Effects of Aging on Luteinizing Hormone Secretion, Ovulation, and Ovarian Tissue-Type Plasminogen Activator Expression

Abstract

This study examined the effects of aging on LH surge magnitude, ovulation, and ovarian expression of tissue-type plasminogen activator (tPA), a protease implicated in follicular rupture. While mean LH levels and ovulation rates were similar in middle-aged cyclic and young groups, there was a significant correlation between peak LH levels and ovulation rates in individual rats, such that females with lower LH surges ovulated fewer ova. In a separate experiment, proestrous LH levels were characterized in young and middle-aged rats, followed by in situ hybridization analysis of ovarian tPA mRNA. In young proestrous rats, tPA expression was observed in thecal-interstitial cells and oocytes, but not granulosa cells, prior to the LH surge. After the LH surge, there was a marked increase in tPA mRNA levels in granulosa cells of preovulatory, but not smaller follicles, peaking at 0200 hr estrus. By 0500 hr estrus, ovarian tPA expression declined, and ovulation had occurred. In contrast, LH-induced follicular tPA mRNA levels were dramatically lower in middle-aged rats with attenuated LH surges, and persisting preovulatory follicles were common in ovaries of these females on estrus morning. These findings suggest that age-related declines in ovulatory function result in part from altered induction of ovarian tPA expression, likely due to decreased proestrous LH secretion.