ISN Forefronts Symposium 2015: Maintaining Balance Under Pressure-Hypertension and the Proximal Tubule

Renal control of effective circulating volume is key for circulatory performance. When renal Na⁺ excretion is inadequate, blood pressure rises and serves as a homeostatic signal to drive natriuresis to re-establish effective circulating volume (ECV). Recognizing that hypertension involves both renal and vascular dysfunction, this report concerns proximal tubule Na⁺/H⁺ exchanger 3 (PT NHE3) regulation during acute and chronic hypertension. NHE3 is distributed in tall microvilli (MV) in the PT where it reabsorbs a significant fraction of the filtered Na⁺. NHE3 redistributes, in the plane of the MV membrane, between the MV body, where NHE3 is active, and MV base where NHE3 is less active. High salt diet and acute hypertension both retract NHE3 to the base and reduce PT Na⁺ reabsorption independent of a change in abundance. The renin angiotensin system provokes NHE3 redistribution independent of blood pressure: the ACE inhibitor captopril redistributes NHE3 to the base and subsequent AngII infusion returns NHE3 to the body of the microvilli and restores reabsorption. Chronic AngII infusion presents simultaneous AngII stimulation and hypertension: NHE3 remains in the body of the MV, due to the high local AngII and inflammation, and exhibits a compensatory decrease in abundance, driven by the hypertension. Genetically modified mice with blunted hypertensive responses to chronic AngII infusion (due to lack of PT AngII receptors, IL-17A or IFN-γ expression) exhibit reduced local AngII accumulation and inflammation and larger decreases in NHE3 abundance which improve the pressure natriuresis responses and reduces the need for elevated BP to facilitate circulating volume balance.