Reabsorption of nitro-L-arginine infused into the late proximal tubule participates in modulation of TGF responsiveness

Concurrent activation of multiple vasoactive signaling pathways in vasoconstriction caused by tubuloglomerular feedback: a quantitative assessment

Tubuloglomerular feedback (TGF) describes the negative relationship between (a) NaCl concentration at the macula densa and (b) glomerular filtration rate or glomerular capillary pressure. TGF-induced vasoconstriction of the afferent arteriole results from the enhanced effect of several vasoconstrictors with an effect size sequence of adenosine = 20-HETE > angiotensin II > thromboxane = superoxide > renal nerves > ATP. TGF-mediated vasoconstriction is limited by the simultaneous release of several vasodilators with an effect size sequence of nitric oxide > carbon monoxide = kinins > adenosine. The sum of the constrictor effects exceeds that of the dilator effects by the magnitude of the TGF response. The validity of the additive model used in this analysis can be tested by determining the effect of combined inhibition of some or all agents contributing to TGF. Multiple independent contributors to TGF are consistent with the variability of TGF and of the factors contributing to TGF resetting.

Systemic arterial and venous determinants of renal hemodynamics in congestive heart failure

Heart and kidney interactions are fascinating, in the sense that failure of the one organ strongly affects the function of the other. In this review paper, we analyze how principal driving forces for glomerular filtration and renal blood flow are changed in heart failure. Moreover, renal autoregulation and modulation of neurohumoral factors, which can both have repercussions on renal function, are analyzed. Two paradigms seem to apply. One is that the renin-angiotensin system (RAS), the sympathetic nervous system (SNS), and extracellular volume control are the three main determinants of renal function in heart failure. The other is that the classical paradigm to analyze renal dysfunction that is widely applied in nephrology also applies to the pathophysiology of heart failure: pre-renal, intra-renal, and post-renal alterations together determine glomerular filtration. At variance with the classical paradigm is that the most important post-renal factor in heart failure seems renal venous hypertension that, by increasing renal tubular pressure, decreases GFR. When different pharmacological strategies to inhibit the RAS and SNS and to assist renal volume control are considered, there is a painful lack in knowledge about how widely applied drugs affect primary driving forces for ultrafiltration, renal autoregulation, and neurohumoral control. We call for more clinical physiological studies.

Tubular control of renin synthesis and secretion

The intratubular composition of fluid at the tubulovascular contact site of the juxtaglomerular apparatus serves as regulatory input for secretion and synthesis of renin. Experimental evidence, mostly from in vitro perfused preparations, indicates an inverse relation between luminal NaCl concentration and renin secretion. The cellular transduction mechanism is initiated by concentration-dependent NaCl uptake through the Na-K-2Cl cotransporter (NKCC2) with activation of NKCC2 causing inhibition and deactivation of NKCC2 causing stimulation of renin release. Changes in NKCC2 activity are coupled to alterations in the generation of paracrine factors that interact with granular cells. Among these factors, generation of PGE2 in a COX-2-dependent fashion appears to play a dominant role in the stimulatory arm of tubular control of renin release. [NaCl] is a determinant of local PG release over an appropriate concentration range, and blockade of COX-2 activity interferes with the NaCl dependency of renin secretion. The complex array of local paracrine controls also includes nNOS-mediated synthesis of nitric oxide, with NO playing the role of a modifier of the intracellular signaling pathway. A role of adenosine may be particularly important when [NaCl] is increased, and at least some of the available evidence is consistent with an important suppressive effect of adenosine at higher salt concentrations.

Assessment of renal autoregulation

The kidney displays highly efficient autoregulation so that under steady-state conditions renal blood flow (RBF) is independent of blood pressure over a wide range of pressure. Autoregulation occurs in the preglomerular microcirculation and is mediated by two, perhaps three, mechanisms. The faster myogenic mechanism and the slower tubuloglomerular feedback contribute both directly and interactively to autoregulation of RBF and of glomerular capillary pressure. Multiple experiments have been used to study autoregulation and can be considered as variants of two basic designs. The first measures RBF after multiple stepwise changes in renal perfusion pressure to assess how a biological condition or experimental maneuver affects the overall pressure-flow relationship. The second uses time-series analysis to better understand the operation of multiple controllers operating in parallel on the same vascular smooth muscle. There are conceptual and experimental limitations to all current experimental designs so that no one design adequately describes autoregulation. In particular, it is clear that the efficiency of autoregulation varies with time and that most current techniques do not adequately address this issue. Also, the time-varying and nonadditive interaction between the myogenic mechanism and tubuloglomerular feedback underscores the difficulty of dissecting their contributions to autoregulation. We consider the modulation of autoregulation by nitric oxide and use it to illustrate the necessity for multiple experimental designs, often applied iteratively.

