C-type natriuretic peptide (CNP), the third member of the natriuretic peptide family, has been found at its highest tissue concentrations in the anterior pituitary, where it is localised in gonadotrophs. Its specific guanylyl cyclase-containing receptor, GC-B, is also expressed on several anterior pituitary cell types, and CNP potently stimulates cGMP accumulation in rat pituitary cell cultures and pituitary cell lines. The mouse gonadotroph-derived alpha T3-1 cell line has been shown to express CNP as well as GC-B (but not GC-A) receptors, suggesting that CNP may well be an autocrine regulator of gonadotrophs. Comparing effects of three natriuretic peptides (atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP) and CNP) on cGMP accumulation in four pituitary cell lines (alpha T3-1, TtT-GF, AtT-20 and GH(3)) we find that CNP is most potent and effective in alpha T3-1 cells. In these cells, CNP-stimulated cGMP accumulation was found to desensitise during a 30 min exposure to CNP. Pretreatment with CNP for up to 6 h also caused a significant reduction in the ability of CNP to subsequently stimulate cGMP accumulation. This effect was receptor specific, because pretreatment with sodium nitroprusside (an activator of nitric oxide-sensitive guanylyl cyclase), or with ANP or BNP, did not cause desensitisation of CNP-stimulated cGMP accumulation. Protein kinase C activation with phorbol esters also inhibited CNP-stimulated cGMP accumulation and such inhibition was also seen in cells desensitised by pretreatment with CNP. Thus it appears that the endogenous GC-B receptors of alpha T3-1 cells are subject to both homologous and heterologous desensitisation, that the mechanisms underlying these forms of desensitisation are distinct, and that cGMP elevation alone is insufficient to desensitise GC-B receptors.
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