Mechanisms for macula densa cell release of renin.

The juxtaglomerular apparatus in the kidney is important in controlling extracellular fluid volume and renin release. The fluid load to the distal tubule is first sensed at the macula densa site via the entry of NaCl, through a Na, K, 2Cl co-transport mechanism. The next step is unclear, but there is recent evidence of an increased macula densa cell calcium concentration with a reduction in fluid load to the macula densa. An increase in macula densa cell calcium could activate phospholipase A2 to release arachidonic acid, the rate-limiting step in the formation of prostaglandins. Recent evidence suggests that the prostaglandin formed is PGE2, a potent stimulator for renin release. Recent evidence has also shown that adenosine has an important function in the juxtaglomerular apparatus. It stimulates calcium release in afferent arteriolar smooth muscle cells, leading to contraction of the afferent arteriole as part of the tubuloglomerular feedback mechanism, and inhibits renin release. Thus, renin release from the afferent arteriole is mediated partly through formation of PGE2, and partly through the reduction of adenosine formation that inhibits renin production.

Nitric oxide modulates and adenosine mediates the tubuloglomerular feedback mechanism.

The tubuloglomerular feedback (TGF) mechanism is an important regulator of the glomerular filtration rate (GFR) and urine excretion rate. It operates by sensing the distal delivery of fluid at the macula densa site and adjusting the tone of the glomerular arterioles to control GFR. We found evidence that nitric oxide is an important modulator of the setting of the sensitivity of the TGF mechanism. Studies on adenosine A1 receptor deficient mice have shown that these animals lack the TGF response and have an increased renin release. These findings show the important role of adenosine as a mediator of the signal for the TGF mechanism and as an inhibitor of renin release.

Reduced rat renal vascular response to angiotensin II after chronic inhibition of nNOS.

The neuronal isoform of nitric oxide synthase (nNOS) in the kidney is predominantly located in the macula densa (MD) cells. These cells are known to be the sensor in the tubuloglomerular feedback, which influences the tonus of the afferent arteriole. This study investigated the effect of angiotensin II (Ang II) after chronic inhibition of nNOS on renal blood flow (RBF) and cytosolic calcium concentration [Ca(2+)]i in smooth muscle cells from afferent arterioles. Measurements of RBF were made in two control groups and two groups treated with a nNOS inhibitor, 7-nitro indazole (7-NI), for 1 and 4 weeks. At the time of the experiment Ang II bolus was given in the renal artery before and during i.v. l-NNA. [Ca(2+)]i was measured in arterioles from control rats and from rats treated for 1 week with 7-NI. RBF decreased after bolus Ang II by 60 +/- 11% in the control vs. 23 +/- 8% in the 1 week 7-NI treated group. The decreased sensitivity to Ang II after 1 week of 7-NI treatment compared with control rats persisted after l-NNA infusion. There were no differences from control in the group treated for 4 weeks. Ang II gave a transient [Ca(2+)]i increase in vessels from control rats whereas this response was absent in 1 week 7-NI-treated rats. A possible explanation for these findings could be a down regulation of Ang II receptors. The renal vasculature of rats exhibits a diminished RBF and [Ca(2+)]i response to Ang II after 1 week blockade of nNOS.

Abolished tubuloglomerular feedback and increased plasma renin in adenosine A1 receptor-deficient mice.

The hypothesis that adenosine acting on adenosine A1 receptors (A1R) regulates several renal functions and mediates tubuloglomerular feedback (TGF) was examined using A1R knockout mice. We anesthetized knockout, wild-type, and heterozygous mice and measured glomerular filtration rate, TGF response using the stop-flow pressure (P(sf)) technique, and plasma renin concentration. The A1R knockout mice had an increased blood pressure compared with wild-type and heterozygote mice. Glomerular filtration rate was similar in all genotypes. Proximal tubular P(sf) was decreased from 36.7 +/- 1.2 to 25.3 +/- 1.6 mmHg in the A1R+/+ mice and from 38.1 +/- 1.0 to 27.4 +/- 1.1 mmHg in A1R+/- mice in response to an increase in tubular flow rate from 0 to 35 nl/min. This response was abolished in the homozygous A1R-/- mice (from 39.1 +/- 4.1 to 39.2 +/- 4.5 mmHg). Plasma renin activity was significantly greater in the A1R knockout mice [74.2 +/- 14.3 milli-Goldblatt units (mGU)/ml] mice compared with the wild-type and A1R+/- mice (36.3 +/- 8.5 and 34.1 +/- 9.6 mGU/ml), respectively. The results demonstrate that adenosine acting on A1R is required for TGF and modulates renin release.

Effects of long-term inhibition of neuronal nitric oxide synthase on blood pressure and renin release.

