| The exercise pressor reflex, which arises from the contraction-induced stimulation of group III and IV muscle afferents, is believed to be evoked by metabolic stimuli signaling a mismatch between blood/oxygen demand and supply in the working muscles. Nevertheless, mechanical stimuli may also play a role in evoking this reflex. We, therefore, examined the effect of gadolinium, which blocks mechanosensitive channels, on the muscle reflex in cats. Gadolinium significantly attenuated the reflex pressor responses to static contraction and to tendon stretch. In contrast, gadolinium had no effect on the reflex pressor response to femoral arterial injection of capsaicin. Gadolinium significantly attenuated the responses to both contraction and stretch of group III muscle afferents, many of which are mechanically sensitive. Gadolinium, however, had no effect on the responses to contraction or to capsaicin injection of group IV muscle afferents, many of which are metabolically sensitive. We conclude that mechanical stimuli arising in contracting muscles contribute to the elicitation of the muscle reflex.;Cardiorespiratory responses to exercise are smaller in women than in men, a difference which has been attributed to estrogen. In decerebrate male cats, we examined the effect of estrogen on the cardiorespiratory responses to central command and the muscle reflex. 17beta-estradiol attenuated the pressor, cardioaccelerator, and phrenic nerve responses to stimulation of the mesencephalic locomotor region (i.e., central command). In contrast, 17beta-estradiol had no effect on the responses to static contraction or tendon stretch. We conclude that estrogen attenuates cardiorespiratory responses to central command but has no effect on the exercise pressor reflex.;Both central command and the exercise pressor reflex have been shown to increase whole nerve renal sympathetic activity. However, these mechanisms may activate either the same or different populations of renal sympathetic postganglionic efferents. In decerebrate cats, we examined the responses of single renal sympathetic postganglionic efferents to central command and contraction. We found that in 24 of 28 fibers either central command or the muscle reflex, but not both, significantly increased the discharge of these efferents. This finding supports the theory that central command and the exercise pressor reflex activate separate renal sympathetic pathways. |
