Characterization of natriuretic peptide receptor subtypes in the AtT-20 pituitary tumour cell line

Receptors for the natriuretic peptide family have been characterized in the adrenocorticotrophic hormone (ACTH)-secreting AtT-20 pituitary tumour cell line. Northern blot analysis detected mRNA transcripts for the guanylate cyclase-linked GC-B receptor subtype. There was no evidence for the expression of either guanylate cyclase-linked GC-A receptor or atrial natriuretic peptide (ANP)-C (clearance) receptor mRNAs. Cyclic GMP production in AtT-20 cells was stimulated up to 200-fold by C-type natriuretic peptide (CNP), which was 10- and 20 times as effective as equivalent concentrations of brain natriuretic peptide and ANP respectively. Cyclic GMP dose-response curves to CNP failed to show any signs of saturation even at concentrations up to 30 microM, indicating a relatively low affinity of CNP for the GC-B receptor. Although CNP induced large stimulations in cyclic GMP production, specific binding of [125I-Tyr0]CNP could not be demonstrated in AtT-20 cells. The absence of specific binding with this radiolabelled analogue is possibly due to a reduced affinity for the GC-B receptor, as CNP analogues with N-terminal modifications such as [Tyr0]CNP and [127I-Tyr0]CNP exhibited reduced abilities to stimulate cyclic GMP production in these cells. Despite elevating cyclic GMP levels, CNP had no effect on basal or corticotrophin-releasing factor-stimulating ACTH release from the cells. These results show that the guanylate cyclase-coupled GC-B receptor is the only natriuretic peptide receptor subtype expressed in AtT-20 cells. Although CNP can markedly stimulate cyclic GMP production in these cells, there is incomplete expression of the normal natriuretic peptide-induced inhibitory pathway of ACTH secretion at some point distal to the production of cyclic GMP.