Oral contraceptives modulate the muscle metaboreflex in healthy young women

Consideration of absolute intensity when examining sex differences in blood pressure responses during static exercise

Low- to moderate-intensity submaximal static contractions are commonly used to study the effects of biological sex on the cardiovascular response to exercise. Under this paradigm, premenopausal females frequently demonstrate smaller blood pressure responses than age-matched males. These differences are preserved during postexercise circulatory occlusion, implicating the muscle metaboreflex as an important driver of sex differences in the blood pressure response to static exercise. The mechanisms responsible for these differences are incompletely understood but often attributed to innate sex differences in skeletal muscle fiber type distribution, muscle metabolism, and/or sympathetic control of the circulation. However, one potential confounding factor is that the majority of studies use relative intensity exercise (e.g., 30% of maximal voluntary contraction), such that on average, females are completing static contractions at a lower absolute intensity. In this review, we summarize human evidence showing that sex differences in blood pressure responses to static exercise are attenuated or abolished when controlling for absolute intensity and muscle strength, either by statistical methods or strength-matched cohorts. We highlight evidence that the effect of higher absolute contraction intensity on exercise blood pressure likely occurs through increased mechanical occlusion of skeletal muscle microvasculature, leading to greater activation of the muscle metaboreflex. These findings highlight an important need to account for absolute intensity when studying and interpreting sex differences in cardiovascular responses to exercise.

Role of the peripheral chemoreceptors in cardiovascular and metabolic control in type 2 diabetes

Preclinical work supports a role for the peripheral chemoreceptors in the progression of cardiovascular and metabolic pathologies. In the present study, we examined peripheral chemosensitivity in adults with type 2 diabetes (T2D) and the contribution of the peripheral chemoreceptors to resting cardiovascular and metabolic control. We hypothesized that: (1) adults with T2D exhibit exaggerated peripheral chemoreflex sensitivity; (2) the peripheral chemoreceptors contribute to cardiovascular dysfunction in T2D; and (3) attenuation of peripheral chemoreceptor activity improves glucose tolerance in T2D. Seventeen adults with diagnosed T2D [six males/11 females; aged 54 ± 11 years; glycated haemoglobin (HbA1c) 7.6 ± 1.5%] and 20 controls without T2D (9 males/11 females; aged 49 ± 13 years, HbA1c 5.2 ± 0.4%) participated in the study. The hypoxic ventilatory response (HVR) was assessed as an index of peripheral chemosensitivity. Resting heart rate, blood pressure and minute ventilation were measured when breathing normoxic followed by hyperoxic air (1.0F I O 2 ${{F}_{{\mathrm{I}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$ ) to acutely attenuate peripheral chemoreceptor activity. A subset of participants (n = 9 per group) completed two additional visits [normoxia (0.21F I O 2 ${{F}_{{\mathrm{I}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$ ), hyperoxia (1.0F I O 2 ${{F}_{{\mathrm{I}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$ )] where glucose and insulin were measured for 2 h following an oral glucose challenge. HVR was augmented in adults with T2D (-0.84 ± 0.49 L min-1/%) vs. control (-0.48 ± 0.40 L min-1/%, P = 0.021). Attenuation of peripheral chemoreceptor activity decreased heart rate (P < 0.001), mean blood pressure (P = 0.009) and minute ventilation (P = 0.002); any effect of hyperoxia did not differ between groups. There was no effect of hyperoxia on the glucose (control, P = 0.864; T2D, P = 0.982), nor insulin (control, P = 0.763; T2D, P = 0.189) response to the oral glucose challenge. Peripheral chemoreflex sensitivity is elevated in adults with T2D; however, acute attenuation of peripheral chemoreflex activity with hyperoxia does not restore cardiometabolic function. KEY POINTS: Preclinical work supports a role for the peripheral chemoreceptors in the progression of cardiovascular and metabolic pathologies. In the present study, we examined peripheral chemosensitivity in adults with type 2 diabetes and the contribution of the peripheral chemoreceptors to resting cardiovascular control and glucose tolerance. We observed elevated peripheral chemoreflex sensitivity in adults with diabetes which was associated with glycaemic control (i.e. glycated haemoglobin). Notably, acute attenuation of peripheral chemoreflex activity with hyperoxia did not restore cardiometabolic function in the individuals studied.

