The internal calcium concentration of human platelets increases during chilling

Human platelets must be stored at 22 degrees C in blood banks, because of t he well-known phenomenon of cold-induced activation. When human platelets a re chilled below room temperature, they undergo shape change and vesicle se cretion that resembles physiological agonist-mediated activation. The trigg er for the cascade of events leading to platelet activation at hypothermic temperatures is not known, although an increase in the internal calcium con centration ([Ca](i)) due to passage of the platelet membranes through their thermotropic phase transition has been proposed. We report here that the f luorescent calcium-sensitive probe, Indo-1, has been used to estimate the i nternal calcium concentration of human platelets during a reduction in temp erature from 20 degrees C to 5 degrees C at a rate of 0.5 degrees C/min. An increase on the order of 100 nM was recorded. Almost all of the increase i n [Ca2+](i) occurs during the chilling process, as incubation of platelets for 1 h at low temperature did not lead to a continued calcium concentratio n increase. The increase in [Ca2+](i) during chilling is likely to be due t o more than a single mechanism, but might include some release of the calci um stores from the dense tubule system. Loading platelets with the calcium chelator BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) d ramatically reduced the increase in [Ca2+](i) seen during chilling. Antifre eze glycoproteins (AFGPs) isolated from the blood serum of Antarctic fishes , which are known to protect platelets from cold-induced activation, did no t eliminate the rise in [Ca2+](i) during chilling, suggesting that signalin g mechanisms are likely to be involved in cold-induced activation. (C) 1999 Elsevier Science B.V. All rights reserved.