Factors which modify lymphoid distribution of tissues were found to modify the adenosine triphosphatase activity of these tissues. Starvation or cortisone injection, which produces destructive changes in lymphoid tissues, was found to increase the enzyme activity of spleen and thymus tissues. The greater increment of enzyme activity of the thymus as compared to that of the spleen was correlated with its normally higher content of lymphoid tissue.

The increase in adenosine triphosphatase activity of hematopoietic tissues appears to be associated with the type of cells present in the assay medium. With respect to peripheral blood leukocytes of the rat, the cell type is confined largely to lymphocytes and granulocytes. The increase in adenosine triphosphatase activity of the leukocytes after total-body x-ray was seen to parallel the increase in granulocytes present in the assay medium. The ratio of granulocytes to lymphocytes is not appreciably altered in dog peripheral blood after exposure to total-body x-ray; the adenosine triphosphatase activity similarly was not significantly altered. After total-body x-ray (390 r and 780 r), cells isolated from the rat bone marrow displayed a fivefold increase in adenosine triphosphatase activity. This increase was seen to correspond with an increase in the ratio of segmented leukocytes and reticuloendothelial cells and a decrease in the immature forms of the erythroid and myeloid cells.

The heterogeneous cell mixtures used for our assay procedures permit the observation that total-body x-irradiation results in an increased enzyme activity of the isolated cells of the peripheral blood, bone marrow and spleen tissue of the rat. The increased enzyme activity was associated with the increased ratio of cells with high enzyme activity present in the assay medium.

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