Corticotropin-releasing factor desensitization of adrenocorticotropic hormone release is augmented by arginine vasopressin

Abstract

The desensitization of corticotropin-releasing factor (CRF)-stimulated ACTH release from an intracellular cyclic AMP accumulation in anterior pituitary cells was investigated in primary cultures of rat pituitary cells and in a mouse tumor cell line (AtT 20/D16-16). CRF potently stimulated ACTH secretion and cyclic AMP accumulation in both preparations. When primary cultures were preincubated with 100 nM CRF, 50% desensitization of ACTH release occurred within 4 hr while similar desensitization of cyclic AMP accumulation occurred by 1 hr. This desensitization was manifested as a reduced maximal CRF response. Concentrations of CRF as low as 1 to 10 pM induced densensitization. Pretreatment did not affect ACTH content nor did it alter the ability of epinephrine or forskolin to stimulate ACTH release. Pretreatment of AtT 20 cells, which are a homogeneous population of corticotrophs with 100 nM CRF reduced the subsequent ability of CRF, but not of isoproterenol or of forskolin to stimulate ACTH release and cyclic AMP formation. Preincubation of AtT 20 cells with isoproterenol did not affect the cyclic AMP or ACTH release responses induced by CRF, but did desensitize β-adrenergic receptors. Arginine vasopressin (AVP) was a weak ACTH-releasing factor, but AVP enhanced CRF-directed ACTH release from primary cultures. When these cells were preincubated with both CRF and AVP, CRF desensitization occurred with lower concentrations of CRF than when CRF desensitization was induced by preincubating the cells with CRF alone. The ED50 for CRF-induced desensitization was 700 ± 150 pM when the cells were exposed to CRF alone, and 20 ± 15 pM when 1 μM AVP was added during the preincubation. This enhancement of CRF desensitization was also seen when lower concentrations of AVP were added. In conclusion: (1) ovine CRF rapidly desensitizes its ability to stimulate ACTH secretion and cyclic AMP accumulation in primary monolayer cultures of rat pituitary cells and in cultured mouse pituitary tumor cells; (2) this desensitization is specific, allowing the cells to respond to other ACTH-secretagogues; (3) AVP enhances the ability of CRF to stimulate ACTH release from primary cultures during brief incubation periods; and (4) CRF desensitization is augmented when primary cultures of rat pituitary cells are preincubated with both CRF and AVP.