In this study, we found that PCOS women with blunted HRR had lower values of circulating APN than those women with normal HRR. Furthermore, in multivariate analysis, age, BMI, hypertension and APN concentration were independent risk factors of HRR impairment.
PCOS women were characterized as elevated androgen levels, anovulation, and menstrual irregularity. Substantial evidences prove that sympathetic nervous system hyperactivity are associated with those vital symptoms . Moreover, accumulating clinical data demonstrate that PCOS women have increased risks of cardiovascular diseases and metabolic disorders, compared with age-matched healthy women. Meanwhile, hypertension, exaggerated SBP response to exercise, increased musclesympathetic nerve activity, increased levels of adrenergic metabolites in the serum and urine, increased HRrest and lower HR variability were reported as different manifestations of a generalized increase in sympathetic tone in PCOS women . HRR, the quantification of HR decrease after exercise, is a straightforward method and a highly reproducible tool in assessingautonomic nervous function. Extensive investigations have recognized that it is not only a powerful factor predicting all-cause mortality, but also a potential marker predicting health outcomes including cardiovascular diseases . It was reported that attenuated HRR was a precursor to hyperglycemia as well as an indicator of cardiovascular dysfunction [21–23]. In the past decades, HRR has been gradually evaluated in clinical researches of PCOS women. Giallauria F et al. found that HRR was lower in young PCOS women compared with healthy subjects (12.9 ± 1.8 beats vs. 20.4 ± 3.1 beats) . Similarly, Kaya C et al. found HRR was decreased in PCOS women compared with age- and BMI-matched women (15.4 ± 1.9 beats vs. 24.2 ± 3.4 beats) . Additionally, it was reported that 89 out of 243 young PCOS patients without known risk factors for cardiovascular diseases presented abnormal HRR, in which study abnormal HRR was defined as ≤ 18 beats for standard exercise testing. In the present study, our definition of blunted HRR was ≤ 12 beats, and 23 out of 89 women exhibited blunted HRR.
Because HRR possesses such important predictive values, researchers made efforts to explore its influencing factors. Previous studies showed age was one of those factors, and our multivariate analysis result confirmed it. The prevalence of abnormal HRR was reported to be greater in older than younger individuals [13, 27–28]. This age-effect was also supported by Buchheit et al. , whose study revealed HRR in children were faster compared with adolescents and adults. Physical fitness and training may also influence HRR. Several cross-sectional investigations showed that athletes or physically trained individuals had faster HRR than sedentary individuals [6, 30], so we collected the information about exercise habits of those PCOS women at enrollment. Although exercise habit wasn’t associated with HRR independently, the ratio of women taking regular exercise in the Blunted HRR Group was significantly lower than the Normal HRR group.
We found increased BMI was another risk factor for blunted HRR in PCOS women. As early as in 2008, it was reported abnormal HRR was inversely correlated with BMI in overweight PCOS women . Then it was found that weight loss in overweight and obese women with PCOS was associated significantly with improvements in HRR . Another positive finding in the current study was hypertension was also an independent determinant of attenuated HRR in PCOS women. A recent research based in China reported that HRR was lower in hypertensive patients than non-hypertensive patients, and it was even lower in hypertensive patients with uncontrolled BP than hypertensive patients with controlled BP . Moreover, a study enrolling 1,855 participants who were healthy and normotensive initially indicated that slow HRR was independently associated with the development of hypertension after average 4-year’s follow-up . Hypertension, as a common coexisting condition in PCOS, has complicated interactions with HRR, which was out of the present study’s scope. However, it was affirmative that longitudinal HRR assessment in PCOS women was helpful both in prehypertension phase and different grades of hypertension.
Besides these above-discussed clinical parameters, some circulating factors were found to be closely correlated with HRR in PCOS, such as C-reactive protein [26, 34]white blood cells count  and homocysteine . Elevated C-reactive protein and white blood cells counts are classic inflammatory markers, and high level of homocysteine is confirmed as having close involvement in endothelial dysfunction. Furthermore, adipokines, which are produced by adipose tissue, have been found to mediate the crosstalk betweenmetabolic function and autonomic nervous function in different populations during the past two decades’ researches . APN, the most abundant adipose-released cytokine, has an important role in metabolism, primarily through reducing insulin resistance. It not only plays an important role in whole-body energy homeostasis, but also results in protection of the vasculature, heart, lung and kidney because of its anti-apoptotic properties. In addition to those effects, it was reported that higher APN levels were associated with a more favorable development of cardiovascular autonomic function in individuals with type 2 diabetes independently of multiple confounders . Another clinical observation reported that lower APN levels were related to sympathetic activation and severity of obstructive sleep apnea. On the other side, Łukasz Nowak et al. found that blockade of sympathetic nervous system activity by rilmenidine increased plasma APN concentration in patients with essential hypertension. Recently, it was shown that increased sympathetic activity was associated with and may modulate high-molecular-weight APN levels in premenopausal women with PCOS .
Although our study showed the significance difference of APN levels between Blunted HRR Group and Normal HRR Group, it was found HRR wasn’t an independent factor of APN concentration after multiple linear regression analysis. However, age and hyperlipemia were correlated with APN levels independently. Previous studies have revealed that serum APN levels rise with increasing age and an elevation of ~ 1 µg/mL of circulating APN for every 10 years of age, in healthy individuals . Meanwhile, the significant relationship between hyperlipemia and APN was consistent with former reports. It was found serum APN was negatively related to TC, LDL-C and TG concentrations, and positively related to HDL-C concentration in nondiabetic subjects. Although the mechanism that accounts for the relation between APN and serum lipids is not fully understood, it was revealed that APN was directly associated with enzymes that regulate lipid metabolism, and those associations were independent of age, sex, BMI, insulin resistance and systemic inflammation.
Despite those interesting findings in the current study, certain limitations should be considered. First, this was a single-center study in which only a small number of PCOS patients were available for analysis; Second, our work was cross-sectional, it would be more informative to have serial values of APN and HRR to explore causality; Third, there were some factors which were noted to influence serum APN levels, such as smoking habit  and moderate alcohol consumption . There was no women having smoking habit or alcohol habit in our study. Current evidence about the effect of dietary fatty acids on APN values is equivocal , and we didn’t collect the information of those women’s oral daily supplements, like fish oil or conjugated linoleic acid, which might have potential effect on circulating APN values.