Role of nitric oxide and endothelium-derived hyperpolarizing factor (EDHF) in the regulation of blood pressure by leptin in lean and obese rats

Leptin in reproduction and hypertension in pregnancy

Leptin has important roles in numerous physiological functions, including those in the regulation of energy balance, and in immune and reproductive systems. However, in the recent years, evidence has implicated it in a number of obesity-related diseases, where its concentrations in serum are significantly elevated. Elevated serum leptin concentrations and increased placental leptin secretion have been reported in women with hypertensive disorders of pregnancy. Whether leptin is responsible for this disorder remains to be established. Leptin injections in healthy rats and mice during pregnancy result in endothelial activation, increased blood pressure and proteinuria. A potential role for leptin in the pathogenesis of pre-eclampsia is hypothesised, particularly in women who are overweight or obese where serum leptin concentrations are often elevated. This review summarises pertinent information in the literature on the role of leptin in puberty, pregnancy, and hypertensive disorders of pregnancy. In particular, the possible mechanism that may be involved in leptin-induced increase in blood pressure and proteinuria during pregnancy and the potential role of marinobufagenin in this disease entity. We hypothesise a significant role for oxidative stress in this, and propose a conceptual framework on the events that lead to endothelial activation, raised blood pressure and proteinuria following leptin administration.

Vasoactive Effects of Chronic Treatment with Fructose and Slow-Releasing H2S Donor GYY-4137 in Spontaneously Hypertensive Rats: The Role of Nitroso and Sulfide Signalization

Increased fructose consumption induces metabolic-syndrome-like pathologies and modulates vasoactivity and the participation of nitric oxide (NO) and hydrogen sulfide (H₂S). We investigated whether a slow-releasing H₂S donor, GYY-4137, could exert beneficial activity in these conditions. We examined the effect of eight weeks of fructose intake on the blood pressure, biometric parameters, vasoactive responses, and NO and H₂S pathways in fructose-fed spontaneously hypertensive rats with or without three weeks of GYY-4137 i.p. application. GYY-4137 reduced triacylglycerol levels and blood pressure, but not adiposity, and all were increased by fructose intake. Fructose intake generally enhanced endothelium-dependent vasorelaxation, decreased adrenergic contraction, and increased protein expression of interleukin-6 (IL-6), tumor necrosis factor alpha (TNFα), and concentration of conjugated dienes in the left ventricle (LV). Although GYY-4137 administration did not affect vasorelaxant responses, it restored disturbed contractility, LV oxidative damage and decreased protein expression of TNFα in fructose-fed rats. While the participation of endogenous H₂S in vasoactive responses was not affected by fructose treatment, the expression of H₂S-producing enzyme cystathionine β-synthase in the LV was increased, and the stimulation of the NO signaling pathway improved endothelial function in the mesenteric artery. On the other hand, chronic treatment with GYY-4137 increased the expression of H₂S-producing enzyme cystathionine γ-lyase in the LV and stimulated the beneficial pro-relaxant and anti-contractile activity of endogenous H₂S in thoracic aorta. Our results suggest that sulfide and nitroso signaling pathways could trigger compensatory vasoactive responses in hypertensive rats with metabolic disorder. A slow H₂S-releasing donor could partially amend metabolic-related changes and trigger beneficial activity of endogenous H₂S.

Leptin, Obesity, and Hypertension: A Review of Pathogenetic Mechanisms

The adipokine leptin is expressed at higher concentrations in obese subjects, who also incidentally have a higher prevalence of hypertension. The pathogenesis of this obesity-related hypertension is controversial and is believed to be related to many factors including increased sympathetic activity, abnormalities of the renin-angiotensin system, sodium retention, and an endotheliopathy acting independently or in concert with increased circulating leptin. This review discusses the potential mechanisms through which changes in leptin signal transduction pathways in tissues with the leptin receptor, especially the hypothalamus, mediate the pathogenetic relationships between obesity and hypertension. The hypothesis is explored that leptin effects on blood pressure (BP) are meditated by the downstream effects of hypothalamic leptin signaling and ultimately result in activation of specific melanocortin receptors located on sympathetic neurons in the spinal cord. The physiological consequences of this sympathetic activation of the heart and kidney are activation of the renin-angiotensin system, sodium retention and circulatory expansion and finally, elevated BP. This sequence of events has been elegantly demonstrated with leptin infusion and gene knockout studies in animal models but has not been convincingly reproducibly confirmed in humans. Further studies in human subjects on the specific roles of hypothalamic leptin in essential hypertension are indicated as elucidation of the signaling pathways should provide better understanding of the role of weight loss in BP control and afford an additional mechanism for pharmacologic control of BP in adults and children at risk of cardiovascular disease.

