Studies on prolactin-secreting cells in aging rats of different strains. I. Alterations in pituitary histology and serum prolactin levels as related to aging.

Serum PRL levels and histologically tumor-free pituitary glands of 91 aging rats of the BN/BiRij strain, the WAG/Rij strain and their F1 hybrid were studied. In rats with pituitary glands without signs of hyperplasia, serum PRL levels were, in comparison to rats of 15-24 months, increased 25-29-month-old female BN/BiRij rats and showed a decline with further aging. This rise and decline during aging correlated with changes in the PRL cell volume density and in ultrastructural signs of their synthetic activity. Rats with hyperplastic pituitaries showed similar age-related changes in serum PRL levels, but these levels were higher. Concerning the hyperplasia, some strain differences were found. In BN/BiRij rats anti-r-PRL positive hyperplasia, and in WAG/Rij and F1 rats, anti-r-PRL negative and anti-r-PRL positive hyperplasia were present. All foci of hyperplasia were negative for anti-ACTH and anti-h-GH. In male rats no age-related changes of serum PRL levels could be established, although a decline of PRL cell volume density in the oldest rats is indicated. We conclude that the absence of a continuous age-related rise of serum PRL levels in our animals is caused by exclusion of animals with pituitary tumors.