Mediators of tubuloglomerular feedback regulation of glomerular filtration: ATP and adenosine

In the juxtaglomerular apparatus of the kidney the loop of Henle gets into close contact to its parent glomerulus. This anatomical link between the tubular system and the vasculature of the afferent and efferent arteriole enables specialized tubular cells, the macula densa (MD) cells, to establish an intra-nephron feedback loop designed to control preglomerular resistance and thereby single nephron glomerular filtration rate. This review focuses on the signalling mechanisms which link salt-sensing MD cells and the regulation of preglomerular resistance, a feedback loop known as tubuloglomerular feedback (TGF). Two purinergic molecules, ATP and adenosine, have emerged over the years as most likely candidates to serve as mediators of TGF. Data will be reviewed supporting a role of either ATP or adenosine as mediators of TGF. In addition, a concept will be discussed that integrates both ATP and adenosine into one signalling cascade that includes (i) release of ATP from MD cells upon increases in tubular salt concentration, (ii) extracellular degradation of ATP to form adenosine, and (iii) adenosine-mediated vasoconstriction of the afferent arteriole.

Mechanisms for Renal Blood Flow Control Early in Diabetes as Revealed by Chronic Flow Measurement and Transfer Function Analysis

The purpose of this study was to establish the roles of the myogenic response and the TGF mechanism in renal blood flow (RBF) control at the very earliest stages of diabetes. Mean arterial pressure (MAP) and RBF were measured continuously, 18 h/d, in uninephrectomized control and diabetic rats, and transfer function analysis was used to determine the dynamic autoregulatory efficiency of the renal vasculature. During the control period, MAP averaged 91 +/- 0.5 and 89 +/- 0.4 mmHg, and RBF averaged 8.0 +/- 0.1 and 7.8 +/- 0.1 ml/min in the control and diabetic groups, respectively. Induction of diabetes with streptozotocin caused a marked and progressive increase in RBF in the diabetic rats, averaging 10 +/- 6% above control on day 1 of diabetes and 22 +/- 3 and 34 +/- 1% above control by the end of diabetes weeks 1 and 2. MAP increased approximately 9 mmHg during the 2 wk in the diabetic rats, and renal vascular resistance decreased. Transfer function analysis revealed significant increases in gain to positive values over the frequency ranges of both the TGF and myogenic mechanisms, beginning on day 1 of diabetes and continuing through day 14. These very rapid increases in RBF and transfer function gain suggest that autoregulation is impaired at the very onset of hyperglycemia in streptozotocin-induced type 1 diabetes and may play an important role in the increase in RBF and GFR in diabetes. Together with previous reports of decreases in chronically measured cardiac output and hindquarter blood flow, this suggests that there may be differential effects of diabetes on RBF versus nonrenal BF control.

NO dependency of RBF and autoregulation in the spontaneously hypertensive rat

In the spontaneously hypertensive rat (SHR), renal blood flow (RBF) has been reported to be very dependent on nitric oxide (NO); however, autoregulation is normal, albeit shifted to higher perfusion pressures. To test the hypothesis that in the SHR NO dependency of RBF autoregulation is diminished, we investigated RBF autoregulation in anesthetized young male SHR and normotensive Wistar-Kyoto (WKY) rats before and during acute intravenous NO synthase (NOS) inhibition with N(omega)-nitro-L-arginine (L-NNA) and urinary excretion of nitrate plus nitrite (U(NOx)V) at different renal perfusion pressures (RPP). Under baseline conditions, SHR had higher mean arterial pressure (147 +/- 4 mmHg) and renal vascular resistance (16 +/- 1 U) than WKY (105 +/- 4 mmHg and 10 +/- 0.5 U, respectively, P < 0.05). RBF was similar (9.4 +/- 0.5 vs. 10.3 +/- 0.1 ml x min(-1)x g kidney wt(-1)). Acute NOS blockade increased mean arterial pressure similarly, but there was significantly more reduction in RBF and hence an enhanced increase in renal vascular resistance in SHR (to 36 +/- 3 vs. 17 +/- 1 U in WKY, P < 0.001). The renal vasculature of SHR is thus strongly dependent on NO in maintaining basal RBF. The lower limit of autoregulation was higher in SHR than WKY in the baseline situation (85 +/- 3 vs. 71 +/- 2 mmHg, P < 0.05). Acute L-NNA administration did not decrease the lower limit in the SHR (to 81 +/- 3 mmHg, not significant) and decreased the lower limit to 63 +/- 2 mmHg (P < 0.05) in the WKY. The degree of compensation as a measure of autoregulatory efficiency attained at spontaneous perfusion pressures was comparable in SHR vs. WKY but with a shift of the curve toward higher perfusion pressures in SHR. Acute NOS blockade only increased the degree of compensation in WKY. Remarkably, U(NOx)V was significantly lower at spontaneous RPP in SHR. After reduction of RPP, the observed decrease in U(NOx)V was significantly more pronounced in WKY than in SHR. In conclusion, the renal circulation in SHR is dependent on high levels of NO; however, the capacity to modulate NO in response to RPP-induced changes in shear stress seems to be limited.

