Diabetes-related changes in cAMP-dependent protein kinase activity and decrease in relaxation response in rat mesenteric artery

Cardiac Phosphodiesterases Are Differentially Increased in Diabetic Cardiomyopathy

AIM

Diabetic cardiomyopathy (DCM) accomodates a spectrum of cardiac abnormalities. This study aims to investigate whether DCM is associated with changes in cyclic adenosine 3'-5' monophosphate (cAMP) signaling, particularly cyclic nucleotide phosphodiesterases (PDEs).

MAIN METHODS

Type 1 diabetes (T1D) was induced in rats by streptozotocin (STZ, 65 mg/kg) injection. Myocardial remodeling, structure and function were evaluated by histology and echocardiography, respectively. We delineated the sequential changes affecting cAMP signaling and characterized the expression pattern of the predominant cardiac PDE isoforms (PDE 1-5) and β-adrenergic (β-AR) receptors at 4, 8 and 12 weeks following diabetes induction, by real-time quantitative PCR and Western blot. cAMP levels were measured by immunoassays.

KEY FINDINGS

T1D-induced DCM was associated with cardiac remodeling, steatosis and fibrosis. Upregulation of β₁-AR receptor transcripts was noted in diabetic hearts at 4 weeks along with an increase in cAMP levels and an upregulation in the ejection fraction and fraction shortening. However, β₂-AR receptors expression remained unchanged regardless of the disease stage. Moreover, we noted an early and specific upregulation of cardiac PDE1A, PDE2A, PDE4B, PDE4D and PDE5A expression at week 4, followed by increases in PDE3A levels in diabetic hearts at week 8. However, DCM was not associated with changes in PDE4A gene expression irrespective of the disease stage.

SIGNIFICANCE

We show for the first time differential and time-specific regulations in cardiac PDEs, data that may prove useful in proposing new therapeutic approaches in T1D-induced DCM.

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.

Enhanced Integrin α4β1-Mediated Adhesion Contributes to a Mobilization Defect of Endothelial Progenitor Cells in Diabetes

Diabetes is associated with a deficit of circulating endothelial progenitor cells (EPCs), which has been attributed to their defective mobilization from the bone marrow. The basis for this mobilization defect is not completely understood, and we sought to determine if hyperglycemic conditions enhanced EPC adhesion. We found that culturing EPCs in high glucose media increased adhesion to bone marrow stromal cells. This enhanced adhesion was associated with decreased expression of protein kinase A regulatory subunit 1β (PRKAR1β), activation of protein kinase A (PKA), and phosphorylation of α4-integrin on serine 988. This potentiated adhesion was reversed by treatment with a PKA inhibitor, overexpression of PRKAR1β, or expression of a phosphorylation-defective α4-integrin variant (α4[S988A]). Using a model of type 1 diabetes, we showed that α4(S988A)-expressing mice have more circulating EPCs than their wild-type counterparts. Moreover, diabetic α4(S988A) mice demonstrate enhanced revascularization after hind limb ischemia. Thus, we have identified a novel signaling mechanism activating PKA in diabetes (downregulation of an inhibitory regulatory subunit) that leads to deficits of circulating EPCs and impaired vascular repair, which could be reversed by α4-integrin mutation.

Kv7 channels critically determine coronary artery reactivity: left-right differences and down-regulation by hyperglycaemia

AIMS

Voltage-gated potassium channels encoded by KCNQ genes (Kv7 channels) are emerging as important regulators of vascular tone. In this study, we analysed the contribution of Kv7 channels to the vasodilation induced by hypoxia and the cyclic AMP pathway in the coronary circulation. We also assessed their regional distribution and possible impairment by diabetes.

METHODS AND RESULTS

We examined the effects of Kv7 channel modulators on K+ currents and vascular reactivity in rat left and right coronary arteries (LCAs and RCAs, respectively). Currents from LCA were more sensitive to Kv7 channel inhibitors (XE991, linopirdine) and activators (flupirtine, retigabine) than those from RCA. Accordingly, LCAs were more sensitive than RCAs to the relaxation induced by Kv7 channel enhancers. Likewise, relaxation induced by the adenylyl cyclase activator forskolin and hypoxia, which were mediated through Kv7 channel activation, were greater in LCA than in RCA. KCNQ1 and KCNQ5 expression was markedly higher in LCA than in RCA. After incubation with high glucose (HG, 30 mmol/L), myocytes from LCA, but not from RCA, were more depolarized and showed reduced Kv7 currents. In HG-incubated LCA, the effects of Kv7 channel modulators and forskolin were diminished, and the expression of KCNQ1 and KCNQ5 was reduced. Finally, vascular responses induced by Kv7 channel modulators were impaired in LCA, but not in RCA, from type 1 diabetic rats.

CONCLUSION

Our results reveal that the high expression and function of Kv7 channels in the LCA and their down-regulation by diabetes critically determine the sensitivity to key regulators of coronary tone.

Mechanisms underlying reduced P2Y(1) -receptor-mediated relaxation in superior mesenteric arteries from long-term streptozotocin-induced diabetic rats

AIM

Extracellular nucleotides activate cell-surface purinergic (P2) receptors, contribute to the local regulation of vascular tone and play important roles in pathophysiological states. However, little is known about the vasodilator effects of P2Y(1) -receptor activation in diabetic states. We hypothesized that in a model of established type 1 diabetes, long-term streptozotocin (STZ)-induced diabetic rats, the arterial relaxation elicited by a P2Y(1) -receptor agonist would be impaired.

METHODS

Relaxations to adenosine 5'-diphosphate sodium salt (ADP), 2-MeSADP (selective P2Y(1) -receptor agonist) and adenosine 5'-triphosphate disodium salt (ATP) were examined in superior mesenteric artery rings from long-term STZ-induced diabetic rats (at 50-57 weeks after STZ injection). ADP-stimulated nitric oxide (NO) production in the superior mesenteric artery was assessed by measuring the levels of NO metabolites. Mesenteric artery expressions of P2Y(1) receptor, and ADP-stimulated levels of phosphorylated endothelial NO synthase (eNOS) (at Ser(1177) and at Thr(495) ) and eNOS were detected by Western blotting.

RESULTS

Arteries from diabetic rats exhibited (vs. those from age-matched control rats): (i) reduced ADP-induced relaxation, which was partly or completely inhibited by endothelial denudation, by NOS inhibitor treatment and by a selective P2Y(1) -receptor antagonist, (ii) reduced 2-MeSADP-induced relaxation, (iii) reduced ADP-stimulated release of NO metabolites and (iv) impaired ADP-induced stimulation of eNOS activity (as evidenced by reduced the fold increase in eNOS phosphorylation at Ser(1177) with no difference in fold increase in eNOS phosphorylation at Thr(495) ). The protein expression of P2Y(1) receptor did not differ between diabetic and control arteries.

CONCLUSIONS

These results suggest that P2Y(1) -receptor-mediated vasodilatation is impaired in superior mesenteric arteries from long-term type 1 diabetic rats. This impairment is because of reduced P2Y(1) -receptor-mediated NO signalling, rather than to reduced P2Y(1) -receptor expression.

