The effects of castration and sex steroids on ventilatory control in male guinea pigs

Male guinea pigs (N = 7) given progesterone hyperventilate with Pa_CO2 fallings from control levels of 39.0 ± 0.6 (SEM) Torr to 32.0 ± 0.7 Torr after 7 days of treatment. This response was associated with a rise in plasma progesterone concentration to approximately 15 ng ml^-1 and a transient rise in plasma 17β-estradiol concentrations. To determine the role of the testes in generating teh transient estrogen increase as well as the significance of estrogens to the progesterone response, male guinea pigs were castrated and treated either with estradiol or a placebo. Estrogen-treated castrates (N = 15) had a mean Pa_CO2 of 36.1 ± 0.8 Torr and the castrates given the placebo (N = 7) had an average Pa_Co2 of 43.8 ± 1.2 Torr (P < 0.001). Both of these castrate groups were also different from intact, untreated males (P < 0.01). Progesterone concentrations were very low and not different. When progesterone was additionally administered, the Pa_CO2's fell to 33.0 ± 0.8 and 38.2 ± 0.6 Torr for the estrogen- and placebo-treated castrate groups, respectively. The male guinea pig hyperventilates when given progesterone with the magnitude and time course of his response comparable to the human's. The response to progesterone is not critically dependent on the testes or on plasma estrogen concentrations; however, both castration and exogenous estrogen appear to influence Pa_CO2 without altering plasma progesterone concentrations.