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Biomedical subjects

D G Walker

Publications and source records attributed to D G Walker.

At least 109 records · Page 6Linked to original sources
PubMed1978

Glucose metabolism in the newborn rat: the role of insulin.

The effect of the administration of anti-insulin serum to newborn rats, surgically delivered under ether anaesthesia at term, was examined with respect to liver glycogen concentration and plasma concentrations of glucose, lactate and free fatty acids. Newborn rats thus treated showed decreased liver glycogen concentrations and elevated plasma concentrations of glucose, lactate and free fatty acids compared to untreated control animals one hour later. These effects were dose-dependent with respect to the amount of anti-insulin serum administered. The simultaneous administration of glucagon with anti-insulin serum at birth was no more effective in mobilising glycogen stores than anti-insulin serum alone, although plasma glucose concentrations in these animals were higher and plasma lactate concentrations were lower. Either anti-insulin serum or glucagon abolished the postnatal hypoglycaemia observed in untreated neonatal rats. The rate of fall in plasma lactate concentrations after birth was stimulated in glucagon-treated rats but was retarded in rats treated with anti-insulin serum. Hormonal control over the initiation of glycogenolysis and gluconeogenesis in the newborn rat appears to be different, a fall in plasma insulin being the prime factor involved in triggering glycogen mobilization and a rise in plasma glucagon the prime event that initiates gluconeogenesis.

Animals↗
PubMed1978

A comparison of lipid metabolism in hepatocytes isolated from fed and starved neonatal and adult rats.

1. The utilization of [1-14C]palmitate by hepatocytes prepared from fed and starved neonatal and adult rats has been examined by measuring isotopic incorporation into various products. 2. In cells from fed adult rats the principal products were esters (triglycerides and phospholipids) but ketone bodies were the main metabolic end products in cells from starved adult and fed and starved neonatal rats. Production of triglycerides exceeded that of phospholipids in fed adult cells whereas phospholipid formation always predominated in neonatal cells. 3. The high rate of fatty acid oxidation and hence NADH formation by neonatal cells is reflected by a lower acetoacetate--3-hydroxybutyrate ratio at the earlier stages of incubation of neonatal cells. 4. The addition of glycerol modified quantitatively the products of palmitate metabolism by adult hepatocytes but no such effects were observed with neonatal cells. 5. Compared with adult cells, neonatal hepatocytes showed very low rates of lipogenesis that were only enhanced a little by addition of lactate/pyruvate and did not show any effects of glucose concentration upon incorporation of tritium from 3H2O into lipids.

Aging↗
PubMed1978

A comparison of gluconeogenesis in hepatocytes from neonatal and adult rats.

1. A method for the preparation of hepatocytes from livers of 11-15-day old rats is described. These cells in general behave similarly to hepatocytes made from adult rats with respect to stimulation of gluconeogenesis by glucagon and adrenaline and the effects of added oleate. 2. Significant differences in the behaviour of hepatocytes from neonatal and adult rats were nevertheless seen in certain situations, e.g. with alanine as gluconeogenic substrate, and appeared to be related to the redox state of the cells. 3. The importance of redox state upon gluconeogenesis was examined in more detail by determining the effects of oleate, ethanol and DL-3-hydroxybutyrate alone and in combinations. Major differences between neonatal and adult hepatocytes were again observed with alanine as substrate. 4. A discussion concludes that, while some relevant differences in the enzyme complements of neonatal and adult rat livers are known, it is the high capacity of the neonatal liver to generate reducing power by oxidation of fatty acid that can explain the observed differences.

Aging↗
PubMed1976

Studies on the use of sepharose-N-(6-aminohexanoyl)-2-amino-2-deoxy-D-glucopyranose for the large-scale purification of hepatic glucokinase.

The synthesis of N-(6-aminohexanoyl)-2-amino-2-deoxy-D-glucose is described and it was shown to be a competitive inhibitor (Ki, 0.75 mM) with respect to glucose of rat hepatic glucokinase (EC 2.7.1.2). After attachment to CNBr-activated Sepharose 4B, this derivative was able to remove glucokinase quantitatively from crude liver extracts and release it when the columns were developed with glucose, glucosamine, N-acetyl-glucosamine or KC1. Repeated exposure of the columns to liver extracts led to rapid loss in their effectiveness as affinity matrices because proteins other than glucokinase are bound to the columns. The nature of such protein binding and methods for the rejuvenation of "used" columns are discussed along with the effect of the mode of preparation of the Sepharose-ligand conjugate and the concentration of bound ligand on the purification of glucokinase. Glucose 6-phosphate dehydrogenase is cited as an example of both non-specific protein binding to the affinity column and of the importance of the control of ligand concentration in removing such non-specifically bound proteins. Some guidelines emerged that should be generally applicable to other systems, particularly those which involve affinity chromatography of enzymes that are present in tissue extracts in very low amounts and possess only a relatively low association constant for the immobilized ligand.

Animals↗
PubMed1976

The purification in high yield and characterization of rat hepatic glucokinase.

A new improved procedure for the purification of rat hepatic glucokinase (ATP-d-glucose 6-phosphotransferase, EC 2.7.1.2) is given. A key step is affinity chromatography on Sepharose-N-(6-aminohexanoyl)-2-amino-2-deoxy-d-glucopyranose. A homogeneous enzyme, specific activity 150 units/mg of protein, is obtained in about 40% yield. The molecular weight of the pure enzyme was determined by several procedures. In particular, sedimentation-equilibrium studies under a variety of conditions indicate a molecular weight of 48000 and no evidence for dimerization; reports in the literature of other values are discussed in the light of this evidence on the pure enzyme. The amino acid composition suggests that hepatic glucokinase is closely related to rat brain hexokinase and also the wheat "light" hexokinases.

