Synaptosomes from rat brain: morphology, compartmentation, and transmembrane pH and electrical gradients.
Morphological studies of synaptosomes isolated from rat brains show that approximately 68% of the synaptosomes in these preparations contain synaptic vesicles (range, 62--72.5%). Approximately 30% of the synaptosomes contain mitochondria, and only less than 20% of the total mitochondria in good preparations are free and not enclosed in synaptic structures. The mitochondrial volume percent calculated on the basis of the measured cytochrome c content is 5% for synaptosomes isolated from anesthetized animals and 11% for synaptosomes isolated from unanesthetized animals. These numbers bracket the value of 8.7% obtained from electron micrographs. The volume percent of intrasynaptic vesicles is 1.5% as calculated from electron micrographs. The pH gradient between the extracellular pH and the mean intracellular pH is --0.45, as measured by equilibrium distributions of methylamine and dimethylamine, and --0.05, as determined by equilibrium distributions of 5,5-dimethyloxazolidine-2,4-dione and trimethylacetic acid. Analysis of these data shows that there cannot be a large pH gradient (alkaline inside) across the mitochondria, nor can the synaptic vesicle compartment be very large (less than 1.85%). Equilibrium distribution of [3H]triphenylmethylphosphonium ion in synaptosomal preparations gives a calculated apparent potential of --85 mV, in agreement with our previous value. Analysis of these data using the measured volumes of mitochondrial and intrasynaptic vesicular compartments (8.7 and 1.5%, respectively) gives a maximum possible transmitochondrial membrane potential of --59 mV.
D000818:Animals; D001921:Brain / Q000166:cytology*; D002413:Cations, Divalent / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008297:Male; D008564:Membrane Potentials; D008854:Microscopy, Electron; D008928:Mitochondria / Q000648:ultrastructure; D009861:Onium Compounds / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D051381:Rats; D013572:Synaptic Vesicles / Q000648:ultrastructure; D013574:Synaptosomes / Q000648:ultrastructure*; D014320:Triphenylmethyl Compounds / Q000378:metabolism
D002413:Cations, Divalent; D009861:Onium Compounds; D014320:Triphenylmethyl Compounds; C011561:triphenylmethylphosphonium; D011188:Potassium