Altered platelet calsequestrin abundance, Na+/Ca2 + exchange and Ca2 + signaling responses with the progression of diabetes mellitus

IntroductionDownregulation of calsequestrin (CSQ), a major Ca2 + storage protein, may contribute significantly to the hyperactivity of internal Ca2 + ([Ca2 +]i) in diabetic platelets. Here, we investigated changes in CSQ-1 abundance, Ca2 + signaling and aggregation responses to stimulation with the progression of diabetes, especially the mechanism(s) underlying the exaggerated Ca2 + influx in diabetic platelets.Materials and methodsType 1 diabetes was induced by streptozotocin in rats. Platelet [Ca2 +]i and aggregation responses upon ADP stimulation were assessed by fluorescence spectrophotometry and aggregometry, respectively. CSQ-1 expression was evaluated using western blotting.ResultsDuring the 12-week course of diabetes, the abundance of CSQ-1, basal [Ca2 +]i and ADP-induced Ca2 + release were progressively altered in diabetic platelets, while the elevated Ca2 + influx and platelet aggregation were not correlated with diabetes development. 2-Aminoethoxydiphenyl borate, the store-operated Ca2 + channel blocker, almost completely abolished ADP-induced Ca2 + influx in normal and diabetic platelets, whereas nifedipine, an inhibitor of the nicotinic acid adenine dinucleotide phosphate receptor, showed no effect. Additionally, inhibition of Na+/Ca2 + exchange induced much slower Ca2 + extrusion and more Ca2 + influx in normal platelets than in diabetic platelets. Furthermore, under the condition of Ca2 +-ATPase inhibition, ionomycin caused greater Ca2 + mobilization and Ca2 + influx in diabetic platelets than in normal platelets.ConclusionsThese data demonstrate that platelet hyperactivity in diabetes is caused by several integrated factors. Besides the downregulation of CSQ-1 that mainly disrupts basal Ca2 + homeostasis, insufficient Na+/Ca2 + exchange also contributes, at least in part, to the hyperactive Ca2 + response to stimulation in diabetic platelets.