ACTH secretion in mouse pituitary tumor cells in culture: Inhibition of CRF-stimulated hormone release by somatostatin

Synthetic corticotropin-releasing factor (CRF) stimulates ACTH secretion in the clonal mouse pituitary cell strain AtT20/D16v (D16) in a dose-dependent manner with a half-maximal effect at 2×10^?9M. A single dose of 5×10^?9M CRF maximally stimulates the rate of ACTH secretion during the initial two hrs of treatment. During the period of maximal CRF stimulation intracellular hormone concentration declines progressively to a nadir at 4 hrs. During the ensuing 24 hrs of incubation intracellular hormone levels in CRF-stimulated cells increase gradually toward control values. Somatostatin (SRIF) inhibits the secretory response to CRF. This action of SRIF is dose-dependent with a half-maximal effect at 1×10^?9M and results in decreased maximal ACTH secretion with little effect on the ED₅₀ for CRF.