Regulation of NKCC2B by TNF in response to salt restriction.

We have previously shown that TNF-alpha produced by renal epithelial cells inhibits Na+-K+-2Cl(-) cotransporter (NKCC2) activity as part of a mechanism that attenuates increases in blood pressure in response to high NaCl intake. As the role of TNF-alpha in the kidney is still being defined, the effects of low salt intake on TNF-alpha and NKCC2B expression were determined. Mice given a low-salt (0.02% NaCl) diet (LSD) for 7 days exhibited a 62 +/- 7.4% decrease in TNF-alpha mRNA accumulation in the renal cortex. Mice that ingested the LSD also exhibited an similar to 63% increase in phosphorylated NKCC2 expression in the cortical thick ascending limb of Henle's loop and a concomitant threefold increase in NKCC2B mRNA abundance without a concurrent change in NKCC2A mRNA accumulation. NKCC2B mRNA levels increased fivefold in mice that ingested the LSD and also received an intrarenal injection of a lentivirus construct that specifically silenced TNF-alpha in the kidney (U6-TNF-ex4) compared with mice injected with control lentivirus. Administration of a single intrarenal injection of murine recombinant TNF-alpha (5 ng/g body wt) attenuated the increases of NKCC2B mRNA by similar to 50% and inhibited the increase in phosphorylated NKCC2 by similar to 54% in the renal cortex of mice given the LSD for 7 days. Renal silencing of TNF-alpha decreased urine volume and NaCl excretion in mice given the LSD. effects that were reversed when NKCC2B was silenced in the kidney. Collectively, these findings demonstrate that downregulation of renal TNF-alpha production in response to low-salt conditions contributes to the regulation of NaCl reabsorption via an NKCC2B-dependent mechanism.