ALTERATIONS IN PLASMA AND OVARIAN IMMUNOREACTIVE INHIBIN DURING REPRODUCTIVE AGING IN THE FEMALE RAT

Previously, we showed that ovarian inhibin alpha- and beta(A)-subunit mRNAs are elevated in middle-aged and old persistent-estrous (PE) fema le rats. To determine whether higher inhibin subunit mRNA expressions result in increased circulating inhibins during reproductive aging, pl asma immunoreactive inhibin alpha (ir-inh alpha) and gonadotropins wer e measured in young, middle-aged and PE rats. Plasma LH profiles were distinctly divergent in the middle-aged rats with some showing LH surg es indistinguishable from young rats and others showing significantly attenuated LH surges. Plasma ir-inh alpha in middle-aged rats with LH surges were similar to those of young rats. However, animals with atte nuated LH surges had higher peak ir-inh alpha levels than young and mi ddle-aged animals with LH surges. Immunohistochemistry revealed increa sed levels of ovarian inhibin alpha staining in those animals with att enuated LH surges. Overall, the highest plasma and ovarian inhibin alp ha were found in PE rats which lack LH surges. However, significant de creases of plasma and ovarian inhibin alpha were seen after reinstatem ent of estrous cyclicity with progesterone implant treatment. Thus, in creases in both plasma and ovarian inhibin alpha appear to be closely associated with attenuation or loss of the preovulatory gonadotropin s urge that occurs during aging.