Nitric oxide (NO) produced by neuronal NO-synthase (nNOS) in macula densa cells may be involved in the control of renin release. 7-Nitro indazole (7-NI) inhibits nNOS, and we investigated the effect of short- (4 days) and long-term (4 weeks) 7-NI treatment on blood pressure (BP), plasma renin concentration (PRC) and glomerular filtration rate (GFR) in rats on different salt diets. Rats were divided into three groups and given low-salt (LS), normal (C) and high-salt (HS) diets. Each diet group was subdivided into two groups treated either with 7-NI or vehicle. Long-term 7-NI-treated rats (LS and C) showed increased BP compared with controls (LS: 149 +/- 4 vs. 133 +/- 3; C: 146 +/- 4 vs. 127 +/- 4 mmHg). Blood pressure in HS rats did not differ from that in controls. Plasma renin concentration was stimulated in LS-rats (251 +/- 64 mGU mL(-1)) compared with C and HS rats (42 +/- 8 and 39 +/- 5 mGU mL(-1), respectively) but was not significantly affected by chronic 7-NI treatment (350 +/- 103, 49 +/- 10 and 50 +/- 15 mGU mL(-1) in LS, C and HS, respectively). In rats treated with 7-NI for 4 days, no effect on BP was seen, but PRC was increased in 7-NI treated LS rats compared with vehicle treated LS rats (107 +/- 15 vs. 56 +/- 1 mGU mL(-1)). Stimulation of PRC in LS rats was further enhanced by 7-NI after 4 days of treatment, but not affected in rats treated for 4 weeks. This suggests that inhibition of nNOS stimulates renin release but that this stimulatory effect in the long run might be depressed by the increase in blood pressure.

Increased tubuloglomerular feedback reactivity is associated with increased NO production in the streptozotocin-diabetic rat.

The characteristics of the tubuloglomerular feedback (TGF) mechanism were examined in streptozotocin-diabetic rats. This model is known to induce damage in the distal tubular system and thus Tamm-Horsfall protein (THP) secretion. Three groups of male Sprague-Dawley rats were studied: (A) diabetic rats with blood glucose levels (BG)<19 mmol/l, (B) with BG>/=19 mmol/l, and (C) control rats. After 50 days, the diabetic rats had higher arterial blood pressure and increased TGF reactivity (delta P(SF)) than control rats. The proximal tubular free-flow pressure (P(T)) and stop-flow pressure (P(SF)) were reduced, while the glomerular filtration was normal. This indicates that the diabetic animals of this study were severely vasoconstricted. Inhibition of renal nitric oxide synthase (NOS) resulted in a greater increase of TGF reactivity in diabetic rats than in control rats. Diabetic rats also showed increased excretion rates of albumin and THP. The excretion rate of THP was associated with P(SF) (r=-0.88, p<0.01). In conclusion, diabetes mellitus was associated with an increased blood pressure and an increased TGF reactivity, which indicates that the diabetic rats were vasoconstricted. NOS inhibition increased the reactivity of TGF to greater extent in diabetic animals than in controls, indicating that the renal vasoconstriction was compensated for by an increased NO production.

Impaired effect by NO synthase inhibition on tubuloglomerular feedback in rats after chronic renal denervation.

Acute unilateral renal denervation (aDNX) is associated with reduced tubuloglomerular feedback (TGF) sensitivity. Six days after denervation (cDNX) TGF sensitivity is somewhat restored, but TGF reactivity increased. This study aimed to investigate if the increased TGF reactivity that was seen in cDNX kidneys was owing to reduced production of nitric oxide (NO). TGF characteristics were determined with micropuncture experiments in anaesthetized rats, using the stop-flow pressure (PSF) technique. Maximal drop in PSF (DeltaPSF) was used as an index of TGF reactivity and the loop of Henle perfusion rate that elicited half-maximal DeltaPSF, the turning point (TP) was used as a measure of TGF sensitivity. In cDNX kidneys, TP was higher than in control rats (25.4 +/- 1.5 nL min-1 vs. 19.1 +/- 1.1 nL min-1), but clearly lower than in aDNX rats (37. 3 +/- 3.1 nL min-1). TGF was more reactive in cDNX rats (DeltaPSF=14. 7 +/- 1.1 mmHg) than in aDNX (7.9 +/- 1.1 mmHg) and control rats (9. 6 +/- 0.9 mmHg). Intratubular inhibition of NO synthase N omega-nitro-L-arginine (L-NA) in sham-DNX animals, decreased TP to 13.9 +/- 2.2 nL min-1 and DeltaPSF was increased with 92%. In cDNX kidneys TP was not significantly reduced by L-NA, and TGF reactivity was only moderately increased by 31%. Intratubular infusion of L-arginine (L-Arg) reduced DeltaPSF from 10.2 +/- 0.7 to 6.5 +/- 0.6 mmHg in sham-DNX kidneys, but TP was unaffected. In cDNX kidneys, there was no effect on either DeltaPSF or TP by the addition of L-Arg. However, when NO was delivered via sodium nitroprusside in the tubular perfusate, a clear reduction of DeltaPSF was seen in both sham-DNX and cDNX kidneys (from 9.9 +/- 0.5 to 4.4 +/- 1.0 and from14.9 +/- 1.3 to 8.1 +/- 1.5 mmHg, respectively). This indicates that cDNX is a state of low renal NO production and that this low level of NO resets TGF to a higher sensitivity and more pronounced reactivity.

