Attenuated proestrous luteinizing hormone surges in middle-aged rats are associated with decreased pituitary luteinizing hormone-beta messenger ribonucleic acid expression.

Abstract

This study determined whether attenuated preovulatory luteinizing hormone surges in aging rats are associated with a decrease in pituitary luteinizing hormone content or luteinizing hormone beta-messenger ribonucleic acid expression on proestrus. Blood samples were taken every 90 minutes from 1:30 to 10:30 PM on proestrus in young (n = 8) and middle-aged (n = 12), regularly cyclic rats for plasma luteinizing hormone determination. On the next proestrus at 12 noon, rats were killed and the pituitaries were removed for luteinizing hormone content determination by radioimmunoassay and luteinizing hormone beta-messenger ribonucleic acid expression by dot blot analysis. Results were analyzed by one-way analysis of variance. Seven of the middle-aged rats had attenuated luteinizing hormone surges while the remaining five females had surges similar to those of young rats. On the next proestrus, all rats had similar quantities of pituitary luteinizing hormone. However, luteinizing hormone beta-messenger ribonucleic acid expression in middle-aged rats with attenuated luteinizing hormone surges was lower (p less than 0.05) than that of middle-aged and young rats with normal surges. Decreased luteinizing hormone beta-messenger ribonucleic acid expression, but not pituitary luteinizing hormone content at 12 noon on proestrus is correlated with attenuated luteinizing hormone surges in middle-aged rats.