Vasopressin-induced activation of human blood platelets: prominent role of Mg²⁺

Summary

Arginine-vasopressin (AVP) caused a marked shape change reaction and rise in [Ca²⁺]_i in human blood platelets only when the extracellular buffer contained Mg²⁺ or Ca²⁺. At physiological concentrations of the cations the potency of AVP was higher in the presence of Mg²⁺ than of Ca²⁺. The amplitude of the shape change reaction was also greater with Mg²⁺ than with Ca²⁺, although the [Ca²⁺]_i-rise was slightly more marked with extracellular Ca²⁺. The concentration of Mg²⁺ at which AVP showed half of its maximal effects was below the physiological plasma level of the cation, whereas the corresponding value for Ca²⁺ was higher. Addition of Ca²⁺ to the Mg²⁺ containing medium did not further enhance the action of AVP on platelet shape. In platelet-rich plasma the potency and efficacy of AVP in causing a shape change were similar in the presence and absence of EGTA, whereas with EDTA in the medium AVP had no effect. In conclusion, Mg²⁺ has an essential physiological role in AVP-induced platelet activation, which is brought about partly by release of intracellular calcium and partly by some other intracellular mechanism.