Effects of uroguanylin, an intestinal natriuretic peptide, on tubuloglomerular feedback

Uroguanylin is an endogenous peptide that stimulates cyclic guanosine monophosphate (cGMP) production via the activation of guanylate cyclase C (GC-C) in the intestine and kidney. A high salt diet, but not intravenous salt load, enhances the secretion of biologically active uroguanylin from the intestine and increases its concentration in plasma and urine. Our purpose is to clarify the effect of uroguanylin on renal microcirculation and the tubuloglomerular feedback (TGF) mechanism. Clearance and micropuncture experiments were performed in anesthetized rats. TGF responsiveness was assessed in superficial nephrons by measuring the changes of early proximal flow rate (EPFR) in response to orthograde loop perfusion at 40 nl/min with artificial tubular fluid (ATF). Reductions in EPFR induced by loop perfusion during intravenous infusion of uroguanylin at the rate of 10 and 50 nmol/kg/h were similar yet significantly less than that during the control period (33+/-3% and 35+/-3% vs. 47+/-3%, p<0.05). Intraluminal application of uroguanylin at 10(-7) and 10(-5) mol/l in ATF decreased EPFR by 40+/-3% and 33+/-7%, respectively, with the latter value being significantly less than the control (p<0.05). Intravenous infusion of uroguanylin did not significantly change whole kidney function. Administration of atrial natriuretic peptide (ANP), which activates GC-A and B, significantly suppressed TGF-mediated EPFR reduction either intravenously (10 nmol/kg/h) or intraluminally (10(-5) mol/l in ATF) (9+/-3% and 13+/-2% vs. 47+/-3% of the control, p<0.05). In conclusion, uroguanylin clearly suppresses TGF both through intravenous and intraluminal routes, although the effects on glomerular microcirculation and whole kidney function are far less than those of ANP.

Nitric oxide produced by THAL nitric oxide synthase inhibits TGF

Nitric oxide (NO) produced by neuronal NO synthase (nNOS) in the macula densa decreases tubuloglomerular feedback (TGF). NO produced by NOS in the thick ascending limb (THAL) inhibits NaCl transport. We hypothesized that NO produced by NOS in the THAL reaches the macula densa and inhibits TGF. Rabbit afferent arterioles and attached macula densa were simultaneously microperfused in vitro. TGF response was determined by measuring afferent arteriole diameter before and after increasing NaCl in the macula densa perfusate. When the nNOS inhibitor 7-nitroindazole (7-NI) (10 micromol/L) was added to the macula densa lumen, it increased TGF from 2.3 +/- 0.2 to 3.5 +/- 0.5 microm (P<0.02; n=6). In the presence of 7-NI, N(omega)-nitro-L-arginine methyl ester (L-NAME) (1 mmol/L) enhanced TGF from 2.6 +/- 0.3 to 4.0 +/- 0.5 microm (P<0.02; n=6) when the macula densa was perfused orthograde via the THAL, whereas it had no effect on TGF when the macula densa was perfused retrograde via the distal tubule (DT). Inhibition of macula densa soluble guanylate cyclase with LY83583 (1 micromol/L) blocked the effect of NO produced by THAL NOS when the macula densa was perfused via the THAL. We concluded that NO produced by THAL NOS acts as a paracrine factor, reaching the macula densa and inhibiting TGF.

Feedback control of glomerular vascular tone in neuronal nitric oxide synthase knockout mice

For further elucidation of the role of neuronal nitric oxide synthase (nNOS) in macula densa (MD) cells, experiments were performed in anesthetized nNOS knockout mice (nNOS -/-). At comparable levels of arterial BP, renal blood flow was not significantly different between nNOS +/+ and nNOS -/- (1.7 +/- 0.2 versus 1.4 +/- 0.1 ml/min), and autoregulation of renal blood flow was maintained to a pressure level of approximately 85 mmHg in both groups of mice (n = 6 in each group). The fall in proximal tubular stop-flow pressure in response to an increase in loop of Henle perfusion rate from 0 to 30 nl/min was comparable in nNOS +/+ and -/- mice (40.7 +/- 1.6 to 32 +/- 2 mmHg versus 40.6 +/- 1.6 to 31.6 +/- 2 mmHg; not significant; n = 13 versus 18 nephrons). Luminal application of the nonselective NOS inhibitor nitro-L-arginine (10(-3) and 10(-2) M) enhanced the perfusion-dependent fall in stop-flow pressure in nNOS +/+ (7 +/- 1 to 13 +/- 2 mmHg; P < 0.05) but not in nNOS -/- (7 +/- 1 to 8 +/- 1 mmHg; not significant) mice. nNOS -/- mice exhibited a lower nephron filtration rate, compared with nNOS +/+, during free-flow collections from early distal tubules (influence of MD intact, 7 +/- 0.7 versus 10.9 +/- 1 nl/min; P = 0.002) but not from late proximal tubule (influence of MD minimized, 10.1 +/- 1 versus 11.7 +/- 1 nl/min; not significant; n = 16 nephrons). Distal Cl concentration and fractional absorption of fluid or chloride up to the early distal tubule was not different between nNOS -/- and +/+ mice. The data indicate that nNOS in MD tonically attenuates the GFR-lowering influence of ambient luminal NaCl, which may serve to increase the fluid and electrolyte load to the distal tubule, consistent with a role of MD nNOS in tubuloglomerular feedback resetting.

