Background: Adrenomedullin (ADM) exerts antiproliferative effects on rat mesangial cells in vitro and, therefore is a possible renoprotective agent. In contrast, nitric oxide (NO) is capable of exerting both cytoprotective and cytotoxic actions. It was the objective of the present study to examine whether NO stimulates the ADM system.
Methods: Rat mesangial cells were incubated with the NO donors GSNO and SNAP, the guanylate cyclase inhibitor ODQ, and the cGMP analog 8-bromo-cGMP. ADM radioligand binding, ADM-induced intracellular cAMP-accumulation (radioimmunoassay) and ADM receptor gene expression (TaqMan real time PCR) were measured.
Results: Twenty-four hour treatment of mesangial cells with GSNO and SNAP (100 micromol/L each) increased the maximal binding of ADM to its receptor from 52%+/- 4% to 101%+/- 4% (P < 0.001) and 81%+/- 2% (P < 0.001), respectively. GSNO, SNAP (both 100 micromol/L) and 8-bromo-cGMP (50 micromol/L) increased EC50 from 9.9 x 10-8 to 7.0 x 10-10, 4.8 x 10-10, 1.1 x 10-9, respectively. In contrast, combined pretreatment with GSNO (100 micromol/L) and ODQ (100 micromol/L) reduced EC50 to values similar to the control cells (2.4 x 10-8). In contrast, ADM receptor gene expression was reduced significantly by different concentrations of GSNO, SNAP, and by 50 micromol/L 8-bromo-cGMP, but not by 8-bromo-cAMP.
Conclusions: NO increases ADM signal transduction via a cGMP dependent pathway. This effect is caused, at least in part, by an increase in ADM receptor availability and is counteracted in a feedback manner on the mRNA level. This mechanism might direct the impact of NO on mesangial cell function toward cytoprotection.