Effect of NO on EDHF response in rat middle cerebral arteries

Endothelial TrkB receptor activation controls vascular tone of rat middle cerebral artery

Little is known on the cerebrovascular BDNF (brain-derived neurotrophic factor)/TrkB (tropomyosin related kinase B) pathway. This study investigated the contribution of endogenous endothelial BDNF to the control of vascular tone of rat middle cerebral artery (MCA) and the capacity of exogenous agonist of TrkB receptors to induce their relaxation. Endothelial cells constitutively expressed both BDNF and activated TrkB receptors. Supporting endothelial BDNF as an autocrine regulator of basal myogenic tone, incubation of MCA with the TrkB antagonist cyclotraxin B induced contraction as observed with incubation in the presence of inhibitors of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) production. Exposure of MCA with the TrkB agonist LM22A-4 that increased expression of TrkB receptors phosphorylated at tyrosine 816 induced relaxation of preconstricted MCA (EC₅₀ 6.7 × 10^-8 mol/L) as efficiently than acetylcholine (EC₅₀ 5.3 × 10^-8 mol/L). Finally, endothelium removal, exposure to a TrkB antagonist or to inhibitors of NO and EDHF production prevented the relaxant effect of LM22A-4. In conclusion, our study identified endothelial BDNF as a new autocrine regulator of vascular tone of MCA, thus making the endothelial BDNF/TrkB pathway an attractive target for strategies aiming to improve blood supply to the brain.

Apelin inhibits an endothelium-derived hyperpolarizing factor-like pathway in rat cerebral arteries

Apelin has complex vasomotor actions inasmuch as the peptide may cause either vasodilation or vasoconstriction depending on the vascular bed and experimental conditions. In cerebral arteries, apelin inhibits endothelium-dependent relaxations mediated by nitric oxide (NO); however, its effects on relaxation to other endothelium-derived substances (e.g. prostacyclin, endothelium-derived hyperpolarizing factors(s) (EDHF)) are unknown. The present study was designed to determine effects of apelin on endothelium-dependent relaxations that are independent of NO in rat cerebral arteries. In arterial rings contracted with 5-HT, A23187 caused endothelium-dependent relaxation that was unaffected by inhibitors of eNOS, guanylyl cyclase or cyclooxygenase, but was attenuated by MS-PPOH, a selective inhibitor of cytochrome P450 catalyzed synthesis of epoxyeicosatrienoic acids (EETs) and by 14,15-EE(Z)E, an EET-receptor antagonist. Apelin inhibited A23187-induced relaxation, as well as relaxations evoked by exogenous 11,12- and 14,15-EET. These effects of apelin were mimicked by the selective BK_Ca channel blocker, iberiotoxin. The APJ receptor antagonist, F13A abolished the effects of apelin on A23187-induced relaxations. Both 11,12- and 14,15-EET also increased BK_Ca channel current density in isolated cerebral artery smooth muscle cells, effects that were inhibited in a similar manner by apelin and iberiotoxin. These findings provide evidence that apelin impairs endothelium-dependent relaxation of cerebral arteries by inhibiting an NO-independent pathway (i.e. "EDHF-like") involving activation of smooth muscle cell BK_Ca channels by endothelium-derived EETs. Inhibition of such pathway may create an environment favoring vasoconstriction in cerebral arteries.

Potassium inhibits nitric oxide and adenosine arteriolar vasodilatation via K(IR) and Na(+)/K(+) ATPase: implications for redundancy in active hyperaemia

Redundancy, in active hyperaemia, where one vasodilator can compensate for another if the first is missing, would require that one vasodilator inhibits the effects of another; therefore, if the first vasodilator is inhibited, its inhibitory influence on the second vasodilator is removed and the second vasodilator exerts a greater vasodilatory effect. We aimed to determine whether vasodilators relevant to skeletal muscle contraction [potassium chloride (KCl), adenosine (ADO) and nitric oxide] inhibit one another and, in addition, to investigate the mechanisms for this interaction. We used the hamster cremaster muscle and intravital microscopy to directly visualize 2A arterioles when exposed to a range of concentrations of one vasodilator [10(-8) to 10(-5) M S-nitroso-N-acetyl penicillamine (SNAP), 10(-8) to 10(-5) M ADO, 10 and 20 mM KCl] in the absence and then in the presence of a second vasodilator (10(-7) M ADO, 10(-7) M SNAP, 10 mM KCl). We found that KCl significantly attenuated SNAP-induced vasodilatations by ∼65.8% and vasodilatations induced by 10(-8) to 10(-6) M ADO by ∼72.8%. Furthermore, we observed that inhibition of KCl vasodilatation, by antagonizing either Na(+)/K(+) ATPase using ouabain or inward rectifying potassium channels using barium chloride, could restore the SNAP-induced vasodilatation by up to ∼53.9% and 30.6%, respectively, and also restore the ADO-induced vasodilatations by up to ∼107% and 76.7%, respectively. Our data show that vasodilators relevant to muscle contraction can interact in a way that alters the effectiveness of other vasodilators. These data suggest that active hyperaemia may be the result of complex interactions between multiple vasodilators via a redundant control paradigm.