Endothelium-Dependent Hyperpolarization (EDH) in Diabetes: Mechanistic Insights and Therapeutic Implications

Diabetes mellitus is one of the major risk factors for cardiovascular disease and is an important health issue worldwide. Long-term diabetes causes endothelial dysfunction, which in turn leads to diabetic vascular complications. Endothelium-derived nitric oxide is a major vasodilator in large-size vessels, and the hyperpolarization of vascular smooth muscle cells mediated by the endothelium plays a central role in agonist-mediated and flow-mediated vasodilation in resistance-size vessels. Although the mechanisms underlying diabetic vascular complications are multifactorial and complex, impairment of endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells would contribute at least partly to the initiation and progression of microvascular complications of diabetes. In this review, we present the current knowledge about the pathophysiology and underlying mechanisms of impaired EDH in diabetes in animals and humans. We also discuss potential therapeutic approaches aimed at the prevention and restoration of EDH in diabetes.

Spironolactone treatment attenuates vascular dysfunction in type 2 diabetic mice by decreasing oxidative stress and restoring NO/GC signaling

Type 2 diabetes (DM2) increases the risk of cardiovascular disease. Aldosterone, which has pro-oxidative and pro-inflammatory effects in the cardiovascular system, is positively regulated in DM2. We assessed whether blockade of mineralocorticoid receptors (MR) with spironolactone decreases reactive oxygen species (ROS)-associated vascular dysfunction and improves vascular nitric oxide (NO) signaling in diabetes. Leptin receptor knockout [LepR(db)/LepR(db) (db/db)] mice, a model of DM2, and their counterpart controls [LepR(db)/LepR(+), (db/+) mice] received spironolactone (50 mg/kg body weight/day) or vehicle (ethanol 1%) via oral per gavage for 6 weeks. Spironolactone treatment abolished endothelial dysfunction and increased endothelial nitric oxide synthase (eNOS) phosphorylation (Ser(1177)) in arteries from db/db mice, determined by acetylcholine-induced relaxation and Western Blot analysis, respectively. MR antagonist therapy also abrogated augmented ROS-generation in aorta from diabetic mice, determined by lucigenin luminescence assay. Spironolactone treatment increased superoxide dismutase-1 and catalase expression, improved sodium nitroprusside and BAY 41-2272-induced relaxation, and increased soluble guanylyl cyclase (sGC) β subunit expression in arteries from db/db mice. Our results demonstrate that spironolactone decreases diabetes-associated vascular oxidative stress and prevents vascular dysfunction through processes involving increased expression of antioxidant enzymes and sGC. These findings further elucidate redox-sensitive mechanisms whereby spironolactone protects against vascular injury in diabetes.

Renal hemodynamic and morphological changes after 7 and 28 days of leptin treatment: the participation of angiotensin II via the AT1 receptor

The role of hyperleptinemia in cardiovascular diseases is well known; however, in the renal tissue, the exact site of leptin’s action has not been established. This study was conducted to assess the effect of leptin treatment for 7 and 28 days on renal function and morphology and the participation of angiotensin II (Ang II), through its AT₁ receptor. Rats were divided into four groups: sham, losartan (10 mg/kg/day, s.c.), leptin (0.5 mg/kg/day for the 7 days group and 0.25 mg/kg/day for the 28 days group) and leptin plus losartan. Plasma leptin, Ang II and endothelin 1 (ET-1) levels were measured using an enzymatic immuno assay. The systolic blood pressure (SBP) was evaluated using the tail-cuff method. The renal plasma flow (RPF) and the glomerular filtration rate (GFR) were determined by p-aminohippuric acid and inulin clearance, respectively. Urinary Na⁺ and K⁺ levels were also analyzed. Renal morphological analyses, desmin and ED-1 immunostaining were performed. Proteinuria was analyzed by silver staining. mRNA expression of renin-angiotensin system (RAS) components, TNF-α and collagen type III was analyzed by quantitative PCR. Our results showed that leptin treatment increased Ang II plasma levels and progressively increased the SBP, achieving a pre-hypertension state. Rats treated with leptin 7 days showed a normal RPF and GFR, but increased filtration fraction (FF) and natriuresis. However, rats treated with leptin for 28 showed a decrease in the RPF, an increase in the FF and no changes in the GFR or tubular function. Leptin treatment-induced renal injury was demonstrated by: glomerular hypertrophy, increased desmin staining, macrophage infiltration in the renal tissue, TNF-α and collagen type III mRNA expression and proteinuria. In conclusion, our study demonstrated the progressive renal morphological changes in experimental hyperleptinemia and the interaction between leptin and the RAS on these effects.

