Temperature-dependence of LDL binding and activation of human platelets

Abstract

The effect of low density lipoprotein (LDL) on intracellular free calcium ion concentration ([Ca 2+] i), taken as an index of the degree of platelet activation, was investigated in normal volunteers. At 37°C LDL, in a dose of 20 μg of protein / ml, increased [Ca 2+] i in all subjects tested (basal 57±11 to 113±19 nM). In contrast, when measurements were performed at 20°C, no effect on [Ca 2+] i was seen following LDL. Thrombin (0.2 U / ml) increased [Ca 2+] i to 455±98 nM. When platelets had been exposed to LDL before thrombin stimulation, this increase was less pronounced (to 301±43 nM). Our finding of a temperature dependence of LDL induced increase in platelet [Ca 2+] i supports the concept of a platelet-LDL receptor mediated mechanism. Furthermore, the lower thrombin response following LDL exposure suggests a LDL-thrombin interaction, possibly at the thrombin receptor level and/or calcium recruitment from the same stores.