Influence of endogenous and exogenous hormones on the cardiovascular response to lower extremity exercise and group III/IV activation in young females

Oral contraceptive (OC) use can increase resting blood pressure (BP) in females as well as contribute to greater activation of group III/IV afferents during upper body exercise. It is unknown, however, whether an exaggerated BP response occurs during lower limb exercise in OC users. We sought to elucidate the group III/IV afferent activity-mediated BP and heart rate responses while performing lower extremity tasks during early and late follicular phases in young, healthy females. Females not taking OCs (NOC: n = 8; age: 25 ± 4 yr) and those taking OCs (OC: n = 10; age: 23 ± 2 yr) completed a continuous knee extension/flexion passive stretch (mechanoreflex) and cycling exercise with subsystolic cuff occlusion (exercise pressor reflex), which was followed by a 2-min postexercise circulatory occlusion (PECO) (metaboreflex). Data collection occurred on two occasions: once during the early follicular phase (days 1-4) and once during the late follicular phase (days 10-14) of their menstrual cycle (NOC) or during the placebo and active pill phases (OC). Resting mean arterial BP and heart rate were not different between phases in NOC and OC participants (P > 0.05). Hemodynamic responses to metaboreflex, mechanoreflex, and collective exercise pressor reflex activation were not different between phases in both groups (P > 0.05). In conclusion, although OCs are known to increase BP at rest, our findings indicate that neither endogenous nor exogenous (OC) sex hormones modulate BP during large, lower limb muscle exercise with or without group III/IV afferent activation in young, healthy females.NEW & NOTEWORTHY Sex differences in the cardiovascular response to exercise have been demonstrated and may be dependent on sex hormone levels. Furthermore, oral contraceptives (OCs) have been shown to exaggerate the blood pressure response to upper extremity exercise. The results of this study indicate that neither endogenous nor exogenous (OC) sex hormones modulate BP during lower extremity dynamic exercise or with group III/IV afferent activation in young, healthy females.

Cardiovascular and hematological responses to a dry dynamic apnea in breath hold divers

The mammalian dive reflex, characterized by bradycardia and peripheral vasoconstriction, occurs in all mammals, including humans, in response to apnea. However, the dive reflex to a single, maximal, dry, dynamic apnea (DYN) and how it compares to a time-matched exercise control trial (EX) or dry static apnea (SA) has not been studied. We examined the hypotheses that, compared with EX and SA, the magnitude of the 1) cardiovascular response and 2) hematological response to DYN would be greater. Cardiovascular parameters [heart rate (HR), systolic (SBP), diastolic (DBP), and mean arterial (MAP) blood pressure] were continuously collected in 23 (F = 6 females) moderate and elite freedivers, first during a maximal DYN, then during a time-matched SA and EX on a swimming ergometer in randomized order. Venous blood draws were made before and following each trial. The change in calculated oxygen saturation (DYN: -17 ± 13%, EX: -2 ± 1%, ΔSA: -2 ± 1%; P < 0.05, all comparisons) was greater during DYN compared with EX and SA. During DYN, ΔSBP (DYN: 104 ± 31 mmHg; EX: 38 ± 23 mmHg; and SA: 20 ± 11 mmHg), ΔDBP (DYN: 45 ± 12 mmHg; EX: 14 ± 10 mmHg; and SA: 15 ± 8 mmHg), and ΔMAP (DYN: 65 ± 17 mmHg; EX: 22 ± 13 mmHg; and SA: 16 ± 9 mmHg) were increased compared with EX and SA, while ΔHR was greater during EX (DYN: -24 ± 23 beats/min; EX: 33 ± 13 beats/min; and SA: -1 ± 10 beats/min) than either DYN or SA (P < 0.0001, all comparisons). Females had a greater pressor response to EX (ΔSBP: 59 ± 30 mmHg; ΔDBP: 24 ± 14 mmHg; and ΔMAP: 35 ± 8 mmHg) than males (ΔSBP: 31 ± 15 mmHg; ΔDBP: 11 ± 6 mmHg; and ΔMAP: 18 ± 8 mmHg; P < 0.01, all comparisons). Together, these data indicate that DYN elicits a distinct, exaggerated cardiovascular response compared with EX or SA alone.NEW & NOTEWORTHY This study performed a dry dynamic apnea with sport-specific equipment to closely mimic the physiological demands of competition diving. We found the cardiovascular and hematological responses to dynamic apnea were more robust compared with time-matched exercise and dry static apnea control trials.