Differential activations of PKC/PKA related to microvasculopathy in diabetic GK rats

Microvasculopathy is the most serious and predictable threat to the health of diabetic patients, which often results in end-stage renal disease, blindness, and limb amputations. Up to the present, the underlying mechanisms have remained elusive. Here, it was found that the differential activations of PKC/PKA were involved in diabetic microvasculopathy in diabetic GK rats. By real-time PCR, Western blot, immunohistochemistry, and enzyme activity assay, upregulation of PKC was prominent in kidney but was not significant in liver and brain. The expression and activity of PKA were lowered in kidney but comparable in brain and liver during diabetic nephropathy. Furthermore, the generation of reactive oxygen species, production of nitric oxide, and expression of inducible nitric oxide synthase induced by advanced glycation end products were inhibited by PKCβ inhibitor LY-333531 or a PKA agonist in rat glomerular microvascular endothelial cells. Finally, albuminuria was significantly lowered by a PKA agonist and boosted by a PKA antagonist. It suggested that the differential activations of PKC/PKA related to microvasculopathy in diabetes and that activation of PKA may protect the diabetic microvasculature.

Principal role of adenylyl cyclase 6 in K⁺ channel regulation and vasodilator signalling in vascular smooth muscle cells

AIMS

Membrane potential is a key determinant of vascular tone and many vasodilators act through the modulation of ion channel currents [e.g. the ATP-sensitive potassium channel (K(ATP))] involved in setting the membrane potential. Adenylyl cyclase (AC) isoenzymes are potentially important intermediaries in such vasodilator signalling pathways. Vascular smooth muscle cells (VSMCs) express multiple AC isoenzymes, but the reason for such redundancy is unknown. We investigated which of these isoenzymes are involved in vasodilator signalling and regulation of vascular ion channels important in modulating membrane potential.

METHODS AND RESULTS

AC isoenzymes were selectively depleted (by >75%) by transfection of cultured VSMCs with selective short interfering RNA sequences. AC6 was the predominant isoenzyme involved in vasodilator-mediated cAMP accumulation in VSMCs, accounting for ∼60% of the total response to β-adrenoceptor (β-AR) stimulation. AC3 played a minor role in β-AR signalling, whereas AC5 made no significant contribution. AC6 was also the principal isoenzyme involved in β-AR-mediated protein kinase A (PKA) signalling (determined using the fluorescent biosensor for PKA activity, AKAR3) and the substantial β-AR/PKA-dependent enhancement of K(ATP) current. K(ATP) current was shown to play a vital role in setting the resting membrane potential and in mediating the hyperpolarization observed upon β-AR stimulation.

CONCLUSION

AC6, but not the closely related AC5, plays a principal role in vasodilator signalling and regulation of the membrane potential in VSMCs. These findings identify AC6 as a vital component in the vasodilatory apparatus central to the control of blood pressure.

Vasodilator effect of Cassiarin A, a novel antiplasmodial alkaloid from Cassia siamea, in rat isolated mesenteric artery

The aim of this study was to investigate the vasorelaxant effect induced by cassiarin A, a novel antiplasmodial alkaloid from Cassia siamea, in rings cut from rat superior mesenteric arteries. In rings precontracted with phenylephrine, cassiarin A induced a concentration-dependent relaxation. This relaxation was attenuated: 1) after removal of the endothelium or after pretreatment of rings with 100 microM of N(G)-nitro-L-arginine (nitric oxide synthase inhibitor) or 10 microM of 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (guanylyl cyclase inhibitor), but not after pretreatment with 10 microM of indomethacin (cyclooxygenase inhibitor); and 2) after pretreatment of preparations with either a nonselective or selective inhibitor of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels [1 mM of tetraethylammonium or 100 nM of iberiotoxin, respectively]. The cassiarin A-induced relaxation was also attenuated by these BK(Ca) inhibitors in endothelium-denuded preparations. The cassiarin A-induced relaxation was not altered by treatment with the ATP-sensitive K(+)-channel inhibitor glibenclamide (10 microM) or with the voltage-dependent K(+)-channel inhibitor 4-aminopyridine (1 mM). In isolated mesenteric artery rings, cassiarin A tended to increase nitric oxide (NO) levels. These results suggest that in the rat mesenteric artery, cassiarin A-induced relaxation may be mediated by endothelial NO and may occur partly via BK(Ca)-channel activation.

[Therapeutic target for endothelium-derived hyperpolarizing factor signaling in diabetic vascular complication]

Vascular tone is tightly regulated by endothelium-derived factors. These include relaxing factors (EDRFs) such as nitric oxide (NO), hyperpolarizing factors (EDHFs), and contracting factors (EDCFs). Although EDHF is a prominent vasodilator, particularly in smaller arteries, little attention has been paid to the potential role of EDHF responses in diabetes. EDHF signaling may involve various factors, including several diffusible factors and non-diffusible factors (e.g., gap junctions). It has been demonstrated that the alterations in EDHF relaxation seen in mesenteric arteries from diabetic rats may be attributable to an increase in phosphodiesterase 3 (PDE3) activity, leading to a reduction in the action of adenosine 3',5'-cyclic monophosphate (cAMP), and consequently the activity of protein kinase A (PKA) is decreased in such arteries. Moreover, it has been suggested that the imbalance between EDRFs and EDCFs is present in mesenteric arteries from type 2 diabetic rats and the EDHF relaxation can be partly reversed by suppression of EDCF signaling. Indeed, chronic treatment with metformin, eicosapentaenoic acid, or thromboxane synthase inhibitor can reduce EDCF signaling and normalize EDHF signaling in mesenteric arteries from type 2 diabetic rats. Although the improvement or restoration of EDHF responses has not been the direct subject of any pharmaceutical effort, increasing cAMP/PKA signaling (e.g., by inhibiting PDE3 activity) or reducing EDCFs signaling has potential as an interesting therapeutic target in diabetic vasculopathy.

Connexins and gap junctions in the EDHF phenomenon and conducted vasomotor responses

It is becoming increasingly evident that electrical signaling via gap junctions plays a central role in the physiological control of vascular tone via two related mechanisms (1) the endothelium-derived hyperpolarizing factor (EDHF) phenomenon, in which radial transmission of hyperpolarization from the endothelium to subjacent smooth muscle promotes relaxation, and (2) responses that propagate longitudinally, in which electrical signaling within the intimal and medial layers of the arteriolar wall orchestrates mechanical behavior over biologically large distances. In the EDHF phenomenon, the transmitted endothelial hyperpolarization is initiated by the activation of Ca(2+)-activated K(+) channels channels by InsP(3)-induced Ca(2+) release from the endoplasmic reticulum and/or store-operated Ca(2+) entry triggered by the depletion of such stores. Pharmacological inhibitors of direct cell-cell coupling may thus attenuate EDHF-type smooth muscle hyperpolarizations and relaxations, confirming the participation of electrotonic signaling via myoendothelial and homocellular smooth muscle gap junctions. In contrast to isolated vessels, surprisingly little experimental evidence argues in favor of myoendothelial coupling acting as the EDHF mechanism in arterioles in vivo. However, it now seems established that the endothelium plays the leading role in the spatial propagation of arteriolar responses and that these involve poorly understood regenerative mechanisms. The present review will focus on the complex interactions between the diverse cellular signaling mechanisms that contribute to these phenomena.

