A marker for megakaryocytes: serotonin accumulation in guinea pig megakaryocytes

Serotonin accumulation was studied in guinea pig megakaryocytes and platelets. Megakaryocyte suspensions, 80% to 90% pure, and platelet-rich plasma were used. 14C-serotonin was used to assay serotonin uptake. Maximum capacity for serotonin storage in 2.5 x 10(5) megakaryocytes was 4.2 nmol and in 2.5 x 10(8) platelets was 3.1 nmol. One thousand times as many platelets as megakaryocytes were used to normalize for protein content of the two cells. Thus the megakaryocyte's capacity for accumulating serotonin is equivalent to, if not greater than, that of the platelet. Serotonin accumulation in individual megakaryocytes and other hematopoietic cells was studied by autoradiography. Whole marrow suspensions were incubated with 3H-serotonin. Cells in whole marrow suspensions, cells in the pellet of the albumin density gradient, and purified megakaryocytes were then analyzed by autoradiography. The pellet of the albumin density gradient was inspected because it is known to contain megakaryocytes that are not recovered in the purified megakaryocyte suspensions. Examination of whole marrow cell suspensions revealed that virtually all megakaryocytes had taken up serotonin, but there was no evidence of serotonin accumulation in other hematopoietic cells. Over 95% of megakaryocytes in the pellet of the albumin density gradient had retained serotonin. The megakaryocytes that had not retained serotonin or not accumulated the amine did not comprise a specific population in respect to maturity. Evaluation of purified megakaryocytes showed that megakaryocytes at various stages of maturation, as defined by diverse size and cytoplasmic/nuclear maturation, had accumulated equivalent amounts of serotonin. Thus the capacity for serotonin accumulation is established in the immature megakaryocyte. Serotonin uptake can serve as a marker for the identification of the immature megakaryocyte.