Phosphorylation of neuronal and endothelial nitric oxide synthase in the kidney with high and low salt diets

Regulation of carbohydrate metabolism by nitric oxide and hydrogen sulfide: Implications in diabetes

Nitric oxide (NO) and hydrogen sulfide (H₂S) are two gasotransmitters that are produced in the human body and have a key role in many of the physiological activities of the various organ systems. Decreased NO bioavailability and deficiency of H₂S are involved in the pathophysiology of type 2 diabetes and its complications. Restoration of NO levels have favorable metabolic effects in diabetes. The role of H₂S in pathophysiology of diabetes is however controversial; H₂S production is decreased during development of obesity, diabetes, and its complications, suggesting the potential therapeutic effects of H₂S. On the other hand, increased H₂S levels disturb the pancreatic β-cell function and decrease insulin secretion. In addition, there appear to be important interactions between NO and H₂S at the levels of both biosynthesis and signaling pathways, yet clear an insight into this relationship is lacking. H₂S potentiates the effects of NO in the cardiovascular system as well as NO release from its storage pools. Likewise, NO increases the activity and the expression of H₂S-generating enzymes. Inhibition of NO production leads to elimination/attenuation of the cardioprotective effects of H₂S. Regarding the increasing interest in the therapeutic applications of NO or H₂S-releasing molecules in a variety of diseases, particularly in the cardiovascular disorders, much is to be learned about their function in glucose/insulin metabolism, especially in diabetes. The aim of this review is to provide a better understanding of the individual and the interactive roles of NO and H₂S in carbohydrate metabolism.

Calmodulin-induced Conformational Control and Allostery Underlying Neuronal Nitric Oxide Synthase Activation

Nitric oxide synthase (NOS) is the primary generator of nitric oxide signals controlling diverse physiological processes such as neurotransmission and vasodilation. NOS activation is contingent on Ca²⁺/calmodulin binding at a linker between its oxygenase and reductase domains to induce large conformational changes that orchestrate inter-domain electron transfer. However, the structural dynamics underlying activation of full-length NOS remain ambiguous. Employing hydrogen-deuterium exchange mass spectrometry, we reveal mechanisms underlying neuronal NOS activation by calmodulin and regulation by phosphorylation. We demonstrate that calmodulin binding orders the junction between reductase and oxygenase domains, exposes the FMN subdomain, and elicits a more dynamic oxygenase active site. Furthermore, we demonstrate that phosphorylation partially mimics calmodulin activation to modulate neuronal NOS activity via long-range allostery. Calmodulin binding and phosphorylation ultimately promote a more dynamic holoenzyme while coordinating inter-domain communication and electron transfer.

Overexpression myocardial inducible nitric oxide synthase exacerbates cardiac dysfunction and beta-adrenergic desensitization in experimental hypothyroidism

BACKGROUND

Altered nitric oxide synthase (NOS) has been implicated in the pathophysiology of heart failure (HF). Recent evidence links hypothyroidism to the pathology of HF. However, the precise mechanisms are incompletely understood. The alterations and functional effects of cardiac NOS in hypothyroidism are unknown. We tested the hypothesis that hypothyroidism increases cardiomyocyte inducible NOS (iNOS) expression, which plays an important role in hypothyroidism-induced depression of cardiomyocyte contractile properties, [Ca(2+)]i transient ([Ca(2+)]iT), and β-adrenergic hyporesponsiveness.