Leptin-induced endothelium-dependent vasorelaxation of peripheral arteries in lean and obese rats: role of nitric oxide and hydrogen sulfide

Adipose tissue hormone leptin induces endothelium-dependent vasorelaxation mediated by nitric oxide (NO) and endothelium-derived hyperpolarizing factors (EDHF). Previously it has been demonstrated that in short-term obesity the NO-dependent and the EDHF-dependent components of vascular effect of leptin are impaired and up-regulated, respectively. Herein we examined the mechanism of the EDHF-dependent vasodilatory effect of leptin and tested the hypothesis that alterations of acute vascular effects of leptin in obesity are accounted for by chronic hyperleptinemia. The study was performed in 5 groups of rats: (1) control, (2) treated with exogenous leptin for 1 week to induce hyperleptinemia, (3) obese, fed highly-palatable diet for 4 weeks, (4) obese treated with pegylated superactive rat leptin receptor antagonist (PEG-SRLA) for 1 week, (5) fed standard chow and treated with PEG-SRLA. Acute effect of leptin on isometric tension of mesenteric artery segments was measured ex vivo. Leptin relaxed phenylephrine-preconstricted vascular segments in NO- and EDHF-dependent manner. The NO-dependent component was impaired and the EDHF-dependent component was increased in the leptin-treated and obese groups and in the latter group both these effects were abolished by PEG-SRLA. The EDHF-dependent vasodilatory effect of leptin was blocked by either the inhibitor of cystathionine γ-lyase, propargylglycine, or a hydrogen sulfide (H2S) scavenger, bismuth (III) subsalicylate. The results indicate that NO deficiency is compensated by the up-regulation of EDHF in obese rats and both effects are accounted for by chronic hyperleptinemia. The EDHF-dependent component of leptin-induced vasorelaxation is mediated, at least partially, by H2S.

Leptin and the regulation of endothelial function in physiological and pathological conditions

Obesity and the accompanying metabolic syndrome are among the most important causes of cardiovascular pathologies associated with endothelial dysfunction, such as arterial hypertension and atherosclerosis. This detrimental effect of obesity is mediated, in part, by excessive production of the adipose tissue hormone leptin. Under physiological conditions leptin induces endothelium-dependent vasorelaxation by stimulating nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). Leptin activates endothelial NO synthase (eNOS) through a mechanism involving AMP-activated protein kinase (AMPK) and protein kinase B/Akt, which phosphorylates eNOS at Ser(1177) , increasing its activity. Under pathological conditions, such as obesity and metabolic syndrome, the NO-mediated vasodilatory effect of leptin is impaired. Resistance to the acute NO-mimetic effect of leptin is accounted for by chronic hyperleptinaemia and may result from different mechanisms, such as downregulation of leptin receptors, increased levels of circulating C-reactive protein, oxidative stress and overexpression of suppressor of cytokine signalling-3. In short-lasting obesity, impaired leptin-induced NO production is compensated by EDHF; however, in advanced metabolic syndrome, the contribution of EDHF to the haemodynamic effect of leptin becomes inefficient. Resistance to the vasodilatory effects of leptin may contribute to the development of arterial hypertension owing to unopposed stimulation of the sympathetic nervous system by this hormone.

Endothelium-derived hyperpolarizing factor and diabetes

In addition to its role as a barrier between blood and tissues, the vascular endothelium is responsible for the synthesis and released of a number of vasodilators including prostaglandins, nitric oxide and endothelium-derived hyperpolarizing factor (EDHF). As one of these vasodilators, the specific nature of EDHF has not been fully elucidated, although a number of roles have been proposed. Importantly, many conditions, such as hypertension, hyperlipidemia, heart failure, ischemia-reperfusion and diabetes mellitus comprise vascular endothelial dysfunction with EDHF dysregulation. This article reviews reports on the role of EDHF in diabetes-related endothelial dysfunction.

