Key words: Macula densa Afferent and efferent arterioles Sodium balance Angiotensin II Autoregulation Nitric oxide Adenosine

Transcription

1 Special Article Tubuloglomerular Feedback Sadayoshi ITO, MD, and Keishi ABE, MD SUMMARY The macula densa, a plaque of specialized tubular epithelial cells located in the distal tubule, monitors the NaCl concentration of the tubular fluid and sends an as of yet unidentified signal to control glomerular hemodynamics. In this mechanism, called tubuloglomerular feedback (TGF), an increase in NaCl concentration at the macula densa constricts the glomerular afferent arteriole and thus decreases the single-nephron GFR. Along with the myogenic response, TGF significantly contributes to renal autoregulation. In addition, the macula densa also controls the rate of renin release, and hence the level of angiotensin II. Studies indicate that an appropriate interaction between TGF and the renin-angiotensin system is essential for body fluid and electrolyte homeostasis in the face of rather big variations in daily salt intake. Thus, alterations in TGF may play an important role in the pathogenesis/pathophysiology of various diseases such as hypertension, diabetes mellitus and congestive heart failure. (Jpn Heart J 1996; 37: ) Key words: Macula densa Afferent and efferent arterioles Sodium balance Angiotensin II Autoregulation Nitric oxide Adenosine LTERATIONS in glomerular hemodynamics and the renin angiotensin system (RAS) are important in the pathogenesis and pathophysiology of many diseases. Indeed, recent clinical studies have proven that interventions improving glomerular hemodynamics and/or the RAS are effective therapeutic modalities in such diseases as hypertension, congestive heart failure, diabetic nephropathy and chronic glomerulonephritis.1-4) Based on substantial progress made in recent years, this article reviews the mechanisms that control glomerular hemodynamics with a particular focus on tubuloglomerular feedback (TGF), as well as their alterations in various physiological and pathological conditions. Only a limited number of references is given. From the Second Department of Internal Medicine, Tohoku University School of Medicine, Sendai, Japan. Address for correspondence: Sadayoshi Ito, MD, Second Department of Internal Medicine, Tohoku University School of Medicine, 1-1, Seiryo-cho, Aoba-ku, Sendai , Japan. Received for publication November 6, Accepted November 6,

2 154 ITO AND ABE Jpn Heart J March 1996 JUXTAGLOMERULAR APPARATUS In each nephron of the mammalian kidney, the tubule returns to the parent glomerulus, forming the juxtaglomerular apparatus (JGA). The JGA displays a unique arrangement of the glomerular afferent (Af-) and efferent arteriole (Ef- Art), extraglomerular mesangial cells and specialized tubular epithelial cells, called the macula densa.5) It is now clear that the macula densa somehow senses changes in the composition of tubular fluid and sends an as of yet undefined signal to control renin release. More recently the macula densa was also found to control glomerular hemodynamics independently of renin release, a mechanism called tubuloglomerular feedback (TGF). Micropuncture studies have shown that increasing the rate of infusion of an isotonic solution from the proximal tubule through Henle's loop lowers glomerular capillary pressure and thus single nephron GFR (SNGFR). Calculated regional vascular resistance indicates that increases in Af-Art resistance can account for most of the changes in SNGFR, suggesting the Af-Art to be the major effecter site of TGF responses.6) However, Figure 1. (a) Simultaneous perfusion of an afferent arteriole (Af-Art) and attached macula densa (MD). Ef-Art, efferent arteriole; DCT, distal convoluted tubule; TAL, thick ascending limb of loop of Henle. (b) After perfusion has been established, both the MD and distal Af-Art can be visualized.

3 Vol 37 No 2 TUBULOGLOMERULAR FEEDBACK 155 Figure 2. Afferent arteriolar constriction induced by high NaCl. studies with orthograde perfusion were unable to define whether the change in osmolality, sodium or chloride concentration elicits TGF responses, since a long tubular segment between the site of infusion and the macula densa modifies the composition of tubular fluid reaching the macula densa.7,8) Although the macula densa was perfused in a retrograde direction from the distal tubule (closer to the macula densa) to circumvent such limitations, clarification of the tubular signal has proven to be difficult.9,10) Studies have employed in vitro microperfusion techniques in order to study the JGA more directly. Perfusion/cannulation of the Af- and Ef-Art was first described by Edwards.11) In the same year, Osgood et al12) described a method of perfusing a single isolated glomerulus, which was subsequently modified for the study of renin release.13) In 1985 we reported that microdissected rabbit Af-Arts, either alone or with macula densa attached, can be used for the study of renin release.14) In the same year, Kirk et al15) first reported the feasibility of microdissecting and perfusing the macula densa segment with attached but nonperfused glomerulus. This technique was further extended for the study of renin release by Skott, Briggs and Schnermann et al,16) who have further provided strong evidence that chloride is the tubular signal for macula densa control of renin release.17) Considering the facts that the macula densa has Na-K-2Cl