The influence of oral contraceptives on the exercise pressor reflex in the upper and lower body

Previous research has demonstrated that oral contraceptive (OC) users have enhanced cardiorespiratory responses to arm metaboreflex activation (i.e., postexercise circulatory occlusion, PECO) and attenuated pressor responses to leg passive movement (PM) compared to non-OC users (NOC). We investigated the cardiorespiratory responses to arm or leg metaboreflex and mechanoreflex activation in 32 women (OC, n = 16; NOC, n = 16) performing four trials: 40% handgrip or 80% plantarflexion followed by PECO and arm or leg PM. OC and NOC increased mean arterial pressure (MAP) similarly during handgrip, plantarflexion and arm/leg PECO compared to baseline. Despite increased ventilation (VE) during exercise, none of the women exhibited higher VE during arm or leg PECO. OC and NOC similarly increased MAP and VE during arm or leg PM compared to baseline. Therefore, OC and NOC were similar across pressor and ventilatory responses to arm or leg metaboreflex and mechanoreflex activation. However, some differences due to OC may have been masked by disparities in muscle strength. Since women increase VE during exercise, we suggest that while women do not display a ventilatory response to metaboreflex activation (perhaps due to not reaching a theoretical metabolite threshold to stimulate VE), the mechanoreflex may drive VE during exercise in women.

The human skeletal muscle metaboreflex contribution to cardiorespiratory control in males and females in dynamic exercise

There is a significant effect of sex and muscle mass on the cardiorespiratory response to the skeletal muscle metaboreflex during isometric exercise. We therefore tested the hypothesis that sex differences would be present when isolated following dynamic exercise. We also tested the hypothesis that single and double leg post-exercise circulatory occlusion (PECO) following heavy exercise would elicit a cardiorespiratory response proportional to the absolute muscle mass. Healthy (24 ± 4 years) males (n = 10) and females (n = 10) completed pulmonary function and an incremental cycle test to exhaustion. Participants completed two randomized, 6 min bouts of intense cycle exercise (84 ± 7% V̇O2peak). One exercise bout was immediately followed by 3 min PECO (220 mmHg) of the legs while the other exercise bout was followed by passive recovery. Males completed an additional session of testing with single leg PECO. The mean arterial pressure during PECO and control was greater in males compared to females (p = 0.004). The was a significant time by condition by sex interaction in the heart rate response to PECO (p = 0.027). There was also a significant condition by sex interaction in the ventilatory response to PECO (p = 0.026). In males, we observed a dose-dependent cardiovascular, but not ventilatory, response to muscle mass occluded (all p < 0.05). Our findings suggest the metaboreflex contribution to cardiorespiratory control during dynamic exercise is greater in males compared to females. The ventilatory response induced by double-leg occlusion but not single-leg occlusion, suggests that the ventilatory influence of the metaboreflex is less sensitive than the cardiovascular response and may be linked to the greater afferent activation induced by double-leg occlusion.

Quadriceps fatigue during hypoxic and ischemic knee-extension exercise is similar in males and females

Hypoxia is known to increase muscle fatigue via both central and peripheral mechanisms. Females are typically less fatigable than males during isometric fatiguing contractions due to greater peripheral blood flow. However, sex differences in fatigue are blunted during dynamic fatiguing tasks. Thus, this study determined the interactions of sex and hypoxia on knee extensor muscle contractile function during a dynamic, ischemic fatiguing contraction. Electrical stimulation was used to determine contractile properties of the knee extensor muscles in eight males and eight females before and after an ischemic, dynamic fatiguing task while inspiring room air or a hypoxic gas mixture (10% O2:90% N2). Fatigue (assessed as time-to-task failure) was ∼10% greater during the hypoxic condition (94.3 ± 33.4 s) compared with normoxic condition (107.0 ± 42.8 s, P = 0.041) and ∼40% greater for females than males (77.1 ± 18.8 vs. 124.2 ± 38.7, P < 0.001). Immediately after the dynamic fatiguing task, there were reductions in maximal voluntary contraction force (P = 0.034) and electrically evoked twitch force (P < 0.001), and these reductions did not differ based on sex or inspirate. Cerebral tissue oxygenation showed a significant interaction of time and inspirate (P = 0.003) whereby it increased during normoxia and remained unchanged in hypoxia. No sex-related differences in the changes of cerebral tissue oxygenation were observed (P = 0.528). These data suggest that acute hypoxia increases central fatigue during ischemic single-leg exercise resulting in earlier exercise termination, but the effect does not differ based on sex.NEW & NOTEWORTHY Hypoxia exacerbates fatigue via central mechanisms after ischemic single-leg exercise. The greater fatigue observed during ischemic dynamic fatiguing exercise with hypoxia inspirate did not differ between the sexes. Hypoxia-induced central limitations are present in acute ischemic exercise and do not appear different in males and females.

