Elevated ovarian expression and serum concentration of α inhibin in the luteal phase during follicular development in the squirrel monkey (Saimiri boliviensis) compared to the human

Abstract

The goal of the present investigation was to determine in the squirrel monkey the source and pattern of inhibin, a hormone known to effect reproductive steroid levels via pituitary and ovarian mechanisms. Since this seasonally polyestrous species is known to have elevated serum levels of reproductive steroids compared to other primates, the levels of ovarian alpha subunit mRNA expression and serum total alpha inhibin, estradiol, progesterone, and luteinizing hormone were measured and compared to human levels. Expression of the alpha subunit was robust in monkey luteal tissue compared to expression in human luteal tissue. Squirrel monkey serum inhibin peaked 4 days after the luteinizing hormone surge and correlated with progesterone changes. These luteal serum levels of inhibin were greater than 12 times higher than the human levels yet bio-LH activities were less than in the human during the luteal phase. Inhibin concentrations during the nonbreeding season were generally half the levels measured in the breeding season and undetectable in ovariectomized animals. However, exogenous FSH stimulation induced a marked rise in inhibin, which correlated with an estradiol rise. In conclusion, abundant alpha inhibin subunit expression in the luteal ovary of the squirrel monkey and loss of serum delectability in ovariectomized animals indicates that the principle source of inhibin in the squirrel monkey is the ovary. Elevated serum inhibin levels during the luteal phase concurrent with ovulatory-size follicular development is unique among species studied thus far. Possible simultaneous inhibin production from both follicular and luteal tissue may be responsible for the exceptionally high inhibin levels.

Midcycle and luteal elevations of follicle stimulating hormone in squirrel monkeys (Saimiri boliviensis) during the estrous cycle

“…The presently observed inverse relationship between the levels of FSH and estradiol suggests an inhibitory feedback similar to that in sheep [Findlay et al, 1990]. Our recent report of inhibin patterns in the squirrel monkey shows a fall starting three days before the LH surge and then rising the second day after the LH surge [Yeoman et al, 1999]. These elevations of FSH and inhibin within the luteal phase suggest that inhibin is not the major inhibitory feedback to FSH during this phase in the squirrel monkey.…”

Midcycle and luteal elevations of follicle stimulating hormone in squirrel monkeys (Saimiri boliviensis) during the estrous cycle

“…Our recent report of inhibin patterns in the squirrel monkey shows a fall starting three days before the LH surge and then rising the second day after the LH surge [Yeoman et al, 1999]. These elevations of FSH and inhibin within the luteal phase suggest that inhibin is not the major inhibitory feedback to FSH during this phase in the squirrel monkey.…”