Modulation of Na+/H+ exchanger 3 trafficking and activity in rat proximal tubule

The study by Healy et al. (2014) in the present issue entitled ‘The adrenergic regulation of proximal tubular Na/H exchanger 3 in the rat’ provides evidence of a new player in the modulation of apical Na/H exchanger 3 (NHE3) activity and abundance in proximal tubules. The authors demonstrated that acute denervation triggers natriuresis and diuresis, which are accompanied by a reduction in NHE3 abundance and activity in the brush border membrane. The effects of low level electrical stimulation of the renal innervation are antinatriuresis and antidiuresis, which are accompanied by enhanced NHE3 activity in brush border membrane. Apical NHE3 activity of proximal tubule has a central role in pressure diuresis and tubuloglomerular feedback (TGF) in renal blood flow autoregulation. Using a non-obstructing optical technique to measure proximal tubular flow in free-flow nephron, Chou and Marsh showed that acute hypertension inhibited proximal tubular fluid re-absorption in denervated rat kidney (Chou & Marsh 1988). The rapid increase in fluid load signals the macula densa to sustain afferent arteriolar constriction during acute hypertension via TGF and provides significant fraction of the increased fluid and salt excretion in pressure natriuresis and diuresis. However, persistent changes in body salt (fluid) balance alter renin secretion in juxtaglomerular apparatus, which optimizes the generation of angiotensin II to maintain long-term body salt balance. One function of angiotensin II is to provoke TGF sensitivity adaptation to serve the needs of body salt balance (Schnermann 1998). Zhang et al. (1996) utilized sorbitol density gradient membrane fractionation and immunoblotting of renal cortical membrane and demonstrated that acute hypertension provokes a redistribution of apical NHE3 from brush border membrane and inhibition of basolateral Na pumps. Acute hypertension not only triggers a redistribution of apical NHE3 from brush border to intermicrovillar cleft (Yip et al. 1998), but also inhibits the apical Na/H exchange activity when measured in microperfused proximal tubule in vivo (Yip et al. 2000). As the NHE3 transport activity in membrane vesicles derived from fractionated renal cortical membrane is not affected by acute hypertension, it was concluded that decrease in renal proximal tubular Na transport during acute hypertension is mediated by removal of transport-competent NHE3 from the apical brush border to subapical and internal reserves (Yang et al. 2002). Adrenergic regulation of proximal tubular NHE3 was first implicated in a rat hypertension model induced by intrarenal injection of phenol. Renal injury induced by phenol activates renal sympathetic afferent pathways, increases noradrenaline release and elicit a rapid and persistent hypertension. Yang et al. (2003) showed that acute phenol injury-induced hypertension was associated with redistribution of NHE3 from intermicrovillar cleft and intracellular membrane pools to apical brush border in proximal tubules. Leong et al. (2006) confirmed that acute phenol injury increased apical Na/H exchanger activity and tubular reabsorption in microperfused proximal tubule in vivo. These observations are consistent with the notion that enhanced renal sympathetic activity contributes to phenol injuryinduced hypertension by augmenting Na and fluid reabsorption in proximal tubules. However, they did not detect an increase of Na/H exchanger activity per transporter in isolated brush border membrane vesicles, or an obvious redistribution of NHE3 to the brush border. Phenol-induced renal injury not only activates renal sympathetic nerves but also increases arterial blood pressure. Hypertension per se triggers NHE3 retraction from brush border. It is speculated that activation of sympathetic nervous system alone might lead to a more obvious redistribution of NHE3 to the brush border. In the present issue of Acta Physiologica, Healy and co-workers demonstrated that low level of electrical stimulation of renal innervation increases Na/H exchanger activity per transporter by 70% in isolated brush border membrane vesicles (Healy et al. 2014). Renal denervation decreases Na/H exchanger activity by 78% and NHE3 abundance by sevenfold. As low level of electrical stimulation did not alter arterial blood pressure, the current study provides compelling evidence that renal sympathetic activity regulates NHE3 activity and abundance in brush border of proximal tubules. Similar observations were found in primary culture of proximal tubular cells when stimulated with noradrenaline and the selective a1-adrenoceptor agonist. Although the effects of electrical stimulation on the abundance of NHE3 in brush border were not reported in the current study, it is most likely that the activation of a1-adrenoceptor will also increase NHE3 abundance based on the observations from the effects of renal denervation.