METHODS AND RESULTS

We simultaneously evaluated LV functional performance and compared myocyte three NOS, β-adrenergic receptors (AR) and SERCA2a expressions and assessed cardiomyocyte contractile and [Ca(2+)]iT responses to β-AR stimulation with and without pretreatment of iNOS inhibitor (1400 W, 10(-5)mol/L) in 26 controls and 26 rats with hypothyroidism induced by methimazole (~30 mg/kg/day for 8 weeks in the drinking water). Compared with controls, in hypothyroidism, total serum T3 and T4 were significantly reduced followed by significantly decreased LV contractility (EES) with increased LV time constant of relaxation. These LV abnormalities were accompanied by concomitant significant decreases in myocyte contraction (dL/dtmax), relaxation (dR/dtmax), and [Ca(2+)]iT. In hypothyroidism, isoproterenol (10(-8)M) produced significantly smaller increases in dL/dtmax, dR/dtmax and [Ca(2+)]iT. These changes were associated with decreased β1-AR and SERCA2a, but significantly increased iNOS. Moreover, only in hypothyroidism, pretreatment with iNOS inhibitor significantly improved basal and isoproterenol-stimulated myocyte contraction, relaxation and [Ca(2+)]iT.

CONCLUSIONS

Hypothyroidism produces intrinsic defects of LV myocyte force-generating capacity and relaxation with β-AR desensitization. Up-regulation of cardiomyocyte iNOS may promote progressive cardiac dysfunction in hypothyroidism.

Female sex hormones protect against salt-sensitive hypertension but not essential hypertension

Initial studies found that female Dahl salt-sensitive (DS) rats exhibit greater blood pressure (BP) salt sensitivity than female spontaneously hypertensive rats (SHR). On the basis of the central role played by NO in sodium excretion and BP control, we further tested the hypothesis that blunted increases in BP in female SHR will be accompanied by greater increases in renal inner medullary nitric oxide synthase (NOS) activity and expression in response to a high-salt (HS) diet compared with DS rats. Gonad-intact and ovariectomized (OVX) female SHR and DS rats were placed on normal salt (NS; 0.4% salt) or HS (4% salt) diet for 2 wk. OVX did not alter BP in SHR, and HS diet produced a modest increase in BP. OVX significantly increased BP in DS rats on NS; HS further increased BP in all DS rats, although OVX had a greater increase in BP. Renal inner medullary NOS activity, total NOS3 protein, and NOS3 phosphorylated on serine residue 1177 were not altered by salt or OVX in either strain. NOS1 protein expression, however, significantly increased with HS only in SHR, and this corresponded to an increase in urinary nitrate/nitrite excretion. SHR also exhibit greater NOS1 and NOS3 protein expression than DS rats. These data indicate that female sex hormones offer protection against HS-mediated elevations in BP in DS rats but not SHR. We propose that the relative resistance to HS-mediated increases in BP in SHR is related to greater NOS expression and the ability to increase NOS1 protein expression compared with DS rats.

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.

Phosphorylation of neuronal nitric oxide synthase at Ser1412 in the dentate gyrus of rat brain after transient forebrain ischemia

We previously demonstrated that calmodulin-dependent protein kinase IIα (CaM-KIIα) phosphorylates nNOS at Ser(847) in the hippocampus after forebrain ischemia; this phosphorylation attenuates NOS activity and might contribute to resistance to post-ischemic damage. We also revealed that cyclic AMP-dependent protein kinase (PKA) could phosphorylate nNOS at Ser(1412)in vitro. In this study, we focused on chronological and topographical changes in the phosphorylation of nNOS at Ser(1412) after rat forebrain ischemia. The hippocampus and adjacent cortex were collected at different times, up to 24h, after 15min of forebrain ischemia. NOS was partially purified from crude samples using ADP agarose gel. Neuronal NOS, phosphorylated (p)-nNOS at Ser(1412), PKA, and p-PKA at Thr(197) were studied in the rat hippocampus and cortex using Western blot analysis and immunohistochemistry. Western blot analysis revealed that p-nNOS at Ser(1412) significantly increased between 1 and 6h after reperfusion in the hippocampus, but not in the cortex. PKA was cosedimented with nNOS by ADP agarose gel. Immunohistochemistry revealed that phosphorylation of nNOS at Ser(1412) and PKA at Thr(197) occurred in the subgranular layer of the dentate gyrus. Forebrain ischemia might thereby induce temporary activation of PKA at Thr(197), which then phosphorylates nNOS at Ser(1412) in the subgranular layer of the dentate gyrus.

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.

