Neuronal nitric oxide synthase-deficient mice have impaired renin release but normal blood pressure

The Role of Macula Densa Nitric Oxide Synthase 1 Beta Splice Variant in Modulating Tubuloglomerular Feedback

Abnormalities in renal electrolyte and water excretion may result in inappropriate salt and water retention, which facilitates the development and maintenance of hypertension, as well as acid-base and electrolyte disorders. A key mechanism by which the kidney regulates renal hemodynamics and electrolyte excretion is via tubuloglomerular feedback (TGF), an intrarenal negative feedback between tubules and arterioles. TGF is initiated by an increase of NaCl delivery at the macula densa cells. The increased NaCl activates luminal Na-K-2Cl cotransporter (NKCC2) of the macula densa cells, which leads to activation of several intracellular processes followed by the production of paracrine signals that ultimately result in a constriction of the afferent arteriole and a tonic inhibition of single nephron glomerular filtration rate. Neuronal nitric oxide (NOS1) is highly expressed in the macula densa. NOS1β is the major splice variant and accounts for most of NO generation by the macula densa, which inhibits TGF response. Macula densa NOS1β-mediated modulation of TGF responses plays an essential role in control of sodium excretion, volume and electrolyte hemostasis, and blood pressure. In this article, we describe the mechanisms that regulate macula densa-derived NO and their effect on TGF response in physiologic and pathologic conditions. © 2023 American Physiological Society. Compr Physiol 13:4215-4229, 2023.

A cardiac-null mutation of Prdm16 causes hypotension in mice with cardiac hypertrophy via increased nitric oxide synthase 1

Hypertension or hypotension prevails as a comorbidity in patients with heart failure (HF). Although blood pressure (BP) is an important factor in managing the mortality of HF, the molecular mechanisms of changes in BP have not been clearly understood in cases of HF. We and others have demonstrated that a loss in PRDM16 causes hypertrophic cardiomyopathy, leading to HF. We aimed to determine whether BP is altered in mice that experience cardiac loss of Prdm16 and identify the underlying mechanism of BP-associated changes. BP decreased significantly only in female mice with a cardiac-null mutation of Prdm16 compared with controls, by an invasive protocol under anesthesia and by telemetric method during conscious, unrestrained status. Mice with a cardiac loss of Prdm16 had higher heart-to-body weight ratios and upregulated atrial natriuretic peptide, suggesting cardiac hypertrophy. Plasma aldosterone-to-renin activity ratios and plasma sodium levels decreased in Prdm16-deficient mice versus control. By RNA-seq and in subsequent functional analyses, Prdm16-null hearts were enriched in factors that regulate BP, including Adra1a, Nos1, Nppa, and Nppb. The inhibition of nitric oxide synthase 1 (NOS1) reverted the decrease in BP in cardiac-specific Prdm16 knockout mice. Mice with cardiac deficiency of Prdm16 present with hypotension and cardiac hypertrophy. Further, our findings suggest that the increased expression of NOS1 causes hypotension in mice with a cardiac-null mutation of Prdm16. These results provide novel insights into the molecular mechanisms of hypotension in subjects with HF and contribute to our understanding of how hypotension develops in patients with HF.

Diurnal Regulation of Renal Electrolyte Excretion: The Role of Paracrine Factors

Many physiological processes, including most kidney-related functions, follow specific rhythms tied to a 24-h cycle. This is largely because circadian genes operate in virtually every cell type in the body. In addition, many noncanonical genes have intrinsic circadian rhythms, especially within the liver and kidney. This new level of complexity applies to the control of renal electrolyte excretion. Furthermore, there is growing evidence that paracrine and autocrine factors, especially the endothelin system, are regulated by clock genes. We have known for decades that excretion of electrolytes is dependent on time of day, which could play an important role in fluid volume balance and blood pressure control. Here, we review what is known about the interplay between paracrine and circadian control of electrolyte excretion. The hope is that recognition of paracrine and circadian factors can be considered more deeply in the future when integrating with well-established neuroendocrine control of excretion.