Normal TGF responsiveness during chronic treatment with angiotensin-converting enzyme inhibition: role of AT1 receptors

Acute inhibition of angiotensin II formation by angiotensin-converting enzyme inhibition (ACE-I) attenuates tubuloglomerular feedback (TGF) responsiveness. This has been proposed to facilitate sodium excretion, which contributes to the antihypertensive effects of ACE-I. However, in previous experiments in spontaneously hypertensive Fawn-hooded rats, TGF responses were normal during chronic ACE-I treatment. In the present study, we investigated TGF responsiveness during chronic ACE-I treatment in normotensive rats and the involvement of changes in nitric oxide or angiotensin II activity. Maximum TGF responses were assessed in control Sprague-Dawley rats and in rats acutely (acute ACE-I, 3 microgram/min IV) and chronically (chronic ACE-I, 100 mg/L PO 2 to 3 weeks+acute 3 microgram/min enalaprilat IV) treated with ACE-I. In all groups, TGF responses were also assessed during late proximal tubular perfusion with 1 mmol/L nitro-L-arginine. In a last group, the chronic ACE-I treatment was combined with acute ACE-I and high doses of intrarenal losartan (acute 3 microgram/min enalaprilat IV+50 mg/kg losartan). The maximum TGF responses in acutely treated ACE-I rats were strongly attenuated (0.7+/-0.4 mm Hg versus 6.5+/-0.8 mm Hg in control rats, P<0.05). Mean arterial pressure was lower in the chronically treated ACE-I group (107+/-5 mm Hg versus 126+/-5 mm Hg in control rats, P<0.05); however, TGF responses were normal (6. 4+/-0.9 mm Hg). Intraluminal nitro-L-arginine infusion did not influence TGF responses during acute ACE-I (2.3+/-0.4 mm Hg) but enhanced TGF responses during chronic ACE-I to the same extent as in control rats (14.5+/-2.3 versus 16.7+/-1.9 mm Hg, NS). In the rats chronically treated with ACE-I with superimposed acute infusion of losartan or chronically treated with losartan, TGF responses were significantly attenuated (1.8+/-0.8 mm Hg and 2.6+/-0.8 mm Hg, respectively; P:<0.05 versus chronic ACE-I and control). Prolonged administration with ACE-I is associated with normal TGF responses. This phenomenon appears to be mediated by AT1 receptors, because acute treatment with losartan in rats chronically treated with ACE-I and chronic treatment with losartan lead to strong attenuation of TGF responses.

Impaired renal blood flow autoregulation in two-kidney, one-clip hypertensive rats is caused by enhanced activity of nitric oxide

Increases in renal perfusion pressure will induce shear stress-mediated nitric oxide (NO) release, which could oppose autoregulation of renal blood flow (RBF). Although cardiac, cerebral, and mesenteric autoregulation is enhanced during nitric oxide (NO) synthesis inhibition, this has not been reported for renal autoregulation of blood flow. In the present study, the lower limit and efficiency of RBF autoregulation (as assessed by the degree of compensation) were studied before and during NO inhibition in normotensive Sprague Dawley rats (control; n = 9) and in the non-clipped kidney of two-kidney, one-clip Goldblatt hypertensive animals (2K1C; n = 9; 3 wk; 0.25-mm silver clip). In both groups, renal autoregulation curves were obtained before and during infusion of N(G) -nitro-L-arginine (L-NNA) (bolus 1.5 mg/kg intravenously, infusion 10 microg/kg per min intravenously), using a transit-time flow probe around the left renal artery. In control rats, mean arterial pressure (MAP) increased, RBF decreased, and renal vascular resistance (RVR) increased in response to L-NNA infusion. The lower limit of autoregulation in control animals did not significantly change during L-NNA infusion (78 +/- 3 to 70 +/- 2 mmHg). The degree of compensation in these rats slightly increased during L-NNA infusion, however, this was only significant below 90 mmHg. The 2K1C rats had elevated MAP under baseline conditions. L-NNA infusion resulted in a decrease in RBF and an increase in MAP and RVR. During L-NNA infusion, RVR in 2K1C rats greatly exceeded RVR in control rats. A significant decrease was observed in the lower limit of autoregulation from 85 +/- 3 to 72 +/- 5 mmHg (P < 0.05). In the contralateral kidney of 2K1C rats, the degree of compensation was lower than in control rats under baseline conditions. L-NNA infusion resulted in significantly higher degrees of compensation compared to baseline. In conclusion, the contralateral kidney displayed a high NO dependency, as RBF greatly decreased and RVR dramatically increased in response to L-NNA infusion. The contralateral kidney of 2K1C rats exhibited impaired RBF autoregulation, which was improved by NO inhibition, as judged from a decrease in the lower limit of autoregulation and an increase in the degree of compensation. This study indicates that perfusion pressure-dependent NO release can oppose autoregulation in the kidney. However, the enhanced influence of NO on pressure-dependent RBF may facilitate the preservation of renal function in the nonclipped kidney of 2K1C rats.

