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Elevated platelet‐free calcium in uraemia

Bleeding complications in uraemia are not uncommon. The pathogenesis of haemorrhage in uraemia is still a matter of controversy and the pattern of bleeding suggests a defect of primary haemostasis. Platelet aggregation and biochemistry, including calcium levels, have been studied; however, the results are controversial. We have examined platelet aggregation, platelet‐free calcium and calmodulin in platelet‐rich plasma because of the significant role of calcium and calmodulin in regulating platelet and other cells’functions. Platelet aggregation in uraemic subjects was similar to that of controls. Platelet basal free cytosolic calcium and platelet calcium in response to 10 μM Ca + + ionophore A23187 in eight subjects with uraemia were 117 ± 33 nM and 2025 ± 398 nM (mean ± SEM) respectively. By contrast in seven matched healthy controls basal calcium and ionophore‐stimulated calcium values were 47 ± 14 nM and 1354 ± 414 nM, significantly less than in the patients with uraemia (P < 0.05). The sensitivity of uraemic platelets to A23187 was similar to that of controls. Calmodulin activity in platelet‐rich plasma of 12 subjects with uraemia showed no significant difference from that of controls [1.86 ± 0.29 μg/ml (mean ± SEM) and 2.0 ± 0.37 μg/ml (mean ± SEM) respectively] We conclude that despite elevation of platelet calcium in uraemia, which may be due to a plasma factor such as parathyroid hormone, platelet aggregation is normal and bleeding in uraemia is more likely to be due to other factors, including the effect of reduced haematocrit on platelet endothelial interaction. Disturbances in platelet calcium cannot explain the bleeding manifestations in uraemia but warrant further investigation in order to identify the pathogenic mechanisms responsible.