The study was undertaken to investigate the possible correlation of total volume of mitochondria per cell with the rate of succinate oxidation in isolated nerve cell bodies, after various functional stresses in the experimental animals. Significant cytological effects were found in the nerve cells of the supraoptic nucleus in rats which had been thirsting for 4–12 days or had been given 2% sodium chloride solution as a substitute for drinking water for a few weeks. Quantitation of mitochondria was done from electron micrographs. The cell volumes were calculated from sections of Epon-embedded tissue under phase-contrast microscopy. Succinate oxidation was measured on groups of 10 nerve cells with the microdiver technique. As a result of either thirst or sodium chloride load, the volume of mitochondria per nerve cell more than doubled. The rate of succinate oxidation was not changed after the rats had been thirsting but was enhanced by over 100% after they had drunk sodium chloride. A linear relationship was found for the amount of mitochondria versus respiration in the supraoptic neurons for all experimental groups except the thirsting animals. The mitochondria in the supraoptic neurons from thirsting animals were of the same size or smaller than those in controls, whereas in animals given sodium chloride solution the mitochondria were considerably enlarged. The observed effects were specific for the supraoptic nucleus.

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