Synaptosomes (isolated nerve-endings) have been prepared from the rat cerebral cortex according to Bradford et al. (1975) and used to study the effects of anoxia and hypoxia on cerebral metabolism. Experiments using a Rank oxygen-electrode have shown that synaptosomes behave in a manner reflecting the presence of the intra-synaptosomal mitochondria when placed in a mitochondrial incubation medium, e.g. (1) they respond to the addition of ADP by increasing O2 consumption when supported by exogenous respiratory substrates such as glutamate and succinate, but not to the extent expected of pure mitochondria, (2) hjrpotonic shock reduces O2 consumption and does not improve the ADP response, (3) ADP stimulation is blocked by oligomycin which can be overcome by DNP, and (4) the presence of a Na+, K+-ATPase in the synaptosomal limiting membrane is indicated by increased O2 consumption on addition of Na+ which is reversed by ouabain. Synaptosomes behave in a similar manner in a hypoxic medium ([O2] 4muM) as in anair-saturated Krebs phosphate medium, e.g. (1) a critical O2 concentration of 4muM was found where O2 consumption became dependent on the PO2 below this concentration, regardless of the initial PO2 , and (2) glucose and bovine serum albumin stimulate O2 consumption though by different, independent means. Any pre-incubation of synaptosomes should be carefully considered as should the composition of the medium since they will determine the rate of O2 uptake. There is no evidence that anoxia and hypoxia alters the accumulation and distribution of long-chain fatty acids and the increased respiration and glycolysis in a Ca2+-free medium indicates the dependency of cerebral metabolism on the Ca2+ concentration in hypoxia. It is concluded therefore that other oxygen-dependent processes such as monoamine synthesis, are responsible for the behavioural and functional alterations occurring at degrees of hypoxia too mild to affect ATP production.
|Date of Award||1979|