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

The goal of the present investigation was to determine in the squirrel monk ey the source and pattern of inhibin, a hormone known to effect reproductiv e steroid levels via pituitary and ovarian mechanisms. Since this seasonall y polyestrous species is known to have elevated serum levels of reproductiv e 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 expres sion in human luteal tissue, Squirrel monkey serum inhibin peaked 4 days af ter the luteinizing hormone surge and correlated with progesterone changes. These luteal serum levels of inhibin were greater than 12 times higher tha n the human levels yet bio-LH activities were less than in the human during the luteal phase, Inhibin concentrations during the nonbreeding season wer e generally half the levels measured in the breeding season and undetectabl e in ovariectomized animals. However, exogenous FSH stimulation induced a m arked rise in inhibin, which correlated with an estradiol rise. In conclusi on, abundant alpha inhibin subunit expression in the luteal ovary of the sq uirrel monkey and loss of serum delectability in ovariectomized animals ind icates that the principle source of inhibin in the squirrel monkey is the o vary. 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 lut eal tissue may be responsible for the exceptionally high inhibin levels. Am . J. Primatol, 47:165-179, 1999. (C) 1999 Wiley-Liss, Inc.