Concerted actions of renal endothelial and macula densa NO systems in the maintenance of extracellular fluid volume

It is now clear that nitric oxide (NO) exerts a substantial influence on renal function and that the kidney has a high capacity to produce NO. However, there are at least two different NO systems in the kidney. The interplay between NO generated by the endothelium and by the macula densa is considered in this review. It seems that endothelial NO increases in response to an increase in perfusion pressure and an increase in distal delivery, whereas macula densa NO decreases upon a sustained increase in distal delivery. Furthermore, evidence is accumulating that macula densa NO may well mediate renin release. Though seemingly in contrast, both the response of the endothelial NO and of the macula densa NO system seem appropriate to restore a perturbation of fluid balance. The function of the tubuloglomerular feedback (TGF) mechanism is likely to be influenced by both sources of NO, because of the close proximity of these NO producing cells to the vascular smooth muscle cells of the afferent arteriole. The endothelial NO system seems to be responsible for short-term, dampening actions to increased afferent arteriolar tone elicited by activation of the TGF system. The macula densa NO system, on the other hand, is probably adapting TGF responses to sustained increases in distal delivery. The analysis presented in this paper is an attempt to integrate the function of the two NO systems into physiological regulation. The exact role of the medullary NOS enzymes remains to be further elucidated.

Increased availability of nitric oxide leads to enhanced nitric oxide dependency of tubuloglomerular feedback in the contralateral kidney of rats with 2-kidney, 1-clip Goldblatt hypertension

The contralateral kidney of 2-kidney, 1-clip hypertensive (2K1C) rats is unable to escape the renal vasoconstrictive and sodium-retaining effects of increased circulating angiotensin II levels. Evidence is accumulating that renal function is relatively preserved by enhanced influence of NO in the contralateral kidney. In this study, we investigated (1) whether the high NO dependency of renal hemodynamics in the contralateral kidney is due to increased availability of NO or increased sensitivity to NO and (2) whether elevated NO activity dampens the actions of angiotensin II to enhance tubuloglomerular feedback (TGF) responses in the nonclipped kidney of 2K1C rats. To estimate whether the available NO is increased, the NO clamp technique was applied in rats that underwent sham operation (n=6) and in the contralateral kidney of 2K1C Sprague-Dawley rats (3 weeks old; 0.25-mm silver clip; n=6). During systemic infusion of nitro-L-arginine (L-NNA; 50 microg/kg. min(-1)), sodium nitroprusside (SNP) was infused in the renal artery and the rate was adjusted so that renal vascular resistance (RVR) was restored to baseline levels. In sham rats, RVR increased during L-NNA treatment from 17.2+/-2.0 to 33.0+/-3.6 U (P<0.01) and was restored to baseline values during SNP infusion (17.1+/-2.3 U); 9. 2+/-1.8 nmol/min of SNP was needed to restore RVR to baseline values. In 2K1C rats, RVR increased during L-NNA treatment from 16.7+/-1.1 to 53.4+/-3.5 U (P<0.01). This increase of RVR was significantly larger than in sham rats. RVR was restored to baseline values during SNP infusion (17.4+/-0.9 U); 26.0+/-4.3 nmol/min of SNP was needed to restore RVR to baseline values (P<0.05 versus sham). Furthermore, maximum TGF responses were assessed before and during late proximal tubular infusion of L-NNA in the kidneys of sham rats and the nonclipped kidneys of 2K1C rats. Control maximum TGF responses were 4.7+/-0.7 and 5.1+/-0.4 mm Hg in sham and 2K1C rats, respectively. During intraluminal L-NNA infusion, maximum TGF responses were 15. 4+/-0.9 mm Hg in sham rats and 22.2+/-2.5 mm Hg in 2K1C rats (P<0.05 versus sham). Finally, urinary NO(2)+NO(3) excretion in the nonclipped kidney was significantly higher than in the clipped kidney (P<0.05). In conclusion, (1) as assessed using the NO clamp, ambient intrarenal NO levels are increased in the contralateral kidney of 2K1C rats and (2) the NO dependency of the TGF system is enhanced. These experiments indicate that adaptations in NO activity lead to relatively low TGF responsiveness, which will offset the simultaneous sodium-retaining actions of angiotensin II on proximal tubular reabsorption and TGF responsiveness.