Leptin regulates ACE activity in mice

Leptin is a hormone related to metabolism. It also influences blood pressure, but the mechanisms triggered in this process are not yet elucidated. Angiotensin-I converting enzyme (ACE) regulates cardiovascular functions and recently has been associated with metabolism control and obesity. Here, we used ob/ob mice, a model lacking leptin, to answer the question whether ACE and leptin could interact to influence blood pressure, thereby linking the renin-angiotensin system and obesity. These mice are obese and diabetic but have normal 24 h mean arterial pressure. Our results show that plasma and lung ACE activities as well as ACE mRNA expression were significantly decreased in ob/ob mice. In agreement with these findings, the hypotensive effect produced by enalapril administration was attenuated in the obese mice. Plasma renin, angiotensinogen, angiotensin I, bradykinin, and angiotensin 1-7 were increased, whereas plasma angiotensin II concentration was unchanged in obese mice. Chronic infusion of leptin increased renin activity and angiotensin II concentration in both groups and increased ACE activity in ob/ob mice. Acute leptin infusion restored ACE activity in leptin-deficient mice. Moreover, the effect of an ACE inhibitor on blood pressure was not changed in ob/+ mice during leptin treatment but increased four times in obese mice. In summary, our findings show that the renin-angiotensin system is altered in ob/ob mice, with markedly reduced ACE activity, which suggests a possible connection between the renin-angiotensin system and leptin. These results point to an important interplay between the angiotensinergic and the leptinergic systems, which may play a role in the pathogenesis of obesity, hypertension, and metabolic syndrome.

Protective effects of prostaglandin E1 on human umbilical vein endothelial cell injury induced by hydrogen peroxide

AIM

To investigate the protective effects of prostaglandin E(1) (PGE(1)) against H(2)O(2)-induced oxidative damage on human umbilical vein endothelial cells (HUVECs).

METHODS

HUVECs were pretreated with PGE(1) (0.25, 0.50, and 1.00 micromol/L) for 24 h and exposed to H(2)O(2) (200 micromol/L) for 12 h, and cell viability was measured by the MTT assay. LDH, NO, SOD, GSH-Px, MDA, ROS, and apoptotic percentage were determined. eNOS expression was measured by Western blotting and real-time PCR.

RESULTS

PGE(1) (0.25-1.00 micromol/L) was able to markedly restore the viability of HUVECs under oxidative stress, and scavenged intracellular reactive oxygen species induced by H(2)O(2). PGE(1) also suppressed the production of lipid peroxides, such as MDA, restored the activities of endogenous antioxidants including SOD and GSH-Px, and inhibited cell apoptosis. In addition, PGE(1) significantly increased NO content, eNOS protein, and mRNA expression.

CONCLUSION

PGE(1) effectively protected endothelial cells against oxidative stress induced by H(2)O(2), an activity that might depend on the up-regulation of NO expression.

Muscle blood flow responses to dynamic exercise in young obese humans

Exercise is a common nonpharmacological way to combat obesity; however, no studies have systematically tested whether obese humans exhibit reduced skeletal muscle blood flow during dynamic exercise. We hypothesized that exercise-induced blood flow to skeletal muscle would be lower in young healthy obese subjects (body mass index of >30 kg/m(2)) compared with lean subjects (body mass index of <25 kg/m(2)). We measured blood flow (Doppler Ultrasound of the brachial and femoral arteries), blood pressure (auscultation, Finapress), and heart rate (ECG) during rest and two forms of single-limb, steady-state dynamic exercise: forearm exercise (20 contractions/min at 4, 8, and 12 kg) and leg exercise (40 kicks/min at 7 and 14 W). Forearm exercise increased forearm blood flow (FBF) similarly in both groups (P > 0.05; obese subjects n = 9, lean subjects n = 9). When FBF was normalized for perfusion pressure, forearm vascular conductance was not different between groups at increasing workloads (P > 0.05). Leg exercise increased leg blood flow (LBF) similarly in both groups (P > 0.05; obese subjects n = 10, lean subjects n = 12). When LBF was normalized for perfusion pressure, leg vascular conductance was not different between groups at increasing workloads (P > 0.05). These results were confirmed when relative blood flow was expressed at average relative workloads. In conclusion, our results show that obese subjects exhibited preserved FBF and LBF during dynamic exercise.

Resistance to acute NO-mimetic and EDHF-mimetic effects of leptin in the metabolic syndrome

AIMS

We examined mechanisms leading to the impairment of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF)-dependent vasorelaxation in response to acutely administered leptin in rats with the metabolic syndrome.