Short-term angiotensin-1 receptor antagonism in type 2 diabetic Goto-Kakizaki rats normalizes endothelin-1-induced mesenteric artery contraction

Endothelin (ET)-1 and angiotensin II (Ang II) are likely candidates for a key role in diabetic vascular complications. We demonstrated previously that an enhanced ET-1-induced contraction is present in mesenteric arteries from Goto-Kakizaki (GK) rats at the chronic stage of type 2 diabetes. Here, we investigated whether short-term treatment of such rats with losartan, an angiotensin type 1 receptor antagonist, might normalize the ET-1-induced contraction. In mesenteric arteries from GK rats at the chronic stage (34-38 weeks) (vs. those from age-matched control Wistar rats): (1) the ET-1-induced contraction was enhanced, (2) the levels of ET-1 and Ang II were increased, (3) ET-1-stimulated ERK2 phosphorylation was increased, and (4) the ACh-induced endothelium-dependent relaxation was reduced. Mesenteric arteries isolated from such GK rats following treatment with losartan (25mg/kg/day for 2 weeks) exhibited reduced ET-1- and Ang II-induced contractions, suppressed ET-1-stimulated ERK phosphorylation, and increased ACh-induced relaxation, while the rats exhibited normalized plasma NO metabolism and their mesenteric arteries exhibited increased basal NO formation. However, such losartan treatment did not alter the increased levels of ET-1 and Ang II seen in GK mesenteric arteries. Our data suggest that within the timescale studied here, losartan normalizes ET-1-induced mesenteric artery contraction through a suppression of ERK activities and/or by normalizing endothelial function.

Physical activity maintains aortic endothelium-dependent relaxation in the obese type 2 diabetic OLETF rat

We tested the hypothesis that physical activity can attenuate the temporal decline of ACh-induced endothelium-dependent relaxation during type 2 diabetes mellitus progression in the Otsuka Long-Evans Tokushima fatty (OLETF) rat. Sedentary OLETF rats exhibited decreased ACh-induced abdominal aortic endothelium-dependent relaxation from 13 to 20 wk of age (20-35%) and from 13 to 40 wk of age (35-50%). ACh-induced endothelium-dependent relaxation was maintained in the physically active OLETF group and control sedentary Long-Evans Tokushima Otsuka (LETO) group from 13 to 40 wk of age. Aortic pretreatment with N(G)-nitro-l-arginine (l-NNA), indomethacin (Indo), and l-NNA + Indo did not alter the temporal decline in ACh-induced endothelium-dependent relaxation. Temporal changes in the protein expression of SOD isoforms in the aortic endothelium or smooth muscle did not contribute to the temporal decline in ACh-induced endothelium-dependent relaxation in sedentary OLETF rats. A significant increase in the 40-wk-old sedentary LETO and physically active OLETF rat aortic phosphorylated endothelial nitric oxide (p-eNOS)-to-eNOS ratio was observed versus 13- and 20-wk-old rats in each group that was not seen in the 40- versus 13- and 20-wk-old sedentary OLETF rats. These results suggest that temporal changes in the antioxidant system, EDHF, and cycloxygenase metabolite production in sedentary OLETF rat aortas do not contribute to the temporal decline in sedentary OLETF rat aortic ACh-induced endothelium-dependent relaxation seen with type 2 diabetes mellitus progression. We also report that physical activity in conjunction with aging in the OLETF rat results in a temporal increase in the aortic endothelial p-eNOS-to-eNOS ratio that was not seen in sedentary OLETF rats. These results suggest that the sustained aortic ACh-induced endothelium-dependent relaxation in aged physically active OLETF rats may be the result of an increase in active aortic eNOS.

Diabetes-associated changes and role of N epsilon-(carboxymethyl)lysine in big ET-1-induced coronary vasoconstriction

Using perfused hearts from streptozotocin-induced long-term diabetic rats, we studied the coronary vasoconstrictor effect of the endothelin-1 (ET-1) precursor big ET-1 and also whether this response was modulated by N(epsilon)-(carboxymethyl)lysine (CML; a representative advanced glycation end product that is implicated in the pathogenesis of diabetic vasculopathy). The big ET-1-induced vasoconstriction (a) developed more rapidly (i.e., was greater in the first 30 min) in the diabetic group than in the age-matched controls, and (b) in each group was largely suppressed by phosphoramidon [nonselective endothelin-converting enzyme (ECE)/neutral endopeptidase (NEP) inhibitor] or CGS35066 (selective ECE inhibitor), but not by thiorphan (selective NEP inhibitor). The ET-1 release occurring after treatment with big ET-1, which was greater in diabetic coronary arteries than in the controls, was reduced by CGS35066. The dose-response curve for ET-1 was shifted to the left in the diabetics, so that at some lower doses of ET-1 the vasoconstriction was greater than in the controls. CML enhanced big ET-1- or ET-1-induced vasoconstriction in the controls, but not in the diabetics. Finally, the plasma level of CML was higher in diabetic than in control rats. These findings suggest (a) that the increased responsiveness to big ET-1 shown by diabetic coronary arteries may be attributable both to a more rapid conversion of big ET-1 to ET-1 (by ECE), allowing it to exert its contractile activity, and to an increased vascular sensitivity to ET-1, and (b) that CML may be at least partly responsible for the diabetes-associated enhancement of big ET-1-mediated coronary vasoconstriction.

Effect of N-epsilon-(carboxymethyl)lysine on coronary vasoconstriction in isolated perfused hearts from control and streptozotocin-induced diabetic rats

Advanced glycation end products (AGEs) derived from glucose are implicated in the pathogenesis of diabetic vascular disease. However, their direct modulatory effects on coronary vascular tone remain unclear. We previously reported that coronary vasoconstriction was induced by acetylcholine (ACh) infusion of the isolated perfused rat heart and that sensitivity was greater in perfused hearts from streptozotocin (STZ)-induced diabetic rats than in those from age-matched controls (Kamata et al., 2008). Here, we investigated the effect of N(epsilon)-(carboxymethyl)lysine (CML), which has one of the main AGE structures, on ACh-induced vasoconstriction in perfused hearts isolated from control and diabetic rats. ACh-induced vasoconstriction was significantly greater in the STZ-induced diabetic group than in the age-matched controls. CML enhanced the ACh-induced vasoconstriction in coronary arteries from control rats, but not in those from STZ-induced diabetic rats. In the controls, the vasoconstriction induced by the calcium-channel activator Bay K 8644 was also enhanced by CML. These CML-mediated enhancements of the vasoconstrictions induced by ACh and Bay K 8644 were significantly suppressed by tempol, a superoxide dismutase mimetic. The plasma CML and glucose levels were each significantly elevated in STZ-induced diabetic rats. These findings suggest (a) that CML augments ACh-induced coronary vasoconstriction, an effect that may be attributable to increased superoxide and to activation of voltage-gated Ca(2+) channels and (b) that this modulating effect may be desensitized in the STZ-induced diabetic heart.