Amino Acids↗
PubMed1976

The topographical location and unique nature of a glucokinase associated with the Golgi apparatus of rat liver.

A particulate glucokinase was recovered in the Golgi-rich fraction of rat liver prepared by the method of Morré [Methods Enzymol. (1971) 22, 130-148], thus extending the demonstration by Berthillier et al. [Biochim. Biophys. Acta (1973), 293, 370-378] of particulate glucokinase activity in a microsomal subfraction that showed enrichment in Golgi characteristics. The purity of this fraction was examined and it was then subjected to several treatments, the action of Triton X-100, freezing and thawing, and sonication to establish the topographical location of the glucokinase activity thus solubilized. The evidence suggests that the glucokinase activity is either soluble in the lumen of the Golgi apparatus or loosely associated with the inside of the Golgi membranes.

Animals↗
PubMed1975

Bone resorption restored in osteopetrotic mice by transplants of normal bone marrow and spleen cells.

Capacity to resorb bone and calcified cartilage was restored permanently in mice with inherited osteopetrosis by the intravenous administration of cell suspensions prepared from spleen and bone marrow of normal littermates. Beginning near active growth plates as early as 2 weeks after transplantation, replacement of the abnormal spongiosa continued until medullary cavitites were fully expanded.

Age Factors↗
PubMed1975

Spleen cells transmit osteopetrosis in mice.

Osteopetrosis was induced in lethally irradiated, normal mice of grey-lethal and microphthalmic stocks by cell infusions prepared from the spleens of osteopetrotic littermates. Failure of skeletal remodeling became evident within a few weeks after transplantation as calcified cartilage and bone accumulated excessively in the active metaphyses of the long bones. The massive lesions produced were extensively infiltrated with abnormal osteoclasts.

Animals↗
PubMed1975

Control of bone resorption by hematopoietic tissue. The induction and reversal of congenital osteopetrosis in mice through use of bone marrow and splenic transplants.

The reciprocal transplantation of hematopoietic tissues was carried out on young, lethally irradiated mice of inbred, microphthalmic stock. The cell infusions prepared from the bone marrow or spleen of a normal littermate fully restored capacity to resorb bone and cartilage in the osteopetrotic recipients. Conversely, cell infusions prepared from the spleen of microphthalmic mice induced osteopetrosis in their irradiated, normal littermates. It is concluded that resorption of skeletal matrix is controlled by migratory cells, possibly osteoclastic progenitors, derived from the myelogenous tissues. No evidence was obtained to suggest that skeletal changes observed in the experimental animals were mediated by a graft-vs.-host reaction. The earliest skeletal changes in the experimental mice were detected 2 wk after onset which may represent the length or time required to replace the osteoclast population of the mouse.

Animals↗
PubMed1974

Enzyme synthesis in the regulation of hepatic "malic" enzyme activity.

A homogeneous preparation of ;malic' enzyme (EC 1.1.1.40) from livers of thyroxine-treated rats was used to prepare in rabbits an antiserum to the enzyme that reacts monospecifically with the ;malic' enzyme in livers of rats in several physiological states. Changes in enzyme activity resulting from modification of the state of the animal are hence due to an altered amount of enzyme protein. The antiserum has been used to precipitate out ;malic' enzyme from heat-treated supernatant preparations of livers from both adult and neonatal rats, in a number of physiological conditions, that had been injected 30min earlier with l-[4,5-(3)H]leucine. The low incorporations of radioactivity into the immunoprecipitable enzyme have permitted the qualitative conclusion that changed enzyme activity in adult rats arises mainly from alterations in the rate of enzyme synthesis. The marked increase in ;malic' enzyme activity that occurs naturally or as a result of thyroxine treatment of the weanling rat is likewise due to a marked increase in the rate of enzyme synthesis possibly associated with a concurrent diminished rate of enzyme degradation.

Age Factors↗
PubMed1974

Regulation of hepatic L-serine dehydratase and L-serine-pyruvate aminotransferase in the developing neonatal rat.

1. The activities of l-serine dehydratase and l-serine-pyruvate aminotransferase were determined in rat liver during foetal and neonatal development. 2. l-Serine-pyruvate aminotransferase activity begins to develop in late-foetal liver, increases rapidly at birth to a peak during suckling and then decreases at weaning to the adult value. 3. l-Serine dehydratase activity is very low prenatally, but increases rapidly after birth to a transient peak. After a second transient peak around the time weaning begins, activity gradually rises to the adult value. Both of these peaks have similar isoenzyme compositions. 4. In foetal liver both l-serine dehydratase and l-serine-pyruvate aminotransferase activities are increased after injection in utero of glucagon or dibutyryl cyclic AMP. Cycloheximide or actinomycin D inhibited the prenatal induction of both enzymes and actinomycin D blocked the natural increase of l-serine dehydratase immediately after birth. Glucose or insulin administration also blocked the perinatal increase of l-serine dehydratase. 5. After the first perinatal peak of l-serine dehydratase, activity is increased by cortisol and this is inhibited by actinomycin D. After the second postnatal peak, activity is increased by amino acids or cortisol and this is insensitive to actinomycin D inhibition. Glucose administration blocks the cortisol-stimulated increase in l-serine dehydratase and also partially lowers the second postnatal peak of activity. 6. The developmental patterns of the enzymes are discussed in relation to the pathways of gluconeogenesis from l-serine. The regulation of enzyme activity by hormonal and dietary factors is discussed with reference to the changes in stimuli that occur during neonatal development and to their possible mechanisms of action.

Animals↗