Human circadian system causes a morning peak in prothrombotic plasminogen activator inhibitor-1 (PAI-1) independent of the sleep/wake cycle

Abstract

Serious adverse cardiovascular events peak in the morning, possibly related to increased thrombosis in critical vessels. Plasminogen activator inhibitor-1 (PAI-1), which inhibits fibrinolysis, is a key circulating prothrombotic factor that rises in the morning in humans. We tested whether this morning peak in PAI-1 is caused by the internal circadian system or by behaviors that typically occur in the morning, such as altered posture and physical activity. Twelve healthy adults underwent a 2-week protocol that enabled the distinction of endogenous circadian effects from behavioral and environmental effects. The results demonstrated a robust circadian rhythm in circulating PAI-1 with a peak corresponding to ∼6:30 am. This rhythm in PAI-1 was 8-times larger than changes in PAI-1 induced by standardized behavioral stressors, including head-up tilt and 15-minute cycle exercise. If this large endogenous morning peak in PAI-1 persists in vulnerable individuals, it could help explain the morning peak in adverse cardiovascular events.

Figure 1

Presence of endogenous circadian rhythm (independent of behavioral effects) and absence of significant effect of behavioral stressors (independent of circadian effects) on PAI-1. PAI-1 had a significant endogenous circadian rhythm with the circadian peak corresponding to approximately 6:30 am, at the beginning of the vulnerable time of 6 am to noon (light gray bars in A,C). The behavioral states, including head up tilt and exercise, had no significant effect on PAI-1 (B,D). There were no statistical interactions between the circadian and behavioral effects. Panels A and B show individual data, while panels C and D show group analysis. Cosine models (black lines) and 95% confidence intervals (dark gray areas) are based on mixed-model analyses and use precise circadian phase data. To show that these models adequately fit the actual data, we also plot average data grouped into 60-circadian-degree windows with error bars representing standard error of the mean. Data are normalized according to each individual’s average across the FD protocol. Bottom x-axes (A,C), circadian phase with 0° indicating the timing of the fitted circadian core body temperature minimum (average ∼4:30 am in these participants); bottom x-axes (B,D), behavioral states; top x-axes (A,C), corresponding average clock time in these participants; right y-axes, percentage of each individual’s mean across the protocol; left y-axes, absolute values; light gray bars (A,C), most vulnerable period for adverse cardiovascular events observed in epidemiologic studies (∼6:00 am to noon). P values, significance of circadian effect from cosinor analyses.

Figure 2

Comparison between the endogenous circadian rhythm in PAI-1 and day/night rhythm in PAI-1. Comparison between the endogenous circadian rhythm in PAI-1 assessed during the twelve 20-hour cycles of the FD protocol (A) and the day/night rhythm in PAI-1 assessed during the second 24-hour baseline day including sleep/wake, fasting/feeding, supine/upright, rest/activity, and dark/light cycle (B). Figures are aligned according to the approximate average corresponding clock time of the circadian profile (ie, 0 circadian degrees equates to ∼4:30 am and 3 hours prior to habitual scheduled awakening in these participants). In order to allow comparison of absolute values, subjects’ data were not normalized (in contrast to Figure 1). Timing of meals, sleep, and test battery (gray bars) are indicated for the baseline day (data for FD protocol were all collected during the test battery). B, breakfast; L, lunch; D, dinner; S, snack. M, mental stress test (10-minute computerized serial addition test); T, head-up tilt; E, exercise. Top x-axis (A), corresponding average clock time; top x-axis (B), average clock time. During the baseline day, participants were in semirecumbent posture until T, in varied postures throughout the remainder of the day, and remained lying down throughout the sleep episode. Error bars represent standard error of the mean.