Discussion
hypertension, cardiovascular diseases
ZONA, HEART, HYPERTENSION, CARDIOVASCULAR DISEASES
This post hoc analysis demonstrated that, in univariate comparison, there was no significant difference between auricular tVNS at the tragus and sham treatment at the earlobe with respect to alterations in serum aldosterone concentrations from pre- to post-stimulation. In both younger and older participants, aldosterone levels decreased during the treatment period and, although this decline was more pronounced during active tVNS, the difference was not enough to conclude relevant effects compared to sham stimulation. Irrespective of the stimulation conditions, aldosterone concentrations depended significantly on the gender and age group. Among the study participants, younger women had the highest aldosterone levels, which were significantly elevated compared to younger men and older women, with the latter group having the lowest serum concentrations.It is well known that the risk of developing cardiovascular diseases and hypertension differs between men and womenFor example, estrogen has been shown to reduce angiotensin-II-receptor 1 expression, while increasing the expression of angiotensin-converting enzyme 2, which stimulates angiotensin (1–7) production and angiotensin-II-receptor-2 activationAll these data suggest an inhibitory role of estrogen on RAAS in younger women, which would imply lower aldosterone concentrations compared to men. Consistent with this view, a population-based study with over 3000 participants reported lower plasma aldosterone levels in younger and older women compared with menFurthermore, a drop in estrogen after menopause should have a stimulatory effect on aldosterone, which however was neither observed in the present nor in other studies. Consistent with our results, data from 2891 subjects enrolled in the Framingham Heart Community Study showed that postmenopausal women not using hormonal replacement therapy had significantly lower serum aldosterone levels than premenopausal womenThese discrepancies in results could be due to the influence of progesterone on aldosterone production during the female menstruation cycle. Interestingly, progesterone, but not estrogen, stimulated the secretion of aldosterone in zona glomerulosa cells from female rats in vitroTo the best of our knowledge, this is the first study to investigate tVNS effects on circulating aldosterone levels in humans. Two recent animal studies have reported that rats exposed to chronic mild stress showed significantly reduced circulating ACTH and corticosterone concentrations after auricular tVNS treatment compared to controls, indicating inhibition of the HPA axis and possibly aldosteroneAlthough the earlobe is intensively used as a sham location in tVNS studies, its usage is still under debateSeveral limitations must be considered when interpreting the results of this analysis. Firstly, the generalizability of our results is considerably limited, since the entire study cohort consisted of white participants and the sample size was very small, which particularly restricts the interpretability of ethnic and gender differences. Secondly, salt intake was not controlled, hence it cannot be excluded that significant differences in aldosterone levels were caused by prior sodium intake. Thirdly, detailed information about the menstrual cycle or menopausal status of our female participants was not available. Therefore, it cannot be excluded that increased progesterone concentrations in younger women may have caused their high aldosterone levels. Fourthly, the concentrations of potassium, angiotensin II, renin and ACTH were not assessed, restricting any conclusions on the effects of gender and tVNS treatment on the HPA axis and the RAAS. Furthermore, blood was drawn in a seated position, therefore aldosterone levels were probably higher than in a supine position