Abstract
Although an ovariectomy is the routine approach used to study the role of ovarian hormones on respiratory control, the results have often been contradictory. We tested the hypothesis that the ventilatory response to hypoxia is modified by the age at which the ovariectomy is performed. Female rats were ovariectomized either atan early (3 weeks old, i.e., prepubertal) or late (10 weeks old, i.e., adult) stage, and ventilation was then assessed at 12 weeks of age using whole-body plethysmography. The control group included sham-operated rats that had undergone the same surgical procedure but were not ovariectomized. Independent of the age at which surgery was performed, ovariectomy significantly decreased circulating progesterone and 17-b-estradiol levels without re-ducing them below their detection threshold. Despite that decrease, there was no difference in baseline minute ventilation or in the ventilatory response to hypoxia (FiO2 = 12%, 20 min; expressed as the percentage of increase from baseline) between ovariectomized and shamoperated rats. These results suggest that ovariectomy at either a young or at an adult age is insufficient to completely suppress circulating hormones and disrupt the regulation of ventilation.
Highlights
In adult mammals, including humans, an important body of evidence shows that the ovarian hormones progesterone and estradiol are potent respiratory stimulants [1,2]
We tested the hypothesis that performing ovariectomy at an early age and not at an adult age disrupts the hypoxic response in rats
The findings observed in this study clearly show that the hypoxic ventilatory response remains unaffected in adult female rats (12 weeks old), independent of the age at which the ovariectomy was performed (3 or 10 weeks), despite a significant decrease in plasma estradiol and progesterone levels
Summary
In adult mammals, including humans, an important body of evidence shows that the ovarian hormones progesterone and estradiol are potent respiratory stimulants [1,2]. Both hormones act on peripheral (via carotid body chemoreceptors) and central respiratory control systems (via the medullary nucleus) to increase breathing in response to either hypoxia or hypercapnia [2,3]. That contradiction may partly depend on the delay between the surgery modulating the level of circulating ovarian hormones and the assessments of ventilation In this regard, we tested the hypothesis that performing ovariectomy at an early age and not at an adult age disrupts the hypoxic response in rats. The plasma levels of ovarian hormones were assessed to evaluate the potential relationship between hormone levels and ventilation
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