ABNORMALITIES OF CYTOPLASMIC [Ca++] IN PLATELETS FROM UREMIC PATIENTS STUDIED WITH AEQU0RIN AND INDO-1

Abstract

Uremic patients have a hemorrhagic tendency, often with prolonged bleeding times and abnormalities of platelet function in vitro. Whether these defects result from plasma factors, abnormalities in platelet surface receptors, or intracellular mediators is unknown. Accordingly, blood was obtained from 16 patients with severe uremia (BUN >90), and platelets were washed, loaded with aequorin or indo-1, gel-filtered, and resuspended in either plasma or buffer. Of the 16 patients, 4 had template bleeding times greater than 12 minutes, but platelet aggregation in plasma was not consistently impaired. However, the rise in cytoplasmic [Ca++] in response to the Ca++-ionophore A23187 or ADP in aequorin-loaded platelets from the 4 patients with long bleeding times was much lower than in uremic patients with normal bleejljLng times or in normal volunteers. The reduced [Ca++] response was associated with decreased aggregation of gel-filtered platelets in buffer. Prolonged bleeding time was less consistently correlated with decreased responses to epinephrine or arachidonate. Suspending washed aequorin-loaded uremic platelets in normal plasma for 10-20 min did not reverse the decreased agonist-induced rise in [Ca++]; platelets from a normal donor resuspended in uremic pla^iya responded normally. The agonist-induced rise in [Ca++] shown by indo-1 was not abnormal in patients with prolonged bleeding times; however, uremic patients generally had higher indo-l-indicated basal platelet cytoplasmic [Ca++] than normal. We conclude that the hemorrhagic tendency in some patients with uremia (|s associated with abnormal intracellular platelet [Ca++] regulation marked by elevated resting [Ca++] and a decreased rise in cytoplasmic [Ca++] in response to certain agonists; this latter abnormality appears to be correlated with prolonged bleeding times.