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R L Davidson

Publications and source records attributed to R L Davidson.

At least 91 records · Page 5Linked to original sources
PubMed1975

Requirement of bromodeoxyuridine for the maintenance of "transformed" characteristics in bromodeoxyuridine dependent cells.

A bromodeoxyuridine(BUdr) dependent cell line, called B4, Which requires BUdr not only for optimal growth but also for the maintenance of the non-contact inhibited state was described previously. We have now shown that contact inhibition in the B4 cells in the absence of BUdr is associated with a marked decrease in the percent of cells synthesizing DNA. The transition to the contact inhibited state in the absence of BUdr does not seem to be due to changes in cyclic AMP levels. It has also been shown that several but not all of the characteristics which distinguish transformed from untransformed cells also distinguish B4 cells grown in the presence of BUdr from B4 cells grown in the absence of BUdr. In addition to being contact inhibited, B4 cells grown in the absence of BUdr have a higher serum requirement, grow less well in soft agar, and are less agglutinable by wheat germ agglutinin than B4 cells grown in the presence of BUdr. Agglutinability by concanavalin A, however, is the same for B4 cells grown in the presence and absence of BUdr. Dependent cells maintained in the presence of BUdr do not form tumors and it is not yet clear how the transformed characteristics of the dependent cells are related to malignancy.

Agglutination Tests↗
PubMed1975

Control of the expression of a herpes simplex virus thymidine kinase gene incorporated into thymidine kinase-deficient mouse cells.

When thymidine kinase-deficient mouse cells "transformed" by in activated herpes simplex virus and expressing the viral thymidine kinase (TK) are grown in nonselective medium, there is an exponential decay in the proportion of cells that continue to express the viral enzyme. However, the viral TK can be reactivated at a frequency of approximately 1 cell in 10(6) in every population that has lost TK activity. When cells in which the viral TK has been reactivated are grown in nonselective medium, a decay in the expression of the viral enzyme occurs again at the same rate as in the initial transformed population. Studies on the reactivation of viral TK indicate that reappearance of the enzyme is not induced by the selective medium (HAT) used to detect cells in which the enzyme has reappeared. Furthermore, treatments known to induce latent viruses in other systems--eg, exposure of the cells to mutagens or cell fusion--do not affect the frequency with which viral TK is reactivated.

Animals↗
PubMed1975

Replication of human chromosomes in human-mouse hybrids: evidence that the timing of DNA synthesis is determined independently in each human chromosome.

The terminal phase of DNA replication was studied by autoradiography in hybrids between human lymphocytes and mouse fibroblasts. The hybrids contained on the average only 11 human chromosomes. It was found that the sequence of terminal DNA replication for the human chromosomes in the hybrids was the same as the sequence of terminal replication for the corresponding chromosomes in the human lymphocytes. Furthermore, it was shown that the maintenance of the normal terminal replication sequence of the human chromosomes in the hybrids was not dependent on the presence of any specific human chromosome. The results suggest that the timing of terminal DNA replication is determined independently in each human chromosome.

Animals↗
PubMed1974

Centric fusion, satellite DNA, and DNA polarity in mouse chromosomes.

A fluorescent staining technique has demonstrated a contralateral arrangement of fluorescent spots in the centromeric region of mouse metacentric chromosomes which have resulted from centric fusion. The results suggest that centric fusion involves the maintenance of DNA polaritv through the centromere and that the thymidine-rich chain of satellite DNA in the centromeric region is associated with the same DNA chain in every mouse autosome.

Animals↗
PubMed1974

Total substitution of bromodeoxyuridine for thymidine in the DNA of a bromodeoxyuridine-dependent cell line.

In an attempt to isolate cells that could survive with total replacement of thymidine by bromodeoxyuridine in nuclear DNA, cells of a bromodeoxyuridine-dependent Syrian hamster line were cultured in medium containing aminopterin and bromodeoxyuridine but no thymidine. A line of cells, called HAB, was isolated. The HAB cells have been maintained in continuous cultivation for over nine months and have undergone more than 125 population doublings. Direct base analysis showed that the level of substitution of bromodeoxyuridine for thymidine in nuclear DNA was at least 99.8%, and possibly 100%. The existence of such cells raises many questions. The expected high frequency of bromodeoxyuridine-induced base transitions, including errors in both replication and transcription, would seem to be incompatible with the apparently stable transmission and expression of the genetic information in these cells.

Aminopterin↗
PubMed1974

Reversible "transformation" of bromodeoxyuridine-dependent cells by bromodeoxyuridine.

