Joyner, Michael J.

Background: Fatiguing inspiratory muscle work can trigger the sympathetically-mediated respiratory muscle metaboreflex. Activating this reflex has been shown to increase blood pressure and heart rate while decreasing peripheral blood flow. Studies have shown females have an attenuated mean arterial pressure response (MAP) compared to males despite similar workloads to fatiguing inspiratory muscle work, suggesting females may have an attenuated respiratory muscle metaboreflex. However, the sex difference effects of respiratory muscle metaboreflex effects on resting limb vascular conductance (LVC) remains unknown. The purpose of this study was to investigate whether sex differences exist in LVC responses to fatiguing inspiratory muscle work. We hypothesize that males would exhibit a greater decrease in LVC compared to females. Methods: Eight healthy females (F; 35±11 years) and 8 healthy males (M; 31±6 years) completed the study. Participants completed a 5-minute baseline followed by a 5-minute inspiratory pressure threshold loading (IPTL) trial. The breathing pattern was set to 60% of their maximal inspiratory mouth pressure with a breathing frequency of 15 breaths/minute and an inspired duty cycle of 0.7 to activate the respiratory muscle metaboreflex. CO 2 was introduced into the inspired circuit to prevent hypocapnia. Continuous MAP (finger photoplethysmography) and heart rate (electrocardiogram) were recorded. Continuous blood velocity and diameters of the superficial femoral artery were measured using duplex ultrasonography. Leg blood flow (LBF) was calculated as the product of mean blood velocity and femoral artery cross-sectional area. LVC was calculated as LBF/mean arterial pressure. These data were averaged in 30-second segments and reported from 5-minute mark of IPTL (End-Task % of baseline). Groups were compared using Student’s t -test and analyzed using a two-way repeated-measures ANOVA. Results: Pulmonary function was within predicted ranges for all participants (FEV 1 /FVC: [F vs. M]: 100±8 vs. 100±9%predicted; P=0.919). Heart rate ([Baseline: F vs. M]: 70±13 vs. 73±8 bpm; P=0.571) and MAP (Baseline: [F vs. M]: 90±12 vs. 95±8 mmHg; P=0.378) were not different between females and males at baseline or throughout the IPTL trial (Group Main Effect: p>0.05 for both). Additionally, LBF decreased in response to IPTL but there was no difference between groups (End-Task: [F vs. M]: -31±33 vs. -21±24%baseline; Time Main Effect: P=0.012, Group Main Effect: P=0.352). When accounting for blood pressure, LVC also decreased during IPTL but was not different between females and males (End-Task: [F vs. M]: -37±30 vs. -33±24%baseline; Time Main Effect: p<0.001, Group Main Effect: P=0.655). Conclusion: These preliminary data suggest LVC decreases during a 5-minute period of fatiguing inspiratory work using IPTL; however, IPTL does not elicit variable LVC decreases between females and males. NIH (HL139854); NSERC (PDF-577750-2023) This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Background: During acute hypoxia, Thehuman brainemploys complex regulatory mechanisms to compensate for high O 2 demand and low O 2 blood saturation. High hemoglobin-oxygen (Hb-O 2 ) affinity improves arterial oxygen saturation (S a O 2 ) in low oxygen environments providing a potential physiologic benefit in humans. However, the impact of high Hb-O 2 affinity on cerebral blood flow (CBF) and cerebral tissue oxygenation (S t O 2 ) has yet to be examined. This work aimed to determine the influence of an acute increase in Hb-O 2 affinity from Etavopivat, a selective red blood cell pyruvate kinase activator, on CBF regulation and S t O 2 during handgrip exercise in normoxia and hypoxia. Methods: 16 healthy adults (31±8 yr, 5 females) completed two visits; before and after Etavopivat (400 mg daily, 7 days) administration,. Each visit included graded handgrip (~3 min stages each) exercise (rest, 10% maximal voluntary contraction (MVC), 20% MVC) while participants inspired normoxic (NORM, 21% O 2 ), moderate (MH, 15% O 2 ), and severe (SH, 10% O 2 ) hypoxic gas mixtures. Hb-O 2 binding affinity (P 50 ), was measured using dual-wavelength spectrophotometry (pH~7.4 and 37ºC). Blood sampling through catheterization of the non-exercising arm measured S a O 2 . CBF was approximated by the peak middle cerebral artery velocity, measured with a 2MHz transcranial doppler ultrasound through the transtemporal window. S t O 2 was measured using frequency-domain near infrared spectroscopy of the frontal cortex. CBF and S t O 2 between visits were compared using separate two-way RM ANOVA models to determine the effects of Etavopivat [before (baseline) or after 7d Etavopivat exposure (post)] and exercise intensity [rest, 10% MVC, 20% MVC] for each inspirate. Results: P 50 was reduced after Etavopivat exposure (Baseline: 28.6±1.6 mmHg, Etavo: 24.8±1.3, P <0.001) without impacting hematocrit ( P =0.97). S a O 2 was higher following Etavopivat exposure during each inspirate ( P <0.001 for each). There was no significant effect of handgrip exercise on CBF or S t O 2 within any inspirate. Cerebral S t O 2 was higher during each inspirate (all P <0.002)– NORM (Baseline: 65.3±12.3%, Post: 68.8±9.8%), MH (Baseline: 61.0±11.7 %, Post: 66.1±10.5%) and SH (Baseline: 47.7±13.5%, Post: 56.6±11.5%). In addition, CBF was increased during each inspirate (all P <0.001)– NORM (Baseline: 62.2±12.2 cm/s, Post: 69.2±12.5 cm/s), MH (Baseline: 60.8±13.1 cm/s, Post: 67.9±12.3 cm/s) and SH (Baseline: 69.8±16.3 cm/s, Post: 79.2±18.0 cm/s). Conclusion: The acute pharmacological induction of high Hb-O 2 affinity leads to increased CBF and cerebral S t O 2 during acute hypoxic exposure in conditions of mild muscular exertion. Whether the improvement of cerebral S t O 2 is associated increased S a O 2 or CBF (or both) is unclear. These findings are the first to investigate the complex determinants of cerebral S t O 2 as a function of an acute increase in Hb-O 2 affinity. These improvements in S t O 2 during hypoxic exposure may lead to a host of theoretical benefits such as improved hypoxia tolerance, and mitigated decrements in cognitive function during hypoxic exposure. This study was funded by Forma Therapeutics Inc, which was acquired by Novo Nordisk on October 14, 2022, Novo Nordisk A/S. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