Cyclic AMP-dependent phosphorylation of neuronal nitric oxide synthase mediates penile erection

Nitric oxide (NO) generated by neuronal NO synthase (nNOS) initiates penile erection, but has not been thought to participate in the sustained erection required for normal sexual performance. We now show that cAMP-dependent phosphorylation of nNOS mediates erectile physiology, including sustained erection. nNOS is phosphorylated by cAMP-dependent protein kinase (PKA) at serine(S)1412. Electrical stimulation of the penile innervation increases S1412 phosphorylation that is blocked by PKA inhibitors but not by PI3-kinase/Akt inhibitors. Stimulation of cAMP formation by forskolin also activates nNOS phosphorylation. Sustained penile erection elicited by either intracavernous forskolin injection, or augmented by forskolin during cavernous nerve electrical stimulation, is prevented by the NOS inhibitor L-NAME or in nNOS-deleted mice. Thus, nNOS mediates both initiation and maintenance of penile erection, implying unique approaches for treating erectile dysfunction.

L-NAME treatment enhances exercise-induced content of myocardial heat shock protein 72 (Hsp72) in rats

BACKGROUND/AIM

Nitric oxide (NO) modulates the expression of the chaperone Hsp72 in the heart, and exercise stimulates both NO production and myocardial Hsp72 expression. The main purpose of the study was to investigate whether NO interferes with an exercise-induced myocardial Hsp72 expression.

METHODS

Male Wistar rats (70-100 days) were divided into control (C, n=12), L-NAME-treated (L, n=12), exercise (E, n=13) and exercise plus L-NAME-treated (EL, n=20) groups. L-NAME was given in drinking water (700 mg·L(-1)) and the exercise was performed on a treadmill (15-25 m·min(-1), 40-60 min.day(-1)) for seven days. Left ventricle (LV) protein Hsp content, NOS and phosphorylated-NOS (p-NOS) isoforms were measured using Western blotting. The activity of NOS was assayed in LV homogenates by the conversion of [(3)H]L-arginine to [(3)H]L-citrulline.

RESULTS

Hsp72 content was increased significantly (223%; p < 0.05) in the E group compared to the C group, but exercise alone did not alter the NOS content, p-NOS isoforms or NOS activity. Contrary to our expectation, L-NAME enhanced (p < 0.05) the exercise-induced Hsp72 content (EL vs. C, L and E groups = 1019%, 548% and 457%, respectively). Although the EL group had increased stimulatory p-eNOS(Ser1177) (over 200%) and decreased inhibitory p-nNOS(Ser852) (ñ50%) compared to both the E and L groups (p < 0.05), NOS activity was similar in all groups.

CONCLUSIONS

Our results suggest that exercise-induced cardiac Hsp72 expression does not depend on NO. Conversely, the in vivo L-NAME treatment enhances exercise-induced Hsp72 production. This effect may be due to an increase in cardiac stress.

Nitric oxide signaling in brain function, dysfunction, and dementia

Nitric oxide (NO) is an important signaling molecule that is widely used in the nervous system. With recognition of its roles in synaptic plasticity (long-term potentiation, LTP; long-term depression, LTD) and elucidation of calcium-dependent, NMDAR-mediated activation of neuronal nitric oxide synthase (nNOS), numerous molecular and pharmacological tools have been used to explore the physiology and pathological consequences for nitrergic signaling. In this review, the authors summarize the current understanding of this subtle signaling pathway, discuss the evidence for nitrergic modulation of ion channels and homeostatic modulation of intrinsic excitability, and speculate about the pathological consequences of spillover between different nitrergic compartments in contributing to aberrant signaling in neurodegenerative disorders. Accumulating evidence points to various ion channels and particularly voltage-gated potassium channels as signaling targets, whereby NO mediates activity-dependent control of intrinsic neuronal excitability; such changes could underlie broader mechanisms of synaptic plasticity across neuronal networks. In addition, the inability to constrain NO diffusion suggests that spillover from endothelium (eNOS) and/or immune compartments (iNOS) into the nervous system provides potential pathological sources of NO and where control failure in these other systems could have broader neurological implications. Abnormal NO signaling could therefore contribute to a variety of neurodegenerative pathologies such as stroke/excitotoxicity, Alzheimer's disease, multiple sclerosis, and Parkinson's disease.