Renal endothelial and macula densa NOS: integrated response to changes in extracellular fluid volume

If, only 20 years ago, anyone had postulated that the absence of nitric oxide gas (NO) would lead to severe hypertension and destruction of the vascular bed of the kidney within weeks, it is not unlikely that smiles of pity would have appeared on the faces of fellow researchers. By now, this has become common knowledge, and hundreds of reports have appeared on the regulation of vascular and renal function by nitric oxide. The amount of information complicates the design of a concept on how NO participates in control of extracellular fluid volume (ECFV) by the kidney. This review analyzes the function of endothelial and macula densa NO synthase (NOS) in the regulation of renal function. From this analysis, endothelial NOS (eNOS)-derived NO is considered a modulator of vascular responses and of renal autoregulation in particular. Increases in renal perfusion pressure and sodium loading will increase eNOS activity, resulting in vasodilatation and depression of tubuloglomerular feedback system responsiveness. Endothelium-derived NO seems important to buffer minute-to-minute variations in perfusion pressure and rapid changes in ANG II activity. In contrast, macula densa NOS is proposed to drive adaptations to long-term changes in distal delivery and is considered a mediator of renin formation. Increases in perfusion pressure and distal delivery will depress the activity and expression of the enzyme that coincides with, and possibly mediates, diminished renin activity. Together, the opposite responses of eNOS and macula densa NOS-derived NO to changes in ECFV lead to an appropriate response to restore sodium balance. The concept that the two enzymes with different localizations in the kidney and in the cell are producing the same product, displaying contrasting responses to the same stimulus but nevertheless exhibiting an integrated response to perturbation of the most important regulated variable by the kidney, i.e., the ECFV, may be applicable to other tissues.

Endothelin mediates renal vascular memory of a transient rise in perfusion pressure due to NOS inhibition

We investigated the renal responses to NO synthase (NOS) inhibition with N-monomethyl-L-arginine (L-NMA; 30 mg/kg) in anesthetized rats in which renal perfusion pressure (RPP) to the left kidney was mechanically adjusted. Acute L-NMA increased blood pressure (BP, approximately 20%) and renal vascular resistance (RVR) rose ( approximately 50%) in the right kidneys that were always exposed to high RPP. In group 1, the left kidney was exposed to a transient increase (5 min) in RPP which was then normalized, and the rise in RVR was similar to the right kidney. In group 2 the left kidney was never exposed to high RPP, and the rise in RVR was attenuated relative to the right kidney. In group 3, rats were pretreated with the endothelin (ET) receptor antagonist Bosentan, immediately before exposure of the left kidney to a transient increase in RPP, and the rise in RVR was also attenuated relative to the right kidney. NOS inhibition resulted in a natriuresis and diuresis in the right kidneys, and approximately 50% of the natriuresis persisted in the left kidney of group 2, in the absence of any rise in RPP. ET antagonism completely prevented the natriuresis and diuresis in response to acute L-NMA in both left and right kidneys. These data suggest that transient exposure to high RPP by NOS inhibition prevents an appropriate vasodilatory response when RPP is lowered, due to the intrarenal action of ET.

Tonic and phasic influences of nitric oxide on renal blood flow autoregulation in conscious dogs

The aim of this study was to investigate the influence of the mean level and phasic modulation of NO on the dynamic autoregulation of renal blood flow (RBF). Transfer functions were calculated from spontaneous fluctuations of RBF and arterial pressure (AP) in conscious resting dogs for 2 h under control conditions, after NO synthase (NOS) inhibition [NG-nitro-L-arginine methyl ester hydrochloride (L-NAME)] and after L-NAME followed by a continuous infusion of an NO donor [S-nitroso-N-acetyl-DL-penicillamine (SNAP)]. After L-NAME (n = 7) AP was elevated, heart rate (HR) and RBF were reduced. The gain of the transfer function above 0.08 Hz was increased, compatible with enhanced resonance of the myogenic response. A peak of high gain around 0.03 Hz, reflecting oscillations of the tubuloglomerular feedback (TGF), was not affected. The gain below 0.01 Hz, was elevated, but still less than 0 dB, indicating diminished but not abolished autoregulation. After L-NAME and SNAP (n = 5), mean AP and RBF were not changed, but HR was slightly elevated. The gain above 0.08 Hz and the peak of high gain at 0.03 Hz were not affected. The gain below 0.01 Hz was elevated, but smaller than 0 dB. It is concluded that NO may help to prevent resonance of the myogenic response depending on the mean level of NO. The feedback oscillations of the TGF are not affected by NO. NO contributes to the autoregulation below 0.01 Hz due to phasic modulation independent of its mean level.

