Marked temperature dependence of the platelet calcium signal induced by human von Willebrand factor

Interaction of von Willebrand factor (VWF) with the platelet is essential t o hemostasis when vascular injury occurs. This interaction elevates the int racellular free calcium concentration ([Ca2+](i)) and promotes platelet act ivation. The present study investigated the temperature dependence of vWF-i nduced [Ca2+](i) signaling in human platelets. The influence of temperature can provide invaluable insight into the underlying mechanism. Platelet [Ca 2+](i) was monitored with Fura-PE3. Ristocetin-mediated binding of vWF indu ced a transient platelet [Ca2+](i) increase at 37 degrees C, but no respons e at lower temperatures (20 degrees C to 25 degrees C). This temperature de pendence could not be attributed to a reduction in vWF binding, as ristocet in-mediated platelet aggregation and agglutination were essentially unaffec ted by temperature. Most other platelet agonists (U-46619, alpha-thrombin, and adenosine 5'-diphosphate [ADP]) induced a [Ca2+](i) signal whose amplit ude did not diminish at lower temperatures. The [Ca2+](i) signal in respons e to arachidonic acid, however, showed similar temperature dependence to th at seen with VWF. Assessment of thromboxane A(2) production showed a strong temperature dependence for metabolism of arachidonic acid by the cyclo-oxy genase pathway, vWF induced thromboxane A(2) production in the platelet. As pirin treatment abolished the vWf-induced [Ca2+](i) signal. These observati ons suggest that release of arachidonic acid and its conversion to thrombox ane A(2) play a central role in vWf-mediated [Ca2+](i) signaling in the pla telet at physiological temperatures. (C) 1999 by The American Society of He matology.