Effect of the Concentration of Ca²⁺ in the Suspending Medium on the Responses of Human and Rabbit Platelets to Aggregating Agents

Summary

The effect of the concentration of Ca²⁺ in the suspending medium of human and rabbit platelets on aggregation, release of ¹⁴C-serotonin, and TXB₂ formation in response to ADP, thrombin, l-0-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine (PAF), collagen and arachidonic acid was studied in either platelet-rich plasma anticoagulated with D-phenylalanyl-prolyl-arginyl chloromethylketone (PPACK) or citrate, or suspensions of washed platelets in modified Tyrode-albumin solutions containing 1 mM Mg²⁺ and concentrations of added Ca²⁺ ranging from 0 to 5 mM. In response to ADP, thrombin, or PAF, human platelets were stimulated to form TXA₂ by close platelet contact in a low- Ca²⁺ medium; at physiological concentrations of Ca²⁺, TXB ₂formation was much less and declined progressively as the concentration of Ca²⁺ was raised. When the formation of TXA ₂was blocked with aspirin or indomethacin, aggregation and release by human platelets were strongest at physiological concentrations of Ca²⁺. Rabbit platelet responses differed markedly from those of human platelets because close contact of rabbit platelets in a low-Ca²⁺ medium did not promote TXA₂ formation. Rabbit platelet responses were more strongly inhibited by the lack of added Ca²⁺ in the medium than the responses of human platelets, possibly because rabbit platelets do not contain releasable Ca²⁺.

In all studies of human platelets in media with low concentrations of Ca²⁺, the additional contribution to platelet responses of TXA₂ formed because of close platelet contact should be considered because TXA₂ formation is not usually stimulated in this way at physiological concentrations of Ca²⁺. When TXA₂ formation is blocked, aggregation and release responses to all agonists are greatest at physiological concentrations of Ca²⁺. Thus, the responses of human platelets in media with low concentrations of Ca²⁺ (citrated platelet-rich plasma or artificial media to which no Ca²⁺ has been added) are abnormal in at least two ways, and do not correspond to the responses at physiological concentrations of Ca²⁺.

Keywords

Human platelets - Rabbit platelets - Calcium - Platelet aggregation

• References

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