Mechanisms underlying enhanced vasorelaxant response to protease-activated receptor 2-activating peptide in type 2 diabetic Goto-Kakizaki rat mesenteric artery

Protease-activated receptor 2 (PAR2) is a G-protein-coupled receptor that is proteolytically activated by certain endogenous proteases, such as trypsin, tryptase, and factor Xa. PAR2 can also be activated by synthetic peptides if their sequence mimics the tethered ligand exposed after receptor cleavage. Although it is known that PAR2 modulates vascular reactivity, it is unclear whether at the chronic stage of type 2 diabetes there are alterations in PAR2-mediated vascular responses. We investigated this issue by exposing mesenteric artery rings to PAR2-activating peptide (PAR2-AP; SLIGRL-NH(2)), the arteries used being obtained from later-stage (32-40-week-old) type 2 diabetic Goto-Kakizaki (GK) rats. The PAR2-AP-induced relaxation was enhanced in GK rats (vs. age-matched Wistar rats), whereas the ACh-induced relaxation was weaker in GK than in Wistar rats. In both groups, the PAR2-AP-induced relaxation was largely blocked by endothelial denudation or by N(G)-nitro-L-arginine [nitric oxide (NO) synthase inhibitor] treatment, but it was unaffected by indomethacin (cyclooxygenase inhibitor) treatment. Both the NO production induced by PAR2-AP and the PAR2 protein expression were significantly increased in mesenteric arteries from GK rats (vs. Wistar rats). These data are the first to indicate that the PAR2-AP-induced endothelium-dependent relaxation is enhanced in mesenteric arteries isolated from type 2 diabetic GK rats at the chronic stage, and they further suggest that the enhancement may be due to an increased expression of PAR2 receptors in this artery.

Involvement of NO and MEK/ERK pathway in enhancement of endothelin-1-induced mesenteric artery contraction in later-stage type 2 diabetic Goto-Kakizaki rat

Endothelin (ET)-1 is a likely candidate for a key role in diabetic vascular complications. However, no abnormalities in the vascular responsiveness to ET-1 have been identified in the chronic stage of type 2 diabetes. Our goal was to look for abnormalities in the roles played by ET receptors (ETA and ETB) in the mesenteric artery of the type 2 diabetic Goto-Kakizaki (GK) rat and to identify the molecular mechanisms involved. Using mesenteric arteries from later-stage (32–38 wk old) individuals, we compared the ET-1-induced contraction and the relaxation induced by the selective ETB receptor agonist IRL1620 between GK rats and control Wistar rats. Mesenteric artery ERK activity and the protein expressions for ET receptors and MEK were also measured. In GK rats (vs. age-matched Wistar rats), we found as follows. 1) The ET-1-induced contraction was greater and was attenuated by BQ-123 (ETA antagonist) but not by BQ-788 (ETB antagonist). In the controls, BQ-788 augmented this contraction. 2) Both the relaxation and nitric oxide (NO) production induced by IRL1620 were reduced. 3) ET-1-induced contraction was enhanced by NG-nitro-l-arginine (l-NNA; NO synthase inhibitor) but suppressed by sodium nitroprusside (NO donor). 4) The enhanced ET-1-induced contraction was reduced by MEK/ERK pathway inhibitors (PD-98059 or U0126). 5) ET-1-stimulated ERK activation was increased, as were the ETA and MEK1/2 protein expressions. 6) Mesenteric ET-1 content was increased. These results suggest that upregulation of ETA, a defect in ETB-mediated NO signaling, and activation of the MEK/ERK pathway together represent a likely mechanism mediating the hyperreactivity to ET-1 examined in this study.

The renal protective effects of cilostazol on suppressing pathogenic thrombospondin-1 and transforming growth factor-beta expression in streptozotocin-induced diabetic rats

Diabetic nephropathy is a major complication facing patients with diabetes mellitus. The renal protective effects of the phosphodiesterase III inhibitor, cilostazol, were investigated in rats with streptozotocin-induced diabetes. Expression of thrombospondin-1 (TSP-1) and transforming growth factor-beta (TGF-beta) in the kidney was measured by immunohistochemistry and real-time reverse transcription-quantitative polymerase chain reaction analysis in diabetic rats, cilostazol-treated diabetic rats and control rats. Ultrastructural changes in the kidney were also analysed using microscopy. Four weeks after the induction of diabetes, TSP-1 and TGF-beta expression was significantly increased in the kidneys of diabetic rats compared with the control and was significantly lower in cilostazol-treated diabetic rats than in the untreated diabetic rats. Microscopy revealed characteristic renal pathology in the diabetic group, which was rarely seen in the cilostazol-treated diabetic rats. In conclusion, this study indicates that cilostazol treatment of diabetic rats effectively prevents pathological kidney changes, possibly via the down-regulation of TSP-1 and TGF-beta expression compared with untreated rats.

Altered mechanism of adenosine-induced coronary arteriolar dilation in early-stage metabolic syndrome

Onset of the combined metabolic syndrome (MetS) is a complex progressive process involving numerous cardiovascular risk factors. Although patients with established MetS exhibit reduced coronary flow reserve and individual components of the MetS reduce microvascular vasodilation, little is known concerning the impact of early-stage MetS on the mechanisms of coronary flow control. Therefore, we tested the hypothesis that coronary arteriolar dilation to adenosine is attenuated in early-stage MetS by reduced A2 receptor function and diminished K+ channel involvement. Pigs were fed control or high-fat/cholesterol diet for 9 weeks to induce early-stage MetS. Coronary atheroma was determined in vivo with intravascular ultrasound. In vivo coronary dilation was determined by intracoronary adenosine infusion. Further, apical coronary arterioles were isolated, cannulated and pressurized to 60 cmH2O for in vitro pharmacologic assessment of adenosine dilation. Coronary atheroma was not different between groups, indicating early-stage MetS. Coronary arteriolar dilation to adenosine (in vivo) and 2-chloroadenosine (2-CAD; in vitro) was similar between groups. In control arterioles, 2-CAD-mediated dilation was reduced only by selective A(2A) receptor inhibition, whereas only dual A(2A/2B) inhibition reduced this response in MetS arterioles. Arteriolar A(2B), but not A(2A), receptor protein expression was reduced by MetS. Blockade of voltage-dependent K+ (K(v)) channels reduced arteriolar sensitivity to 2-CAD in both groups, whereas ATP-sensitive K+ (K(ATP)) channel inhibition reduced sensitivity only in control arterioles. Our data indicate that the mechanisms mediating coronary arteriolar dilation to adenosine are altered in early-stage MetS prior to overt decrements in coronary vasodilator reserve.

