The internal calcium concentration of human platelets increases during chilling

Human platelets must be stored at 22°C in blood banks, because of the well-known phenomenon of cold-induced activation. When human platelets are chilled below room temperature, they undergo shape change and vesicle secretion that resembles physiological agonist-mediated activation. The trigger for the cascade of events leading to platelet activation at hypothermic temperatures is not known, although an increase in the internal calcium concentration ([Ca](i)) due to passage of the platelet membranes through their thermotropic phase transition has been proposed. We report here that the fluorescent calcium-sensitive probe, Indo-1, has been used to estimate the internal calcium concentration of human platelets during a reduction in temperature from 20°C to 5°C at a rate of 0.5°C/min. An increase on the order of 100 nM was recorded. Almost all of the increase in [Ca²⁺](i) occurs during the chilling process, as incubation of platelets for 1 h at low temperature did not lead to a continued calcium concentration increase. The increase in [Ca²⁺](i) during chilling is likely to be due to more than a single mechanism, but might include some release of the calcium 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) dramatically reduced the increase in [Ca²⁺](i) seen during chilling. Antifreeze glycoproteins (AFGPs) isolated from the blood serum of Antarctic fishes, which are known to protect platelets from cold-induced activation, did not eliminate the rise in [Ca²⁺](i) during chilling, suggesting that signaling mechanisms are likely to be involved in cold-induced activation. Copyright (C) 1999 Elsevier Science B.V.