Dexamethasoneの抗脳浮腫作用に関する実験的研究 ―凍結損傷脳における(3)H-dexamethasoneの経時的濃度変化と,組織内の分布についての検討―

脳神経外科的疾患に随伴する脳浮腫の予防および治療を目的として, glucocorticoid剤が,臨床上広く用いられている.しかし,現在のところ, glucocorticoid剤の脳浮腫抑制効果発現の作用機序については,いまだ明確な結論がえられず,ただ単に臨床経験的に使用されているにすぎないのが現状である.近年ようやくglucocorticoid剤の大量投与時に,血液中,髄液中の濃度を測定し,その使用方法,投与量についての再検討がなされつつある(1),2)).しかし,その抗脳浮腫作用の面に注目して,実際に,脳組織中濃度変化に関して検討を加えた報告は,未だ数少ない(3),4)).そこで,実験動物に,凍結損傷法による脳浮腫を作成し,臨床上よく用いられるglucocorticoid剤であるdexamethasoneの(3)H標識化合物をとりあげ,浮腫脳組織中におけるその経時的濃度変化と分布について検索を行ない,本剤の最適な投与方法,投与量を決定するとともに,その抗脳浮腫作用の作用機序の一端を明らかにせんと試みたので若干の文献的考察を加え報告する.

Dexamethasone has been used for prevention of cerebral edema, however its most suitable way of administration has not been fully clarified yet. Experimental cerebral edema was produced on the right parietal lobe of Wistar male rats with a cold metal probe cooled by liquid nitrogen. Twenty hour later, (3)H-dexamethasone was either intramuscularly or intravenously injected into rats in three different ways, classified as Group I.M.A, Group I.M.B and Group I.V., Then, (3)H-dexamethasone was estimated in the brain tissue by the liquid scintillation counting method. In Group I.M.A, dexamethasone 0.5mg/kg B.W., containing (3)H-dexamethasone 100uCi/mg of dexamethasone, was intramuscularly injected into 25 rats. Then, 5 rats were sacrified 1,2,3,4 and 6 hours later. Subsequently a portion of edematous brain was obtained from the right parietal lobe. Simultaneously the left parietal lobe was sampled as a control. In Group I.M.B, a dose of dexamethasone injected was 1.0mg/kg B.W.. Otherwise, the procedure was exactly the same as in Group I.M.A.. In Group I.V., dexamethasone 0.5mg/kg B.W. was intravenously, not intramuscularly, injected into 30 rats. Then 5 rats were sacrificed 5 minutes, 1,2,3,4 and 6 hours later. Edematous brain generally ocntained much higher (3)H-activity than the control. Futhermore, I.V. injection (Group I.V.) showed higher (3)H-activity than I.M. injection (both Group I.M.A and Group I.M.B) in both edematnus and control brains at all times. Therefore, intravenous administration is apparently better than I.M. administration for maintaining high levels of dexamethasone in brain tissue. For examination of the subcellular distribution of (3)H-dexamethasone in edematous brain, 20 rats were sacrificed 2 hours following intramuscular injection. Homogenates of the edematous portion and control portion were separately prepared. The subcellular fraction in each homogenate was separated by differential centrifugation and equillibrium fraction method. As a result, (3)H-dexamethasone was most strongly detected in the supernatant fraction (63% ), followed by the heavy mitchondrial fraction (25.4% ) and the nuclear fraction (8.4% ). Although edematous brain tissue constantly demonstrated higher (3)H-activity than the control, its supernatant fraction conversely had less activity. Therefore high concentrations of (3)H-dexamethasone in the edmatous brain is most likely due to its high accumulation in both the mitchondrial fraction and nuclear fraction rather than in the cytoplasm and intracellular space of the supernatant fraction. As a next step, distribuiton of (3)H-dexamethasone in the supernatant fraction was studied. Fourteen brian homogenates were similarly prepared from 7 rats and were incubated with 10 uCi of (3)H-dexamethasone for 2 hours at 2℃. Each homogenate was separately centrifuged for 60 minutes at 1.12×10(5)g at 2℃. Then, aliquots of each sample were gently layered on 4.6ml linear gradients of 10 to 30% sucrose solution and centrifuged for 16 hours at 3.0×10(5)g at 2℃. Following centrifugation, about 20 fractions were identified in each sample and the (3)H-activity was counted in each. The result was that the high molecular weight fraction in the edematous brain showed higher radioactivity than the control in 6 out of 7 rats. From these findings, unequivocal distribution of dexamethasone in the supernatant fraction of edematous brain tissue could be correlated with its biochemical action for preventing brain edema.