WITHDRAWN: Effect of cilostazol in alleviating cardiovascular complications through regulation of type 1 plasminogen activator inhibitor and transforming growth factor-β1 overexpression in experimental rats

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Eicosapentaenoic acid improves imbalance between vasodilator and vasoconstrictor actions of endothelium-derived factors in mesenteric arteries from rats at chronic stage of type 2 diabetes

Accumulating evidence demonstrates that dietary intake of n-3 polyunsaturated fatty acids (PUFAs) is associated with a reduced incidence of several cardiovascular diseases that involve endothelial dysfunction. However, the molecular mechanism remains unclear. We previously reported that mesenteric arteries from type 2 diabetic Otsuka Long-Evans Tokushima fatty (OLETF) rats exhibit endothelial dysfunction, leading to an imbalance between endothelium-derived vasodilators [namely, nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF)] and vasoconstrictors [endothelium-derived contracting factors (EDCFs)] [namely cyclooxygenase (COX)-derived prostanoids] (Am J Physiol Heart Circ Physiol 293:H1480-H1490, 2007). We hypothesized that treating OLETF rats with eicosapentaenoic acid (EPA), a major n-3 PUFA, may improve endothelial dysfunction by correcting this imbalance. In OLETF rats [compared with age-matched control Long-Evans Tokushima Otsuka (LETO) rats]: 1) acetylcholine (ACh)-induced (endothelium-dependent) relaxation was impaired, 2) NO- and EDHF-mediated relaxations and nitrite production were reduced, and 3) ACh-induced EDCF-mediated contraction, production of prostanoids, and the protein expressions of COX-1 and COX-2 were all increased. When OLETF rats received chronic EPA treatment long-term (300 mg/kg/day p.o. for 4 weeks), their isolated mesenteric arteries exhibited: 1) improvements in ACh-induced NO- and EDHF-mediated relaxations and COX-mediated contraction, 2) reduced EDCF- and arachidonic acid-induced contractions, 3) normalized NO metabolism, 4) suppressed production of prostanoids, 5) reduced COX-2 expression, and 6) reduced phosphoextracellular signal-regulated kinase (ERK) expression. Moreover, EPA treatment reduced both ERK2 and nuclear factor (NF)-kappaB activities in isolated OLETF aortas. We propose that EPA ameliorates endothelial dysfunction in OLETF rats by correcting the imbalance between endothelium-derived factors, at least partly, by inhibiting ERK, decreasing NF-kappaB activation, and reducing COX-2 expression.

Cilostazol improves endothelial dysfunction by increasing endothelium-derived hyperpolarizing factor response in mesenteric arteries from Type 2 diabetic rats

Diabetes mellitus impairs endothelial function, an effect that can be considered a hallmark of the development of cardiovascular diseases in diabetics. Cilostazol, a selective phosphodiesterase 3 inhibitor, is currently used to treat patients with diabetic vascular complications. However, the effects of cilostazol on responses mediated by endothelium-derived relaxing [in particular, nitric oxide (NO) and hyperpolarizing factors (EDHF)] and contracting factors remain unclear. Here, we hypothesized that cilostazol could improve endothelial dysfunctions in mesenteric arteries isolated from type 2 diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Using cilostazol-treated (100 mg/kg/day for 4 weeks) or -untreated OLETF and control (Long Evans Tokushima Otsuka) rats, we examined the acetylcholine-induced endothelium-dependent responses and the cell-permeant cyclic adenosine monophosphate (cAMP) analog-induced relaxations in the superior mesenteric artery. We also determined blood parameters in these animals. In OLETF rats, chronic treatment with cilostazol reduced the blood levels of triglyceride, non-esterified fatty acids, and leptin, and increased antioxidant capacity, but did not alter the blood glucose or insulin levels. In studies on mesenteric arteries from cilostazol-treated OLETF animals, the cilostazol treatment improved: (a) the acetylcholine-induced EDHF-mediated relaxation and (b) the cAMP-mediated relaxation. However, cilostazol did not alter the NO-mediated relaxation or the endothelium-derived contracting factor-mediated contraction. These results suggest that cilostazol improves endothelial functions in OLETF mesenteric arteries by increasing EDHF signaling, and that it normalizes some metabolic abnormalities in OLETF rats. On that basis, cilostazol may prove to be a potent drug for the clinical treatment of diabetic vasculopathy.

Metformin normalizes endothelial function by suppressing vasoconstrictor prostanoids in mesenteric arteries from OLETF rats, a model of type 2 diabetes

We previously reported that in mesenteric arteries from aged Otsuka Long-Evans Tokushima fatty (OLETF) rats (a type 2 diabetes model) endothelium-derived hyperpolarizing factor (EDHF)-type relaxation is impaired while endothelium-derived contracting factor (EDCF)-mediated contraction is enhanced (Matsumoto T, Kakami M, Noguchi E, Kobayashi T, Kamata K. Am J Physiol Heart Circ Physiol 293: H1480-H1490, 2007). Here we investigated whether acute and/or chronic treatment with metformin might improve this imbalance between the effects of the above endothelium-derived factors in mesenteric arteries isolated from OLETF rats. In acute studies on OLETF mesenteric arteries, ACh-induced relaxation was impaired and the relaxation became weaker at high ACh concentrations. Both metformin and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside [AICAR, an AMP-activated protein kinase (AMPK) activator that is also activated by metformin] 1) diminished the tendency for the relaxation to reverse at high ACh concentrations and 2) suppressed both ACh-induced EDCF-mediated contraction and ACh-stimulated production of prostanoids (thromboxane A2 and PGE2). In studies on OLETF arteries from chronically treated animals, metformin treatment (300 mg.kg(-1).day(-1) for 4 wk) 1) improved ACh-induced nitric oxide- or EDHF-mediated relaxation and cyclooxygenase (COX)-mediated contraction, 2) reduced EDCF-mediated contraction, 3) suppressed production of prostanoids, and 4) reduced superoxide generation. Metformin did not alter the protein expressions of endothelial nitric oxide synthase (eNOS), phospho-eNOS (Ser1177), or COX-1, but it increased COX-2 protein. These results suggest that metformin improves endothelial functions in OLETF mesenteric arteries by suppressing vasoconstrictor prostanoids and by reducing oxidative stress. Our data suggest that within the timescale studied here, metformin improves endothelial function through this direct mechanism, rather than by improving metabolic abnormalities.

