C-TYPE NATRIURETIC PEPTIDE (CNP) IN THE PITUITARY - IS CNP AN AUTOCRINE REGULATOR OF GONADOTROPES

Natriuretic peptides act via receptors with intrinsic guanylate cyclas e activity to stimulate cGMP production and are thought to be importan t regulators of neuroendocrine systems. C-Type natriuretic peptide (CN P) is of particular interest in this regard because the highest tissue concentrations of CNP occur in the anterior pituitary, where it is a highly potent stimulator of cGMP production. Here we show that pituita ries of rats and mice contain abundant CNP prohormone messenger RNA (m RNA), but no atrial natriuretic peptide or B-type natriuretic peptide prohormone mRNAs. Using reverse transcriptase-polymerase chain reactio n, both A- and B-type natriuretic peptide receptor (GC-A and GC-B, res pectively) transcripts were detected in rat and mouse pituitaries, alt hough only the GC-B mRNA was measurable by Northern blotting. Immunohi stochemistry revealed CNP-positive cells in the anterior, but not post erior, pituitaries of rats, and the vast majority of these cells were identified as gonadotropes by colocalization of CNP and LH immunoreact ivities. Targeted toxicity using GnRH conjugated to the ricin-A chain was used to test whether gonadotropes are also direct targets for GnRH action. The conjugate dose dependently inhibited the proliferation of alpha T3-1 cells (gonadotrope-derived cells with GnRH receptors), but had no such effect on GH(3) cells (which do not have GnRH receptors). Culture of rat pituitary cells with the conjugate caused comparable r eductions in CNP-stimulated cGMP production, GnRH-stimulated LH releas e, and Ca2+ ionophore (A23187)-stimulated LH release, but did not meas urably alter cAMP production in response to pituitary adenylate cyclas e-activating polypeptide. We conclude that CNP is synthesized in the p ituitary, where it is located predominantly in gonadotropes, and GC-B receptors expressed in the pituitary mediate the direct effects of CNP in gonadotropes. Together with the recent demonstration of CNP synthe sis and action in alpha T3-1 cells, the data suggest CNP to be a novel autocrine regulator of gonadotropes.