The effects of oral contraceptives on resting autonomic function and the autonomic response to physiological stressors: a systematic review

PURPOSE

This systematic review aimed to summarize how oral contraceptives (OC) affect resting autonomic function and the autonomic response to a variety of physiological stressors.

METHODS

A search strategy was created to retrieve citations investigating physiological responses comparing OC users to non-users (NOC) in response to autonomic reflex activation.

RESULTS

A total of 6148 citations were identified across databases from inception to June 2, 2022, and 3870 citations were screened at the abstract level after deduplication. Then, 133 texts were assessed at full-text level, and only 40 studies met eligibility requirements. Included citations were grouped by the aspect of autonomic function assessed, including autonomic reflex (i.e., baroreflex, chemoreflex, mechanoreflex, metaboreflex, and venoarterial reflex), or indicators (i.e., heart rate variability, pulse wave velocity, and sympathetic electrodermal activity), and physiological stressors that may alter autonomic function (i.e., auditory, exercise, mental or orthostatic stress, altitude, cold pressor test, sweat test, and vasodilatory infusions).

CONCLUSION

OC influence the physiological responses to chemoreflex, mechanoreflex, and metaboreflex activation. In terms of autonomic indices and physiological stressors, there are more inconsistencies within the OC literature, which may be due to estrogen dosage within the OC formulation (i.e., heart rate variability) or the intensity of the stressor (exercise intensity/duration or orthostatic stress). Further research is required to elucidate the effects of OC on these aspects of autonomic function because of the relatively small amount of available research. Furthermore, researchers should more clearly define or stratify OC use by duration, dose, and/or hormone cycling to further elucidate the effects of OC.

Effects of biological sex and oral contraception on the sympathetic neurocirculatory adjustments to static handgrip exercise in humans

While biological sex affects the neurocirculatory adjustments to exercise, the effects of sex hormones on sympathetic action potential (AP) patterns and ensuing vascular transduction remain unknown. We tested the hypothesis that males, and females using oral contraceptive pills (OCPs), would demonstrate larger increases in sympathetic activation and sympathetic vascular transduction compared with naturally menstruating females during static handgrip exercise (SHG) and postexercise circulatory occlusion (PECO). Young males [n = 14, 25 (5) yr], females using OCPs [n = 16, 24 (6) yr], and naturally menstruating females [n = 18, 26 (4) yr] underwent assessments of multiunit muscle sympathetic nerve activity (MSNA)/AP discharge patterns (microneurography) and femoral artery blood flow (ultrasound) during fatiguing SHG at 40% maximum voluntary contraction and 2-min PECO. Sympathetic vascular transduction was determined as the quotient of the change in leg vascular conductance (LVC) and MSNA/AP discharge. Males demonstrated greater increases in APs/burst [males: Δ7 (6) vs. midluteal: Δ2 (3), P = 0.028] and total AP clusters [males: Δ5 (3) vs. midluteal: Δ2 (3), P = 0.008] compared with naturally menstruating females only but not those using OCPs during exercise (APs/burst: P = 0.171, total clusters: P = 0.455). Sympathetic vascular transduction of MSNA burst amplitude, APs/burst, and total AP clusters was greater in males and females using OCPs compared with naturally menstruating females (range: P = 0.004-0.044). In contrast, during PECO no group differences were observed in AP discharge (range: P = 0.510-0.872), and AP discharge was not related to LVC during PECO (range: P = 0.08-0.949). These data indicate that biological sex and OCP use impact the central generation of AP discharge, as well as the transduction of these neuronal messages into peripheral vasoconstriction during static exercise.

Cardiovascular responses to combined mechanoreflex and metaboreflex activation in healthy adults: effects of sex and low- versus high-hormone phases in females