MAIN METHODS

Effects of leptin on blood pressure and NO and cGMP in the aortic wall were studied in four groups of rats: (1) lean control, (2) obese, fed "cafeteria diet" for 3months (hyperleptinemia and hyperinsulinemia), (3) hyperleptinemia induced by administration of exogenous leptin for 8days, and (4) fructose-fed, receiving 20% fructose in the drinking water for 8weeks (hyperinsulinemia with slightly elevated leptin).

KEY FINDINGS

Stimulatory effect of leptin on NO and cGMP production in the aortic wall was impaired in obese and hyperleptinemic groups but not in the fructose group. In contrast, EDHF-mimetic effect of leptin was impaired in obese and fructose-fed but not in the hyperleptinemic group. Leptin increased tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) in the aortic wall, and this effect was impaired in obese and fructose-fed animals. The EDHF-mimetic effect of leptin was abolished by phosphoinositide 3-kinase inhibitor, wortmannin, whereas its effect on NO was not. In addition, IRS-1 phosphorylation at Ser(307) and Ser(612) was enhanced in obese and fructose-fed but not in hyperleptinemic rats.

SIGNIFICANCE

These results indicate that: (1) long-term hyperleptinemia induces resistance to acute vascular NO-mimetic effect of leptin in obesity/metabolic syndrome, (2) leptin stimulates EDHF in IRS-1 and PI3K-dependent manner, and this effect is impaired in obesity due to excessive serine phosphorylation of IRS-1.

Leptin-stimulated endothelial nitric-oxide synthase via an adenosine 5'-monophosphate-activated protein kinase/Akt signaling pathway is attenuated by interaction with C-reactive protein

The AMP-activated protein kinase (AMPK) lies upstream of Akt in the pathway leading to endothelial NO synthase (eNOS) activation. Whether leptin promotes eNOS activation via AMPK-dependent activation of Akt, and which of the two AMPKalpha catalytic subunits is involved, remains unknown. Leptin resistance may be partly attributed to interaction between leptin and C-reactive protein (CRP). We hypothesized that leptin effect on eNOS activation in human aortic endothelial cells might be blunted by direct interaction with human recombinant CRP. Small interfering RNAs (siRNAs) were used to knock down expression of alpha1- or alpha2-AMPK in transient transfection assay to evaluate which is involved in this pathway and whether leptin effect on eNOS activation in human aortic endothelial cells might be blunted by direct interaction with human CRP. siRNA-mediated down-regulation of AMPKalpha1, but not AMPKalpha2, abolished leptin-induced Akt-Ser(473) phosphorylation, eNOS-Ser(1177) phosphorylation, eNOS activation, and cGMP accumulation. By contrast, siRNA-mediated knockdown of Akt1 did not affect AMPKalpha1 phosphorylation, but it abolished leptin-induced phosphorylation of Akt-Ser(473) and eNOS-Ser(1177), suggesting that Akt functions downstream of AMPKalpha1. Preincubation of leptin with human recombinant CRP impaired leptin-induced AMPK activation, eNOS-Ser(1177) phosphorylation, eNOS activity, and intracellular cGMP accumulation. The data are consistent with a model implicating an AMPKalpha1-->Akt-->eNOS pathway leading to NO production in response to leptin supporting the idea that interaction between leptin and CRP may have a role in impairing leptin effect on eNOS activation, suggesting a link between leptin resistance, low-grade inflammation, and endothelial dysfunction.

Impact of high-protein diets with either moderate or low carbohydrate on weight loss, body composition, blood pressure and glucose tolerance in rats

One approach to achieve weight loss and decrease both obesity and associated morbidities involves high-protein, low-carbohydrate (HPLC) diets. This study compares the impact on metabolic health of HPLC and high-protein, medium-carbohydrate (HPMC) diets offered to diet-induced obese (DIO) rats. Weanling male rats were fed either a 37 % fat diet (n48) or stock pellets (n12) for 22 weeks. Rats fed the 37 % fat diet accumulated more body fat (26·6versus14·8 % body weight,P < 0·001) compared with those on stock diet. The DIO rats had higher systolic blood pressure (+6·6 mmHg,P = 0·002), fasting insulin (+63 %P = 0·006) and areas under the glucose (+21 %,P < 0·001) and insulin (+81 %,P < 0·001) curves following an oral glucose tolerance test. DIO rats were then separated into four groups and offered for 8 weeks either: (1) the 37 % fat diet; (2) an HPLC or (3) HPMC diet; or (4) fed the 37 % fat diet to the intake of the HPMC group. Rats offered the 37 % fat or HPLC diets gained while those on HPMC lost body fat. Blood pressure was not altered by the dietary switch. Both HPLC and HPMC rats had lowered fasting insulin (P = 0·027) and improved homeostatic assessment (HOMA;P = 0·011) that was not different from those of stock animals. These improvements occurred despite differences in fat gain, and indicate that both weight loss and macronutrient intake can impact favourably on obesity-associated morbidities.

