Estrogen attenuates the exercise pressor reflex in female cats

In humans, the pressor and muscle sympathetic nerve responses to static exercise are less in women than in men. The difference has been attributed to the effect of estrogen on the exercise pressor reflex. Estrogen receptors are abundant in areas of the dorsal horn receiving input from group III and IV muscle afferents, which comprise the sensory limb of the exercise pressor reflex arc. These findings prompted us to investigate the effect of estrogen on the spinal pathway of the exercise pressor reflex arc. Previously, we found that the threshold concentration of 17β-estradiol needed to attenuate the exercise pressor reflex in male decerebrate cats was 10 μg/ml (Schmitt PM and Kaufman MP. J Appl Physiol 94: 1431-1436, 2003). The threshold concentration for female cats, however, is not known. Consequently, we applied 17β-estradiol to a well covering the L₆-S₁ spinal cord in decerebrate female cats. The exercise pressor reflex was evoked by electrical stimulation of the L₇ or S₁ ventral root, a maneuver that caused the hindlimb muscles to contract statically. We found that the pressor response to contraction averaged 38 ± 7 mmHg before the application of 17β-estradiol (0.01 μg/ml) to the spinal cord, whereas it averaged only 23 ± 4 mmHg 30 min after application (P < 0.05). Recovery of the pressor response to contraction was not obtained for 2 h after application of 17β-estradiol. Application of 17β-estradiol in a dose of 0.001 μg/ml had no effect on the exercise pressor reflex (n = 5). We conclude that the concentration of 17β-estradiol required to attenuate the exercise pressor reflex is 1,000 times more dilute in female cats than that needed to attenuate this reflex in male cats.