form-B DNA-dependent RNA polymerase activity. 3. The endogenous RNA polymerase activity associated with nuclear chromatin is also stimulated by food intake. Both purified and chromatin-associated form-B enzyme activities exhibit different ionic strength requirements after food intake. 4. The sensitivity of exogenous (added) form-B-enzyme to changes in ionic strength changes after feeding when chromatin is used as template. 5. Chromatin extracted from the liver of fed rats is a better template for form-B-enzyme than chromatin extracted from starved rats.