Imbalance between endothelium-derived relaxing and contracting factors in mesenteric arteries from aged OLETF rats, a model of Type 2 diabetes

We investigated whether the balance between endothelium-derived relaxing factors (EDRFs) and endothelium-derived contracting factors (EDCFs) might be altered in mesenteric arteries from aged Otsuka Long-Evans Tokushima Fatty (OLETF) rats (a Type 2 diabetic model) [vs. age-matched control Long-Evans Tokushima Otsuka (LETO) rats]. ACh-induced relaxation was impaired in the OLETF group, and a tendency for the relaxation to reverse at high ACh concentrations was observed in both groups. This tendency was abolished by indomethacin. Nitric oxide- and/or endothelium-derived hypolarizing factor-mediated relaxation and the protein expressions of phospho-endothelial nitric oxide synthase (Ser1177) and extracellular superoxide dismutase were also reduced in OLETF. An ACh-induced contraction was observed at higher ACh concentrations in the presence of NG-nitro-l-arginine (l-NNA) but was greater in OLETF rats. This contraction in OLETF rats was reduced by cyclooxygenase (COX) inhibitors and by prostanoid-receptor antagonists. The ACh-induced productions of thromboxane A2 and PGE2 were greater in OLETF than LETO rats, as were the mesenteric artery COX-1 and COX-2 protein expressions. Moreover, tert-butyl hydroperoxide ( t-BOOH) (membrane-permeant oxidant) induced a concentration-dependent contraction that was greater in OLETF rats. The t-BOOH-mediated contraction was increased both by l-NNA and by endothelium removal in LETO but not OLETF rats, suggesting that a negative modulatory role of the endothelium was lost in OLETF rats. These results suggest that an imbalance between EDRFs and EDCFs may be implicated in the endothelial dysfunction seen in aged OLETF mesenteric arteries, and may be attributable to increased oxidative stress.

Mechanisms underlying the chronic pioglitazone treatment-induced improvement in the impaired endothelium-dependent relaxation seen in aortas from diabetic rats

The objectives of this study were to determine the effects of chronic treatment with pioglitazone, a peroxisome proliferator-activated receptor gamma agonist, on the impaired endothelium-dependent relaxation seen in aortas from established streptozotocin (STZ)-induced diabetic rats, and to identify some of the molecular mechanisms involved. Starting at 8 weeks of diabetes, pioglitazone (10 mg/kg) was administered to STZ-induced diabetic rats for 4 weeks. In untreated STZ rats (vs age-matched control rats): (1) ACh-induced relaxation, cGMP accumulation, phosphorylation of the cGMP-dependent protein kinase substrate vasodilator-stimulated phosphoprotein at Ser-239 [an established biochemical end-point of nitric oxide (NO)/cGMP signaling], and Cu/Zn-superoxide dismutase (SOD) expression and SOD activity were all reduced; (2) aortic superoxide generation, nitrotyrosine expression, and NAD(P)H oxidase activity were increased; (3) plasma endothelin-1 (ET-1) and aortic c-Jun (AP-1 component) protein expressions were increased. Pioglitazone treatment markedly corrected the above abnormalities. Collectively, these results suggest that pioglitazone treatment improves endothelium-dependent relaxation by reducing oxidative stress via increased SOD activity, decreased NAD(P)H oxidase activity, and a decreased ET-1 level, and that this decreased ET-1 level may be attributable to an inhibition of the AP-1 signaling pathway.

Impaired cAMP signaling does not account for the attenuated EDHF-mediated dilations in female rat middle cerebral artery

Dilations to endothelium-derived hyperpolarizing factor (EDHF) are significantly attenuated in the middle cerebral artery (MCA) isolated from female compared to male rats. Since gap junctions appear to be involved in the EDHF pathway and cAMP has been shown to enhance gap junction permeability, we tested the hypothesis that elevation of cAMP would enhance EDHF-mediated dilations in female rat MCA. Vascular diameter was measured in perfused MCA segments using videomicroscopy in the presence and absence of IBMX, an inhibitor of cAMP phosphodiesterase. In the presence of L-NAME and indomethacin, dilation to 10(-4) M ATP was significantly reduced in females (48+/-12%) compared to males (92+/-2%). IBMX, an inhibitor of cAMP phosphodiesterase, had no significant effect on ATP-mediated dilations in both males and females. Basal cAMP levels were comparable in male and female MCAs (1.7 pmol/mg protein). Incubation with IBMX (2 x 10(-4) M) significantly elevated cAMP in both male (12.8 pmol/mg protein) and female (11.2 pmol/mg protein) MCAs. Our results demonstrate that reduced EDHF dilations in female rat MCA cannot be solely attributed to impaired cAMP signaling. Future studies will target other potential sites along the EDHF pathway in order to identify why EDHF dilations are reduced in the female compared to the male rat MCA.

Enhancement of methoxamine-induced contractile responses of rat ventricular muscle in streptozotocin-induced diabetes is associated with alpha1A adrenoceptor upregulation

AIM

To clarify the time-related changes in cardiac function and the mechanism underlying the cardiac dysfunction present in diabetes mellitus, we studied mechanical responses induced by alpha(1)- and beta-adrenoceptors, the Ca(2+)-entry promoter Bay K 8644- and ryanodine (an agent known to inhibit Ca(2+) release from the sarcoplasmic reticulum) in papillary muscles from streptozotocin (STZ)-induced diabetic and age-matched control rats.

METHODS

Male Wistar rats received a single injection of STZ (60 mg kg(-1)) via the tail vein to induce diabetes. For the mechanical studies, papillary muscle preparations were suspended in an organ bath and isometric contractions were measured in 1-, 4-, and 10-week STZ-induced diabetic and age-matched control rats.

RESULTS

In 1-week diabetic rats, the contractions induced by isoproterenol, methoxamine and Bay K 8644 were unchanged (vs. age-matched controls). In 4-week diabetic rats, (a) the isoproterenol- and Bay K 8644-induced contractions were impaired, (b) sensitivity to ryanodine was reduced, whereas (c) the methoxamine-induced contraction was unchanged. In 10-week diabetic rats, the isoproterenol- and Bay K 8644-induced contractile responses were impaired and the sensitivity to ryanodine was reduced, but in sharp contrast the methoxamine-induced contraction was enhanced. Both the mRNA level for the alpha(1A) adrenoceptor (but not the alpha(1B) or alpha(1D) mRNAs) and alpha(1A) adrenoceptor protein were increased in 10-week diabetic rats (vs. age-matched controls).

CONCLUSION

These results suggest that impairments of beta-adrenergic and Ca(2+)-handling mechanisms occur early in the development of cardiomyopathy in STZ-induced diabetic rats, and that this is followed by augmentation of alpha(1A) adrenoceptor-mediated inotropy due to alpha(1A) adrenoceptor upregulation.

Apocynin normalizes hyperreactivity to phenylephrine in mesenteric arteries from cholesterol-fed mice by improving endothelium-derived hyperpolarizing factor response

We studied the relationship among endothelial function, oxidative stress, and phenylephrine (PE; alpha(1)-adrenoceptor agonist)-induced contraction in mesenteric arteries from high-cholesterol (HC)-diet-fed mice. In HC mice (vs age-matched normal-diet-fed mice): (1) PE-induced contraction in endothelium-intact rings was enhanced (endothelial denudation increased contraction in "normal-diet" rings, but did not enhance it further in "HC" rings); (2) the enhanced PE-induced contraction was further enhanced in the presence of N(G)-nitro-L-arginine (L-NNA; nitric oxide synthase inhibitor) or L-NNA plus indomethacin (cyclooxygenase inhibitor) [to preserve endothelium-derived hyperpolarizing factor (EDHF)], but unchanged in the presence of charybdotoxin plus apamin (to block EDHF); (3) ACh-induced EDHF-type relaxation was reduced; and (4) oxidative stress [indicated by the plasma 8-isoprostane level (reliable systemic marker) and aortic superoxide production] was greater. In HC mice, PE-induced contraction was normalized by apocynin [NAD(P)H oxidase inhibitor] or tempol (superoxide dismutase mimetic), but enhanced by NADH [NAD(P)H oxidase substrate]. Oral dietary supplementation with apocynin (30 mg/kg/day for 4 weeks) corrected the above abnormalities. Hence: (1) PE-induced contraction is modulated by the endothelium, and the enhanced contractility in HC mice results from defective EDHF signaling and elevated oxidative stress, and (2) apocynin normalizes PE-induced contraction in HC mice by improving EDHF signaling.

