Low-dose whole-body exposure of mice to less than 0.01 Gy gamma-rays causes inhibition of incorporation of thymidine or 5-iodo-2-deoxyuridine into DNA of bone-marrow cells in vitro; the effect is maximal in cells at 4 hours after exposure and then subsides within about 10 hours. This is due to the inhibition of cellular thymidine kinase, which gradually develops to a maximum at 4 hours after exposure and again subsides within the next 10 hours. This inhibition involves only 35 per cent of the entire cellular enzyme activity and, analogous to the depression of thymidine incorporation into the cells, is only seen when the cells are collected into medium that is buffered to 7.2-7.4 and contains about 1350 mg NaHCO3 per litre. Addition of NaHCO3 to the cell homogenate or to the high speed supernatant containing the enzyme, but not to intact cells, failed to produce enzyme inhibition. There is also no depression of 5-iodo-2-deoxyuridine uptake into the intact cells in vitro when the mice are irradiated either shortly before or after i.v. injection of 0.02 mg of procain chloride. The reversibility of the effect in vivo and in vitro suggests a particular enzyme control mechanism that may be non-specifically triggered by intracellular charges, such as peroxides, and may enhance repair.
Die Elemente La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Cp, Hf, W, Re, Os, Ir, Pt, Au, Tl, Pb, Bi wurden mit Hilfe von Kernplatten auf schwache α-Aktivität hin untersucht. Dabei zeigte sich bei Nd, W, Pt und Bi ein positiver Effekt. Bei Nd konnte Nd
The reduction of the incorporation of IUdR in bone marrow cells depends on the time after irradiation and on te microenvironment of the cells. The strongest effect is observed 4 hours after irradiation. For absorbed doses above 40 rad, whole-body irradiated mice were more sensitive with respect to depression of IUdR incorporation in bone marrow cells, when the bone marrow cells were labelled in vivo, and less sensitive for in vitro labelling. The converse was observed for very small doses of 1 rad and below. Such small doses resulted in a significant depression of IUdR incorporation after in vivo irradiation and in vitro labelling and showed no effect at all after in vivo irradiation and in vivo labelling. The least effect of radiation was observed after both irradiation and labelling in vitro. Although the mechanisms are not fully understood, the biological results and microdosimetric considerations indicate that at the smallest doses the effect is due to functional changes of cellular organelles which control intracellular mechanisms. A working hypothesis is proposed for the reduction of IUdR incorporation at low doses as being due to functional changes of the cellular membranes.
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