4 156 ITO AND ABE Jpn Heart J March 1996 cotransporters,18) with their activities depending more on chloride than sodium, and that loop diuretics block both renin and Af-Art responses to increased NaCl at the macula densa, it is likely that chloride is also the signal for the TGF. Finally, we have shown that both Af-Art and the macula densa can be microperfused simultaneously in vitro,19,20) demonstrating that increasing the NaCl concentration at the macula densa causes constriction of the terminal segment of the Af-Art. (Figures 1 and 2). AUTOREGULATION The GFR and renal blood flow are maintained at a constant level over a wide range of renal perfusion pressure (autoregulation). Such stability of GFR seems to be the basic requirement for a complex tubular system to function in a well-integrated manner and hence maintain homeostasis of body fluid and electrolytes. Along with the myogenic response, TGF is one of the two intrinsic mechanisms for autoregulation. In myogenic responses, the Af-Art responds to changes in perfusion pressure per se, with increased pressure causing constriction which prevents a rise in glomerular capillary pressure.21,22) In the TGF, the macula densa senses changes in NaCl concentration resulting from changes in SNGFR of the parent glomerulus. The TGF mechanism is exquisitely intricate, Figure 3. Simultaneous measurement of early distal tubule Cl- concentration and proximal tubule hydrostatic pressure in the same nephron.

5 Vol 37 No 2 TUBULOGLOMERULAR FEEDBACK 157 Figure 4. Tubuloglomerular feedback. When the NaCl concentration at the macula densa increases, the single-nephron glomerular filtration rate decreases, primarily due to constriction of the afferent arteriole. Volume expansion shifts the tubuloglomerular feedback response curve to the right. showing a synchronous oscillation of Cl- concentration and SNGFR (Figure 3). Thus, any small changes in SNGFR that are not prevented by the myogenic response are well compensated for by the TGF. Perhaps this is the reason why the kidney exhibits the most efficient autoregulation compared with any other organ in the body. It should be pointed out, however, that the myogenic response and TGF may interact with each other at the level of the Af-Art.24) Thus the degree of contribution of each mechanism to autoregulation may differ in various physiological and pathological conditions. For example, TGF may be primarily responsible for autoregulation during low salt intake, while the myogenic response becomes more important during high salt intake or volume expansion. NaCl concentration at the macula densa increases during volume expansion. According to the TGF phenomenon, such increases in NaCl at the macula densa would cause constriction of the Af-Art and hence decrease GFR. However, volume expansion does not change or even increase GFR in normal animals and humans. Such an apparent contradiction can be resolved by understanding that the TGF is a flexible mechanism that changes its sensitivity depending on the status of body fluid volume.25-27) As shown in Figure 4, the TGF-response curve shifts to the right, with a smaller maximal response during volume expansion. Thus, despite an increase in NaCl concentration at the macula densa, GFR is well maintained, allowing continued excretion of unnecessary sodium. While the mechanism of such shifts is not entirely clear, suppression of angiotensin (Ang II) seems to play an important role.26)