The growth characteristics of a bromodeoxyuridine-dependent cell line that was derived from a non-contact-inhibited Syrian hamster melanoma line were studied. The dependent cells require high concentrations of bromodeoxyuridine not only for optimal growth but also for the maintenance of the non-contact-inhibited state. When grown in the absence of bromodeoxyuridine, the dependent cells become contact-inhibited. The transition to the contact-inhibited state is reversed when bromodeoxyuridine is added back to the medium, if the bromodeoxyuridine is incorporated into dividing cells. The effects of bromodeoxyuridine on growth rate and on contact inhibition are separable.

Animals↗
PubMed1973

Bromodeoxyuridine dependence--a new mutation in mammalian cells.

When cells of a Syrian hamster melanoma were grown in increasing concentrations of bromodeoxyuridine (BrdU), lines of mutant cells able to grow well at high concentrations of BrdU were isolated. The mutant cells were characterized by BrdU dependence. In the absence of BrdU, the cells grew very poorly. The requirement for BrdU was specific for both the bromine and the deoxy sugar. The mutant cells incorporated BrdU into the DNA, replacing about 50% of the thymine residues with bromouracil. The reason for the BrdU dependence is not known.

Aminopterin↗
PubMed1973

Herpes simplex virus as a source of thymidine kinase for thymidine kinase-deficient mouse cells: suppression and reactivation of the viral enzyme.

Thymidine kinase (EC 2.7.1.21)-deficient mouse cells were infected with inactivated herpes simplex virus, after which "transformed" cells that produce viral thymidine kinase were isolated. Shortly after transformation, the expression of the viral enzyme could be suppressed and reactivated with high efficiency. On continued multiplication in nonselective medium, the proportion of cells producing the viral enzyme decayed exponentially. This decay seemed to represent a change in the expression of the viral gene for thymidine kinase rather than the loss of the gene from the cells, since the viral enzyme could be apparently reactivated in every cell, albeit at a very low frequency.

Aminopterin↗
PubMed1973

The interrelationships and stability of Coulter S-determined blood indices.

The blood of 104 medical inpatients was examined at various intervals during storage for 72 hours using a Coulter S counter. Over this period remarkable stability of the white cell count, red cell count, haemoglobin, and mean cell haemoglobin was demonstrated. This permits a useful interpretation of indices obtained in routine postal samples sent to the laboratory by general practitioners for screening. Furthermore, the implications of high correlations between red cell indices in freshly examined blood are considered and the regression line MCV = 2.5 MCH + 16 is derived. Using this relationship the normal range of the Coulter S MCHC is defined as 32.6-33.7 g/dl. The significance of the Coulter MCHC in present-day practice is reassessed and the importance of recognizing values for MCV and MCH not coinciding with the regression line is briefly discussed.

Blood Cell Count↗
PubMed1972

Regulation of malanin synthesis in mammalian cells: effect of gene dosage on the expression of differentiation.

Near-tetraploid (2S), pigmented, Syrian hamster cells (3460(2)) were hybridized with unpigmented mouse fibroblasts to produce hybrids that contained two pigment cell genomes and one fibroblast genome per cell (3(2)D). Half of the hybrids were pigmented and showed high activity of the enzyme dihydroxyphenylalanine oxidase; the other half of the hybrids were unpigmented and devoid of the activity of this enzyme. (In contrast, all the previously isolated hybrids containing only one pigment cell genome and one fibroblast genome were unpigmented.) Analyses of the chromosome complement and of a series of enzymes unrelated to melanin synthesis did not reveal any great differences between the pigmented and unpigmented 3(2)D hybrids. Upon the isolation of subclones, unpigmented 3(2)D hybrids produced only unpigmented colonies, whereas pigmented hybrids produced both pigmented and unpigmented colonies. Although the mechanism that results in the formation of some pigmented and some unpigmented hybrids in these experiments is not known, it is clear that the number of pigment cell genomes in a hybrid affects the expression of pigmentation.

Animals↗
PubMed1970

Regulation of specific functions of glial cells in somatic hybrids. II. Control of inducibility of glycerol-3-phosphate dehydrogenase.

Glycerol-3-phosphate dehydrogenase (EC 1.1.1.8) is induced when glial cells are exposed to hydrocortisone in vitro. In contrast, the enzyme activity in fibroblasts is not affected by the steroid. In an attempt to elucidate the mechanisms controlling inducibility, hybrids between glial cells and fibroblasts were studied. It was found that the activity of the enzyme does not increase when the hybrids are exposed to hydrocortisone. It was also shown that inducibility and the noninduced activity of enzyme are controlled independently. Comparisons of S-100 and glycerol phosphate dehydrogenase activity in the hybrids suggest that all the specialized functions characteristics of glial cells are not coordinately controlled.

Animals↗