Blood pressure regulation in stress: focus on nitric oxide-dependent mechanisms

Stress is considered a risk factor associated with the development of various civilization diseases including cardiovascular diseases, malignant tumors and mental disorders. Research investigating mechanisms involved in stress-induced hypertension have attracted much attention of physicians and researchers, however, there are still ambiguous results concerning a causal relationship between stress and long-term elevation of blood pressure (BP). Several studies have observed that mechanisms involved in the development of stress-induced hypertension include increased activity of sympathetic nervous system (SNS), glucocorticoid (GC) overload and altered endothelial function including decreased nitric oxide (NO) bioavailability. Nitric oxide is well known neurotransmitter, neuromodulator and vasodilator involved in regulation of neuroendocrine mechanisms and cardiovascular responses to stressors. Thus NO plays a crucial role in the regulation of the stress systems and thereby in the BP regulation in stress. Elevated NO synthesis, especially in the initial phase of stress, may be considered a stress-limiting mechanism, facilitating the recovery from stress to the resting levels via attenuation of both GC release and SNS activity as well as by increased NO-dependent vasorelaxation. On the other hand, reduced levels of NO were observed in the later phases of stress and in subjects with genetic predisposition to hypertension, irrespectively, in which reduced NO bioavailability may account for disruption of NO-mediated BP regulatory mechanisms and accentuated SNS and GC effects. This review summarizes current knowledge on the role of stress in development of hypertension with a special focus on the interactions among NO and other biological systems affecting blood pressure and vascular function.

Blood Pressure in Healthy Humans Is Regulated by Neuronal NO Synthase

NO is physiologically generated by endothelial and neuronal NO synthase (nNOS) isoforms. Although nNOS was first identified in brain, it is expressed in other tissues, including perivascular nerves, cardiac and skeletal muscle. Increasing experimental evidence suggests that nNOS has important effects on cardiovascular function, but its composite effects on systemic hemodynamics in humans are unknown. We undertook the first human study to assess the physiological effects of systemic nNOS inhibition on basal hemodynamics. Seventeen healthy normotensive men aged 24±4 years received acute intravenous infusions of an nNOS-selective inhibitor, S-methyl-l-thiocitrulline, and placebo on separate occasions. An initial dose-escalation study showed that S-methyl-l-thiocitrulline (0.1–3.0 µmol/kg) induced dose-dependent changes in systemic hemodynamics. The highest dose of S-methyl-l-thiocitrulline (3.0 µmol/kg over 10 minutes) significantly increased systemic vascular resistance (+42±6%) and diastolic blood pressure (67±1 to 77±3 mm Hg) when compared with placebo (both P<0.01). There were significant decreases in heart rate (60±4 to 51±3 bpm; P<0.01) and left ventricular stroke volume (59±6 to 51±6 mL; P<0.01) but ejection fraction was unaltered. S-methyl-l-thiocitrulline had no effect on radial artery flow-mediated dilatation, an index of endothelial NOS activity. These results suggest that nNOS-derived NO has an important role in the physiological regulation of basal systemic vascular resistance and blood pressure in healthy humans.

Inhibition of Nitric Oxide Synthase 1 Induces Salt-Sensitive Hypertension in Nitric Oxide Synthase 1α Knockout and Wild-Type Mice

We recently showed that α, β, and γ splice variants of neuronal nitric oxide synthase (NOS1) expressed in the macula densa and NOS1β accounts for most of the NO generation. We have also demonstrated that the mice with deletion of NOS1 specifically from the macula densa developed salt-sensitive hypertension. However, the global NOS1 knockout (NOS1KO) strain is neither hypertensive nor salt sensitive. This global NOS1KO strain is actually an NOS1αKO model. Consequently, we hypothesized that inhibition of NOS1β in NOS1αKO mice induces salt-sensitive hypertension. NOS1αKO and C57BL/6 wild-type (WT) mice were implanted with telemetry transmitters and divided into 7-nitroindazole (10 mg/kg/d)-treated and nontreated groups. All of the mice were fed a normal salt (0.4% NaCl) diet for 5 days, followed by a high-salt diet (4% NaCl). NO generation by the macula densa was inhibited by >90% in WT and NOS1αKO mice treated with 7-nitroindazole. Glomerular filtration rate in conscious mice was increased by ≈ 40% after a high-salt diet in both NOS1αKO and WT mice. In response to acute volume expansion, glomerular filtration rate, diuretic and natriuretic response were significantly blunted in the WT and knockout mice treated with 7-nitroindazole. Mean arterial pressure had no significant changes in mice fed a high-salt diet, but increased ≈ 15 mm Hg similarly in NOS1αKO and WT mice treated with 7-nitroindazole. We conclude that NOS1β, but not NOS1α, plays an important role in control of sodium excretion and hemodynamics in response to either an acute or a chronic salt loading.

