Circadian Variation in Vascular Function and Regenerative Capacity in Healthy Humans

Abstract

Background: Progenitor cells ( PC s) are mobilized in response to vascular injury to effect regeneration and repair. Recruitment of PC s 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 PC s and their inter‐relation with vascular function remain unknown. We investigated the circadian variation of PC s and vascular function with the hypothesis that this will parallel the pattern observed for cardiovascular events ( CVE s). Methods and Results: In 15 healthy subjects (9 men, 37±16 years), circulating PC s and vascular function were measured at 8 am , noon, 4 pm , 8 pm , midnight, 4 am (only PC s counts), and 8 am the following day. Circulating PC s were enumerated as mononuclear cells ( MNC s; CD 45 ^med ) that express CD 34 as well as CD 133, and their activity was assessed as the number of colonies formed by culturing MNC s. 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 CD 34 ⁺ and CD 34 ⁺ / CD 133 ⁺ cell counts were observed at 8 pm than any other time of the day ( P ‐ ANOVA =0.038 and 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 CVE s 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.