Renal NOS activity, expression, and localization in male and female spontaneously hypertensive rats

The goal of this study was to examine the status of the renal nitric oxide (NO) system by determining NO synthase (NOS) isoform activity and expression within the three regions of the kidney in 14-wk-old male and female spontaneously hypertensive rats (SHR). NOS activity, and NOS1 and NOS3 protein expressions and localization were comparable in the renal cortex and outer medulla of male and female SHR. In contrast, male SHR had significantly less NOS1 and NOS3 enzymatic activity (0 +/- 5 and 53 +/- 7 pmol.mg(-1).30 min(-1), respectively) compared with female SHR (37 +/- 16 and 172 +/- 40 pmol.mg(-1).30 min(-1), respectively). Lower levels of inner medullary NOS1 activity in male SHR were associated with less NOS1 protein expression [45 +/- 7 relative densitometric units (RDU)] and fewer NOS1-positive cells in the renal inner medulla compared with female SHR (79 +/- 12 RDU). Phosphorylation of NOS3 is an important determinant of NOS activity. Male SHR had significantly greater phosphorylation of NOS3 on threonine 495 in the renal cortex compared with females (0.25 +/- 0.05 vs. 0.15 +/- 0.06 RDU). NOS3 phosphorylation was comparable in males and females in the other regions of the kidney. cGMP levels were measured as an indirect index of NO production. cGMP levels were significantly lower in the renal cortex (0.08 +/- 0.01 pmol/mg) and inner medulla (0.43 +/- 0.02 pmol/mg) of male SHR compared with females (cortex: 0.14 +/- 0.02 pmol/mg; inner medulla: 0.56 +/- 0.02 pmol/mg). Our data suggest that the effect of the sex of the animal on NOS activity and expression is different in the three regions of the SHR kidney and supports the hypothesis that male SHR have lower NO bioavailability compared with females.

Effect of chronic nNOS inhibition on blood pressure, vasoactivity, and arterial wall structure in Wistar rats

While the unequivocal pattern of endothelial nitric oxide (NO) synthase (eNOS) inhibition in cardiovascular control has been recognised, the role of NO produced by neuronal NOS (nNOS) remains unclear. The purpose of the present study was to describe the cardiovascular effects of NO production interference by inhibition of nNOS with 7-nitroindazole (7-NI). Wistar rats (10 weeks old) were used: control and experimental rats were administered 7-NI 10 mg/kg b.w./day in drinking water for 6 weeks. Systolic blood pressure (BP) was measured by the tail-cuff plethysmographic method. Isolated thoracic aortas (TAs) were used to study vasomotor activity of the conduit artery in vitro. The BP response of anaesthetised animals was used to follow the cardiovascular-integrated response in vivo. Geometry of the TA was measured after perfusion fixation (120 mm Hg) by light microscopy. Expression of eNOS was measured in the TA by immunoblot analysis. Although 6 weeks of nNOS inhibition did not alter systolic BP, the heart/body weight ratio was decreased. Relaxation of the TA in response to acetylcholine (10(-9)-10(-5)mol/L) was moderately inhibited. However, no difference in the BP hypotensive response after acetylcholine (0.1, 1, 10 microg) was observed. The contraction of TA in response to noradrenaline (10(-10)-10(-5)mol/L), and the BP pressor response to noradrenaline (0.1, 1 microg) was attenuated. The inner diameter of the TA was increased, and the wall thickness, wall cross-sectional area, and wall thickness/inner diameter ratio were decreased. The expression of eNOS in the TA was increased. In summary, cardiac and TA wall hypotrophy, underlined by decreased contractile efficiency, were observed. The results suggested that two constitutive forms of NOS (nNOS, eNOS) likely participate in regulation of cardiovascular tone by different mechanisms.