6 158 ITO AND ABE Jpn Heart J March 1996 ANGIOTENSIN AND ADENOSINE While the mechanism of signal transmission from the macula densa to the Af-Art is not clear at the present time, there is substantial evidence that Ang II and adenosine are involved in TGF responses. Despite its vasodilator action seen in other vascular segments including the Ef-Art, adenosine is a potent vasoconstrictor of the Af-Art.28) It has been shown that inhibition of adenosine action attenuates TGF responses in vivo29,30) as well as in our in vitro preparation.31) In addition, we have shown that increasing adenosine levels in the juxtaglomerular interstitium but not in the tubular lumen augments TGF responses.31) These results suggest that adenosine, acting in the juxtaglomerular interstitium, plays an important role in macula densa control of glomerular hemodynamics. Recent studies have shown that in normal rats Ang II infused either systemically or directly into the peritubular capillaries enhances TGF sensitivity,32, while blocking Ang II action with either converting enzyme inhibitors or receptor antagonists attenuates the TGF response.34-37) Taken together with our finding that TGF responses are intact in our preparation devoid of angiotensinogen (and hence Ang II), these results suggest that Ang II is a modulator but not a mediator of the TGF. In addition, Hall and Granger,38) reported that intravenous Ang II increased calculated resistance of both the Af-Art and Ef-Art in dogs treated with converting enzyme-inhibitors, while blocking TGF by urethral occlusion abolished the increased Af-Art resistance without affecting the increase in Ef-Art resistance. Furthermore, using our preparation, we have recently reported that Ang II-induced constriction of the Af-Art becomes stronger when the NaCl concentration at the macula densa is elevated,39) demonstrating a significant interaction between Ang II and macula densa control of glomerular hemodynamics. It may be that Ang II increases tubular transport at the macula densa, thereby elevating levels of the vasoconstrictor signal (as yet undefined) sent from the macula densa to the Af-Art. It is also possible that Ang II and the vasoconstrictor signal interact at the level of the Af-Art in a synergistic manner. In this regard, it is interesting to note that Ang II and adenosine may enhance each other's action in the Af-Art,40) while adenosine seems to play an important role in the signal transmission of the TGF. The interaction between Ang II and macula densa control of Af-Art resistance may play an important role in various physiological and pathological conditions. Under physiological conditions, the activity of the renin-angiotensin system seems to be mainly associated with sodium balances. Despite the potent vasoconstrictor action of Ang II, changes in sodium intake usually cause little change in the GFR. Such stability may be due to the well-integrated action of Ang II on both tubules and the Af-Art, which are controlled by NaCl concentra-

7 Vol 37 No 2 TUBULOGLOMERULAR FEEDBACK 159 tion at the macula densa. For instance, during low NaCl intake, increased Ang II would stimulate proximal tubular reabsorption, leading to some decrease in NaCl delivery to the macula densa (and hence NaCl concentration at the macula densa). Since Ang II action on the Af-Art becomes weaker when the NaCl concentration at the macula densa is decreased, the Af-Art may not constrict very much even in the presence of high Ang II, thereby maintaining the GFR. Thus fine tuning of Ang II action on the Af-Art by NaCl concentration at the macula densa may serve as the basis for GFR stability, and hence body fluid and electrolyte homeostasis despite daily variations in sodium intake. NITRIC OXIDE It has been shown that the macula densa has by far the highest nitric oxide synthase (NOS) levels in the normal kidney.41,42) The isoform present in the macula densa is neuronal NOS which is distinct from the endothelial form. In viv micropuncture studies have shown that NOS inhibitors infused into a proximal tubule or peritubular capillary augment the TGF-mediated decrease in the SNGFR.42) Using isolated microperfused Af-Art and the attached macula densa, we20) examined the effect of L-nitro-L-arginine methyl ester (L-NAME; an inhibitor of NOS) added to the macula densa perfusate on Af-Art diameter. We found that the addition of 10-5 or 10-6M L-NAME to a high-nacl but not a low-nacl macula densa perfusate decreased the diameter (Figure 5). We confirmed that perfusing the macula densa with L-NAME did not affect the vasodilator action of acetylcholine added to the lumen of the afferent arterioles, indicating that NO synthesis by the arteriole was not altered. Furthermore, we observed that 7- nitroindazole, a specific inhibitor of neuronal NOS, had effects similar to those obtained with L-NAME (unpublished observations). These results suggest that NO produced by the macula densa modulates the TGF response of the Af-Art. However, several issues remain to be clarified: 1) whether NO synthesis is regulated by either acute or chronic changes in NaCl transport at the macula densa, or whether NO is released basally regardless of NaCl transport; 2) whether NO alters tubular transport at the macula densa, thereby attenuating afferent arteriolar constriction induced by high NaCl, 3) where in the signal transmission pathway NO is involved in TGF, and 4) whether deterioration in the L-arginine NO pathway of the macula densa is involved in the pathogenesis/pathophysiology of certain diseases such as hypertension. Since NO seems to attenuate the Af-Art constriction induced by the myogenic response43) and TGF,20,42) inhibition of NO synthesis would be expected to decrease RBF and GFR to a greater extent when renal perfusion is high. However, most studies have found that autoregulation is well maintained (but at a

