Estrogen attenuates the exercise pressor reflex in female cats

Abstract

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 17beta-estradiol needed to attenuate the exercise pressor reflex in male decerebrate cats was 10 microg/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 17beta-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 17beta-estradiol (0.01 microg/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 17beta-estradiol. Application of 17beta-estradiol in a dose of 0.001 microg/ml had no effect on the exercise pressor reflex (n = 5). We conclude that the concentration of 17beta-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.

Estrogen’s attenuating effect on the exercise pressor reflex is more opioid dependent in gonadally intact than in ovariectomized female cats

“…The solution remained in the well throughout the experiment and the responses to the exercise pressor reflex were tested 30, 45, and 60 min after the estrogen solution had been applied. The stability of the exercise pressor reflex over this 60-min time period has been established (24,25). Similarly, estrogen has been shown to attenuate the reflex over this time period (24,25).…”

“…Therefore, we choose our dosage of estrogen to be in the range of 70 times the physiological estrogen concentration in blood during peak estrus. Our previous study showed that an estradiol concentration of 0.01 g/ml was sufficient to attenuate the exercise pressor reflex in gonadally intact female cats 30 min after topical application to the cord (24). We choose the naloxone dose (1,000 g) because it was found in a previous study (11) to restore the pressor response to contraction that was attenuated by the ␦-opioid…”

“…The stability of the exercise pressor reflex over this 60-min time period has been established (24,25). Similarly, estrogen has been shown to attenuate the reflex over this time period (24,25). At the end of the procedure, cats were euthanized and presence or absence of ovaries was confirmed.…”

“…Recently, spinally applied estrogen was found to attenuate the arterial pressure component of the exercise pressor reflex in decerebrated cats. This attenuation was shown to be gender specific because the threshold concentration of estrogen needed to attenuate the pressor response to contraction was 1,000 times more dilute in female cats than it was in male cats (24,25).…”

“…Recently, spinally applied estrogen was found to attenuate the arterial pressure component of the exercise pressor reflex in decerebrated cats. This attenuation was shown to be gender specific because the threshold concentration of estrogen needed to attenuate the pressor response to contraction was 1,000 times more dilute in female cats than it was in male cats (24,25).The mechanism by which spinally applied estrogen attenuated the reflex pressor response to static contraction is unknown. Nevertheless, -opioid receptors and ␦-opioid receptors have been implicated in the spinal regulation of the exercise pressor reflex in cats (8,11,18).…”