Isotonic saline infusion increases plasma and urinary levels of tumor necrosis factor‐alpha (TNFα) in mice; evidence for a physiological role of this cytokine in regulating renal function during saline volume expansion.

Although TNFα is considered to play its role in many pathological conditions, the physiological role of this cytokine in regulating many organs’ function including the kidney is increasingly recognized in recent days. Chronic high salt (HS) intake in diet induces an immune response that activates the mononuclear phagocyte system (MPS) cells to release TNFα that appears in the circulation in its soluble form (sTNFα). It has been shown that HS (4% NaCl) diet alone for 2 weeks increased MPS cell infiltration in the renal tissue in mice and this is associated with increases in the circulating sTNFα level in the plasma (Singh et al‐2013). It was also demonstrated that 1% salt added to the drinking water for 3 days in mice increases the urinary excretion rate of sTNFα indicating that its release in the kidney from the infiltrated MPS cells occur even at the early stage of HS intake (Hao et al‐2013). We have previously demonstrated that intravenous infusion of recombinant TNFα in mice induces natriuretic response by inhibiting tubular sodium reabsorption (Shahid et al‐2008; Majid‐2011). We hypothesize that the intravenous saline infusion can also induce the production of sTNFα from activated MPS cells due to an increase in salt content in immune tissues and such increase in sTNF influences saline induced natriuretic response in the kidney. To examine this hypothesis, we measured the changes in sTNFα levels in plasma and urinary excretion rate (U TNFα V) during intravenous infusion of isotonic saline (0.9% NaCl), first at euvolemic conditions (3 μL/min for 60 min; Baseline period) and then at an enhanced infusion rate (12 μL/min for 90 min; saline volume infusion period) in anesthetized mice (n=5). The concentration of sTNFα in plasma and urine samples were determined using ELISA kit (Ebioscience, Woburn, MA) for measuring this cytokine. Baseline level of plasma sTNFα was undetectable, however, the level was increased to 3.7±1.3 pg/mL during saline volume infusion period. Baseline U TNFα V level was 0.01±0.002 pg/min/g of kidney wt, which was increased to 0.11±0.03 pg/min/g (P<0.05) during volume infusion period. In another group of mice (n=5) pretreated with a TNFα inhibitor, etanercept (0.5 mg/kg intraperitoneally once daily for 3 days prior to the experiment day), it was observed that this increase in U TNFα V during saline volume infusion period was markedly attenuated (0.003±0.002 to 0.006±0.004 pg/min/g; P=n.s.). The usual natriuretic response (0.5±0.2 to 3.8 ±1.1 μmol/min/g; P<0.05) to saline volume infusion observed in control mice was seen markedly attenuated (0.4±0.1 to 0.8±0.3 μmol/min/g; P=n.s.) in etanercept pretreated mice. These findings demonstrate for the first time that an intravenous saline volume infusion resulted an increase in sTNFα level in plasma and in urine. These results strongly suggest a physiological natriuretic role for sTNFα in regulating renal excretory function during saline volume expansion.