Females generally have smaller blood pressure (BP) responses to isolated muscle mechanoreflex and metaboreflex activation compared with males, which may explain sex differences in BP responses to voluntary exercise. The mechanoreflex may be sensitized during exercise, but whether mechanoreflex-metaboreflex interactions differ by sex or variations in sex hormones remains unknown. Thirty-one young healthy subjects (females, n = 16) performed unilateral passive cycling (mechanoreflex), active cycling (40% peak Watts), postexercise circulatory occlusion (PECO; metaboreflex), and passive cycling combined with PECO (combined mechanoreflex and metaboreflex activation). Beat-to-beat BP, heart rate, inactive leg vascular conductance, and active leg muscle oxygenation were measured. Ten females underwent exploratory testing during low- and high-hormone phases of their self-reported menstrual cycle or oral contraceptive use. Systolic BP and heart rate responses did not differ between sexes during active cycling [Δ30 ± 9 vs. 29 ± 11 mmHg (males vs. females), P = 0.9; Δ33 ± 8 vs. 35 ± 6 beats/min, P = 0.4] or passive cycling with PECO (Δ26 ± 11 vs. 21 ± 10 mmHg, P = 0.3; Δ14 ± 7 vs. 18 ± 15 beats/min, P = 0.3). Passive cycling with PECO revealed additive, not synergistic, effects for systolic BP [males: Δ23 ± 14 vs. 26 ± 11 mmHg (sum of isolated passive cycling and PECO vs. combined activation); females: Δ26 ± 11 vs. 21 ± 12 mmHg, interaction P = 0.05]. Results were consistent in subset analyses with sex differences in active cycling BP (P > 0.1) and exploratory analyses of hormone phase (P > 0.4). Despite a lack of statistical equivalence, no differences in cardiovascular responses were found during combined mechanoreflex-metaboreflex activation between sexes or hormone levels. These results provide preliminary data regarding the involvement of muscle mechanoreflex-metaboreflex interactions in mediating sex differences in voluntary exercise BP responses.NEW & NOTEWORTHY The muscle mechanoreflex may be sensitized by metabolites during exercise. We show that cardiovascular responses to combined mechanoreflex (passive cycling) and metaboreflex (postexercise circulatory occlusion) activation are primarily additive and do not differ between males and females, or across variations in sex hormones in females. Our findings provide new insight into the contributions of muscle mechanoreflex-metaboreflex interactions as a cause for prior reports that females have smaller blood pressure responses to voluntary exercise.

Blood Pressure Responses to Postexercise Circulatory Occlusion Are Attenuated After Exercise-Induced Muscle Weakness

PURPOSE

Exercise blood pressure (BP) responses are thought to be determined by relative exercise intensity (percent maximal voluntary contraction (MVC) strength). However, cross-sectional studies report that during a static contraction, higher absolute force is associated with greater BP responses to relative intensity exercise and subsequent muscle metaboreflex activation with postexercise circulatory occlusion (PECO). We hypothesized that a bout of unaccustomed eccentric exercise would reduce knee extensor MVC and subsequently attenuate BP responses to PECO.

METHODS

Continuous BP, heart rate, muscle oxygenation, and knee extensor electromyography were recorded in 21 young healthy individuals (female, n = 10) during 2 min of 20% MVC static knee extension exercise and 2 min of PECO, performed before and 24 h after 300 maximal knee extensor eccentric contractions to cause exercise-induced muscle weakness. As a control, 14 participants repeated the eccentric exercise 4 wks later to test whether BP responses were altered when exercise-induced muscle weakness was attenuated via the protective effects of the repeated bout effect.

RESULTS

Eccentric exercise reduced MVC in all participants (144 ± 43 vs 110 ± 34 N·m, P < 0.0001). BP responses to matched relative intensity static exercise (lower absolute force) were unchanged after eccentric exercise ( P > 0.99) but were attenuated during PECO (systolic BP: 18 ± 10 vs 12 ± 9 mm Hg, P = 0.02). Exercise-induced muscle weakness modulated the deoxygenated hemoglobin response to static exercise (64% ± 22% vs 46% ± 22%, P = 0.04). When repeated after 4 wks, exercise-induced weakness after eccentric exercise was attenuated (-21.6% ± 14.3% vs -9.3 ± 9.7, P = 0.0002) and BP responses to PECO were not different from control values (all, P > 0.96).

CONCLUSIONS

BP responses to muscle metaboreflex activation, but not exercise, are attenuated by exercise-induced muscle weakness, indicating a contribution of absolute exercise intensity on muscle metaboreflex activation.

The effect of inspiratory muscle training and detraining on the respiratory metaboreflex

NEW FINDINGS

What is the central question of this study? Is the attenuation of the respiratory muscle metaboreflex preserved after detraining? What is the main finding and its importance? Inspiratory muscle training increased respiratory muscle strength and attenuated the respiratory muscle metaboreflex as evident by lower heart rate and blood pressure. After 5 weeks of no inspiratory muscle training (detraining), respiratory muscle strength was still elevated and the metaboreflex was still attenuated. The benefits of inspiratory muscle training persist after cessation of training, and attenuation of the respiratory metaboreflex follows changes in respiratory muscle strength.

