Hormone‐phase dependence of muscle sympathetic nerve activity in young healthy women using hormonal contraceptives

Abstract

The effects of hormonal contraceptives on control of muscle sympathetic nerve activity (MSNA) are unclear. We tested the hypothesis that high‐ (HH) versus low‐hormone (LH) phases of contraceptive use would not affect MSNA. Integrated MSNA recordings were obtained at rest, during −60 to−80 mmHg lower body negative pressure (LBNP) (n=4), and during combined hypercapnia‐hypoxia stress (HC‐HX) (n=7) during LH and HH. Mean arterial pressure (MAP), heart rate (HR), and cardiac output (Q) were similar between LH and HH across all conditions. Baseline MSNA burst frequency (BF; 16±8 vs 11±6 bursts/min, P=0.04) and burst incidence (BI; 23±9 vs 16±8 bursts/100hb, P=0.056) were greater during HH than LH, respectively. During LBNP, BF was greater during HH than LH (51±10 vs 43±9 at −60, P<0.001; 59±16 vs 44±11 bursts/min at −80, P=0.03), but relative increases in BF or BI were not different. The HC‐HX protocol elicited greater BI during HH than LH (29±10 vs 23±9 bursts/100hb, P=0.047), but similar BF. A subsequent HC‐HX apnea resulted in similarly high absolute BF and BI in LH and HH. Accordingly, HH tended to produce smaller relative changes in BF (+31±11 vs +39±8 bursts/min, P=0.059) and BI (+44±18 vs +54±13 bursts/100hb, P=0.053) than LH. These data indicate that hormone phase, associated with contraceptive use, influences baseline MSNA but not responses to severe reflex stimulation. Supported by NSERC and CIHR.