Hypertension and impaired renal angiotensin II response in rats after chronic neuronal nitric oxide synthase inhibition

Nitric oxide (NO) produced by the macula densa cells is important for the control of tubuloglomerular feedback (TGF). Reduced production of NO by these cells activates TGF and could result in hypertension, although the TGF activity is then normalized in the hypertensive state. The normalization of TGF in this form of hypertension might be explained by an impaired ability of angiotensin II to constrict renal vessels or by up-regulation of some other vasodilator not affected by NO synthase inhibitors.

Nitrite production does not always reflect nitric oxide synthesis by isolated glomeruli

The goal of this paper was the measurement of nitric oxide (NO) production in isolated rat glomeruli using two different techniques. NO production was detected directly by a NO-specific electrode and the results were compared with data measured by the Griess reaction, an indirect index to evaluate NO production. The NO production, determined by both techniques, was dependent on the number of glomeruli. Pretreatment with Nw-nitro-L-arginine methyl ester, an inhibitor of the NO synthesis, reduced the NO concentration detected by the NO-sensor, but increased the NO2-concentration (when both results where compared with glomeruli without treatment). Preincubation with 1 mg/ml of Escherichia coli lipopolysaccharide significantly enhanced both NO and NO2-concentrations. Therefore, the present study provides direct evidence of NO generated in isolated glomeruli under physiological conditions and demonstrates that the measurement of NO2- by the Griess reaction, is not always an adequate technique to evaluate the actual NO production.

Role of macula densa NOS in tubuloglomerular feedback

Recent studies have amply confirmed the robust expression of neuronal nitric oxide synthase (nNOS) in macula densa cells and its function in blunting tubuloglomerular feedback responses. Regulation of nNOS may occur at many levels: (1) transcriptional and translational regulation, which is enhanced by salt restriction and angiotensin II; (2) functional enhancement by L-arginine delivery and uptake via system Y+, which is enhanced during salt loading; (3) structural activation and feedback inhibition provided by postsynaptic density proteins co-expressed with nNOS in the macula densa; (4) competitive inhibition by dimethylarginines, which can be metabolized via NG, NG dimethylarginine dimethylaminohydrolase co-expressed with nNOS in the macula densa; and (5) intracellular activation linked to changes in [Ca++] or pH during luminal Na+ reabsorption. Nitric oxide, once formed, can be degraded by O2- produced principally in the interstitium between the macula densa and afferent arteriole and in the wall of the arteriole. In genetic hypertension, tubuloglomerular feedback responses are enhanced, in part at least because of diminished buffering by macula densa NO and enhanced O2- generation in the juxtaglomerular apparatus. These recent studies highlight the importance of the macula densa nitric oxide-tubuloglomerular feedback system in adapting glomerular hemodynamics and renal function to changes in salt intake, and define potentially important defects in models of genetic hypertension.

Juxtaglomerular cell complex in the regulation of renal salt excretion

Luminal NaCl concentration at the macula densa (MD) has the two established effects of regulating glomerular arteriolar resistance and renin secretion. Tubuloglomerular feedback (TGF), the inverse relationship between MD NaCl concentration and glomerular filtration rate (GFR), stabilizes distal salt delivery and thereby NaCl excretion in response to random perturbations unrelated to changes in body salt balance. Control of vasomotor tone by TGF is exerted primarily by NaCl transport-dependent changes in local adenosine concentrations. During long-lasting perturbations of MD NaCl concentration, control of renin secretion becomes the dominant function of the MD. The potentially maladaptive effect of TGF under chronic conditions is prevented by TGF adaptations, permitting adjustments in GFR to occur. TGF adaptation is mechanistically coupled to the end point targeted by chronic deviations in MD NaCl, the rate of local and systemic angiotensin II generation. MD control of renin secretion is the result of the coordinated action of local mediators that include nitric oxide synthase (NOS) and cyclooxygenase (COX) products. Thus vascular smooth muscle cell activation during high MD transport and granular cell activation during low MD transport is achieved by different extracellular mediators. The coordinated regulation of NOS I and COX-2 expression in MD cells and of renin expression in granular cells suggests that control of juxtaglomerular regulation of gene transcription or mRNA metabolism may be another consequence of a chronic alteration in MD NaCl concentration.