Renal NO production and the development of hypertension.

The juxtaglomerular apparatus (JGA) has the very important functions of detecting the fluid flow rate to the distal tubule and thus controlling the glomerular filtration rate (GFR) (tubuloglomerular feedback mechanism [TGF]) and renin release from the afferent arteriole. In studies of the TGF it has been evident that the sensitivity of this mechanism can be reset. Volume expansion will reset it to a low sensitivity leading to a high GFR and urine excretion rate, while dehydration will sensitize the TGF mechanism, giving rise to a low GFR and low urine excretion rate. Furthermore, we have found that in animals that spontaneously develop hypertension there is initially a sensitization of the TGF, leading to a reduced GFR and urine excretion rate, with fluid volume retention in the body and a consequent rise in blood pressure. When the pressure is raised, the TGF characteristics are normalized. In the macula densa (MD) cells in the JGA, there is a large production of NO from neuronal NOS. This production continuously reduces TGF sensitivity and is apparently impaired in animals that spontaneously develop hypertension. When we added an nNOS inhibitor to the drinking water for several weeks while measuring blood pressure, we found an increase in blood pressure after 3-4 weeks of treatment. This effect was abolished by a high salt diet. From these investigations, it also appeared as if nNOS-derived NO inhibited renin release. Experiments have also indicated that NO may resensitize inhibited G-protein coupled purinergic receptors.

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.

Increased blood pressure in rats after long-term inhibition of the neuronal isoform of nitric oxide synthase.

In the kidney, nitric oxide synthase (NOS) of the neuronal isoform (nNOS) is predominantly located in the macula densa cells. Unspecific chronic NOS inhibition in rats leads to elevated blood pressure (P(A)), associated with increased renal vascular resistance. This study was designed to examine the effect of chronic selective inhibition of nNOS with 7-nitro indazole (7-NI) on P(A), GFR, and the tubuloglomerular feedback (TGF) system. P(A) was repeatedly measured by a noninvasive tail-cuff technique for 4 wk in rats treated orally with 7-NI, and in control rats. After treatment, the animals were anesthetized and renal excretion rates, GFR, and TGF activity were determined. After 1 wk of 7-NI treatment P(A) was increased from 129+/-4 to 143 2 mmHg. GFR (1.85+/-0.1 vs. 1.97+/-0.2 ml/min in controls) was unchanged, but micropuncture studies revealed a more sensitive TGF than in controls. After 4 wk of 7-NI treatment P(A) was 152+/-4 mmHg, but no change in GFR (1.90+/-0.5 ml/min) or TGF sensitivity was detected. Acute administration of 7-NI to nontreated rats did not affect P(A), but decreased GFR (1.49+/-0.1 ml/min) and increased TGF sensitivity. In conclusion, chronic nNOS inhibition leads to increased P(A). Our results suggest that the elevated P(A) could be caused by an initially increased TGF sensitivity, leading to decreased GFR and an increased body fluid volume.

Effects of acute and chronic unilateral renal denervation on the tubuloglomerular feedback mechanism.

We recently observed a time-dependent resetting of the tubuloglomerular feedback (TGF) sensitivity to a subnormal level after acute unilateral renal denervation (aDNX). The present investigation compares the effects of aDNX with those of chronic unilateral renal denervation (cDNX), i.e one week after aDNX. All experiments were performed in anaesthetized rats prepared for micropuncture. cDNX led to increases in urine, sodium and potassium excretion in denervated kidneys, while contralateral kidneys showed reduced excretion of these parameters. GFR was increased in denervated kidneys, but unchanged on the contralateral side. TGF activity was determined by measuring the maximal stop-flow pressure response (delta Psf) and the tubular flow rate at which 50% of the maximal response occurred (turning point; TP). cDNX decreased TGF sensitivity, as indicated by an increased TP from 19.1 nL/min in sham-DNX to 26.1 nL/min. Concomitantly, TP in contralateral kidneys was significantly decreased to 15.9 nL/min, aDNX led to a greater sensitivity reduction: TP increased from 19.8 to 34.0 nL/min and contralaterally TP decreased to 14.0 nL/min. delta Psf in cDNX increased by 63% compared to sham-DNX, while on the contralateral side this was unchanged. No difference in delta Psf was found between control, DNX and contralateral kidneys in the aDNX group. In summary, these experiments show that the previously reported decrease in TGF sensitivity in aDNX kidneys still persists after one week, although less pronounced. As a result of the decreased TGF sensitivity, GFR is kept on a high level in cDNX kidneys. Contralateral kidneys show reversed resetting.