Role of protein kinase G in nitric oxide deficiency-induced supersensitivity to nitrovasodilator in rat pulmonary artery

L-NAME releases nitric oxide and potentiates subsequent nitroglycerin-mediated vasodilation

L-N^G-Nitro arginine methyl ester (L-NAME) has been widely applied for several decades in both basic and clinical research as an antagonist of nitric oxide synthase (NOS). Herein, we show that L-NAME slowly releases NO from its guanidino nitro group. Daily pretreatment of rats with L-NAME potentiated mesenteric vasodilation induced by nitrodilators such as nitroglycerin, but not by NO. Release of NO also occurred with the NOS-inactive enantiomer D-NAME, but not with L-arginine or another NOS inhibitor L-NMMA, consistent with the presence or absence of a nitro group in their structure and their nitrodilator-potentiating effects. Metabolic conversion of the nitro group to NO-related breakdown products was confirmed using isotopically-labeled L-NAME. Consistent with Fenton chemistry, transition metals and reactive oxygen species accelerated the release of NO from L-NAME. Both NO production from L-NAME and its nitrodilator-potentiating effects were augmented under inflammation. NO release by L-NAME can confound its intended NOS-inhibiting effects, possibly by contributing to a putative intracellular NO store in the vasculature.

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NOS-inhibitor L-NAME is also a precursor of NO.

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ROS releases NO from the nitro group of L-NAME via Fenton Chemistry.

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L-NAME potentates nitrodilator-mediated vasodilation.

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Nitroglycerin may cause vasodilation via activation of an intracellular NO store.

Short-term exposure of erythropoietin impairs endothelial function through inhibition of nitric oxide production and eNOS mRNA expression in the rat pulmonary artery

BACKGROUND

Administration of recombinant erythropoietin (rEPO) is often associated with systemic and pulmonary arterial hypertension in animals and human. The present study was conducted to determine whether one-week rEPO-treatment can produce any effect on pulmonary vasomotor function.

METHODS

Male Wistar rats were injected with rEPO (400IU/kg sc) or saline every other day for one week. Tension, biochemical and Real-Time PCR experiments were conducted on left and right branches of pulmonary artery and main pulmonary artery isolated from the rats.

RESULTS

ACh-induced relaxation was significantly (p<0.05) reduced in rEPO-treated rats in comparison to control animals. Relaxation to the NO donor SNP was not different between the groups. EDHF-induced relaxation was remarkably higher in rEPO-treated group in comparison to control. Phenylephrine-induced contraction was significantly (p <0.05) reduced in rings from rEPO-treated rats at the second and third lowest concentrations of phenylephrine and its potency was not significantly reduced. No significant difference was observed in CaCl₂-induced contraction between the groups. Nitric oxide production was significantly reduced in rEPO-treated rats in comparison to control animals. Real-time PCR studies demonstrated a significant decrease (p<0.05) of eNOS transcript. However, peNOS activity was not altered with rEPO treatment.

CONCLUSION

The present study suggests that EPO-treatment for one week attenuates ACh-stimulated NO production. It does not affect the vasodilatory action of SNP. It showed up-regulation of EDHF and decreased potency of phenylephrine. Thus elevated EPO may diversely affect the vasomotor function of pulmonary artery. Clinically, it is important to observe the use of EPO in hypertensive condition.

Diethyl-4,4'-dihydroxy-8,3'-neolign-7,7'-dien-9,9'-dionate exhibits antihypertensive activity in rats through increase in intracellular cGMP level and blockade of calcium channels

