[The nonmonotonicity changes of ornithine decarboxylase activity in spleen following exposure to gamma-irradiation on rats].

Exposure of rats to ionizing radiation caused in spleen nonmonotonous (as a function of time and of dose) changes in the activity of ornithine decarboxylase [ODC EC 4.1.1.17], an initial and rate limiting enzyme in polyamine synthesis. At physiological concentration polyamines (putrescine, spermidine, spermine) act as regulators of growth, proliferation and differentiation. ODC activity as well as spleen mass were studied in 2 h, 1, 7, 30 and 90 days after the exposure of rats to single-dose whole-body gamma-irradiation at the doses of 0.1, 1, 2, 6 Gy as well as 1, 7, 30, 90 day following fractionated one at a dose of 6 Gy (2 Gy three times in a week). Fractionating irradiation dose (2 Gy x 3) results in activation of ODC activity on day 30 after irradiation in contrast to the sharp decrease in the level of ODC activity on the same term after the single-dose exposure to gamma-irradiation at a dose of 6 Gy. The radiation activation of ODC precedes the renewal of organ mass. A possibility to apply a concept about nonmonotonic responses of mammalian cells and tissues to the action of ionizing radiation for analyzing dose-response processes of radiation damage and postradiation reparation of organism is discussed.

[The effect of X-rays and artificial hypotermia on lipids in rat neocortex].

Lipid content of tissue and of fraction of microsomes in neocortex of Wistar rats was studies under artificial hypothermia, after X-ray irradiation in dose 8 Gy under conditions of normothermia and artificial hypothermia in 48 h. The condition of artificial hypothermia get by cooling of rats to 15-18 degrees C. It was shown, that in fraction of microsomes of hypothermia rats the content of phosphatidylinositol was decreased, and in 48 h after cooling of rats the amount of protein, total and individual phospholipids was increased. The lipid content in tissue and in fraction of microsomes of rats, which were irradiated in normotermia, had no changes after 48 h. In fraction of microsomes of rats, which were irradiated after hypothermia, the amount of protein, total phospholipids, sphingomyelin, phosphatidylcholine and phosphatidylserine is increased trustworthy. Thus, we think, that radioprotective effect of hypotermia may be connected with the accumulation of proteins and of phospholipids in the endoplasmic reticulum membranes of neocortex.

[The effect of hypothermia on the rat radioresistance].

The cooling of Wistar rats up to 15-19 degrees C under a condition hypoxia-hypercapnia increased the radioresistance with a dose reduction factor (DRF) of 1.4. To elucidate the mechanisms of hypothermia radioprotective effect was evaluated the functional state of rat neocortex using a electroencephalogram (EEG) as well as was studied the lipid composition of neocortex under the conditions of both normothermia and hypothermia. At 19-20 degrees C the activity within a wide range of frequencies in EEG was suppressed; the nonregular slow waves were recorded against a background of "silence". The reduction of EEG spectrum with increasing temperature began with the low frequencies. At 26-28 egresC the contribution of theta-rhythm (an indicator of brain activity level) in EEG reaches the normothermia value, from this point the rat brain starts to functionate as a whole system. At normothermia the similarity of neocortex lipid composition in nonhibernators (rats) and hibernators (ground squirrels) mammalians was noted. The difference is only in a higher content of phosphatidylinositol in rats. Rats falling into hypothermia state as well as ground squirrels into torpor is followed by a decrease of cholesterol content and the absence of significant changes of the phospholipid composition in neocortex tissues.

Role of lipids in the assembly of endoplasmic reticulum and dictyosomes in neuronal cells from the cerebral cortex of Yakutian ground squirrel (Citellus undulatus) during hibernation.

Phospholipids and cholesterol were assayed in homogenates and microsomal fractions from the cerebral cortex of summer-active, winter-torpid, and winter-active Yakutian ground squirrels (Citellus undulatus). Ultrastructural analysis of both microsomal fraction and intact neurons was performed by serial ultramicrotomy. The levels of sphingomyelin (SM), phosphatidylserine (PS), and phosphatidylethanolamine (PEA) were decreased in homogenates from the cerebral cortex of winter ground squirrels compared with the summer-active animals, while the levels of phosphatidylcholine (PC) and cardiolipin (CL) were increased. The level of cholesterol was decreased in the cerebral cortex of winter-torpid animals compared with both winter-active and summer-active animals, and the level of total phospholipids was decreased in comparison to the summer-active animals. Three-dimensional reconstruction of serial membrane profiles displayed the microsomal fraction to be an interconnected system of cisterns and vesicles, which corresponds to endoplasmic reticulum and dictyosomes (Golgi stacks) of intact neurons. In winter the content of PC was increased in the microsomal fraction, while the contents of lysophosphatidylcholine (LPC), PS, phosphatidylinositol (PI), and SM were decreased. In winter-torpid animals compared with the winter-active ones the contents of total phospholipids, PEA, LPC, and cholesterol were decreased. As for the winter-active ground squirrels, their lipid contents did not differ from those in the summer-active animals, but LPC content was decreased. The changes in microsomal lipid contents in intact pyramidal neurons throughout the hibernation were accompanied by disassembly of dictyosomes and endoplasmic reticulum (ER), including the decomposition of polyribosomes to monosomes. The ultrastructural analysis of nucleoli, ER, and dictyosomes of both winter-active and torpid ground squirrels showed a direct correlation between the increasing contents of both cholesterol and total phospholipids (mainly PEA and LPC) in microsomes and the structural recovery of endoplasmic reticulum, Golgi stacks, and nucleoli in intact pyramidal neurons. A role of seasonal variations in lipid contents of brain cells in their adaptation to low temperature is discussed. We also propose an involvement of cholesterol in the activation of protein-synthesizing function of endoplasmic reticulum and Golgi stacks in intact neurons.