EARLY IN VITRO INDUCTION OF RAT PITUITARY GH mRNA BY T31

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Abstract
Thyroid hormone stimulates rat pituitary GH [growth hormone] synthesis and GH mRNA activity and concentration. The earliest demonstration of increase in GH mRNA activity was 24 h following T3 addition, whereas stimulation of GH synthesis was observed 2 h after treatment with T3. It is unknown whether increase in pituitary GH mRNA is a prerequisite for the stimulation of GH synthesis. In vitro addition of 1.5 .times. 10-10 M T3 to pituitaries isolated form hypothyroid rats resulted in a slight but significant increase of GH mRNA activity within 2 h. Further stimulation of GH mRNA activity was observed over the period of 12 h. No increase of GH mRNA activity occurred in the absence of T3, and T3 had no effect on the PRL [prolactin] mRNA activity. Increase in GH mRNA may be responsible for the observed induction of GH synthesis, and at least one of the primary actions of thyroid hormone may be at the nuclear level.