Circadian variation in vascular function and regenerative capacity in healthy humans

Abstract

Background: Progenitor cells (PCs) are mobilized in response to vascular injury to effect regeneration and repair. Recruitment of PCs requires intact nitric oxide (NO) synthesis by endothelial cells, and their number and activity correlate with cardiovascular disease risk burden and future outcomes. Whereas cardiovascular vulnerability exhibits a robust circadian rhythm, the 24-hour variation of PCs and their inter-relation with vascular function remain unknown. We investigated the circadian variation of PCs and vascular function with the hypothesis that this will parallel the pattern observed for cardiovascular events (CVEs).

Methods and results: In 15 healthy subjects (9 men, 37±16 years), circulating PCs and vascular function were measured at 8 am, noon, 4 pm, 8 pm, midnight, 4 am (only PCs counts), and 8 am the following day. Circulating PCs were enumerated as mononuclear cells (MNCs; CD45(med)) that express CD34 as well as CD133, and their activity was assessed as the number of colonies formed by culturing MNCs. Vascular function was evaluated by measurement of endothelium-dependent, flow-mediated vasodilation (FMD) of the brachial artery and tonometry-derived indices of arterial stiffness. Higher CD34(+) and CD34(+)/CD133(+) cell counts were observed at 8 pm than any other time of the day (P-ANOVA=0.038 and <0.001; respectively) and were lowest at 8 am. PC colony formation was highest at midnight (P-ANOVA=0.045) and lowest in the morning hours. FMD was highest at midnight and lowest at 8 am and 8 pm, and systemic arterial stiffness was greatest at 8 am and lowest at 4 pm and midnight (P-ANOVA=0.03 and 0.01; respectively).

Conclusion: A robust circadian variation in PC counts and vascular function occurs in healthy humans and both exhibit an unfavorable profile in the morning hours that parallels the preponderance of CVEs at these times. Whether these changes are precipitated by awakening and time-dependent physical activity or governed by the endogenous circadian clock needs to be further investigated.

Keywords: arterial stiffness; circadian variation; endothelial function; progenitor cells.

Figure 1.

Circadian variation in hemodynamic parameters. Mean±standard error (SE) of heart rate (HR), diastolic blood pressure (DBP), and systolic blood pressure (SBP). P‐ANOVA: nonsignificant (>0.05) for all, N=15. MAP indicates mean arterial pressure.

Figure 2.

Circadian variation in (A) CD34⁺ and (B) CD34⁺/CD133⁺ (C) colony forming unit (CFU) counts. Bottom box plots: middle band represents median, bottom and top of the box represent lower and upper quartiles, and whiskers represent highest and lowest values that are not outliers. Top: average percent deviation (Δ%) from the individual mean. P values are for ANOVA, N=15 for A and B, N=11 for C.

Figure 3.

Circadian variation in (A) brachial artery FMD and (B) AIX. FMD, brachial artery flow‐mediated dilation (%). AIX, augmentation index. Bottom box plots: middle band represents median, bottom and top of the box represent lower and upper quartiles, and whiskers represent highest and lowest values that are not outliers. Top figures show average percent deviation (Δ%) from the individual mean. P values are for ANOVA, N=15.

Figure 4.

Time points at which peak and nadir values of progenitor cell counts, AIX, and brachial artery FMD were observed. AIX indicates augmentation index; FMD, brachial artery flow‐mediated dilation (%). P values are for paired t tests, N=15.