Macula Densa Nitric Oxide Synthase 1β Protects against Salt-Sensitive Hypertension

Nitric oxide (NO) is an important negative modulator of tubuloglomerular feedback responsiveness. We recently found that macula densa expresses α-, β-, and γ-splice variants of neuronal nitric oxide synthase 1 (NOS1), and NOS1β expression in the macula densa increases on a high-salt diet. This study tested whether upregulation of NOS1β expression in the macula densa affects sodium excretion and salt-sensitive hypertension by decreasing tubuloglomerular feedback responsiveness. Expression levels of NOS1β mRNA and protein were 30- and five-fold higher, respectively, than those of NOS1α in the renal cortex of C57BL/6 mice. Furthermore, macula densa NO production was similar in the isolated perfused juxtaglomerular apparatus of wild-type (WT) and nitric oxide synthase 1α-knockout (NOS1αKO) mice. Compared with control mice, mice with macula densa-specific knockout of all nitric oxide synthase 1 isoforms (MD-NOS1KO) had a significantly enhanced tubuloglomerular feedback response and after acute volume expansion, significantly reduced GFR, urine flow, and sodium excretion. Mean arterial pressure increased significantly in MD-NOS1KO mice (P<0.01) but not NOS1flox/flox mice fed a high-salt diet. After infusion of angiotensin II, mean arterial pressure increased by 61.6 mmHg in MD-NOS1KO mice versus 32.0 mmHg in WT mice (P<0.01) fed a high-salt diet. These results indicate that NOS1β is a primary NOS1 isoform expressed in the macula densa and regulates the tubuloglomerular feedback response, the natriuretic response to acute volume expansion, and the development of salt-sensitive hypertension. These findings show a novel mechanism for salt sensitivity of BP and the significance of tubuloglomerular feedback response in long-term control of sodium excretion and BP.

Distinct regulation of inner medullary collecting duct nitric oxide production from mice and rats

Nitric oxide (NO) and NO synthase 1 (NOS1) maintain sodium and water homeostasis. The NOS1α and NOS1β splice variants are expressed in the rat inner medulla, but only NOS1β is expressed in the mouse. Collecting duct NOS1 is necessary for blood pressure control. We hypothesized that NOS1 splice variant expression and NO production in the inner medullary collecting duct (IMCD) are regulated differently in mice and rats by high dietary sodium. Male C57blk/J6 mice and Sprague-Dawley rats were fed a 0.4% (normal salt; NS), or 4% (high salt; HS) NaCl diet for 2 or 7 days. Mean arterial pressure was not altered by HS, whereas urinary sodium excretion in mice and rats was increased significantly. Urinary excretion of nitrate/nitrite (NO(x)) and IMCD nitrite production were significantly greater in mice compared with rats on the HS diet. Western blotting indicated that only NOS1β and NOS3 were expressed in the mouse IMCD and that expression was unaffected by the HS diet at either time point. In contrast, NOS1α was detected in the IMCD of rats, in addition to NOS1β and NOS3. Feeding of the HS diet for 2 days increased NOS1α and NOS1β expression in the rat IMCD and 7 day feeding of the HS diet further increased NOS1β expression. Expression of NOS3 was unchanged by the HS diet at either time point. In conclusion, IMCD NO production in mice and rats is distinctly regulated under both NS and HS conditions, including expression of NOS1 splice variants.

Nitric oxide and the A and B of endothelin of sodium homeostasis

PURPOSE OF REVIEW

In recent years, renal collecting duct-specific endothelin-1 (ET1), endothelin A (ETA) and endothelin B (ETB) receptors as well as nitric oxide synthase 1 (NOS1) knockout mice have been developed with subsequent identification for an integral role in regulation of sodium water homeostasis and ultimately blood pressure. The focus of this review is to integrate these models and to propose a scheme for the control of sodium excretion by the collecting duct and the endothelin/ETB/NOS system.