8 160 ITO AND ABE Jpn Heart J March 1996 Figure 5. Changes in afferent arteriolar diameter induced by L-NAME added to high-or low-nacl macula densa perfusate. *p<0.001 compared with low NaCl (o). +p<0.001 compared with 0 L-NAME. lower RBF) during NO synthesis inhibition.44) While the reason for this discrepancy is unknown, other compensatory mechanisms may be capable of maintaining RBF in vivo in normal animals. For instance, it is possible that when L-NAME is given to intact animals, the NaCl concentration at the macula densa may fall significantly due to increases in sodium reabsorption by tubular segments proximal to the macula densa,45) so that TGF contributes less to renal autoregulation. TGF AND HYPERTENSION The kidney plays a crucial role in the pathogenesis of hypertension. Studies have shown that in various models of genetic hypertension, transplanting a normal kidney after removal of the native kidneys prevents development of hypertension.46) Moreover, Curtis et al47) reported that renal transplantation cured or dramatically improved hypertension in patients under chronic dialysis due to hypertensive nephrosclerosis. These observations provide strong evidence that some alterations in the kidney are primarily responsible for the development of hypertension in animals and humans. As discussed earlier, an adequate interaction between TGF and Ang II seems to be essential to maintaining GFR at a relatively constant level despite a large variation in daily salt intake. Thus, it is conceivable that alterations in TGF and/or renin release may cause sodium

9 Vol 37 No 2 TUBULOGLOMERULAR FEEDBACK 161 retention, thereby leading to systemic hypertension. Indeed recent preliminary studies have shown that the rightward shift of TGF is impaired in Dahl saltsensitive hypertensive rats.48) In this model, Ef-Art resistance increases upon salt loading, resulting in glomerular hypertension which would cause progressive glomerular damage.49) On the other hand, in the spontaneously hypertensive rat (SHR), a model of non-salt sensitive hypertension, glomerular capillary pressure is within the normal range despite high renal perfusion pressure,50) suggesting that preglomerular resistance is elevated. It has been shown that in the SHR both myogenic and TGF responses of the Af-Art are exaggerated,21,22,51) possibly contributing elevated preglomerular resistance. However, it still remains to be determined whether such alterations are responsible for the development of hypertension. Interestingly, Norrelund et al recently reported that in the F2 generation of SHR and its normotensive control WKY, the diameter of the Af-Art in young animals is inversely correlated with later development of hypertension.52) Thus it would be interesting to study TGF in various animal models in which a specific gene is modified. REFERENCES 1. Lewis EJ, Hunsicker LG, Bain RP, et al, for the Collaborative Study Group. The effect of angiotensinconverting-enzyme inhibitors on diabetic nephropathy. N Engl J Med 1993; 329: Zucchelli P, Zucca A, Borghi M, et al. Long-term comparison between captopril and nifedipine in the progression of renal insufficiency. Kidney Int 1992; 42: Klahr S, Schreiner G, Ichikawa I. The progression of renal disease. N Engl J Med 1988; 318: SOLVD Investigators. Effect of enalapril on survival in patients with reduced left ventricular ejection fraction and congestive heart failure. N Engl J Med 1991; 325: Schnermann J, Briggs JP. Function of the juxtaglomerular apparatus: Control of glomerular hemodynamics and renin secretion. In: Seldin DW, Giebisch G, editors. The Kidney; Physiology and Pathophysiology. New York: Raven Press, 1992: Briggs JP, Wright FS. Feedback control of glomerular filtration rate: Site of the effector mechanism. Am J Physiol 1979; 236: F Briggs JP, Schubert G, Schnermann J. Further evidence for an inverse relationship between macula densa NaCl concentration and filtration rate. Pflugers Arch 1982; 392: Navar LG, Bell PD, Thomas CE, Ploth DW. Influence of perfusate osmolality on stop-flow pressure feedback responses in the dog. Am J Physiol 1978; 235: F Schuermann J, Briggs JP. Concentration-dependent sodium chloride transport as the signal in feedback control of glomerular filtration rate. Kidney Int 1982; 22 (Suppl. 12): S Bell PD, Navar LG. Relationship between tubuloglomerular feedback responses and perfusate hypotonicity. Kidney Int 1982; 22: Edwards RM. Segmental effects of norepinephrine and angiotensin II on isolated renal microvessels. Am J Physiol 1983; 244: F Osgood RW, Patton M, Hanley MJ, Venkatachalam M, Reineck HJ, Stein JH. In vitro perfusion of the isolated dog glomerulus. Am J Physiol 1983; 244: F Bock HA, Hermle M, Fiallo A, Osgood RW, Fried TA. Measurement of renin secretion in single perfused rabbit glomeruli. Am J Physiol 1990; 258: F Itch S, Carretero OA, Murray RD. Possible role of adenosine in the macula densa mechanism of

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