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The frequencies of onset of myocardial infarction and sudden cardiac death are increased between 6 AM and 12 noon. Platelet aggregability, which may play a role in the cause of these disorders, has been observed to increase after the normal morning activities of awakening, arising, and ambulating. To determine which morning activity or activities are responsible for this aggregability increase, we measured platelet aggregation in 16 normal subjects on a control day of delayed arising (i.e., subjects remained supine until 12:30 PM) and on a day in which normal morning activities were divided into three isolated components of awakening (8 AM), assumption of upright posture (9:30 AM), and ambulating (11 AM). Blood samples to assess platelet aggregability were drawn at 8 AM before activity and 90 minutes after the initiation of each activity (i.e., at 9:30 AM, 11 AM, and 12:30 PM). For the group, in vitro platelet responsiveness to adenosine diphosphate and epinephrine increased only after assumption of the upright posture. The lowest concentration of agonist required to produce biphasic platelet aggregation decreased (aggregability increased) between 9:30 and 11 AM (90 minutes after assumption of the upright posture) from 3.3 0.3 to 2.4 + 0.2 ,uM for adenosine diphosphate (p<0.05) and from 2.1 0.5 to 1.0 0.4 ,uM for epinephrine (p<0.05). During the same interval, plasma epinephrine increased from 34+±-7 to 55+±9 pg/ml (p<0.05), and plasma norepinephrine increased from 169±19 to 298 ±+25 pg/ml (p<0.01). There was no significant change in aggregability or catecholamine concentrations on the control day. Thus, the simple assumption of upright posture is capable of causing the morning increase of platelet aggregability. Recognition of the effect of the assumption of upright posture on platelet aggregability will assist in identification of the biochemical cause of the platelet aggregability increase in the morning and may facilitate design of measures to eliminate potentially harmful surges in platelet aggregability. (Circulation 1988;78:35-40) Platelet aggregability increases in the morning, as do the frequencies of onset of myocardial infarction and sudden cardiac death. drinking one cup of coffee, and walking up and down three flights of stairs) was associated with a significant increase of in vitro platelet responsiveness to epinephrine or adenosine diphosphate (ADP). The study also showed that there was no morning increase in platelet responsiveness if the subjects remained inactive. However, the specific morning activity (or activities) responsible for the increase in platelet aggregability was not identified. The purpose of the present study, therefore, was to determine which of the morning activities was associated with the platelet aggregability increase. Subjects and MethodsThe study was conducted in 16 healthy male volunteers aged 20-35 years who were nonsmokers, had taken no aspirin or other medication in the previous 2 weeks, and had given informed consent.

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