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Abnormalities in platelet calcium handling have been reported in spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto (WKY) rats. Furthermore, several reports have indicated that WKY rats differ from both SHR and Wistar rats. The objectives of the present study were to investigate platelet calcium handling in three normotensive stains and in SHR, and to confirm the abnormal calcium mobilization in SHR.We compared calcium handling in fura-2-loaded platelets of SHR, Wistar, Sprague-Dawley (SD) and WKY rats.The basal cytosolic free Ca2+concentration in platelets was significantly higher in SHR and significantly lower in SD rats than in the other strains. The intracellular Ca2 + response to thrombin in the presence of extracellular Ca2+ was greater in SHR than in the three normotensive strains. The thrombin-induced intracellular Ca2+ rise in the absence of extracellular Ca2+ was also greater in SHR and lower in SD rats than in the other strains. However, there was no significant difference among SHR, WKY and Wistar rats at higher doses of thrombin. The intracelluiar ionomycin-released calcium fraction, which may indicate the size of intracellular calcium stores, was similar in SHR, WKY and Wistar rats, and was greater than in SD rats. No difference was detected between WKY and Wistar rats in resting and peak agonist-evoked intracellular Ca2 + concentrations.These results show that calcium handling in WKY rats is similar to that in Wistar rats with respect to platelet calcium metabolism and confirm the abnormality in SHR. Furthermore, the enhanced intracellular Ca2+ response to thrombin in SHR was not dependent on the size of ionomycin-released Ca2+ stores. In addition, substantial differences in platelet calcium handling may occur even among normotensive strains if the strains are not related.