ABSTRACT

Respiratory muscle training (RMT) improves respiratory muscle (RM) strength and attenuates the RM metaboreflex. However, the time course of muscle function loss after the absence of training or 'detraining' is less known and some evidence suggest the respiratory muscles atrophy faster than other muscles. We sought to determine the RM metaboreflex in response to 5 weeks of RMT and 5 weeks of detraining. An experimental group (2F, 6M; 26 ± 4years) completed 5 weeks of RMT and tibialis anterior (TA) training (each 5 days/week at 50% of maximal inspiratory pressure (MIP) and 50% maximal isometric force, respectively) followed by 5 weeks of no training (detraining) while a control group (1F, 7M; 24 ± 1years) underwent no intervention. Prior to training (PRE), post-training (POST) and post-detraining (DETR), all participants underwent a loaded breathing task (LBT) to failure (60% MIP) while heart rate and mean arterial blood pressure (MAP) were measured. Five weeks of training increased RM (18 ± 9%, P < 0.001) and TA (+34 ± 19%, P < 0.001) strength and both remained elevated after 5 weeks of detraining (MIP-POST vs. MIP-DETR: 154 ± 31 vs. 153 ± 28 cmH2O, respectively, P = 0.853; TA-POST vs. TA-DETR: 86 ± 19 vs. 85 ± 16 N, respectively, P = 0.982). However, the rise in MAP during LBT was attenuated POST (-11 ± 17%, P = 0.003) and DETR (-9 ± 9%, P = 0.007) during the iso-time LBT. The control group had no change in MIP (P = 0.33), TA strength (P = 0.385), or iso-time MAP (P = 0.867) during LBT across all time points. In conclusion, RM and TA have similar temporal strength gains and the attenuation of the respiratory muscle metaboreflex remains after 5 weeks of detraining.

Effects of sex and ageing on the human respiratory muscle metaboreflex

Intense inspiratory muscle work evokes a sympathetically mediated pressor reflex, termed the respiratory muscle metaboreflex, in which young females demonstrate an attenuated response relative to males. However, the effects of ageing and female sex hormones on the respiratory muscle metaboreflex are unclear. We tested the hypothesis that the pressor response to inspiratory work would be similar between older males and females, and higher relative to their younger counterparts. Healthy, normotensive young (26 ± 3 years) males (YM; n = 10) and females (YF; n = 10), as well as older (64 ± 5 years) males (OM; n = 10) and females (OF; n = 10), performed inspiratory pressure threshold loading (PTL) to task failure. Older adults had a greater mean arterial pressure (MAP) response to PTL than young (P < 0.001). YF had a lower MAP compared to YM (+10 ± 6 vs. +19 ± 15 mmHg, P = 0.026); however, there was no difference observed between OF and OM (+26 ± 11 vs. +27 ± 11 mmHg, P = 0.162). Older adults had a lower heart rate response to PTL than young (P = 0.002). There was no effect of sex between young females and males (+19 ± 9 and +27 ± 11 bpm, P = 0.186) or older females and males (+17 ± 7 and +20 ± 7 bpm, P = 0.753). We conclude the respiratory muscle metaboreflex response is heightened in older adults, and the sex effect between older males and post-menopause females is absent, suggesting an effect of circulating sex hormones. KEY POINTS: The arterial blood pressure response to the respiratory muscle metaboreflex is greater in older males and females. Compared to sex-matched young individuals, there is no sex differences in the blood pressure response between older males and post-menopause females. Our results suggest the differences between males and females in the cardiovascular response to high levels of inspiratory muscle work is abolished with reduced circulating female sex hormones.

The exercise pressor reflex: An update

The exercise pressor reflex is a feedback mechanism engaged upon stimulation of mechano- and metabosensitive skeletal muscle afferents. Activation of these afferents elicits a reflex increase in heart rate, blood pressure, and ventilation in an intensity-dependent manner. Consequently, the exercise pressor reflex has been postulated to be one of the principal mediators of the cardiorespiratory responses to exercise. In this updated review, we will discuss classical and recent advancements in our understating of the exercise pressor reflex function in both human and animal models. Particular attention will be paid to the afferent mechanisms and pathways involved during its activation, its effects on different target organs, its potential role in the abnormal cardiovascular response to exercise in diseased states, and the impact of age and biological sex on these responses. Finally, we will highlight some unanswered questions in the literature that may inspire future investigations in the field.

