The interaction between the carotid chemoreflex and the muscle mechanoreflex: differential cardiovascular consequences in men and women

Background Although the carotid chemoreflex (CR) and the muscle mechanoreflex (MR) are recognized as sympatho‐excitatory feedback mechanisms, the consequence of their interaction for circulatory control remains unclear in humans. Purpose To evaluate the cardiovascular consequence of the interaction between the CR and the MR in healthy men and women. Method We administered a hypoxic inspirate and passive leg movement (PLM) to activate the CR and the MR, respectively. Thirteen volunteers (7 males, 6 females) completed 2 experimental sessions. In one session, subjects were seated resting in a normoxic control condition (Norm Rest ; S p O 2 ~97%, P ET O 2 ~84 mmHg, P ET CO 2 ~33 mmHg) and in isocapnic hypoxia (Hypo Rest ; S p O 2 ~85%, P ET O 2 ~48 mmHg, P ET CO 2 ~33 mmHg). In the other session, PLM was performed for 1 min in the conditions of normoxia (Norm PLM ) and isocapnic hypoxia (Hypo PLM ). Thus, while Norm Rest was considered a condition with no reflexes activated, Hypo Rest , Norm PLM , and Hypo PLM activated the CR alone, the MR alone, and both of the CR and the MR, respectively. All sessions and conditions were conducted in randomized order. Mean arterial blood pressure (MAP), cardiac output (CO), and femoral blood flow (Q L ) were quantified using finger photoplethysmography and Doppler ultrasound. Results The CR activation alone (i.e., ΔHypo Rest ‐Norm Rest ) significantly increased CO in both men and women (~0.6 L min −1 ). The MR activation alone by PLM (i.e., ΔNorm PLM ‐Norm Rest ) decreased MAP (~−7 mmHg; p < 0.05), but significantly increased CO (~1.1 L min −1 ), Q L (~1.0 L min −1 ), and leg vascular conductance (LVC, ~11 ml min −1 mmHg −1 ) in both groups. During co‐activation of the CR and the MR (i.e., ΔHypo PLM ‐Norm Rest ) in men, the observed Q L and LVC responses were significantly lower when compared with the summated responses evoked by each reflex alone (Q L : 1.1 ± 0.1 vs. 1.4 ± 0.2 L min −1 ; LVC: 11 ± 1 vs. 14 ± 2 ml min −1 mmHg −1 ); there were no differences between the observed and summated responses of MAP and CO. In contrast, CO, Q L , and LVC observed during the reflex co‐activation in women were significantly greater than the summated responses to each reflex alone (CO: 1.5 ± 0.5 vs. 1.0 ± 0.4 L min −1 ; Q L : 0.9 ± 0.1 vs. 0.6 ± 0.1 L min −1 ; LVC: 9 ± 2 vs. 7 ± 2 ml min −1 mmHg −1 ), whereas the observed MAP did not differ from the summated response. Conclusion In men, the interaction of the CR and the MR is additive in terms of MAP and CO, but hypo‐additive in terms of Q L and LVC (i.e., impedes peripheral hemodynamics). In women, the interaction of the CR and the MR is additive in terms of MAP, but hyper‐additive in terms of CO, Q L , and LVC (i.e., facilitates central and peripheral hemodynamics). The sex‐related discrepancy in the peripheral hemodynamic responses to the reflex interaction is perhaps due to greater β ‐adrenoreceptor‐mediated vasodilation in women compared with men. Taken together, despite different cardiovascular consequences, our findings suggest that the interaction between the CR and the MR further augments sympatho‐excitation in healthy men and women. Support or Funding Information National Heart, Lung, and Blood Institute grants (HL‐116579)