Commentary on. . .Neuronal Nitric Oxide Synthase: Its Role and Regulation in Macula Densa Cells

The macula densa control of renin secretion and tubuloglomerular feedback (TGF) both occur in the juxtaglomerular apparatus (JGA) of nephrons. The renin-angiotensin system has basked in attention for 2 decades, while TGF remains arcane to all but those who study it. Nonetheless, the body’s internal environment is regulated, in large part, by these two systems.

A decade ago, Wilcox and colleagues (1) identified nitric oxide synthase (NOS) I in the macula densa. This discovery spawned interest and experimentation. Subsequently, a sound consensus emerged that NO from the macula densa suppresses the TGF response. There is a weaker consensus that NO from the macula densa can stimulate renin secretion. It has been difficult to demonstrate a major consequence of NO-dependent renin secretion. In contrast, physiologic relevance of macula densa NOS as a modulator of TGF is easier to postulate and to prove. This is because NO from the macula densa shifts the TGF response rightward and makes it less steep (2). When macula densa NOS I is active, there will be increased distal salt delivery for any given GFR and increased GFR for any given distal salt delivery. The importance of this is revealed by positive salt balance and increased BP that gradually ensue when macula densa NOS I is inhibited (3).

There are likely multiple determinants of macula densa NOS I activity. On the basis of the ubiquity of homeostasis-as-negative-feedback throughout physiology, one can expect each determinant of macula densa NO will also be affected by NO so as to serve the cause of homeostasis. To understand the role of macula densa NOS I in physiology, it is important to learn what affects it and what is affected by it. It is also important to quantify these relationships to predict the outcome when they come into conflict.

The present study by Kovacs et al. (4) provides new information about certain effectors and affectors of macula densa NOS I. First, these studies confirm that NO from the macula densa suppresses apical Na:2CL:K transport in the same cell. Sodium entry via this symporter is the first step en route to TGF-mediated vasoconstriction, and reduced Na:2Cl:K symport is a leading candidate to explain how NO pushes the TGF response rightward, influencing GFR, distal salt delivery, and BP. Others have inhibited Na:2Cl:K symport using exogenous NO donors in Henle’s loop (5), but this is the first demonstration that endogenous NO can have this effect. These authors also confirm that NO from the macula densa diffuses as far as the vascular pole of the glomerulus, where it could act as a paracrine vasodilator to oppose the influence of all locally active vasoconstrictors, including TGF.

Is there a way to distinguish between the autocrine (transport) and paracrine (vasodilator) effects of macula densa NO as a modulator of TGF? A future approach might be to eliminate the autocrine effect by permeabilizing the macula densa to salt and then determining whether the TGF response to tubular salt is still being modulated by NOS I activity. Another way might become available if someone discovers that the autocrine effect of NO in the macula densa is not mediated by cGMP, because cGMP mediates the overall effect of NO on TGF (6).

A second finding of Kovacs et al. (4) is the converse of the first, and pertains to the effect of intracellular sodium on NOS I activity in the macula densa. The possibility that increased cell sodium activates NOS is important for two reasons. First, combining an inhibitory effect of NO on sodium entry with a positive effect of cell sodium on NO formation would constitute a negative feedback system for stabilizing the macula densa cell. If either of these elements is missing, then macula densa NOS I cannot be considered in this role. Second, if macula densa NOS is activated by cell sodium, this could explain how NOS I contributes to the rightward resetting of TGF that normally occurs during a prolonged increase in salt delivery to the distal nephron. Kovacs et al. (4) are the second group to measure the effect of macula densa salt on NO production. Recently, Liu et al. (7) reported that macula densa NO is salt-dependent. Now, Kovacs et al. (4) do likewise, with one critical caveat, namely that the salt stimulus required to elicit NO is supraphysiologic and may not be relevant to daily life.

If we discard the possibility that macula densa salt normally stimulates NOS I activity as the present findings suggest, then ongoing efforts to explain how the sensitivity of TGF is regulated through events in the JGA become complicated. Those efforts have led us to theorize that NO provides an intermediate link between steady state salt delivery and the amount of salt required to elicit a TGF response. The supporting data for this are complex and eclectic, but one simple observation is worth noting, namely that the vasoconstrictor response to NOS I blockade is usually greater when there is more salt being delivered to the macula densa (1–6⇓⇓⇓⇓⇓). The simplest route to this end would be for NO formation to vary in proportion to physiologic changes in macula densa salt. The present findings call this into question, but one might reconcile these data, the opposite finding from Liu et al. (7), and the micropuncture data from various labs (1–6⇓⇓⇓⇓⇓) by making the reasonable supposition that there are sodium-dependent and sodium-independent pathways competing for control of NOS I in the macula densa and that circumstances can arise where one or the other dominates. In fact, we observed this in micropuncture experiments where NOS I activity was dominated by macula densa salt in normal rats, but not in rats with streptozotocin diabetes (unpublished observation).

A third observation of Kovacs et al. (4) is that angiotensin II (AngII) does not directly affect NO formation in macula densa cells. This is an important direct observation in light of the abundance of phenomenologic data published over the years that stresses AngII–NO interactions in the kidney. However, this negative finding is not necessarily a surprise given that there are models of increased macula densa NOS I expression that run the gamut of renin-angiotensin activities.

Kovacs et al. (4) demonstrate the ability to record important signaling events in the anatomically complex JGA with broad implications for kidney function and salt balance. No doubt, more of this type of information will be forthcoming that could never be revealed by cultured cells or histologic examinations.

• © 2003 American Society of Nephrology