There is a context-dependent trade-off in body size of elite runners, such that smaller body sizes are observed among longer distance runners. However, it is unclear whether this trade-off in body size is observed in other Olympic sports, such as cycling and swimming. To understand the association between body size and athletic competition, we examined metrics of body size from male and female Olympic athletes competing in swimming, running, and cycling. We collected standard anthropometrics (height, mass, body mass index (BMI), and body surface area) of elite male and female athletes competing in the London 2012 Summer Olympics from a public repository. Anthropometric data were compared between sexes (male and female); between sports (swimming, running, and cycling); and between distances within sports (shorter and longer distance). Males were taller ( P < 0.001), heavier ( P < 0.001), and had a larger BMI ( P < 0.001) compared to females. Relative to athletes competing in longer distance events, athletes competing in shorter distance were generally taller (running: P < 0.001, swimming: P ≤ 0.014) and heavier (running: P < 0.001, swimming: P = 0.002 (males) and P = 0.148 (females), cycling: P < 0.001) for both males and females. Height was not different between shorter and longer distance cyclists. There was a trade-off between body size and distance for each of the three Olympics sports, such that smaller athletes were observed in longer distance events. Although there were large sex-based differences in body size, similar trade-offs in size and event were observed for both males and females. These data suggest that there is an optimal combination of skeletal muscle mass and body size to optimize movement economy that is generally preserved across different modes of human locomotion.

Abstract In this article, we highlight the contributions of passive experiments that address important exercise‐related questions in integrative physiology and medicine. Passive experiments differ from active experiments in that passive experiments involve limited or no active intervention to generate observations and test hypotheses. Experiments of nature and natural experiments are two types of passive experiments. Experiments of nature include research participants with rare genetic or acquired conditions that facilitate exploration of specific physiological mechanisms. In this way, experiments of nature are parallel to classical “knockout” animal models among human research participants. Natural experiments are gleaned from data sets that allow population‐based questions to be addressed. An advantage of both types of passive experiments is that more extreme and/or prolonged exposures to physiological and behavioral stimuli are possible in humans. In this article, we discuss a number of key passive experiments that have generated foundational medical knowledge or mechanistic physiological insights related to exercise. Both natural experiments and experiments of nature will be essential to generate and test hypotheses about the limits of human adaptability to stressors like exercise. © 2023 American Physiological Society. Compr Physiol 13:4879‐4907, 2023.

