Circadian variation of endothelial cell function, red blood cell deformability and dehydro-thromboxane B2 in healthy volunteers.

There is a circadian variation in the time of onset of thrombotic disorders. The thrombotic tendency of blood is influenced by many factors including complex interactions between endothelial cells, platelets and red blood cells. We studied the circadian variation of various indices of these cells' functions in 10 young healthy male volunteers. Six blood samples were collected at 4 h intervals from 12.00 until 08.00 the following morning. The volunteers carried out normal daily activities until 00.00 at which time they went to bed. They remained in bed until 08.00 the following morning. The following were measured on each sample: plasma factor VIII von Willebrand factor antigen (FVIII vWF Ag), tissue plasminogen activator activity (tPA), plasminogen activator inhibitor (PAI), prostacyclin stimulating factor (PGI2 SF), fibrinopeptide A (FPA), 11 dehydro-thromboxane B2 (11-dehydro-TXB2) and red cell deformability (RCD). The following parameters demonstrated significant circadian variation: tPA P less than 0.001, PAI P less than 0.04 and 11-dehydro-TXB2 P less than 0.005 (two-way analysis of variance). tPA was highest at 16.00-20.00 and lowest at 08.00; PAI was highest at 08.00 and lowest at 20.00; TXB2 had a peak at 16.00 and trough at 04.00. As the behaviour of endothelial cells and platelets influence the rheological properties of blood, circadian variations in their behaviour may contribute to the time of onset of thrombotic disorders.