Age-associated increases in serum follicle-stimulating hormone levels on estrus are accompanied by a reduction in the ovarian secretion of inhibin

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Abstract
In light of recent data demonstrating age-related alterations in the secretion and production of follicle-stimulating hormone (FSH) during the secondary FSH surge on estrus, the following study was conducted to determine the effects of age on periovulatory inhibin secretion. Ovarian venous blood was collected from groups of ether-anesthetized 3- and 7-month-old rats exhibiting 4-day estrous cycles at the following times: 1200 and 2400 h on proestrus and 1600 h on estrus. Following a 10-min collection period, a terminal blood sample was obtained from the abdominal aorta. Peripheral serum concentrations of luteinizing hormone (LH), FSH, estradiol-17β (E2), progesterone (P) and testosterone (T) were measured by RIA. Inhibin activity in ovarian venous serum (OVS) was assessed by the ability of OVS to suppress basal FSH secretion from dispersed pituitary cells during a 24-hour culture period. At 1200 h on proestrus, serum FSH (and LH) levels were higher in 7-month-old rats than in younger rats while the FSH-suppressing activity of OVS did not differ between age groups at this time. Bioassayable inhibin activity substantially declined between 1200 and 2400 h on proestrus in both groups. By 1600 h on estrus, serum FSH levels and inhibin secretion were higher and lower, respectively, in the older age group compared to 3-month-old rats. Significant increases in inhibin secretion between 2400 h on proestrus and 1600 h on estrus were observed only in younger rats. Blood samples obtained at 1700 h on proestrus from rats whose ovarian venous blood subsequently was collected at 2400 h revealed no age-related difference in peak LH and FSH levels attained during the preovulatory surge. Serum levels of E2, P and T were similar in 3- and 7-month-old rats except at 1200 h on proestrus and 1600 h on estrus when P and T levels respectively, were higher in older rats. These results demonstrate that a reduction in ovarian inhibin secretion accompanies age-associated alterations in FSH secretion on estrus and support the contention that previously documented alterations in FSH synthesis and release may be attributed to age-associated decreases in inhibin secretion.

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