A POSSIBLE MECHANISM OF INCREASE IN SERUM ALKALINE-PHOSPHATASE ACTIVITY IN RATS GIVEN GRANULOCYTE-COLONY-STIMULATING FACTOR

Recombinant human granulocyte colony-stimulating factor (G-CSF) at a d ose of 1 to 300 mu g/kg/day was administered intravenously to rats dai ly for 13 weeks. Serum alkaline phosphatase (ALP) activity increased d ose-dependently with leukocytosis. Most of the increased leukocytes we re segmented neutrophils, and neutrophil alkaline phosphatase (NAP) sc ores were elevated markedly. Serum ALP activity correlated very well w ith the segmented neutrophil counts, and the coefficient of correlatio n was more than 0.97 in both sexes. Pathological examinations revealed splenomegaly and a marked increase in neutrophils in the red pulp of the spleen. In the spleen, phagocytosis of neutrophils by macrophages was observed. These data indicate that the increased ALP was of neutro phil origin. Serum ALP activity may be increased by the direct release of ALP from the high number of neutrophils into the blood, or by the leakage of ALP into the blood mainly from the spleen where many neutro phils are pooled and destroyed by the macrophage system.