Effects of estrogen and progesterone on carotid body neural output responsiveness to hypoxia.

Pregnancy increases ventilatory and carotid body neural output (CBNO) responsiveness to hypoxia in cats (J. Appl. Physiol. 67: 797-803, 1989). To determine whether progesterone and estrogen stimulated hypoxic ventilatory and CBNO responsiveness, we studied 24 castrated male cats before and after 1 wk of placebo, estrogen, progesterone, or estrogen plus progesterone treatment. Estrogen plus progesterone treatment decreased end-tidal PCO2 (-3.8 +/- 0.8 Torr) and increased hypoxic ventilatory responsiveness, whereas estrogen or progesterone alone had no effect. Animals receiving progesterone alone or in combination with estrogen had higher CBNO responsiveness than placebo or estrogen-treated animals (shape parameter A = 45 +/- 7 vs. 27 +/- 4, P less than 0.05). However, the group treated with estrogen plus progesterone did not have greater CBNO responsiveness to hypoxia than the group receiving progesterone alone. The cross plot of the simultaneously measured CBNO and ventilation during progressive hypoxia revealed a greater slope in the estrogen-treated than in the placebo animals, suggesting that estrogen treatment increased central nervous system transduction of CBNO into ventilation. Thus the data taken together suggested that progesterone and estrogen had a combination of peripheral (carotid body) and central sites of action such that the administration of both hormones together had a more consistent stimulatory effect on hypoxic ventilatory responsiveness than either hormone alone.