Ivie, Clayton L.

Sex-related differences in respiratory modulation of sympathetic activity have been observed in rodent models of sleep apnea [intermittent hypoxia (IH)]. In light of sex disparities in the respiratory response to acute IH in humans as well as changes in respiratory modulation of muscle sympathetic nerve activity (MSNA) in clinical sleep apnea, we examined sex-related differences in respiratory modulation of MSNA following acute IH. We hypothesized that respiratory modulation of MSNA would be altered in both male and female participants after IH; however, the respiratory patterning of MSNA following IH would be sex specific. Heart rate, MSNA, and respiration were evaluated in healthy male ( n = 21, 30 ± 5 yr) and female ( n = 10, 28 ± 5 yr) participants during normoxic rest before and after 30 min of IH. Respiratory modulation of MSNA was assessed by fitting polynomials to cross-correlation histograms constructed between sympathetic spikes and respiration. MSNA was elevated after IH in male (20 ± 6 to 24 ± 8 bursts/min) and female (19 ± 8 to 22 ± 10 bursts/min) participants ( P < 0.01). Both male and female participants exhibited respiratory modulation of MSNA ( P < 0.01); however, the pattern differed by sex. After IH, modulation of MSNA within the breath was reduced in male participants ( P = 0.03) but increased in female participants ( P = 0.02). Both male and female adults exhibit changes in respiratory patterning of MSNA after acute IH; however, this pattern differs by sex. These data support sex disparities in respiratory modulation of MSNA and may have implications for conditions such as sleep apnea.

Background Sex‐related differences in respiratory‐sympathetic coupling have been described in preclinical models of intermittent hypoxia (IH) and have been implicated in the development of neurogenic hypertension. Based on sex disparities in the respiratory and blood pressure response to acute IH in humans as well as changes in respiratory‐sympathetic coupling in clinical sleep apnea, we examined sex‐related differences in respiratory modulation of muscle sympathetic nerve activity (MSNA) following acute IH in healthy humans. We hypothesized the magnitude of respiratory‐sympathetic coupling would increase in both men and women following IH; however, the respiratory patterning of MSNA following IH would be sex‐specific. Methods Heart rate, MSNA, and respiration were assessed in 21 healthy young men (30±1 yrs) and 10 healthy young women (28±1 yrs) during normoxic rest prior to and following IH. IH was achieved by alternating between a hypercapnic hypoxic (3% carbon dioxide, 5% oxygen) gas cylinder and room air (21% oxygen) to target 15 hypoxic events over 30‐min. Respiratory modulation of MSNA was assessed by fitting polynomials to cross‐correlation histograms constructed between sympathetic spikes and respiration. Results MSNA was elevated following IH in both men (34±2 to 41±3 bursts/100 heart beats) and women (30±4 to 35±5 bursts/100 heart beats) (p<0.01). When examining the normalized temporal profile of MSNA, both men and women exhibited changes in respiratory patterning of MSNA following IH; however, the pattern of MSNA differed by sex. Following IH, women exhibited more sympathetic activity during late expiration and less during post‐inspiration (interaction of IH and time, p<0.01). In contrast, men exhibited greater sympathetic activity during post‐inspiration following IH (interaction of IH and time, p<0.01). Conclusions Both men and women exhibit changes in respiratory patterning of MSNA following acute IH; however, this pattern differs by sex. These data support sex disparities in respiratory‐sympathetic coupling and advance our understanding of sex‐specific changes in sympathetic activation following IH which may have important implications for the development of neurogenic hypertension.

Repetitive hypoxic apneas, similar to those observed in sleep apnea, result in resetting of the sympathetic baroreflex to higher blood pressures (BP). This baroreflex resetting is associated with hypertension in preclinical models of sleep apnea (intermittent hypoxia, IH); however, the majority of understanding comes from males. There are data to suggest that female rats exposed to IH do not develop high BP. Clinical data further support sex differences in the development of hypertension in sleep apnea, but mechanistic data are lacking. Here we examined sex-related differences in the effect of IH on sympathetic control of BP in humans. We hypothesized that after acute IH we would observe a rise in muscle sympathetic nerve activity (MSNA) and arterial BP in young men ( n = 30) that would be absent in young women ( n = 19). BP and MSNA were measured during normoxic rest before and after 30 min of IH. Baroreflex sensitivity (modified Oxford) was evaluated before and after IH. A rise in mean BP following IH was observed in men (+2.0 ± 0.7 mmHg, P = 0.03), whereas no change was observed in women (−2.7 ± 1.2 mmHg, P = 0.11). The elevation in MSNA following IH was not different between groups (4.7 ± 1.1 vs. 3.8 ± 1.2 bursts/min, P = 0.65). Sympathetic baroreflex sensitivity did not change after IH in either group ( P > 0.05). Our results support sex-related differences in the effect of IH on neurovascular control of BP and show that any BP-raising effects of IH are absent in young women. These data enhance our understanding of sex-specific mechanisms that may contribute to BP changes in sleep apnea.

Background Repetitive hypoxic apneas, similar to what is observed in sleep apnea, result in resetting of the sympathetic baroreflex to higher blood pressures (BP) in humans. This resetting of the baroreflex is associated with increased BP in preclinical models of sleep apnea (chronic intermittent hypoxia, CIH); however, the majority of our understanding comes from men. There are data to suggest female rats exposed to CIH do not develop high BP. With this, we sought to examine potential sex‐related differences in the effect of intermittent hypoxia (IH) on the sympathetic baroreflex. We hypothesized young men would exhibit resetting of the baroreflex to higher BP following IH, with no change in young women. Methods BP and muscle sympathetic nerve activity (MSNA) were assessed in 17 men (30±1 yrs) and 10 women (28±1 yrs) during quiet normoxic rest prior to and following 30‐min of experimental IH [30‐s hypoxic exposures (0.05 F i O 2 ) alternating with 90‐s room air breathing] resulting in 15 hypoxic events (91.7±0.5 % SpO2). Sympathetic baroreflex sensitivity was evaluated using the modified Oxford technique before and after 30‐min of IH. Results Diastolic BP increased in men following IH (71±2 to 74±2 mmHg; p<0.01) with no change in women (75±3 to 75±3 mmHg; p=0.97). Sympathetic baroreflex sensitivity did not change following IH in either group (Men: −1.8±0.2 to −1.9±0.3; Women: −1.6±0.3 to −2.1± 0.5 bursts/100 heart beats/mmHg; Main effect of IH, p=0.14; Interaction of group and IH, p=0.28). Conclusion Acute IH resets the sympathetic baroreflex to higher BP in young men, with no change in young women. Baroreflex sensitivity slopes were unchanged in either group following IH. Our results support sex differences in the effect of IH on sympathetic control of BP and show that young women are protected from the BP‐raising effects of IH. These data enhance our understanding of sex‐specific mechanisms that may contribute to the development of hypertension in sleep apnea. Support or Funding Information NIH HL130339, Mayo Clinic Center for Biomedical Discovery