Nitric oxide and the control of renin secretion

Regulation of the renin-angiotensin-aldosterone system by cyclic nucleotides and phosphodiesterases

The renin-angiotensin-aldosterone system (RAAS) is one of the key players in the regulation of blood volume and blood pressure. Dysfunction of this system is connected with cardiovascular and renal diseases. Regulation of RAAS is under the control of multiple intracellular mechanisms. Cyclic nucleotides and phosphodiesterases are the major regulators of this system since they control expression and activity of renin and aldosterone. In this review, we summarize known mechanisms by which cyclic nucleotides and phosphodiesterases regulate renin gene expression, secretion of renin granules from juxtaglomerular cells and aldosterone production from zona glomerulosa cells of adrenal gland. We also discuss several open questions which deserve future attention.

Glomerular and tubular effects of nitric oxide (NO) are regulated by angiotensin II (Ang II) in an age-dependent manner through activation of both angiotensin receptors (AT1Rs and AT2Rs) in conscious lambs

Renin-angiotensin (RAS) and nitric oxide (NO) systems and the balance and interaction between them are considered of primary importance in maintaining fluid and electrolyte homeostasis. It has been suggested that the effects of NO may be modulated at least in part by the angiotensin (Ang) II, yet the roles of angiotensin receptor type 1 (AT1R) and type 2 (AT2R) are not well understood. Even though both Ang II and NO are elevated at birth and during the newborn period, their contribution to the adaptation of the newborn to life after birth as well as their physiological roles during development are poorly understood. The aim of this study was to determine if NO regulation of renal function during postnatal maturation is modulated by Ang II through activation of AT1R or AT2R or both receptors. Glomerular and tubular effects of either AT1R selective antagonist ZD 7155, AT2R selective antagonist PD 123319, and both antagonists ZD 7155 plus PD 123319, were measured in 1- (N = 9) and 6-week-old (N = 13) conscious, chronically instrumented lambs before and after removal of endogenous NO with L-arginine analogue, L-NAME. Two-way analysis of variance (ANOVA) procedures for repeated measures over time with factors age and treatment were used to compare the effects of the treatments on several glomerular and tubular variables in both groups. This study showed that L-NAME infusion after pre-treatment with ATR antagonists did not alter glomerular function in 1- or 6-week-old lambs. NO effects on electrolytes handling along the nephron during postnatal development were modulated by Ang II through AT1R and AT2R in an age-dependent manner. Selective inhibition of AT1R and AT2R increased excretion of Na⁺, K⁺, and Cl^- in 6- but not in 1-week-old lambs. In 6-week-old lambs, urinary flow rate increased by 200%, free water clearance increased by 50%, and urine osmolality decreased by 40% after L-NAME was added to the pre-treatment with ZD 7155 plus PD 123319. When L-NAME was added either to ZD 7155 or PD 123319, the same trend in the alterations of these variables was observed, albeit to a lower degree. In conclusion, in conscious animals, during postnatal maturation, Ang II modulates the effects of NO on glomerular function, fluid, and electrolyte homeostasis through AT1Rs and AT2Rs in an age-dependent manner. Under physiological conditions, AT2Rs may potentiate the effects of AT1R, providing evidence of a crosstalk between ATRs in modulating NO effects on fluid and electrolyte homeostasis during postnatal maturation. This study provides new insights on the regulation of renal function during early postnatal development showing that, compared with later in life, newborns have impaired capacity to regulate glomerular function, water, and electrolyte balance.

NO and cGMP mediate angiotensin AT2 receptor-induced renal renin inhibition in young rats

We hypothesized that angiotensin subtype-2 receptor (AT(2)R) inhibits renal renin biosynthesis in young rats via nitric oxide (NO). We monitored changes in renal NO, cGMP, renal renin content (RRC), and ANG II in 4-wk-old rats in response to low sodium (LNa(+)) intake alone and combined with 8-h direct renal cortical administration of AT(1) receptor blocker valsartan (VAL), AT(2)R blocker PD123319 (PD), NO synthase inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME), NO donor S-nitroso-N-acetyl penicillamine (SNAP), or guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazolo[4,2-alpha] quinoxaline-1-one (ODQ). In addition, we monitored renal endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) in response to VAL or PD. LNa(+), VAL, PD, l-NAME, and ODQ increased RRC, ANG II, and renin mRNA. PD and l-NAME decreased NO and cGMP, while SNAP reduced RRC, ANG II, renin mRNA, and reversed the effects of PD. PD also reduced eNOS and nNOS protein and mRNA. Combined treatment with PD, l-NAME, or ODQ and VAL reversed the effects of VAL and caused further increase in RRC, ANG II, renin mRNA, and protein. ODQ reversed the effects of SNAP. These data demonstrate that the renal AT(2) receptor decreases renal renin biosynthesis and ANG II production in young rats. Reversal of the PD effects by SNAP and SNAP effects by ODQ confirms that NO and cGMP mediate the AT(2) receptor inhibition of renal renin production.

