Increasing sodium concentrations in human renal medulla enhance ET-1 binding to ETB receptors without affecting ETB mRNA expression

Renal ET-1 participates to sodium and water handling and its urinary excretion is affected by changes in sodium diet. Renal ET-1 synthesis was found to be regulated by local osmolarity and sodium concentration. The present study was designed to investigate whether sodium affects also ET-1 renal receptors. Renal tissue was obtained from 9 patients with normal renal function who underwent renal nefrectomy for polar tumors. During the week preceding surgery patients underwent low sodium (20 mEq/day, n=4) or normal sodium diet (108 mEq/day, n=5). After nefrectomy, cortex and medulla specimens were dissected and immediately stored in liquid nitrogen. The binding of ET-1 to ETA and ETB receptors was investigated using 125I-ET-1 and increasing concentrations of ET-1 (0-1M) or selective ETA (PD 155080) and ETB (BQ788) antagonists. Gene expression of the components of the ET system was investigated by RT-PCR using specific primers for ETA, ETB, ECE-1 and ppET-1. The expression of GAPDH was used as internal standard. Endothelin receptor density is about 2-fold higher in renal medulla than in cortex (12713 vs 646 fmol/mg protein, p<0.05) and receptor population was almost exclusively represented by the ETB subtype in both districts (85%). During low sodium diet ET-1receptor density in renal medulla increased from 12713 to 18426 fmol/mg protein (p<0.05) but mRNA expression for ET-1 receptors remained unchanged. The effects of sodium on ET receptor binding were investigated in vitro by increasing NaCl concentration in binding buffer from 0 to 1.8%. NaCl enhanced both ET-1 receptor density (from 12713 to 17429 fmol/mg, p<0.05) and affinity (4 fold increase vs NaCl-free buffer, p<0.05). In conclusion low sodium diet causes an increased ET-1 binding to ETB receptor without affecting mRNA expression for receptor subtypes and this effect is mediated by local increase in sodium concentration.