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.

Specific impairment of endothelium-derived hyperpolarizing factor-type relaxation in mesenteric arteries from streptozotocin-induced diabetic mice

We hypothesized that the contribution made by endothelium-derived hyperpolarizing factor (EDHF) to acetylcholine (ACh)-induced endothelium-dependent relaxation (EDR) might be altered in mesenteric arteries from streptozotocin (STZ)-induced diabetic mice. In endothelium-intact preparations, the ACh-induced EDR (but not the sodium nitroprusside-induced relaxation) was weaker in the STZ group than in age-matched controls. Indomethacin (10 muM) had no significant effect on EDR in either group, indicating that cyclooxygenase products, including prostacyclin, are not involved. This indomethacin-resistant EDR was weaker in the STZ group than in the controls. To isolate the EDHF-resistant component of EDR, charybdotoxin (100 nM) and apamin (100 nM) were present in the bath solution throughout the next experiment. This EDHF-resistant relaxation did not differ significantly between the two groups. On the other hand, the EDHF-mediated relaxation was significantly weaker in the STZ group than in the controls, and it was completely blocked by lysophosphatidylcholine (LPC, 10 microM) in each group. The eNOS protein expression was similar between the two groups. These results suggest that (a) the endothelial dysfunction present in mesenteric arteries from type 1 diabetic mice is largely attributable to reduced EDHF signaling, and (b) LPC may be involved in this attenuation of EDHF-mediated relaxation.

Mechanisms underlying the impaired EDHF-type relaxation response in mesenteric arteries from Otsuka Long-Evans Tokushima Fatty (OLETF) rats

We previously reported that in mesenteric arteries from streptozotocin-induced diabetic rats, the endothelium-derived hyperpolarizing factor (EDHF)-type relaxation is impaired, possibly due to a reduced action of cAMP. Here, we observed an impairment of acetylcholine-induced EDHF-type relaxation in mesenteric arteries from a type 2 diabetic model, Otsuka Long-Evans Tokushima Fatty (OLETF) rats [vs. age-matched control Long-Evans Tokushima Otsuka (LETO) rats], and we investigated the mechanism underlying this impairment. In the LETO group, this EDHF-type relaxation was attenuated by 18alpha-glycyrrhetinic acid (a gap-junction inhibitor) and by a protein kinase A (PKA) inhibitor. In both groups (OLETF and LETO), it was enhanced by 3-isobutyl-1-methylxanthine, a cAMP-phosphodiesterase (PDE) inhibitor, but following these enhancements it was still weaker in OLETF rats than in LETO rats. The relaxations induced by cilostamide (a selective PDE3 inhibitor) and 8-bromo-cAMP (a cell-permeant cAMP analog) were reduced in OLETF rats, as was PKA activity. The relaxations induced by two activators of Ca(2+)-activated K(+) channels (K(Ca)) [1-ethyl-2-benzimidazolinone (1-EBIO), intermediate-conductance K(Ca) channel (IK(Ca)) activator, and riluzole, small-conductance K(Ca) channel (SK(Ca)) activator] were also impaired in OLETF rats. We conclude that the impairment of EDHF-type relaxation seen in OLETF rats may be attributable not only to a reduction in cAMP/PKA signaling, but also to reduced endothelial K(Ca) channel activities.

ANG II enhances contractile responses via PI3-kinase p110 delta pathway in aortas from diabetic rats with systemic hyperinsulinemia

We investigated the involvement of ANG II and phosphatidylinositol 3-kinase (PI3-K) in the enhanced aortic contractile responses induced by hyperinsulinemia in chronic insulin-treated Type 1 diabetic rats. Plasma ANG II levels were elevated in untreated compared with control diabetic rats and further increased in insulin-treated diabetic rats. Aortic contractile responses and systolic blood pressure were significantly enhanced in chronic insulin-treated diabetic rats compared with the other groups. These insulin-induced increases were largely prevented by cotreatment with losartan (an ANG II type 1 receptor antagonist) or enalapril (an angiotensin-converting enzyme inhibitor). LY-294002 (a PI3-K inhibitor) diminished the increases in contractile responses in ANG II-incubated aortas and aortas from chronic insulin-treated diabetic rats. The norepinephrine (NE)-stimulated levels of p110 delta-associated PI3-K activity and p110 delta protein expression were increased in aortas from insulin-treated diabetic compared with control and untreated diabetic rats, and chronic administration of losartan blunted these increases. Contractions were significantly larger in aortas from diabetic rats incubated with a low concentration (inducing approximately 10% of the maximum contraction) of ANG II or with NE or isotonic K+ than in aortas from nonincubated diabetic rats. NE-stimulated p110 PI3-K activity was elevated in aortas from diabetic rats coincubated with a noncontractile dose of ANG II. These results suggest that, in insulin-treated Type 1 diabetic rats with hyperinsulinemia, chronic ANG II type 1 receptor blockade blunts the increases in vascular contractility and blood pressure via a decrease in p110 delta-associated PI3-K activity.

Effects of anthocyanidin derivative (HK-008) on relaxation in rat perfused mesenterial bed

Anthocyanins, which are responsible for a variety of bright colors (including red, blue, and purple) in fruits, vegetables, and flowers, are consumed as dietary polyphenols. Anthocyanin-containing fruits are thought to decrease coronary heart disease and are used in anti-diabetic preparations. Diabetes is associated with a variety of cardiovascular complications that may be mediated by endothelial dysfunction, and so this study was designed mainly to characterize the influence of a synthesized anthocyanidin derivative (HK-008) over acetylcholine (ACh)-induced relaxation in mesenteric arterial beds isolated from rats. In a glucose-tolerance test in intact rats, HK-008 (30 mg/kg) reduced the glucose level as effectively as the same dose of glibenclamide. The aortic relaxation induced by pinacidil (an ATP-sensitive potassium channel opener) was greatly inhibited by glibenclamide (10 microM), and also significantly inhibited by HK-008 (10 microM). Interestingly, the ACh-induced relaxation in the perfused, preconstricted mesenteric arterial bed was significantly enhanced by HK-008 (10 microM), and this enhancement was significantly attenuated by indomethacin (10 microM). The ACh-induced mesenteric relaxation was impaired by an increase in oxidative stress, viz. superoxide-generating treatment [xanthine oxidase (XO; 0.1 U/ml) plus hypoxanthine (HX; 10 microM)]. However, this impairment was strongly suppressed by HK-008 (10 microM). These results suggest that HK-008 increases endothelium-induced relaxation by suppressing oxidative stress or modulating prostanoids signaling. This compound may therefore be useful against certain cardiovascular disorders.