Role of nitric oxide in tubuloglomerular feedback: effects of dietary salt

1. The tubuloglomerular feedback (TGF) response operates primarily by vasoconstriction of the afferent arteriole and a fall in glomerular capillary pressure (PGC) and single-nephron glomerular filtration rate (SNGFR) during increased NaCl reabsorption in the macula densa (MD). Numerous studies have suggested that nitric oxide (NO) is synthesized by the MD and acts to suppress TGF. As a high-salt (HS) diet has been found to blunt TGF, we tested the effects of salt intake on NO-dependent changes in TGF. 2. In the first series of experiments, values of SNGFR were contrasted from samples of tubular fluid taken from the proximal tubule (PT; MD delivery interrupted) and the distal tubule DT; MD delivery intact). Compared with HS rats, the difference between PT and DT values of SNGFR was increased in low-salt (LS) diet rats (4.3 +/- 0.4 vs 10.3 +/- 1.2 nL/min, respectively; P < 0.001). Intravenous infusion of NG-monomethyl-L-arginine (L-NMMA), in pressor doses increased the difference between PT and DT values of SNGFR of HS rats (4.3 +/- 0.4 vs 9.5 +/- 1.2 nL/min before and during L-NMMA, respectively; P < 0.001) without significantly affecting values in LS rats (10.3 +/- 1.2 vs 12.3 +/- 1.4 nL/min before and during L-NMMA, respectively; NS). 3. A second series of experiments assessed TGF responses directly. Changes in stop-flow pressure (PSF; an index of PGC) were measured in response to graded perfusion of the loop of Henle (LH) with artificial tubular fluid. Loop perfusion with 10(-3) mol/L L-NMMA did not affect the PSF responses of LS rats but did reduce (P < 0.01) the PSF of HS rats during perfusion at 20 nL/min (-1.5 +/- 0.4 mmHg; P < 0.01), 30 nL/min (-1.8 +/- 0.5 mmHg; P < 0.01) and 40 nL/min (-2.2 +/- 0.5 mmHg; P < 0.001). 4. We conclude that the TGF response is increased by suppression of NOS activity during HS but not LS intake.

Macula densa derived nitric oxide in regulation of glomerular capillary pressure

Nitric oxide (NO) is produced by enzymes called nitric oxide synthases (NOS). At least three different isoforms of NOS have been identified in the kidney. This study examines the effects of selective inhibition of the inducible isoform (iNOS) and the neuronal isoform (bNOS) on the glomerular capillary pressure (PGC), through studies of the tubuloglomerular feedback (TGF) mechanism in anaesthetized rats. The proximal tubular stop-flow pressure (PSF) was measured to estimate changes in PGC obtained after activation of the TGF system by varying the loop of Henle perfusion rate with artificial ultrafiltrate including vehicle, NOS inhibition or L-arginine. Infusion of nonspecific NOS inhibition (N omega-Nitro-L-arginine) increased maximal TGF responses (delta PSF) by 84% and L-arginine decreased delta PSF by 37%. Aminoguanidine, a selective iNOS-inhibitor, failed to increase delta PSF, whereas the nonspecific NOS inhibitor methylguanidine increased delta PSF by 64%. 7-Nitro indazole (7-NI), a selective bNOS inhibitor, increased delta PSF by 57% when infused intratubularly, and intraperitoneal administration of 7-NI increased delta PSF by 78%, without any change in blood pressure. Since bNOS is exclusively located in the macula densa (MD) cells, these results confirm and strengthen the obligatory role of MD-produced NO in regulation of TGF and PGC, which has been suggested earlier. iNOS, widely expressed in the kidney, does not seem to play any important role in regulation of PGC.

Nitric oxide antagonizes the actions of angiotensin II to enhance tubuloglomerular feedback responsiveness

The present study was designed to investigate whether nitric oxide (NO) antagonizes angiotensin II (Ang II) in modulating the tubuloglomerular feedback (TGF) system. Maximum TGF responses were assessed by evaluating stop-flow pressure (SFP) responses to late proximal perfusion with artificial tubular fluid (40 nl/min). Peritubular capillary (PTC) infusion of 10(-3) M NG-L-arginine (NLA) at a rate of 20 nl/min, and infusion of 10(-7) and 10(-6) M Ang II at rates that did not decrease SFP under conditions of zero flow to the macula densa (resting SFP), augmented maximum SFP feedback responses to 12.0 +/- 1.7, 12.1 +/- 2.4 and 16.9 +/- 3.0 mm Hg, respectively (all P < 0.01 vs. control response). Combined PTC infusion of NLA and 10(-7) M Ang II at a rate of 20 nl/min resulted in decreases in resting SFP in 7 of the 12 nephrons studied. When the infusion rate was decreased to 15 +/- 3 nl/min, concomitant PTC infusion of NLA and 10(-7) M Ang II was associated with a tremendous increase in maximum TGF responses (23.8 +/- 3.9 mm Hg; P < 0.01 vs. responses during PTC NLA or Ang II) in the absence of a decrease in resting SFP. During AT1 receptor blockade using losartan, SFP feedback responses were attenuated to 1.6 +/- 0.6 mm Hg and PTC infusion of NLA only augmented TGF responses to 3.8 +/- 1.0 mm Hg. These results strongly suggest that local NO antagonizes Ang II with respect to the regulation of TGF responsiveness. Disruption of this balance by NO synthesis inhibition strongly potentiates TGF-independent and TGF-dependent actions of Ang II on the preglomerular vasculature.