Liver tissue from lactating rats produces a factor that stimulates prolactin release and gene expression

Liver tissue from nursing rats produces a substance, termed liver lactogenic factor (LLF), that potently stimulates casein release from isolated mammary cells. Inasmuch as the production of LLF is dependent on PRL, we decided to determine whether it could influence the release of the hormone by dissociated pituitary cells in culture. This was accomplished by measuring PRL release with a reverse hemolytic plaque assay and PRL gene expression with a DNA probe complementary to PRL mRNA. Treatment of pituitary cells from day 10 lactating rats with liver slice incubates from the same type of animal caused a 35.3 +/- 4.3% increase in PRL release during a 3-h incubation. Likewise, the same dose of LLF activity markedly increased (3.5-fold) the steady state levels of PRL mRNA. The responses were reasonably specific for PRL, since neither GH plaque development nor gene expression was affected by identical treatment. Taken together these results demonstrate that LLF can act directly at the pituitary level to exert positive feedback effects on both PRL release and gene expression.