Abstract
The level of estrogen during proestrus and estrus phases (P‐E) in the estrous cycle of most mammalian females is higher than that during metestrus and diestrous phases (M‐D). Muscle contraction stimulates thin fiber muscle afferents and reflexly evokes sympathoexcitation. Previous literature has shown that estrogen attenuates the muscle reflex in female cats. We examined the effect of the estrous cycle on the muscle reflex in female rats. The estrous cycle was judged by vaginal smear. Electrically induced 30 s hindlimb muscle contraction in decerebrate rats during P‐E (n=6) evoked less (P<0.05) increases in mean arterial pressure as compared to rats during M‐D (n=6) (+8 ± 4 vs. +25 ± 4 mmHg; PE vs. M‐D). The renal sympathetic nerve activity (RSNA) response to 1 min intermittent (1–4 s stimulation to relaxation) bouts of static contraction in rats during P‐E (n=6) was also significantly less than that in rats during M‐D (n=6) [+35 ± 10 vs. +96 ± 15 arbitrary unit (a.u.)]. The RSNA elevation seen during 30 s tendon stretch, a stimulus of mechanically sensitive muscle afferents, was significantly less in rats during P‐E (n=12) than that in rats during M‐D (n=10) (+161 ± 26 vs. +266 ± 36 a.u.). Collectively, we suggest that estrous cycle influences the muscle reflex through an effect on mechanically sensitive muscle afferents. Estrogen may play a role in attenuating the muscle reflex seen during P‐E. Supported by KAKEN 21590255.
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