RECENT FINDINGS

NOS1 splice variants are expressed in the kidney, especially in the collecting duct. Mice express predominantly NOS1β in the medulla, with NOS1α and NOS1β in the cortex, whereas rats express NOS1α and NOS1β in both the cortex and medulla. Novel transcription of collecting duct ET1 mediated by epithelial sodium channels, mitochondrial Na/Ca exchangers and glucocorticoids has been determined. ET1 via the ETB receptor increases nitric oxide production in both rat and mouse collecting ducts, suggesting that NOS1β is linked to ET1-dependent NOS activation in the kidney. As well, genetic deletion of NOS1 splice variants in the collecting duct results in a salt-sensitive hypertensive phenotype in mice, much like the collecting duct ET1 and collecting duct ETB knockout mice.

SUMMARY

In the collecting duct, the ET1/nitric oxide pathways are intimately linked, and deletion of collecting duct ET1, ETB receptor or NOS1β results in a salt-sensitive phenotype, which is at least partially dependent on dysregulation of sodium and water reabsorption.

Renal collecting duct NOS1 maintains fluid-electrolyte homeostasis and blood pressure

Nitric oxide is a pronatriuretic and prodiuretic factor. The highest renal NO synthase (NOS) activity is found in the inner medullary collecting duct. The collecting duct (CD) is the site of daily fine-tune regulation of sodium balance, and led us to hypothesize that a CD-specific deletion of NOS1 would result in an impaired ability to excrete a sodium load leading to a salt-sensitive blood pressure phenotype. We bred AQP2-CRE mice with NOS1 floxed mice to produce flox control and CD-specific NOS1 knockout (CDNOS1KO) littermates. CDs from CDNOS1KO mice produced 75% less nitrite, and urinary nitrite+nitrate (NOx) excretion was significantly blunted in the knockout genotype. When challenged with high dietary sodium, CDNOS1KO mice showed significantly reduced urine output, sodium, chloride, and NOx excretion, and increased mean arterial pressure relative to flox control mice. In humans, urinary NOx is a newly identified biomarker for the progression of hypertension. These findings reveal that NOS1 in the CD is critical in the regulation of fluid-electrolyte balance, and this new genetic model of CD NOS1 gene deletion will be a valuable tool to study salt-dependent blood pressure mechanisms.

Control of renin synthesis and secretion

The aspartyl protease renin is the rate limiting activity of the renin-angiotensin-aldosterone system (RAAS). Renin is synthesized as an enzymatically inactive proenzyme which is constitutively secreted from several tissues. Only renin-expressing cells in the kidney are capable of generating active renin from prorenin, which is stored in prominent vesicles and which is released into the circulation upon demand. The acute release of renin is controlled by cyclic adenosine monophosphate (cAMP) and by calcium signaling pathways, which in turn are activated by a number of systemic and local factors. Longer lasting challenges of renin secretion lead to changes in the number of renin-producing cells, which occur by a metaplastic transformation of renin cell precursors such as preglomerular vascular smooth muscle or extraglomerular mesangial cells. This review aims to briefly address the state of knowledge of these various aspects of renin synthesis and secretion and attempts to relate them to the in vivo situation, in particular in men.

Synthesis and secretion of renin in mice with induced genetic mutations

The juxtaglomerular (JG) cell product renin is rate limiting in the generation of the bioactive octapeptide angiotensin II. Rates of synthesis and secretion of the aspartyl protease renin by JG cells are controlled by multiple afferent and efferent pathways originating in the CNS, cardiovascular system, and kidneys, and making critical contributions to the maintenance of extracellular fluid volume and arterial blood pressure. Since both excesses and deficits of angiotensin II have deleterious effects, it is not surprising that control of renin is secured by a complex system of feedforward and feedback relationships. Mice with genetic alterations have contributed to a better understanding of the networks controlling renin synthesis and secretion. Essential input for the setting of basal renin generation rates is provided by β-adrenergic receptors acting through cyclic adenosine monophosphate, the primary intracellular activation mechanism for renin mRNA generation. Other major control mechanisms include COX-2 and nNOS affecting renin through PGE2, PGI2, and nitric oxide. Angiotensin II provides strong negative feedback inhibition of renin synthesis, largely an indirect effect mediated by baroreceptor and macula densa inputs. Adenosine appears to be a dominant factor in the inhibitory arms of the baroreceptor and macula densa mechanisms. Targeted gene mutations have also shed light on a number of novel aspects related to renin processing and the regulation of renin synthesis and secretion.