Endothelial dysfunction in the streptozotocin-induced diabetic apoE-deficient mouse

Endothelial dysfunction plays a role in the development of atherosclerosis and diabetes-associated vascular disease and, in the streptozotocin (STZ)-induced apoE-deficient diabetic mouse, we report that, when compared to the citrate (CIT)-treated nondiabetic apoE-deficient control, acetylcholine (Ach)-mediated endothelium-dependent relaxation was reduced in the small mesenteric arteries (SMA) and the plaque-prone regions of the aorta from the STZ-diabetic mouse. In the SMA the component of Ach-mediated relaxation that was attributed to nitric oxide (NO) from STZ-treated diabetic apoE-deficient mice was enhanced; however, the endothelium-derived hyperpolarizing factor (EDHF)-mediated component was reduced. The EDHF component was assessed by determining the component of the Ach-mediated response that was resistant to the combination of the NO synthase (NOS) inhibitor Nomega-nitro-L-arginine methyl ester, cyclooxygenase inhibitor, indomethacin, and soluble guanylate cyclase inhibitor, ODQ, and inhibited by the combination of the intermediate conductance KCa (IKCa) inhibitor TRAM-34 and the small-conductance KCa (SKCa) inhibitor apamin. Endothelial NOS was increased but SK2, SK3 and connexin (Cx) 37 mRNA expressions were significantly (P<0.05) decreased in the SMA from STZ-treated apoE-deficient mice compared to the CIT-treated controls. There was no difference in the IKCa expression or in Cx 40, 43 and 45 mRNA levels between STZ- and CIT-treated mice. The microvasculature of STZ-induced apoE-deficient mice developed endothelial dysfunction, which may be linked to a decrease in the contribution of the EDHF component due to a decrease in SK2 and 3 and Cx 37 expression.

Cilostazol improves endothelium-derived hyperpolarizing factor-type relaxation in mesenteric arteries from diabetic rats

We previously reported that in mesenteric arteries from streptozotocin (STZ)-induced diabetic rats that 1) endothelium-derived hyperpolarizing factor (EDHF)-type relaxation is impaired, possibly due to a reduced action of cAMP via increased phosphodiesterase 3 (PDE3) activity (Matsumoto T, Kobayashi T, and Kamata K. Am J Physiol Heart Circ Physiol 285: H283-H291, 2003) and that 2) PKA activity is decreased (Matsumoto T, Wakabayashi K, Kobayashi T, and Kamata K. Am J Physiol Heart Circ Physiol 287: H1064-H1071, 2004). Here we investigated whether chronic treatment with cilostazol, a PDE3 inhibitor, improves EDHF-type relaxation in mesenteric arteries isolated from STZ rats. We found that in such arteries 1) cilostazol treatment (2 wk) improved ACh-, A-23187-, and cyclopiazonic acid-induced EDHF-type relaxations; 2) the ACh-induced cAMP accumulation was transient and sustained in arteries from cilostazol-treated STZ rats; 3) the EDHF-type relaxation was significantly decreased by a PKA inhibitor in the cilostazol-treated group, but not in the cilostazol-untreated group; 4) cilostazol treatment improved both the relaxations induced by cAMP analogs and the PKA activity level; and 5) PKA catalytic subunit (Cat-alpha) protein was significantly decreased, but the regulatory subunit RII-beta was increased (and the latter effect was significantly decreased by cilostazol treatment). These results strongly suggest that cilostazol improves EDHF-type relaxations in STZ rats via an increase in cAMP and PKA signaling.

Functional changes in adenylyl cyclases and associated decreases in relaxation responses in mesenteric arteries from diabetic rats

To assess the functional change in adenylyl cyclases (AC) associated with the diabetic state, we investigated AC-mediated relaxations and cAMP production in mesenteric arteries from rats with streptozotocin (STZ)-induced diabetes. The relaxations induced by the water-soluble forskolin (FSK) analog NKH477, which is a putative AC5 activator, but not by the beta-adrenoceptor agonist isoproterenol (Iso) and the AC activator FSK, were reduced in intact diabetic mesenteric artery. In diabetic rats, however, Iso-, FSK-, and NKH477-induced relaxations were attenuated in the presence of inhibitors of nitric oxide synthase and cyclooxygenase. To exclude the influence of phosphodiesterase (PDE), we also examined the relaxations induced by several AC activators in the presence of 3-isobutyl-1-methylxanthine (IBMX; a PDE inhibitor). Under these conditions, the relaxation induced by Iso was greatly impaired in STZ-diabetic rats. This Iso-induced relaxation was significantly attenuated by pretreatment with SQ-22536, an AC inhibitor, in mesenteric rings from age-matched controls but not in those from STZ-diabetic rats. Under the same conditions, the relaxations induced by FSK or NKH477 were impaired in STZ-diabetic rats. Neither FSK- nor A-23187 (a Ca2+ ionophore)-induced cAMP production was significantly different between diabetics and controls. However, cAMP production induced by Iso or NKH477 was significantly impaired in diabetic mesenteric arteries. Expression of mRNAs and proteins for AC5/6 was lower in diabetic mesenteric arteries than in controls. These results suggest that AC-mediated relaxation is impaired in the STZ-diabetic rat mesenteric artery, perhaps reflecting a reduction in AC5/6 activity.

ROLE OF ENDOTHELIUM-DERIVED HYPERPOLARIZING FACTOR IN ENDOTHELIAL DYSFUNCTION DURING DIABETES

1. Under normal conditions, the endothelium plays a major role in the maintenance of vasodilatory tone via the production of endothelium-derived vasodilator agents, such as prostacyclin, nitric oxide and endothelium-derived hyperpolarizing factor (EDHF). Inhibition of endothelium-dependent relaxation features prominently in a range of cardiovascular diseases, including hypertension, coronary artery disease and diabetes. 2. Endothelium-derived hyperpolarizing factor is a prominent vasodilator, particularly in smaller arteries and arterioles. There is now emerging evidence to suggest that EDHF may play a role in the endothelial dysfunction in diabetes. 3. Since the first description of endothelium-dependent hyperpolarization some 20 years ago, it has emerged that EDHF is heterogeneous in nature, consisting of diffusible factors and contact-mediated mechanisms. The specific identity of EDHF in any particular vascular bed may influence the impact of diabetes on vascular function. 4. There is accumulating evidence in diabetic rat models and humans showing impaired EDHF activity in small resistance vessels. In contrast, studies in mice suggest that EDHF activity is actually enhanced under diabetic conditions. 5. It is clear that alterations in EDHF activity may have an important contribution in diabetes, more specifically in contributing to microvascular complications observed under diabetic conditions.