Salt inactivates endothelial nitric oxide synthase in endothelial cells

There is a 1-4 mmol/L rise in plasma sodium concentrations in individuals with high salt intake and in patients with essential hypertension. In this study, we used 3 independent assays to determine whether such a small increase in sodium concentrations per se alters endothelial nitric oxide synthase (eNOS) function and contributes to hypertension. By directly measuring NOS activity in living bovine aortic endothelial cells, we demonstrated that a 5-mmol/L increase in salt concentration (from 137 to 142 mmol/L) caused a 25% decrease in NOS activity. Importantly, the decrease in NOS activity was in a salt concentration-dependent manner. The NOS activity was decreased by 25, 45, and 70%, with the increase of 5, 10, and 20 mmol/L of NaCl, respectively. Using Chinese hamster ovary cells stably expressing eNOS, we confirmed the inhibitory effects of salt on eNOS activity. The eNOS activity was unaffected in the presence of equal milliosmol of mannitol, which excludes an osmotic effect. Using an ex vivo aortic angiogenesis assay, we demonstrated that salt attenuated the nitric oxide (NO)-dependent proliferation of endothelial cells. By directly monitoring blood pressure changes in response to salt infusion, we found that in vivo infusion of salt induced an acute increase in blood pressure in a salt concentration-dependent manner. In conclusion, our findings demonstrated that eNOS is sensitive to changes in salt concentration. A 5-mmol/L rise in salt concentration, within the range observed in essential hypertension patients or in individuals with high salt intake, could significantly suppress eNOS activity. This salt-induced reduction in NO generation in endothelial cells may contribute to the development of hypertension.

Dietary salt activates an endothelial proline-rich tyrosine kinase 2/c-Src/phosphatidylinositol 3-kinase complex to promote endothelial nitric oxide synthase phosphorylation

Although many laboratories have shown that dietary NaCl (salt) intake increases NO production in rodents and humans, the mechanism has not been uncovered. In the present study, pharmacological and dominant-negative strategies were used to show that feeding a formulated diet containing increased amounts of salt to young male Sprague-Dawley rats induced the formation of an endothelial cell-signaling complex that contained proline-rich tyrosine kinase 2, c-Src (also known as pp60(c-src)), and phosphatidylinositol 3-kinase. In the setting of a high-salt diet, proline-rich tyrosine kinase 2 served as the scaffold for c-Src-mediated phosphatidylinositol 3-kinase activation. Phosphatidylinositol 3-kinase was the upstream activator of protein kinase B (Akt), which was responsible for phosphorylation of the rat endothelial isoform of NO synthase at S1176 and thereby promoted the increase in NO production. The combined findings illustrated the crucial role for a proline-rich tyrosine kinase 2-signaling complex in the endothelial response to salt intake.

Modulation of the dystrophin-associated protein complex in response to resistance training in young and older men

The dystrophin-associated protein complex (DAPC) is a scaffold of proteins linking the intracellular cytoskeleton with the extracellular matrix that is integral to structural stability and integrity, signaling and mechanotransduction, and force transmission. We hypothesized that the expression of DAPC component proteins would be altered by resistance loading during progressive resistance training (PRT)-mediated myofiber hypertrophy, and we investigated whether aging influenced these changes. Seventeen young (27 yr) and 13 older (65 yr) men completed 16 wk of PRT with muscle biopsies at baseline (T1), 24 h after bout 1 (T2), and 24 h after the final bout at week 16 (T3). Myofiber hypertrophy in the young (type I 31%, P < 0.005; type II 40%, P < 0.001) far exceeded hypertrophy in the old (type II only, 19.5%, P < 0.05). PRT altered protein expression for caveolin-3 (decreased 24% by T3, P < 0.01), alpha(1)-syntrophin (increased 16% by T3, P < 0.05), alpha-dystrobrevin (fell 23% from T2 to T3, P < 0.01), and dystrophin [rose acutely (30% by T2, P < 0.05) and returned to baseline by T3]. The phosphorylation state of membrane neuronal nitric oxide synthase (Ser(1417)) decreased 70% (P < 0.005) by T3, particularly in the old (81%), whereas p38 MAPK phosphorylation increased twofold by T3 in the old (P < 0.01). We conclude that component proteins of the DAPC are modulated by PRT, which may serve to improve both structural and signaling functions during load-mediated myofiber hypertrophy. The blunted hypertrophic adaptation seen in old vs. young men may have resulted from overstress, as suggested by marked p38 MAPK activation in old men only.