Abstract Objectives The aim of this study was to compare the measured physiological factors that limit running performance with real marathon results from world-class distance runners, evaluating the compatibility between measured data and predicted results based on the previously suggested model. Methods Four world-class East African marathon runners (three male, one female) underwent physiological running assessments to predict marathon performance times using a model based on V ˙ $\dot {\mathrm{V}}$ O 2 peak, percentage of V ˙ $\dot {\mathrm{V}}$ O 2 peak at the second ventilatory threshold, and running economy. Predictions were then compared to participants’ best marathon times. Results The measured V ˙ $\dot {\mathrm{V}}$ O 2 peak of the world-class runners was 75.1 ± 2.7 mL/kg/min. The second ventilatory threshold occurred at 85 ± 3 % of the peak, with a running economy of 63.7 ± 2.4 mL/kg/min at 19.6 ± 0.9 km/h. The predicted marathon performance time was 2:06:51 ± 0:03:17 h:min:s for the males and 2:17:36 h:min:s for the female. Comparing these predictions to their personal best times, the average difference was 00:55 ± 00:51 min:s (range: 00:20-02:08). Conclusions This research provides laboratory data on world-class road running athletes, reinforcing the link between marathon performance and V ˙ $\dot {\mathrm{V}}$ O 2 peak, the percentage of V ˙ $\dot {\mathrm{V}}$ O 2 peak at the second ventilatory threshold, and running economy. The examined athletes had lower V ˙ $\dot {\mathrm{V}}$ O 2 peak compared to predicted values, highlighting the importance of running economy and fractional utilization of V ˙ $\dot {\mathrm{V}}$ O 2 peak in achieving such performances. Future studies should continue to advance the field by including additional bioenergetic parameters measured during race conditions and expanding the participant cohort of elite marathoners, encompassing both sexes.

Abstract The autonomic nervous system is a key regulator of the cardiovascular system. In this review, we focus on how sex and aging influence autonomic regulation of blood pressure in humans in an effort to understand general issues related to the cardiovascular system as a whole. Younger women generally have lower blood pressure and sympathetic activity than younger men. However, both sexes show marked interindividual variability across age groups with significant overlap seen. Additionally, while men across the lifespan show a clear relationship between markers of whole body sympathetic activity and vascular resistance, such a relationship is not seen in young women. In this context, the ability of the sympathetic nerves to evoke vasoconstriction is lower in young women likely as a result of concurrent β 2 ‐mediated vasodilation that offsets α‐adrenergic vasoconstriction. These differences reflect both central sympatho‐inhibitory effects of estrogen and also its influence on peripheral vasodilation at the level of the vascular smooth muscle and endothelium. By contrast postmenopausal women show a clear relationship between markers of whole body sympathetic traffic and vascular resistance, and sympathetic activity rises progressively in both sexes with aging. These major findings in humans are discussed in the context of differences in population‐based trends in blood pressure and orthostatic intolerance. The many areas where there is little sex‐specific data on how the autonomic nervous system participates in the regulation of the human cardiovascular system are highlighted. © 2015 American Physiological Society. Compr Physiol 5:193‐215, 2015.

AbstractMenopause is associated with vascular dysfunction. During the menopausal transition, endogenous oestradiol concentrations diminish. Oestradiol is vasoprotective because it has direct and indirect effects on the vasculature. The present study aimed to determine the effect of acute exogenous oestradiol on endothelium‐dependent, endothelium independent and β2‐adrenergic receptor‐induced vasodilatation in females. Forearm blood flow (venous occlusion plethysmography) was measured during brachial intraarterial infusions of ACh (endothelium‐dependent agonist), sodium nitroprusside (endothelium independent agonist) and terbutaline (β2‐adrenergic receptor agonist) with and without concurrent infusion of 17β‐oestradiol. Nine young premenopausal (age: 26 ± 4 years) and nine postmenopausal (PM, age: 58 ± 4 years, 8 ± 1 years post‐menopause) females completed the study. Concurrent oestradiol infusion augmented the vasodilatory response to ACh, sodium nitroprusside and terbutaline in young premenopausal (all P < 0.05) but not older postmenopausal (all P > 0.05), females. Local infusion of exogenous 17β‐oestradiol augmented endothelial and smooth muscle microvascular vasodilatation in premenopausal but not postmenopausal, females. imageKey points Menopause is associated with vascular dysfunction. Because oestradiol has vasoprotective effects, the menopause‐associated drop in oestradiol concentrations is hypothesized to contribute to vascular dysfunction during the menopause transition. The present study shows that local infusion of exogenous oestradiol augmented microvascular vasodilatation in premenopausal but not postmenopausal females.