Effect of blockade of nitric oxide synthesis on renin secretion in human subjects

Nitric oxide (NO) has been implicated in the control of renin secretion in experimental animals but little information is available concerning its role in humans. The aim of the present study was to investigate the effects of inhibition of NO synthesis on resting renin secretion and on the renin secretory responses to activation of the macula densa and sympathetic neural mechanisms controlling renin secretion. In eight healthy subjects, injection of furosemide increased plasma renin activity (PRA) with little or no change in blood pressure or heart rate. Injection of the NO synthase inhibitor L-NMMA increased blood pressure and decreased heart rate and PRA, but failed to alter the PRA response to furosemide. In another ten subjects, standing increased PRA. L-NMMA again decreased PRA but failed to alter the PRA response to standing. These results suggest that NO participates in the regulation of resting renin secretion in humans, and provide preliminary evidence that NO does not contribute significantly to the renin responses to activation of the macula densa or sympathetic mechanisms controlling renin secretion.

The effect of nitric oxide inhibition on the renin response to frusemide, in man

AIMS

We wished to see if renin release in man was inhibited by nitric oxide blockade, suggesting a role for nitric oxide in renin release. Evidence from animal studies has shown variable effects on renin release depending on the model and stimulus used.

METHODS

Ten normal male volunteers, received either L-NMMA as a front loaded infusion (4 mg kg-1 bolus, with 4 mg kg-1 infusion), or placebo, followed by an intravenous bolus of 5 mg frusemide to stimulate renin. To investigate whether any alteration in renin release was due to the pressor effect of the L-NMMA, the experiment was repeated using an equipressor dose of phenylephrine (0.5 microg kg-1 min-1 ).

RESULTS

L-NMMA caused the expected increase in mean arterial pressure (96+/-2.6 vs 89+/-3.3 mmHg P<0.05 [mean+/-s.e.mean]), and a reduction in heart rate (59+/-3.6 vs 67+/-2.5 beats min-1 P<0.05). L-NMMA completely blocked the renin rise following the bolus of frusemide (1.18+/-0.196 vs 1.96+/-0.333 ng ml-1 h-1 P<0.01). Phenylephrine 0.5 microg kg-1 min-1 produced very similar haemodynamic effects to L-NMMA, and also suppressed the renin response to frusemide (1.43+/-0.290 vs 2.67+/-0.342 ng ml-1 h-1 P<0. 01).

CONCLUSIONS

In man, the renin inhibition seen with NO synthesis inhibition is similar to that seen with a standard pressor stimulus, hence inhibition of renin in man by L-NMMA, may be due to both direct effects on macula densa cells and indirect haemodynamic effects.

Cellular control of renin secretion

Renin secretion at the level of renal juxtaglomerular cells appears to be controlled mainly by classic second messengers such as Ca2+, cyclic AMP and cyclic GMP, which in turn exert their effects through oppositely acting protein kinases and probably also by affecting the activity of ion channels in the plasma membrane. Thus, protein kinase A stimulates renin secretion, whilst protein kinase C and protein kinase G II inhibit renin secretion. Moreover, Cl- channels could be involved in the mediation of the inhibitory action of Ca2+ on renin secretion. This review summarizes our present knowledge about the possible actions of these kinases in renal juxtaglomerular cells and considers pathways in the organ control of renin secretion.

Role of nitric oxide in the control of renin secretion

Because of the significant constitutive expression of NO synthases in the juxtaglomerular apparatus, nitric oxide (NO) is considered as a likely modulator of renin secretion. In most instances, NO appears as a tonic enhancer of renin secretion, acting via inhibition of cAMP degradation through the action of cGMP. Depending on as yet unknown factors, the stimulatory effect of NO on renin secretion may also switch to an inhibitory one that is compatible with the inhibition of renin secretion by cGMP-dependent protein kinase activity. Whether NO plays a direct regulatory role or a more permissive role in the control of renin secretion remains to be answered.

