Human pheochromocytomas express orexin receptor type 2 gene and display anin vitro secretory response to orexins A and B

Orexins A and B are hypothalamic peptides, that act through two receptor su btypes, called OX1-R and M-R. OX1-R selectively binds orexin A, whereas OX2 -R is nonselective for both orexins. High levels of OX1-R mRNA and low leve ls of OX2-R mRNA have been previously detected in the human adrenal cortex and medulla. Here we demonstrated by RT-PCR the expression of the OX2-R, bu t not the OX1-R, gene in 10 benign secreting pheochromocytomas. Both orexin s A and B stimulated catecholamine secretion from pheochromocytoma slices; the maximal effective concentration was 10(-8) mol/liter. Orexins A and B ( 10(-8) mol/liter) increased IP3, but not cAMP production, by tumor slices, and the effect was blocked by the PLC inhibitor U-73122. The catecholamine response to 10(-8) mol/liter orexins A and B was abolished by either U-7312 2 or the PKC antagonist calphostin C and was unaffected by the adenylate cy clase inhibitor SQ-22536 and the PKA inhibitor H-89. Collectively, these fi ndings suggest that orexins stimulate catecholamine secretion from human ph eochromocytomas, acting through OX2-R coupled to the PLC-PKC signaling path way.