We report here the antihypertensive and vasorelaxant potential of some steroidal and non-steroidal compounds identified through a library of compounds. All the novel analogues showed vasorelaxant potential in isolated rat aorta. The most potent lead neolignan1 (Diethyl-4,4'-dihydroxy-8,3'-neolign-7,7'-dien-9,9'-dionate) produced concentration dependent relaxation with [pD₂ 5.16±0.05; n=16 and E_max 96.97%±1.12%; n=16]. The neolignan1 relaxation is independent of endothelium and is sensitive to ODQ (1H-[1, 2, 4] oxadiazolo [4, 3-a] quinoxalin-1-one; a blocker of soluble guanylyl cyclase (sGC) which synthesizes cGMP (cyclic guanosine monophosphate)). ELISA analysis of treated arterial tissues showed concentration-dependent increase in cGMP level in treated tissues compared to control (2.03 and 7.16 fold of control at 10 and 30µM of neolignan1, respectively) and a synergistic increase in cGMP level by 26.66 fold compared to control when used in combination with sildenafil (10µM; a known inducer of cGMP level by selectively blocking cGMP specific phosphodiesterase 5). Our present study reports for the first time that neolignans produce relaxation in isolated rat aorta through increase in intracellular cGMP level. The ODQ resistant relaxation of neolignan1 is mediated by blockade of voltage dependent L-type calcium channel (VDCC) as observed in the experiment with CaCl₂. Neolignan1 upon intravenous administration via tail vein in Spontaneously Hypertensive Rats (SHR) produced significant decrease in systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean arterial blood pressure (MAP). The present study concludes that neolignan1 exhibited antihypertensive potential in rats through rise in intracellular cGMP and blockade of VDCC.

Glabridin-induced vasorelaxation: Evidence for a role of BKCa channels and cyclic GMP

Background and purpose

Glabridin is a major flavonoid in Glycyrrhiza glabra (licorice) root, a traditional Asian medicine. Glabridin is reported to have anti-atherogenic, anti-inflammatory and anti-nephritic properties; however its effects on vascular tone remain unexplored.

Experimental approach

We examined the effect of glabridin on rat main mesenteric artery using isometric myography and also ELISA to measure cGMP levels.

Key results

Glabridin (30 μM) relaxed arteries pre-constricted with the thromboxane A₂ analog U46619 (0.2 μM) by ~ 60% in an endothelium-independent manner. Relaxation to 30 μM glabridin was abolished by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (1 μM) and by the BK_Ca channel blocker tetraethyammonium (1 mM) but was unaffected by the estrogen receptor antagonist ICI182780. The concentration-response curve to glabridin (0.1 to 30 μM) was downshifted by the K_ATP channel blocker glibenclamide (10 μM), the K_V channel blocker 4-aminopyridine (300 μM), and the K_IR blocker BaCl₂ (30 μM). In U46619-contracted arteries partially relaxed by 0.1 μM sodium nitroprusside, application of 10 and 30 nM glabridin caused additional vasorelaxation. Glabridin (30 μM) approximately doubled tissue [cyclic GMP]. Application of the phosphodiesterase inhibitor isobutylmethylxanthine caused a much larger rise in [cyclic GMP], and glabridin failed to cause vasorelaxation or a further rise in [cGMP] when co-applied with IBMX.

Conclusions and implications

Vasorelaxation to glabridin is dependent on the opening of K⁺ channels, particularly BK_Ca, probably caused by a rise in cellular [cyclic GMP] owing to phosphodiesterase inhibition. In the presence of sodium nitroprusside an effect of glabridin is observed at nM concentrations, similar those measured in plasma following human ingestion of licorice flavonoid oil.

BAY 41-2272 Treatment Improves Acetylcholine-Induced Aortic Relaxation in L-NAME Hypertensive Rats

Hypertension, an emerging problem of recent era, and many pathophysiological factors are participating to produce the disease. Nitric oxide (NO) is an important constituent to ameliorate hypertensive condition. Inhibition of endogenous NO synthase by L-N^G-Nitroarginine methyl ester (L-NAME) was responsible for generating hypertension in rats. BAY 41-2272 (5-cyclopropyl-2-[1-(2-fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridine-3-yl]-pyrimidin-4-ylamine), a soluble guanylyl cyclase activator, restricts rise of blood pressure and shows cardioprotective activity. The aim of the present study was to analyze effect of short-term BAY 41-2272 treatment on blood pressure and vascular function. Male Wistar rats were randomly divided into three groups such as control (group-A), hypertensive (group-B), and BAY 41-2272-treated hypertensive (group-C) rats. Normal saline was administered intramuscularly to control rats for last 3 days (days 40, 41, and 42) of total 42 days treatment, whereas rats of group-B and group-C were treated with L-NAME hydrochloride in drinking water at 50 mg/kg body weight daily for 42 days. Also, normal saline and BAY 41-2272 were administered for last 3 days at two different dosages at 1 and 3 mg/kg body weight/day intramuscularly to group-B and group-C rats, respectively. Administration of BAY 41-2272 for 3 days was not sufficient enough to decrease mean arterial pressure of hypertensive rats significantly. BAY at both the treatment dosages significantly ameliorate acetylcholine-induced maximal aortic relaxation compared with BAY-untreated hypertensive rats. Findings of the present study indicate that even shorter period of BAY 41-2272 treatment (3 days) improves vascular relaxation.