The effect of extracellular hyperosmolality on sweat rate during metaboreflex activation in passively heated young men

Metaboreflex activation augments sweating during mild-to-moderate hyperthermia in euhydrated (isosmotic isovolemic) individuals. Recent work indicates that extracellular hyperosmolality may augment metaboreflex-mediated elevations in sympathetic nervous activity. Our primary objective was therefore to test the hypothesis that extracellular hyperosmolality would exacerbate metaboreflex-mediated increases in sweat rate. On two separate occasions, 12 young men (mean (SD): 25 (5) years) received a 90-min intravenous infusion of either 0.9% saline (isosmotic condition, ISO) or 3.0% saline (hyperosmotic condition, HYP), resulting in a post-infusion serum osmolality of 290 (3) and 301 (7) mOsm/kg, respectively. A whole-body water perfusion suit was then used to increase esophageal temperature by 0.8°C above resting. Participants then performed a metaboreflex activation protocol consisting of 90 s isometric handgrip exercise (40% of their pre-determined maximum voluntary contraction), followed by 150 s of brachial occlusion (trapping produced metabolites within the limb). Metaboreflex-induced sweating was quantified as the change in global sweat rate (from pre-isometric handgrip exercise to brachial occlusion), estimated as the surface area-weighted average of local sweat rate on the abdomen, axilla, chest, bicep, quadriceps, and calf, measured using ventilated capsules (3.8 cm²). We also explored whether this response differed between body regions. The change in global sweat rate due to metaboreflex activation was significantly greater in HYP compared to ISO (0.03 mg/min/cm² [95% confidence interval: 0.00, 0.06]; p=0.047), but was not modulated by body region (site*condition interaction: p=0.679). These findings indicate that extracellular hyperosmolality augments metaboreflex-induced increases in global sweat rate, with no evidence for region-specific differences.

Sex Differences in Muscle Metaboreflex Activation following Static Handgrip Exercise

PURPOSE

Larger blood pressure (BP) responses to relative-intensity static exercise in males vs. females is thought to involve altered muscle metaboreflex activation, but whether this is due to an intrinsic sex difference in metabolite production or to differences in muscle strength and absolute load is unknown.

METHODS

Continuous BP and heart rate were recorded in 200 healthy young males and females (females: n = 109) during 2 minutes of static handgrip exercise at 30% of maximal voluntary contraction (MVC), followed by 2 minutes of post-exercise circulatory occlusion (PECO). Muscle sympathetic nerve activity (MSNA) was recorded in a subset of participants (n = 39; female n = 21), permitting calculation of signal-averaged resting sympathetic transduction (MSNA-diastolic BP). Sex differences were examined with and without statistical adjustment for MVC. Multivariate regression analyses were performed to identify predictors of BP responses.

RESULTS

Males had larger systolic BP responses (interactions, P < 0.0001) to static handgrip exercise (24 ± 10 vs. 17 ± 9 mmHg [mean ± SD], P < 0.0001) and PECO (20 ± 11 vs. 16 ± 9 mmHg, P < 0.0001). Adjustment for MVC abolished these sex differences in BP (interactions, P > 0.7). In the subset with MSNA, neither burst frequency or incidence responses to static handgrip exercise or PECO differed between males and females (interactions, P > 0.2). Resting sympathetic transduction was also similar (P = 0.8). Multivariate analysis showed that MVC, the change in MSNA, and sympathetic transduction were predictors of the systolic BP response to static handgrip but only MVC was associated with responses during PECO.

CONCLUSION

Sex differences in absolute contraction load contribute to differences in BP responses during muscle metaboreflex isolation using PECO. These data do not support an intrinsic effect of sex as being responsible for exercise BP differences between males and females.