Role of cGMP-kinase II in the control of renin secretion and renin expression

To investigate the roles of the cGMP-dependent protein kinases (cGKs) in the control of the renin system, we studied the regulation of renin in cGKI- or cGKII-deficient mice in vivo and in vitro. Renal renin mRNA levels both under stimulatory (low-salt diet plus ramipril) and inhibitory (high-salt diet) conditions were not different between wild-type and cGKI-/- mice, but were significantly elevated in cGKII-/- mice under all experimental conditions. In primary cultures of renal juxtaglomerular cells (JG) established from wild-type, cGKI-/-, and cGKII-/- mice, the adenylate cyclase activator forskolin stimulated renin secretion similarly in all genotypes tested. 8-bromo-cGMP attenuated basal and forskolin-stimulated renin secretion in cultures from wild-type and cGKI-/-, but had no effect in cells isolated from cGKII-/- mice. Activation of cGKs by 8-bromo-cGMP decreased renin secretion from the isolated perfused rat kidney, independent of prestimulation by beta-adrenoreceptor activation, macula densa inhibition, reduced perfusion pressure, or by a nominally calcium-free perfusate. Taken together, these findings suggest that activation of cGKII has a general inhibitory effect on renin secretion from renal JG cells.

Effects of nitric oxide on neuroendocrine function and behavior

Nitric oxide (NO) is an unusual chemical messenger. NO mediates blood vessel relaxation when produced by endothelial cells. When produced by macrophages, NO contributes to the cytotoxic function of these immune cells. NO also functions as a neurotransmitter and neuromodulator in the central and peripheral nervous systems. The effects on blood vessel tone and neuronal function form the basis for an important role of NO on neuroendocrine function and behavior. NO mediates hypothalamic portal blood flow and, thus, affects oxytocin and vasopression secretion; furthermore, NO mediates neuroendocrine function in the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes. NO influences several motivated behaviors including sexual, aggressive, and ingestive behaviors. Learning and memory are also influenced by NO. Taken together, NO is emerging as an important chemical mediator of neuroendocrine function and behavior.

Contribution of the renin-angiotensin system to short-term blood pressure variability during blockade of nitric oxide synthesis in the rat

1. The aim of this study was to investigate, by use of spectral analysis, (1) the blood pressure (BP) variability changes in the conscious rat during blockade of nitric oxide (NO) synthesis by the L-arginine analogue NG-nitro-L-arginine methyl ester (L-NAME); (2) the involvement of the renin-angiotensin system in these modifications, by use of the angiotensin II AT1-receptor antagonist losartan. 2. Blockade of NO synthesis was achieved by infusion for 1 h of a low-dose (10 micrograms kg-1 min-1, i.v., n = 10) and high-dose (100 micrograms kg-1 min-1, i.v., n = 10) of L-NAME. The same treatment was applied in two further groups (2 x n = 10) after a bolus dose of losartan (10 mg kg-1, i.v.). 3. Thirty minutes after the start of the infusion of low-dose L-NAME, systolic BP (SBP) increased (+10 +/- 3 mmHg, P < 0.01), with the effect being more pronounced 5 min after the end of L-NAME administration (+20 +/- 4 mmHg, P < 0.001). With high-dose L-NAME, SBP increased immediately (5 min: +8 +/- 2 mmHg, P < 0.05) and reached a maximum after 40 min (+53 +/- 4 mmHg, P < 0.001); a bradycardia was observed (60 min: -44 +/- 13 beats min-1, P < 0.01). 4. Low-dose L-NAME increased the low-frequency component (LF: 0.02-0.2 Hz) of SBP variability (50 min: 6.7 +/- 1.7 mmHg2 vs 3.4 +/- 0.5 mmHg2, P < 0.05), whereas the high dose of L-NAME not only increased the LF component (40 min: 11.7 +/- 2 mmHg2 vs 2.7 +/- 0.5 mmHg2, P < 0.001) but also decreased the mind frequency (MF: 0.2-0.6 Hz) component (60 min: 1.14 +/- 0.3 mmHg2 vs 1.7 +/- 0.1 mmHg2, P < 0.05) of SBP. 5. Losartan did not modify BP levels but had a tachycardic effect (+45 beats min-1). Moreover, losartan increased MF oscillations of SBP (4.26 +/- 0.49 mmHg2 vs 2.43 +/- 0.25 mmHg2, P < 0.001), prevented the BP rise provoked by the low-dose of L-NAME and delayed the BP rise provoked by the high-dose of L-NAME. Losartan also prevented the amplification of the LF oscillations of SBP induced by L-NAME; the decrease of the MF oscillations of SBP induced by L-NAME was reinforced after losartan. 6. We conclude that the renin-angiotensin system is involved in the increase in variability of SBP in the LF range which resulted from the withdrawal of the vasodilating influence of NO. We propose that NO may counterbalance LF oscillations provoked by the activity of the renin-angiotensin system.