Background—Glucocorticoid-induced granulocytosis has been attributed to enhanced release of polymorphonuclear leukocytes (PMNs) from bone marrow, delayed apoptosis, and reduced egress of PMNs into tissues. This study was designed to determine the relative contributions of PMNs released from the bone marrow and those entering the circulation from the marginated pool to the granulocytosis produced by a single dose of dexamethasone (2.0 mg/kg) in rabbits.

Methods and Results—PMN transit through the mitotic and postmitotic pools of the bone marrow and rate of release of PMNs into the circulation were measured by use of the thymidine analogue 5′-bromo-2′-deoxyuridine (BrdU) to pulse-label PMNs in the bone marrow. The shift of PMNs from the marginated to the circulating pool was measured with BrdU-labeled PMNs transferred from donor rabbits to recipients before dexamethasone was delivered. The data show that dexamethasone increased bone marrow release of PMNs and shortened their transit time through the postmitotic pool (P<0.001) but not the mitotic pool of the bone marrow (P>0.05). Dexamethasone slowed the clearance of BrdU-labeled PMNs from the circulation (P<0.05) and lengthened their disappearance (half-life) from the circulation compared with control (half-life, 4.95 versus 9.45 hours). At 6 hours after dexamethasone, bone marrow release contributed ≈10%, mobilization from the marginated pool ≈61%, and a lengthened half-life in the circulation ≈29% to the glucocorticoid-induced granulocytosis.

Conclusions—We conclude that a single dose of dexamethasone causes a granulocytosis primarily by a shift of PMNs from the marginated to the circulating pool, with a minor contribution from marrow release.