Menstrual phase does not influence ventilatory responses to group III/IV afferent signaling in eumenorrheic young females

Estrogen can reduce sympathetic activity, but its effects on minute ventilation (V_E) with group III/IV afferent activation remain unclear. This study examined the influence of estrogen on V_E during lower-extremity exercise with group III/IV activation. Females completed two identical visits in follicular and ovulatory menstrual phases. Nine participants (age 25 ± 4 years) performed three minutes of baseline steady-state cycle ergometry and then group III/IV afferents were further activated with proximal thigh cuffs inflated to 20, 60, and 100 mmHg (randomized) for two minutes and five minutes of cycling between each occlusion. Metaboreflex was isolated by post-exercise circulatory occlusion. Ventilation was measured continuously and rating of perceived exertion (RPE) was recorded for each stage. During rest and exercise, V_E (p < 0.001) and tidal volume (V_T) (p = 0.033) were higher in the follicular than ovulatory phase. Minute ventilation, V_T, and respiratory rate (R_R) with ergoreflex and metaboreflex activation were similar across phases. With cuff occlusion of 100 mmHg, V_E increased from baseline by 26.3 ± 7.0 L/min in the follicular phase (p < 0.001) and by 25.3±7.7 L/min in the ovulatory phase (p < 0.001), with no difference between phases (p> 0.05); R_R and V_T increased similarly with occlusion, also with no phase differences. In eumenorrheic females, menstrual phase influences ventilation but not ventilatory responses to group III/IV isolation.

Gender Differences in Hemodynamic Regulation and Cardiovascular Adaptations to Dynamic Exercise

Exercise is a major challenge for cardiovascular apparatus since it recruits chronotropic, inotropic, pre-load, and afterload reserves. Regular physical training induces several physiological adaptations leading to an increase in both cardiac volume and mass. It appears that several gender-related physiological and morphological differences exist in the cardiovascular adjustments and adaptations to dynamic exercise in humans. In this respect, gender may be important in determining these adjustments and adaptations to dynamic exercise due to genetic, endocrine, and body composition differences between sexes. Females seem to have a reduced vasoconstriction and a lower vascular resistance in comparison to males, especially after exercise. Significant differences exist also in the cardiovascular adaptations to physical training, with trained women showing smaller cardiac volume and wall thickness compared with male athletes. In this review, we summarize these differences.

TRPV1 and BDKRB2 receptor polymorphisms can influence the exercise pressor reflex

KEY POINTS

The mechanisms responsible for the high inter-individual variability in blood pressure responses to exercise remain unclear. Common genetic variants of genes related to the vascular transduction of sympathetic outflow have been investigated, but variants influencing skeletal muscle afferent feedback during exercise have not been explored. Single nucleotide polymorphisms in TRPV1 rs222747 and BDKRB2 rs1799722 receptors present in skeletal muscle were associated with differences in the magnitude of the blood pressure response to static handgrip exercise but not mental stress. The combined effects of TRPV1 rs222747 and BDKRB2 rs1799722 on blood pressure and heart rate responses during exercise were additive, and primarily found in men. Genetic differences in skeletal muscle metaboreceptors may be a risk factor for exaggerated blood pressure responses to exercise.

ABSTRACT

Exercise blood pressure (BP) responses demonstrate high inter-individual variability, which could relate to differences in metabolically sensitive afferent feedback from the exercising muscle. We hypothesized that single-nucleotide polymorphisms (SNPs) in genes encoding metaboreceptors present in group III/IV skeletal muscle afferents can influence the exercise pressor response. Two hundred men and women underwent measurements of continuous BP and heart rate at baseline and during 2 min of static handgrip exercise (30% maximal volitional contraction), post-exercise circulatory occlusion and mental stress (serial subtraction; internal control). Participants were genotyped for SNPs in TRPV1 (rs222747; G/C), ASIC3 (rs2288645; G/A), BDKRB2 (rs1799722; C/T), PTGER2 (rs17197; A/G) and P2RX4 (rs25644; A/G). Exercise systolic BP (19 ± 10 vs. 22 ± 10 mmHg, P = 0.03) was lower in GG versus GC/CC minor allele carriers for TRPV1 rs222747, while exercise diastolic BP (14 ± 7 vs. 17 ± 7 mmHg, P = 0.007) and heart rate (12 ± 8 vs. 15 ± 9 beats min^-1 , P = 0.03) were lower in CC versus CT/TT minor allele carriers for BDKRB2 rs1799722. Individuals carrying both minor alleles for TRPV1 rs222747 and BDKRB2 rs1799722 had greater systolic (22 ± 11 vs. 17 ± 10 mmHg, P = 0.04) and diastolic (18 ± 7 vs. 14 ± 7 mmHg, P = 0.01) BP responses than those with no minor alleles; these differences were larger in men. No differences in BP or heart rate responses were detected during static handgrip with ASIC3 rs2288645, PTGER2 rs17197 or P2RX4 rs25644. None of the selected SNPs were associated with differences during mental stress. These findings demonstrate that variants in TRPV1 and BDKRB2 receptors can contribute to BP differences during static exercise in an additive manner.