Af17 deficiency increases sodium excretion and decreases blood pressure

The putative transcription factor AF17 upregulates the transcription of the epithelial sodium channel (ENaC) genes, but whether AF17 modulates sodium homeostasis and BP is unknown. Here, we generated Af17-deficient mice to determine whether deletion of Af17 leads to sodium wasting and low BP. Compared with wild-type mice, Af17-deficient mice had lower BP (11 mmHg), higher urine volume, and increased sodium excretion despite mildly increased plasma concentrations of aldosterone. Deletion of Af17 led to increased dimethylation of histone H3 K79 and reduced ENaC function. The attenuated function of ENaC resulted from decreased ENaC mRNA and protein expression, fewer active channels, lower open probability, and reduced effective activity. In contrast, inducing high levels of plasma aldosterone by a variety of methods completely compensated for Af17 deficiency with respect to sodium handling and BP. Taken together, these data identify Af17 as a potential locus for the maintenance of sodium and BP homeostasis and suggest that a particular histone modification is directly linked to these processes. Af17-mediated regulation of BP is largely, but not exclusively, the result of modulating ENaC, suggesting it has potential as a therapeutic target for the control of BP.

Neuronal nitric oxide synthase supports Renin release during sodium restriction through inhibition of phosphodiesterase 3

BACKGROUND

Mice with targeted deletion of neuronal nitric oxide (NO) synthase (nNOS⁻(/)⁻) display inability to increase plasma renin concentration (PRC) in response to sodium restriction. nNOS has a distinct expression at the macula densa (MD), and in the present study, it was tested whether nNOS supports renin release by cyclic guanosine monophosphate (cGMP)-mediated inhibition of cyclic adenosine monophosphate (cAMP)-specific phosphodiesterase 3 (PDE3) in juxtaglomerular (JG) cells.

METHODS

The experiments were performed in conscious nNOS⁻(/)⁻ and wild types after 10 days on a low-sodium diet by acute treatment with the PDE3-inhibitor milrinone, the PDE5 inhibitor zaprinast, or vehicle, using a crossover study protocol. PRC was measured with the antibody-trapping technique and blood pressure with telemetry. Glomerular filtration rate (GFR) and renal plasma flow (RPF) were estimated by measurements of inulin- and para-amino hippuric acid (PAH) clearances, respectively.

RESULTS

The basal PRC was reduced in nNOS⁻(/)⁻ compared to the wild types. Administration of milrinone caused a more pronounced PRC increase in nNOS⁻(/)⁻, resulting in normalized renin levels, whereas PDE5 inhibition did not affect PRC in any genotype. The blood pressure was similar in both genotypes, and milrinone did not affect blood pressure compared to vehicle. GFR and RPF were similar at baseline and were reduced by milrinone.

CONCLUSIONS

The present study provides in vivo evidence supporting the view that NO, selectively derived from nNOS, mediates renin release during sodium restriction by inhibiting PDE3, which would increase renin release by elevating cAMP levels in the JG cells.

Targeting cardiac sympatho-vagal imbalance using gene transfer of nitric oxide synthase

Heightened sympathetic excitation and diminished parasympathetic suppression of heart rate, cardiac contractility and vascular tone are all associated with cardiovascular diseases such as hypertension and ischemic heart disease. This phenotype often exists before these disease states have been established and is a strong correlate of mortality in the population. However, the causal role of the autonomic phenotype in the development and maintenance of hypertension and myocardial ischemia remains a subject of debate, as are the mechanisms responsible for regulating sympathovagal balance. Emerging evidence suggests oxidative stress and reactive oxygen species (such as nitric oxide (NO) and superoxide) play important roles in the modulation of autonomic balance, but so far the most important sites of action of these ubiquitous signaling molecules are unclear. In many cases, these mediators have opposing effects in separate tissues rendering conventional pharmacological approaches non-efficacious. Novel techniques have recently been used to augment these signaling pathways experimentally in a targeted fashion to central autonomic nuclei, cardiac neurons, and myocytes using gene transfer of NO synthase. This review article discusses these recent advances in the understanding of the roles of NO and its oxidative metabolites on autonomic imbalance in models of cardiovascular disease.

Regulation of renin release by local and systemic factors

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.