Ca 2+ in the Dense Tubules

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Abstract
—In this work, we explored the relationship between the freely exchangeable Ca 2+ (FECa 2+ ) in the dense tubules (DT) and the sarco(endo)plasmic reticulum (SER) Ca 2+ -ATPase (SERCA) in circulating human platelets and examined the relationship between blood pressure (BP) and these platelet parameters. Studying platelets from 32 healthy men, we showed that the maximal reaction velocity (V max ) of the SERCA significantly correlated with FECa 2+ in the DT and with the protein expressions of SERCA 2 and 3. BP positively correlated with both the V max of the SERCA ( r =.462, P =.010) and the FECa 2+ sequestered in the DT ( r =.492, P =.005). The relationships between these platelet Ca 2+ parameters and BP were in part confounded by increased levels of serum triglycerides and diminished HDL cholesterol with a higher BP. No correlation was observed between the resting cytosolic Ca 2+ and BP. Collectively, these findings indicate that (1) an increase in the cellular Ca 2+ load in platelets is expressed by a higher activity of the SERCA and an increase in the expressions of SERCA 2 and 3 proteins, coupled with an increase in the FECa 2+ in the DT, and (2) a higher BP is associated with an increase in platelet Ca 2+ load in human beings, expressed by a rise in the FECa 2+ in the DT and the upregulation of SERCA activity.