BLOOD-PLATELET HETEROGENEITY - EVIDENCE FOR 2 CLASSES OF PLATELETS INMAN AND RAT

The platelet population of man and rat can be divided into two classes of about equal size on the basis of presence/absence of an acid phosp hatase which acts on paranitrophenylphosphate (a PNPase), at pH 5. The cytochemical reaction product is in the platelet cytoplasmic matrix, without apparent association with organelles or membrane systems. We c ould not relate differences in staining to differences in function: al l cells responded the same to activation by thrombin, ADP. or collagen . in fibrinogen binding to activated platelets, by endocytosis of flui d-phase tracers, and in internalization of latex particles. With respe ct to possible physiological substrates for the PNP-ase, there was no reaction product from beta-glycerophosphate, AMP, ADP, ATP, GTP, CMP, IMP, cAMP, creatine phosphate, and inositol phosphates, and the enzyme was not inhibited by 40 mm lithium. There was reaction product from t yrosine phosphate suggesting that the physiological substrate for PNP- ase is tyrosine phosphate. In rat bone marrow, megakaryocytes also wer e of two classes, PNPase positive and PNPase negative, suggesting that different classes of platelets arise from different classes of megaka ryocytes.