Post-transcriptional regulation of ETB receptor in human kidney. effects of local pH and sodium concentration

Background. Endothelin(ET)-1 participates to sodium and water handling through its binding to two specific receptors ETA and ETB. ETB, the prevalent receptor subtype in human kidney, is highly expressed in epithelial cells of collecting ducts where it stimulates natriuresis and increases free water clearance. During low sodium diet the capability of ETB receptor to bind ET-1 in renal medulla increases at an higher extent than ETB mRNA expression. Aims and Methods. The present study was designed to investigate whether changes in local pH or sodium concentration may exert posttranscriptional effects on ETA and ETB receptors. Binding studies were performed on cell membranes isolated from renal cortex and medulla obtained from 9 patients who underwent renal nefrectomy. The binding of ET-1 to ETA and ETB receptors was investigated using 1251-ET-1 and increasing concentrations of ET-1 (0-1 microM)or selective ETA (BQ123) and ETB (BQ788) antagonists. The effects of sodium and pH on ET receptor binding were investigated by increasing NaCl or NaHCO3 concentration in binding buffer. Results. In renal medulla ET receptor density was about 2-fold higher than in cortex (127±13 vs 59±9 fmol/mg protein, p<0.05) and receptor population was almost exclusively represented by the ETB subtype in both districts (82% and 88% in renal cortex and medulla respectively). The increase in NaCl concentration enhanced both ET-1 receptor density (from 127±13 to 182±18 fmol/mg, p<0.05) and affinity (3.8-fold increase vs NaCl 154 mmol/L, p<0.05). Maximum change occurred in a physiological range of Na concentration (154-308 mmol/L). Also a pH increase from 7 to 8 induced a 2-fold increase in ETB receptor density. Conclusion. Local sodium concentration and pH selectively regulates both affinity and density of ETB receptor in human kidney.