Influence of salt on subcellular localization of nitric oxide synthase activity and expression in the renal inner medulla

1. The aims of this study were: (i) to characterize the subcellular localization of nitric oxide synthase (NOS) 1 and NOS3 activity and expression within the cytosolic, plasma membrane and intracellular membrane subcellular fractions of the renal inner medulla of rats; and (ii) to determine whether NOS1 and NOS3 activity and expression in subcellular fractions of the renal inner medulla are regulated by dietary salt intake. Although the NOS system is important in maintaining Na(+) and water homeostasis, the identity of the NOS isoform that is sensitive to dietary Na(+) remains unclear. In addition, subcellular localization of both NOS1 and NOS3 has been shown to regulate enzymatic activity and influence the ability of NOS to produce nitric oxide (NO). 2. Renal inner medullae were dissected from male Sprague-Dawley rats and separated into cytosolic, plasma membrane and intracellular membrane fractions for measurement of NOS activity and western blot analysis. 3. On a normal-salt diet, NOS activity and NOS1 and NOS3 protein expression were present in all three subcellular fractions, although total NOS activity was enriched in the intracellular membrane fraction. In response to a high-salt diet, urinary nitrate/nitrite (NO(x)) increased. Despite an increase in NO(x) excretion, total NOS activity in the renal inner medullary homogenate was decreased. There were no detectable differences in NOS activity in the subcellular fractions. Expression of NOS1 protein was decreased in the cytoplasmic and plasma membrane fractions, although maintained in the intracellular membrane fraction, in response to high salt. Expression of NOS3 protein was unaffected by high salt. 4. In conclusion, we hypothesize that NOS1 localization in the intracellular membrane is important in increasing NO production to aid Na(+) and water homeostasis.

Role of endothelial-derived nitric oxide in hypertension and renal disease

PURPOSE OF REVIEW

To highlight recent advances in the field of endothelial-derived nitric oxide regulation of blood pressure and renal homeostasis.

RECENT FINDINGS

Many laboratories have dissected a role for nitric oxide in regulating blood pressure and renal function. In models of hypertension, and chronic and acute renal disease, the loss of nitric oxide bioavailability may occur due to inactivation of endothelial nitric oxide synthase, synthesis of endogenous inhibitors or oxidative inactivation of nitric oxide.

SUMMARY

Understanding the molecular mechanisms of nitric oxide synthesis may lead to novel diagnostics and treatments for cardiovascular disorders.

Inhibitors of phosphodiesterase 5 (PDE 5) inhibit the nerve-induced release of nitric oxide from the rabbit corpus cavernosum

BACKGROUND AND PURPOSE

Nitrergic neurons are important for erectile responses in the corpus cavernosum and impaired signalling results in erectile dysfunction, today treated successfully by oral administration of the selective phosphodiesterase 5 (PDE 5) inhibitors sildenafil, tadalafil and vardenafil. Although the importance of nitrergic neurons in urogenital function has become evident, it has not been investigated if the PDE 5 inhibitors affect the nerve-induced release of nitric oxide (NO). In a previous study we found that the soluble guanylate cyclase (sGC)/cyclic guanosine 3',5'-monophosphate (cGMP) pathway might modulate nerve-induced release of NO in isolated cavernous tissue.

