The dynamics of the endothelial cell population was investigated in the rat brain after local irradiation with different doses of X rays. A fluorescent-histochemical technique was used for the visualization of the cells. A decrease in endothelial cell number was observed within 1 day of irradiation with doses of 5-200 Gy. At this time the endothelial cell number had decreased by up to 15% compared with the pre-treatment values. This early dose-independent loss in cell number was maintained for up to 1 month after irradiation. This was then followed by a slow dose-independent decrease in cell density up to 6 months after exposure. Subsequently the depletion of the endothelial cell population exposed to 40 and 60 Gy continued. After a dose of 25 Gy an abortive recovery of cell numbers occurred followed by an abrupt depletion of the endothelial cell population. The possible mechanisms of such changes are discussed.

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The effect of melittin, an activator of phospholipase A 2 , on proliferation and death of rat thymocytes in a broad concentration range was studied. Cell proliferation was estimated by the accumulation of colchicin metaphases, necrotic death was determined from lysis and staining of cells with trypan blue, and apoptosis was assessed from the type of DNA fragmentation, the amount of fragmented DNA, and the percentage of cells with subdiploid DNA. It was shown that low melittin concentrations (below 5 ug/ml) stimulate thymocyte proliferation. At high melittin concentrations, thymocytes die by the primary necrosis type. Throughout the concentration range studied, melittin does not produce apoptosis in thymocytes. Conversely, high melittin concentrations even inhibit thymocyte apoptosis in the control and after irradiation. An inhibitor of RNA synthesis actinomycin D does not affect thymocyte death in the presence of melittin. It is concluded that the activation of phospholipase A 2 can induce necrosis but not apoptosis and thus is not a necessary step in the signaling cascade that initiates apoptosis in thymocytes.

The effect of inhibitors of arachidonic acid metabolism on proliferation and death of tumor P-388 cells in a broad concentration range was studied. Cell proliferation was estimated by the metaphase frequency and the proportion of cells in S phase; cell death was determined from lysis, staining of cells with trypan blue, nuclear damage, percentage of cells with subdiploid DNA and the type of DNA fragmentation. It was shown that low concentrations of phospholipase A P and lipoxygenase inhibitors stimulated the proliferation of P-388 cells. At higher concentrations, the inhibitors suppressed cell proliferation by blocking the G I -S transition and induced cell death of the apoptosis type. Indomethacin, an inhibitor of cyclooxygenase, did not initiate cell death, nor did it affect the proliferation of P-388 cells at concentrations of up to 10 W WM. z 1998 Federation of European Biochemical Societies.

The effects of avermectins on radiation-and dexamethasone-induced apoptosis of rat thymocytes have been studied. In particular, the properties of abamectin (avermectin B 1 ), ivermectin (22,23-dihydroavermectin B 1 , H 2 B 1 ) and aversectin C were examined. Aversectin C is a mixture of eight naturally occurring avermectins of the following composition: B 1 (42%), B 2 (22%), A 1 (13%) and A 2 (23%). This mixture is currently widely used as active ingredient in veterinary antiparasitic formulations produced by Pharmbiomed Co. Apoptosis was estimated by membrane damage, nuclear pycnosis (or chromatin condensation), DNA fragmentation and percentage of cells with subdiploid DNA. It was shown that aversectin C inhibits thymocyte apoptosis induced by both agents. The IC 50 dose was in the range of 0.1-0.3 µg/ml for aversectin C, whereas abamectin and ivermectin produced no effect up to 1 µg/ml. It is suggested that avermectins of the A series are responsible for the high anti-apoptotic activity exhibited by aversectin C. Thus, aversectin C is not only an effective antiparasitic drug but is also capable of exhibiting some other useful activities, e.g. a cytoprotective effect exerted on the mammalian immune cells. Avermectins, antiparasitic drugs, apoptosis, thymocytes, dexamethasone, gamma raysThe avermectins are a family of closely related 16-membered macrocyclic lactones produced by the fungus Streptomyces avermitilis. Nearly all the avermectins exhibit a broad spectrum of activity against nematode and arthropode parasites, with the B 1a compound being the most potent (Campbell 1989). Accordingly, it was used as active ingredient in commercial antiparasitic formulations. Until recently, there were basically two types of such avermectin-based active ingredients, i.e. ivermectin (consisting mainly of avermectin H 2 B 1a ) and abamectin (predominantly containing avermectin B 1a ). However, several years ago the Pharmbiomed Co. started manufacturing the veterinary and plant protection formulations based on a new active ingredient -aversectin C which is an intrinsic mixture of eight naturally occurring avermectins. Preliminary experiments demonstrated that aversectin C (presumably due to the presence of avermectins of both the A-and B-series) possesses some new useful features compared to ivermectin and abamectin, e.g. with the same high efficacy, the spectrum of activity is broader, level of residues is lower and, hence, the shorter withdrawal periods could be recommended (Simetsky et al. 1994;Berezkina et al. 1996). Furthermore, degradation of aversectin C in soil and water was found to be more effective compared to abamectin and ivermectin ( M o sin et al. 1998) and, hence, ecologically aversectin C is safer.

The action of taxifolin on the angiotensinconverting enzyme (ACE) and the formation of reactive oxygen and nitrogen species (ROS/RNS) in the aorta of aging rats and rats treated with nitric oxide synthase inhibitor (N ω -nitro-L-arginine methyl ester (L-NAME)) or dexamethasone have been studied. The ACE activity in aorta sections was determined by measuring the hydrolysis of hippuryl-L-histidyl-L-leucine, and the ROS/ RNS production was measured by oxidation of dichlorodihydrofluorescein. It was shown that taxifolin at a dose of 30-100 μg/kg/day decreases the ACE activity in the aorta of aging rats and of rats treated with L-NAME or dexamethasone to the level of the ACE activity in young control rats. Taxifolin (100 μg/kg/day) was found to also reduce the amount of ROS/RNS in the aorta that increased as a result of L-NAME intake. L-NAME treatment increases the contribution of 5-lipoxygenase and NADPH oxidase to ROS/RNS production in the aorta, while taxifolin (100 μg/kg/day) decreases the contribution of these enzymes to the normal level.

We studied changes in ROS content in the aorta of Wistar rats at early terms after irradiation in doses equal to single fraction used in tumor radiotherapy and the effects of taxifolin and fucoidin, blockers of leukocyte adhesion to endothelium, on ROS content. Male rats were exposed to X-rays (200 kW) in doses of 1-7.5 Gy. ROS production in aorta segments was measured in 1-48 h after irradiation by dichlorodihydrofluorescein oxidation. The content of ROS in the aorta of rats exposed to radiation in doses of 1-2.5 Gy increased in 1-24 h after irradiation, the peak ROS content was found in 2 h after irradiation. Taxifolin (100 μg/kg dihydroquercetin once a day with drinking water) and fucoidin (10 mg/kg, i.v.) abolished ROS accumulation. The content of ROS in rat aorta increased in 1-24 h after irradiation in doses used for tumor radiotherapy and this increase can be determined by leukocyte adhesion to the endothelium.