The inhibitory effect of leptin on angiotensin II-induced vasoconstriction in vascular smooth muscle cells is mediated via a nitric oxide-dependent mechanism

Leptin inhibits the contractile response induced by angiotensin (Ang) II in vascular smooth muscle cells (VSMCs) of the aorta. We studied in vitro and ex vivo the role of nitric oxide (NO) in the effect of leptin on the Ang II-induced vasoconstriction of the aorta of 10-wk-old Wistar rats. NO and nitric oxide synthase (NOS) activity were assessed by the Griess and (3)H-arginine/citrulline conversion assays, respectively. Stimulation of inducible NOS (iNOS) as well as Janus kinases/signal transducers and activators of transcription (JAK/STAT) and phosphoinositide 3-kinase (PI3K)/Akt signaling pathways were determined by Western blot. The contractile responses to Ang II were evaluated in endothelium-denuded aortic rings using the organ bath system. Changes in intracellular Ca(2+) were measured in VSMCs using fura-2 fluorescence. Leptin significantly (P < or = 0.01) stimulated NO release and NOS activity in VSMCs. Leptin's effect on NO was abolished by the NOS inhibitor, N(G)-monomethyl l-arginine, or the iNOS selective inhibitor L-N(6)-(1-iminoethyl)-lysine. Accordingly, leptin increased iNOS protein expression, with a comparable time course with that of NO production and NOS activity. Leptin also significantly increased STAT3 (P < or = 0.01) and Akt (P < or = 0.001) phosphorylation. Moreover, either the JAK2 inhibitor, AG490, or the PI3K inhibitor, wortmannin, significantly (P < or = 0.05) abrogated the leptin-induced increase in iNOS protein. Finally, both N(G)-monomethyl L-arginine and L-N(6)-(1-iminoethyl)-lysine inhibitors completely blunted (P < or = 0.001) the leptin-mediated inhibition of the Ang II-induced VSMC activation and vasoconstriction. These findings suggest that the endothelium-independent depressor action of leptin is mediated by an increase of NO bioavailability in VSMCs. This process requires the up-regulation of iNOS through mechanisms involving JAK2/STAT3 and PI3K/Akt pathways.

Pharmacological characteristics of endothelium-derived hyperpolarizing factor-mediated relaxation of small mesenteric arteries from db/db mice

Endothelial dysfunction is considered as a major risk factor of cardiovascular complications of type I and type II diabetes. Our previous studies have demonstrated that endothelial dysfunction in the small mesenteric arteries from 12-16 week old type II diabetic mice was associated with decreased bio-availability of nitric oxide whereas endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation was preserved. The objective of the present study was to characterize EDHF-mediated relaxations of small mesenteric arteries from db/db mice. A depolarizing concentration of KCl or tetraethylammonium (TEA, 10 mM) significantly inhibited the EDHF-mediated relaxation to acetylcholine and bradykinin in small mesenteric arteries from both db/+ and db/db mice. Charybdotoxin or iberiotoxin alone and a combination of ouabain and barium significantly reduced the maximal relaxation to acetylcholine in small mesenteric arteries from db/db mice and charybdotoxin or iberiotoxin either alone or in combination with apamin reduced the sensitivity to the EDHF-mediated component of the relaxation response to bradykinin. 17-octadecynoic acid, but not catalase, significantly reduced the sensitivity to EDHF-mediated responses to bradykinin in db/db mice; 17-octadecynoic acid had no effect on acetylcholine-mediated relaxations. No differences were, however, detected for mRNA expression levels of calcium-activated potassium channels or connexins 37, 40, 43 and 45. Collectively, these data suggest that bradykinin-induced, EDHF-dependent relaxation in small mesenteric arteries from db/db mice is mediated via cytochrome P450 product that activates the large conductance calcium-activated potassium (BK(Ca) or Slo) channel, whereas the acetylcholine-induced, EDHF-mediated relaxation involves neither cytochrome P450 product nor hydrogen peroxide.