Regulation of sodium glucose co-transporter SGLT1 through altered glycosylation in the intestinal epithelial cells

Inhibition of constitutive nitric oxide (cNO) production inhibits SGLT1 activity by a reduction in the affinity for glucose without a change in Vmax in intestinal epithelial cells (IEC-18). Thus, we studied the intracellular pathway responsible for the posttranslational modification/s of SGLT1. NO is known to mediate its effects via cGMP which is diminished tenfold in L-NAME treated cells. Inhibition of cGMP production at the level of guanylyl cyclase or inhibition of protein kinase G also showed reduced SGLT1 activity demonstrating the involvement of PKG pathway in the regulation of SGLT1 activity. Metabolic labeling and immunoprecipitation with anti-SGLT1 specific antibodies did not show any significant changes in phosphorylation of SGLT1 protein. Tunicamycin to inhibit glycosylation reduced SGLT1 activity comparable to that seen with L-NAME treatment. The mechanism of inhibition was secondary to decreased affinity without a change in Vmax. Immunoblots of luminal membranes from tunicamycin treated or L-NAME treated IEC-18 cells showed a decrease in the apparent molecular size of SGLT1 protein to 62 and 67 kD, respectively suggesting an alteration in protein glycosylation. The deglycosylation assay with PNGase-F treatment reduced the apparent molecular size of the specific immunoreactive band of SGLT1 from control and L-NAME treated IEC-18 cells to approximately 62 kD from their original molecular size of 75 kD and 67 kD, respectively. Thus, the posttranslational mechanism responsible for the altered affinity of SGLT1 when cNO is diminished is secondary to altered glycosylation of SGLT1 protein. The intracellular pathway responsible for this alteration is cGMP and its dependent kinase.

Role of endothelin-1 in the hyper-responsiveness to nitrovasodilators following acute NOS inhibition

BACKGROUND AND PURPOSE

Acute NOS inhibition in humans and animals is associated with hypersensitivity to NO donors. The mechanisms underlying this phenomenon have not been fully elucidated. The purpose of the present study was to assess whether hypersensitivity to NOS-blockade is linked to endothelin-1 (ET-1) signalling.

EXPERIMENTAL APPROACH

Sprague Dawley rats were instrumented with indwelling arterial and venous catheters for continuous assessments of haemodynamic parameters and drug delivery, respectively. Mesenteric arteries were isolated and tested for reactivity by wire myography.

KEY RESULTS

NOS blockade with L-N(G)-nitroarginine methyl ester (L-NAME) caused a pronounced increase in arterial blood pressure (BP) (∼40 mmHg). In L-NAME-treated animals, the dose of sodium nitroprusside (SNP) required to cause a significant reduction in arterial BP was lower than in vehicle-treated rats (P < 0.001), and the magnitude of the reduction in BP was greater. Similar results were obtained with other NO mimetics, but not isoprenaline; moreover, decreasing the BP back to baseline levels with prazosin after L-NAME treatment did not attenuate the hyper-responsiveness to NO donors. The increased responsiveness to NO donors was abolished by pretreatment with the ET(A/B) receptor antagonist, PD145065, or the ET(A) receptor-specific antagonist ABT627. Ex vivo, L-NAME treatment potentiated the constriction induced by big endothelin-1 (bET-1), the precursor to active ET-1, but had no effect on the ET-1-mediated constriction.

CONCLUSIONS AND IMPLICATIONS

These data suggest that the increased sensitivity to NO donors is mediated, at least in part, by ET-1 in vivo, and the mechanism may involve the conversion of bET-1 to ET-1.