Abstract

Atrial natriuretic peptide (ANP)-C receptor activation has been shown to inhibit adenylyl cyclase (AC) activity as well as to stimulate phospholipase C (PLC) signaling pathways. The present studies were undertaken to investigate whether ANP-C receptor-mediated decreased cAMP levels contribute to the activation of PLC signaling. C-ANP_4-23 [des(Gln¹⁸,Ser¹⁹, Glu²⁰,Leu²¹,Gly²²)ANP_4-23-NH₂], a ring-deleted peptide of ANP that interacts specifically with ANP-C receptor, stimulated inositol 1,4,5-tris-phosphate (IP₃) production (PLC activity) in A10 vascular smooth muscle cells in a concentration- and time-dependent manner. The maximal stimulation observed was about 75% at 2 h of treatment, with an apparent EC₅₀ of about 20 to 30 nM. Pertussis toxin treatment of the cells completely abolished the C-ANP_4-23-mediated stimulation of IP₃ production. Forskolin (FSK), a stimulator of adenylyl cyclase, dibutyryl cAMP (db cAMP), and isoproterenol (ISO), a β-adrenergic agonist that stimulates adenylyl cyclase activity and cAMP levels, inhibited IP₃ production by about 35, 30, and 50%, respectively, whereas dideoxyadenosine (DDA), an inhibitor of adenylyl cyclase activity, and oxotremorine stimulated IP₃ production by about 90 and 80%, respectively, in these cells, suggesting a functional interaction between these two signaling pathways. Treatment of the cells with antisense oligonucleotide of ANP-C receptor that attenuated ANP-C receptor-mediated inhibition of adenylyl cyclase resulted in a complete attenuation of C-ANP_4-23-induced stimulation of IP₃ formation, whereas FSK, db cAMP, and ISO-mediated decrease and oxotremorine and endothelin-1 (ET-1)-induced increase in IP₃ production was not affected by this treatment. Furthermore, C-ANP_4-23-induced increase in IP₃ formation was significantly potentiated by DDA and inhibited by FSK and db cAMP, whereas ET-1-induced increase in IP₃ production was not affected by FSK. In addition, N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline (H-89), an inhibitor of protein kinase A, completely abolished C-ANP_4-23 and not ET-1-induced stimulation of IP₃ production. These results indicate that ANP-C receptor activation by C-ANP_4-23 and resulting decrease in cAMP levels may be responsible for the activation of phosphatidylinositol (PI) turnover signaling, suggesting a cross-talk between ANP-C receptor-mediated adenylyl cyclase and PLC signaling pathways.