On the haemodynamic consequence of the chemoreflex and muscle mechanoreflex interaction in women and men: two tales, one story.

The cardiovascular response resulting from the individual activation of the muscle mechanoreflex (MMR) or the chemoreflex (CR) is different between men and women. Whether the haemodynamic consequence resulting from the interaction of these sympathoexcitatory reflexes is also sex-dependent remains unknown. MMR and CR were activated by passive leg movement (LM) and exposure to hypoxia (O2 -CR) or hypercapnia (CO2 -CR), respectively. Twelve young men and 12 young women completed two experimental protocols: (1) resting in normoxia (PET O2 : ∼83 mmHg, PET CO2 : ∼34 mmHg), normocapnic hypoxia (PET O2 : ∼48 mmHg, PET CO2 : ∼34 mmHg) and hyperoxic hypercapnia (PET O2 : ∼524 mmHg, PET CO2 : ∼44 mmHg); (2) LM under the same gas conditions. During the MMR:O2 -CR coactivation, in men, the observed mean arterial pressure (MAP) and cardiac output (CO) were not different (additive effect), while the observed leg blood flow (LBF) and vascular conductance (LVC) were significantly lower (hypo-additive), compared with the sum of the responses elicited by each reflex alone. In women, the observed MAP was not different (additive) while the observed CO, LBF and LVC were significantly greater (hyper-additive), compared with the summated responses. During the MMR:CO2 -CR coactivation, in men, the observed MAP, CO and LBF were not different (additive), while the observed LVC was significantly lower (hypo-additive), compared with the summated responses. In women, the observed MAP was significantly higher (hyper-additive), while the observed CO, LBF and LVC were not different (additive), compared with the summated responses. The interaction of the MMR and CR has a pronounced influence on the autonomic cardiovascular control, with the haemodynamic consequences differing between men and women. KEY POINTS: The cardiovascular response resulting from the activation of the muscle mechanoreflex (MMR) or the chemoreflex (CR) was previously shown to be different between women and men; this study focused on the haemodynamic consequence of the interaction of these two sympathoexcitatory reflexes. MMR and CR were activated by passive leg movement and exposure to hypoxia (O2 -CR) or hypercapnia (CO2 -CR), respectively. Individual and interactive reflex effects on central and peripheral haemodynamics were quantified in healthy young women and men. In men, the MMR:O2 -CR and MMR:CO2 -CR interactions restricted peripheral haemodynamics, likely by potentiating sympathetic vasoconstriction. In women, the MMR:O2 -CR interaction facilitated central and peripheral haemodynamics, likely by potentiating sympathetic vasodilatation; however, the MMR:CO2 -CR interaction was simply additive for the central and peripheral haemodynamics. The interaction between the MMR and the CR exerts a profound influence on the autonomic control of cardiovascular function in humans, with the haemodynamic consequences differing between women and men.