1013-111 Mechanisms Underlying the Morning Increase in Platelet Activation: Study of Whole Blood Platelet Flow Cytometry

Introduction Immediately after arising in the morning there is an increase in the incidence of acute myocardial infarction, stroke and sudden death. Upon assumption of the upright posture platelet aggregation, as measured by conventional techniques using platelet rich plasma, occurs more readily in response to standard aggregating agents. Whether platelet aggregability in the physiological milieu of whole blood also increases and whether this increase in platelet aggregability is also accompanied by activation-dependent platelet surface receptor changes is unknown. In addition, further clarification of the changes in hemostatic and rheological factors that can promote platelet aggregation and that may occur upon standing, are required. Methods and Results We therefore investigated the effect of arising in the morning on whole blood impedance aggregometry in 12 normal subjects and simultaneously measured platelet activation directly by using whole blood flow cytometry (n = 5). Aggregometry was completed and antibodies were added within 10 mins of blood withdrawal. Hemodynamic changes (n = 12), catecholamines (n = 6), fibrinogen (n = 6), platelet count and hematocrit (n = 8) were also measured. Platelet aggregation to ADP and collagen increased by 75% (p l 0.01) and 32% (p l 0.01) respectively. However, this was not associated with evidence of activation using flow cytometry. Six markers (GPlb, P-selectin, activated GPlb-llla, GPIV, fibrinogen and vWF) tested with 10 antibodies showed no change in either percent positivity or flourescence intensity. Platelet count and fibrinogen levels increased by 14% (p l 0.01) and 12% (p l 0.04) respectively which was partly due to hemoconcentration (hematocrit increased by 7%, p l 0.01). Heart rate and norepinephrine levels significantly increased by 20% (p l 0.02) and 163% (p l 0.01) respectively. Conclusions The observed increase in whole blood platelet aggregation on arousal and standing in the morning is not accompanied by activation-dependent platelet surface receptor changes. Thus changes in rheological and hemostatic factors, such as increases in platelet count and fibrinogen level, together with indirect effects of increases in catecholamine levels may play a more important role in promoting platelet aggregation upon standing.