Increased phosphate absorption in mice lacking NHE3 in intestinal epithelial cells

Abstract

The kidneys play a major role in maintaining phosphate (Pi) homeostasis. Patients in later stages of CKD develop hyperphosphatemia. One novel treatment option is tenapanor, an intestinal‐specific NHE3 inhibitor. To gain mechanistic insight into the role of intestinal NHE3 in Pi homeostasis, we studied tamoxifen (Tam) ‐inducible intestinal epithelial cell‐specific NHE3 knockout (NHE3IEC‐KO) mice with the hypothesis that their phenotype should reflect inhibitory effects of tenapanor on intestinal Pi uptake. Mice were challenged by high dietary Pi, and hormones as well as plasma Pi were determined. The in vivo effects of tenapanor were studied via 33P uptake from the intestine. Pi transport was measured ex vivo in everted gut sacs (EGS) and brush border membrane vesicles (BBMV). Intestinal and renal protein expression of Pi transporters were determined. No differences were found between genotypes in terms of Pi homeostasis before Tam. Two weeks after Tam, NHE3IEC‐KO mice had similar Pi homeostasis, but a ~25% reduction in FGF23 compared with control mice on standard diet. Two‐fold greater intestinal Npt2b expression in NHE3IEC‐KO mice (P<0.05) was associated with significantly enhanced Pi uptake in EGS and BBMV. One hour after acute oral Pi loading (0.5 mol/L NaH2PO4), plasma Pi levels increased to a significantly greater extent in NHE3IEC‐KO compared to control mice (2.0±0.2 vs 1.1±0.1 mmol L‐1, P<0.05). Tenapanor treatment caused ~67% lower 33P uptake (P<0.05) after 5 minutes compared with vehicle‐treated control mice. At later time points, tenapanor‐treated control mice showed an increase in intestinal Pi uptake, reaching a maximum of 33P in plasma after 60 minutes. In response to high dietary Pi, plasma Pi slightly but significantly decreased (‐0.25±0.1 mmol/L, P<0.05) in control mice, possibly the consequence of the combination of significantly elevated PTH and FGF23. In contrast to control mice, high dietary Pi significantly increased plasma Pi in NHE3IEC‐KO mice (0.5±0.1 mmol/L, P<0.05), which was associated with significantly increased plasma PTH levels (P<0.05). FGF23 levels also increased to a significantly greater extent in NHE3IEC‐KO vs control mice (552±66 vs 244±14 pg/mL, P<0.05). This was associated with ~2.25‐fold greater Npt2b expression (P<0.05). Fifty percent lower renal Npt2c expression (P<0.05) and a trend for lower Npt2a expression (~25%) were unable to correct for higher plasma Pi. In summary, intestinal NHE3 significantly contributes to Pi homeostasis. In contrast to effects described for tenapanor on Pi homeostasis, NHE3IEC‐KO mice show higher, rather than lower, intestinal Pi uptake.