Somatostatin reduces transcription of the growth hormone gene in rats

To examine whether somatostatin (SS) exerts influences on the steady state levels of GH-releasing factor (GRF), the effect of SS on GH gene transcription was examined in rats. This approach was used because it has been shown that GRF stimulates GH gene transcription independent of GH release, and SS does not inhibit basal or GRF-stimulated GH gene transcription. Therefore, it is assumed that an effect of SS on GH gene transcription would be mediated by a change in GRF levels. Adult female Sprague-Dawley rats were provided with right atrial cannulae. Studies were performed using unanesthetized rats. Pituitary GH gene transcription was measured by transcription assay. An iv administration of antiserum to rat GRF 150 min previously significantly decreased GH gene transcription compared with that in control rats given normal goat serum. A continuous infusion of SS (300 micrograms/kg.h) via the cannula for 150 min significantly decreased GH gene transcription compared with that in control rats receiving 0.9% NaCl. When GRF (3 micrograms/kg.h) was given simultaneously with SS (300 micrograms/kg.h), GH gene transcription increased significantly compared with that in rats receiving SS infusion alone. After the withdrawal of SS infusion, GH gene transcription rapidly and significantly increased. The data suggest that SS reduces the steady state levels of GRF.