EXPERIMENTAL APPROACH

Electrical field stimulation (EFS 5 Hz, 40 V, 0.3 ms pulse duration, 25 pulses at intervals of 2 min) of rabbit isolated cavernous tissue elicited reproducible, nerve-mediated relaxations in the presence of scopolamine (10(-5) M), guanethidine (10(-5) M) and phenylephrine (3 x 10(-6) M). In superfusion experiments, nerve stimulation (20 Hz, 40 V, 1 ms) of the cavernous tissue evoked release of NO/NO2-, measured by chemiluminescence.

KEY RESULTS

Sildenafil, tadalafil and vardenafil decreased the muscular tone and prolonged the relaxations to nerve stimulation. The evoked release of NO decreased to 72+/-11%, 55+/-16% and 61+/-14% of control, respectively after addition of sildenafil, tadalafil or vardenafil (all 10(-4) M, n=6-8, p<0.05).

CONCLUSIONS AND IMPLICATIONS

Selective PDE 5 inhibitors influence the nerve-induced release of NO, probably via cGMP-mediated negative feedback. This negative feedback might explain why priapism is not seen during monotherapy with the PDE inhibitors.

Rosiglitazone treatment enhances acute AMP-activated protein kinase-mediated muscle and adipose tissue glucose uptake in high-fat-fed rats

AMP-activated protein kinase (AMPK) has been implicated in the insulin-sensitizing actions of thiazolidinediones (TZDs), but it is not known whether TZD treatment can enhance tissue glucose uptake in response to AMPK activation. The present study investigated the influence of the TZD rosiglitazone on glucose turnover induced by intravenous infusion of the AMPK activator 5-aminoimidazole 4-carboxamide riboside (AICAR) under euglycemic and iso-insulinemic conditions in insulin-resistant high-fat-fed rats. We found that rosiglitazone treatment significantly enhanced AICAR-stimulated whole-body glucose disposal by 27% in high-fat-fed rats, and a 44% greater glucose infusion rate (both P < 0.01 vs. vehicle control rats) was required to maintain euglycemia. Along with this, both AICAR-stimulated glucose uptake and glucose incorporation into glycogen in muscle and adipose tissue were enhanced (P < 0.05). The enhanced glucose uptake and glycogen synthesis in muscle were associated with increased activity of total AMPK and the AMPKalpha2 subunit. In comparison, these effects were not apparent in rats fed standard rodent diet. Thus, our findings suggest that in addition to ameliorating insulin resistance, TZDs may enhance AMPK-stimulated glucose clearance into peripheral tissues in insulin-resistant states.

Nitric oxide in the kidney: functions and regulation of synthesis

In the kidney nitric oxide (NO) has numerous important functions including the regulation of renal haemodynamics, maintenance of medullary perfusion, mediation of pressure-natriuresis, blunting of tubuloglomerular feedback, inhibition of tubular sodium reabsorption and modulation of renal sympathetic neural activity. The net effect of NO in the kidney is to promote natriuresis and diuresis. Significantly, deficient renal NO synthesis has been implicated in the pathogenesis of hypertension. All three isoforms of nitric oxide synthase (NOS), namely neuronal NOS (nNOS or NOS1), inducible NOS (iNOS or NOS2) and endothelial NOS (eNOS or NOS3) are reported to contribute to NO synthesis in the kidney. The regulation of NO synthesis in the kidney by NOSs is complex and incompletely understood. Historically, many studies of NOS regulation in the kidney have emphasized the role of variations in gene transcription and translation. It is increasingly appreciated, however, that the constitutive NOS isoforms (nNOS and eNOS) are also subject to rapid regulation by post-translational mechanisms such as Ca(2+) flux, serine/threonine phosphorylation and protein-protein interactions. Recent studies have emphasized the role of post-translational regulation of nNOS and eNOS in the regulation of NO synthesis in the kidney. In particular, a role for phosphorylation of nNOS and eNOS at both activating and inhibitory sites is emerging in the regulation of NO synthesis in the kidney. This review summarizes the roles of NO in renal physiology and discusses recent advances in the regulation of eNOS and nNOS in the kidney by post-translational mechanisms such as serine/threonine phosphorylation.