Question: 

Sodium absorption in the aldosterone-sensitive distal nephron (ASDN) plays an important role in the maintenance of body sodium balance and the long term regulation of arterial blood pressure. This absorption is mediated via ENaC and is regulated by hormones and various other factors. Sympathetic innervation of the kidney has been linked to decreased sodium excretion and a recent clinical study demonstrated that renal denervation significantly reduced blood pressure in patients with resistant hypertension. So far it is unclear whether the sympathetic transmitter norepinephrine has a direct effect on tubular sodium transport in the distal nephron.

Methods: 

We investigated the effects of norepinephrine on ENaC-mediated transepithelial Na⁺ transport in cultured mouse mCCD_cl1 cells, a model of renal collecting duct principal cells. ENaC activity was quantified using Ussing chambers to record equivalent short circuit current (I_SC).

Results: 

Addition of norepinephrine (10mM) to the basolateral but not apical bath produced a complex response involving a rapid transient peak in I_SC, followed by an inhibition (15min) below baseline and a subsequent sustained increase in I_SC above baseline over 2.5h. The initial peak response was insensitive to amiloride indicating that it is independent of ENaC activity and may be caused by a transient chloride secretory response. The decline and late increase in I_SC were amiloride-sensitive indicating that altered ENaC activity is responsible for these effects. Addition of aldosterone (3nM) caused a 2-fold stimulation of I_SC confirming that these cells respond to low concentrations of this hormone. When cells were pre-stimulated with aldosterone, the response to norepinephrine was altered with a stronger initial inhibitory phase and a diminished late stimulatory phase. The maximal stimulatory effect of norepinephrine on I_SC was reached more rapidly than that of aldosterone. Half-maximal stimulation occurred after about 65min with norepinephrine and after about 90min with aldosterone.

Conclusion: 

Norepinephrine modulates ENaC activity in mCCD_cl1 cells in a complex manner with a delayed and sustained stimulatory effect. The stimulatory response to norepinephrine appears to be mediated via a different mechanism than that to aldosterone. A sustained stimulatory effect of locally elevated norepinephrine on ENaC may contribute to the hypertensive effect of increased renal sympathetic activity.