Inhibition of α1 Na/K‐ATPase blocks tubuloglomerular feedback (TGF) in mutant mice

Na‐K‐2Cl cotransport is responsible for NaCl entry and initiation of TGF in the macula densa (MD), but it is less clear whether Na‐K‐ATPase (NKA) is responsible for Na + exit from MD cells. Expression of Na‐K‐ATPase is low in MD and it has been proposed that H‐K‐ATPase may account for Na + exit and maintenance of the Na + gradient. Functional analyses have been limited by the fact that the α1 isoform of NKA – the isoform expressed in renal epithelium – is largely insensitive to ouabain in rodents and therefore resistant to pharmacological manipulation. We therefore evaluated TGF responses in gene knock‐in mice in which the ouabain sensitivity of NKA isoforms was altered: α1‐sensitive/α2‐resistant mice (α1s/α2r), and α1‐resistant/α2‐resistant mice (α1r/α2r). In micropuncture experiments, TGF responses were evaluated: 1) before and during i.v. infusion of ouabain (200 ng/min) and 2) before and during luminal application of ouabain. As shown in the figure, i.v. ouabain inhibited TGF in α1s/α2r mice, but not in α1r/α2r mice. Responses were restored by cyclohexyladenosine (CHA, 10μM) in the loop of Henle (LH) perfusate. In the second series, LH perfusion with 50μM ouabain decreased maximal TGF responses from 10.5±1.1 to 4.8±1.3 mmHg in α1s/α2r mice, but had no effect in α1r/α2r mice (10.8±1.3 to 10.7±1.3 mmHg). Responses were restored by CHA in the perfusate. These data indicate that TGF requires activity of the α1NKA. Since TGF could be restored by the adenosine analog CHA – a downstream effector in the TGF signaling cascade – the data also suggest that ouabain exerts its effect at the MD rather than at the afferent arteriole. Supported by NIH‐DK57552 & HL66062.