The functional capacity of guinea pig megakaryocytes. I. Uptake of 3H-serotonin by megakaryocytes and their physiologic and morphologic response to stimuli for the platelet release reaction.

The functional capacity of guinea pig megakaryocytes was tested by studying their ability to concentrate serotonin and their response to agents which trigger the platelet release reaction. Megakaryocytes can concentrate 3H-serotonin as demonstrated by autoradiography after exposure to 0.5 muM 3H-serotonin and by quantitative measurement of isotope incorporation within 60 minutes. Uptake of isotope is rapid and linear within the first 30 minutes and tapers off between 30 and 60 minutes. Incorporation of isotope is diminished during exposure to cold, 2 muM reserpine, and 20 muM imiprimine. The following triggering agents: 10(-5) to 10(-3) M ADP, 1 to 100 units of thrombin, 10(-5) to 10(-3)M epinephrine, and 1 to 12 muM ionophore A23187 all produce significant release of stored 3H-serotonin. In the presence of ADP, albumin and serum completely inhibit the release of serotonin. Scanning microscopic studies show that coincident with serotonin release the triggering agents produce marked changes in cell shape. Transmission electron microscopy on these cells shows that there is the appearance of a prominent contraction zone, which is composed of microfilaments, and also variable diminution of cytoplasmic granules. The specifically induced serotonin release from megakaryocytes coupled with shape change and evidence of cell contraction produced by certain agents demonstrate one aspect of the functional similarly between megakaryocytes and platelets.