Correlation of membrane anisotropy with function in subpopulations of human blood platelets*

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Abstract
Summary. Human blood platelets were fractionated on a discontinuous Percoll gradient into high density (HD), intermediate (ID), and low density (LD) platelets. The subpopulations were characterized with regard to [14C]serotonin uptake and release, cAMP content, aggregation, and membrane anisotropy, which is inversely related to membrane fluidity. Membrane anisotropy, which was high in LD platelets, was found to decrease with increasing density (LD> ID > HD). LD platelets showed significant lower cAMP levels and [14C]serotonin uptake than the total platelet population (TPP) and ID and HD platelets. Upon ADP and serotonin stimulation the cAMP content was reduced in all platelet populations with the exception of HD platelets in which cAMP was unchanged. Upon thrombin stimulation the cAMP content was reduced only in TPP and LD platelet population and it was increased in HD platelet population. Thrombin activation changed the anisotropy only in LD platelets. Thrombin at a concentration of 0·001 U/ml reduced whereas 0·01 and 0·05 U thrombin/ml increased the membrane anisotropy significantly. As compared with TPP and the other subpopulations, LD platelets were most sensitive upon ADP and thrombin stimulated [14C]serotonin release as well as upon ADP, serotonin and thrombin induced aggregation. The findings suggest that the differing functional abilities of the platelet subpopulations are correlated to the various membrane anisotropies observed in these fractions.