Compromised endothelium-dependent hyperpolarization-mediated dilations can be rescued by NS309 in obese Zucker rats

OBJECTIVE

NO and a non-NO/prostacyclin EDH mechanism are major contributors of vascular tone and cerebral blood flow. However, the effect of metabolic syndrome on EDH-mediated responses in cerebral vessels remains unknown and may offer another avenue for therapeutic targeting. The purpose of this study was to investigate EDH-dependent responses in cerebral arteries during metabolic syndrome.

METHODS

EDH-dependent dilations were assessed in MCAs isolated from nondiabetic obese and lean Zucker rats in the presence and absence of NS309, an activator of SKCa and IKCa channels. IKCa channel expression and activity were assessed by western blotting and pressure myography, respectively.

RESULTS

EDH-mediated dilations were significantly attenuated in the obese compared to the lean Zucker rat MCA. Luminal delivery of 1 μM NS309 enhanced EDH-mediated responses in lean and obese Zucker cerebral vessels. Both dose-dependent dilations to luminal NS309 and IKCa protein expression in pooled cerebral arteries were comparable between the two groups.

CONCLUSIONS

Our results suggest that pharmacological targeting of IKCa channels can rescue EDH-mediated dilations in obese Zucker rat MCAs. Compromised EDH-mediated dilations in obesity are not due to impaired IKCa channel expression or activity.

Acetylcholine- and sodium hydrosulfide-induced endothelium-dependent relaxation and hyperpolarization in cerebral vessels of global cerebral ischemia-reperfusion rat

We investigated the effects of endothelium-derived hyperpolarizing factor (EDHF) and the role of hydrogen sulphide (H2S) in the cerebral vasorelaxation induced by acetylcholine (ACh) in global cerebral ischemia-reperfusion (CIR) rats. CIR was induced by occlusion of bilateral carotid and vertebral arteries. Isolated arterial segments from the cerebral basilar (CBA) and middle artery (MCA) of CIR rats were studied in a pressurized chamber. Transmembrane potential was recorded using glass microelectrodes to evaluate hyperpolarization. In the CIR CBAs and MCAs preconstricted by 30 mM KCl, ACh induced concentration-dependent vasorelaxation and hyperpolarization that were partially attenuated by NG-nitro-l-arginine methyl ester (l-NAME, 30 μM) and l-NAME plus indomethacin (10 μM). The residual responses were abolished by the H2S inhibitor dl-propargylglycine (PPG, 100 μM). The H2S donor NaHS and l-Cys, the substrate of endogenous H2S synthase, elicited similar responses to ACh and was inhibited by tetraethylamonine (1 mM) or PPG. ACh induces EDHF-mediated vasorelaxation and hyperpolarization in rat cerebral arteries. These responses are up-regulated by ischemia-reperfusion while NO-mediated responses are down-regulated. Further, the ACh-induced, EDHF-mediated relaxation, and hyperpolarization and the inhibition of these responses are similar to the H2S-induced responses, suggesting that H2S is a possible candidate for EDHF in rat cerebral vessels.

Endothelium-derived hyperpolarizing factor in the brain: influence of sex, vessel size and disease state

The endothelial layer of cells lining the intimal surface of blood vessels is essential for vascular function. The endothelium releases multiple vasodilator and protective factors, including nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor; an imbalance in these factors predisposes individuals to vascular diseases such as stroke. These factors are differentially regulated by vessel size, sex hormones and disease state, therefore playing differential roles in different tissues following vascular injury. In particular, the endothelium-derived hyperpolarizing factor candidate termed epoxyeicosatrienoic acid, plays a prominent role in microvessel function, especially after ischemia, thereby making this signaling pathway an attractive target for therapy in vascular disease, including stroke.

Mechanisms of ATP-mediated vasodilation in humans: modest role for nitric oxide and vasodilating prostaglandins

ATP is an endothelium-dependent vasodilator, and findings regarding the underlying signaling mechanisms are equivocal. We sought to determine the independent and interactive roles of nitric oxide (NO) and vasodilating prostaglandins (PGs) in ATP-mediated vasodilation in young, healthy humans and determine whether any potential role was dependent on ATP dose or the timing of inhibition. In protocol 1 (n = 18), a dose-response curve to intrabrachial infusion of ATP was performed before and after both single and combined inhibition of NO synthase [N(G)-monomethyl-L-arginine (L-NMMA)] and cyclooxygenase (ketorolac). Forearm blood flow (FBF) was measured via venous occlusion plethysmography and forearm vascular conductance (FVC) was calculated. In this protocol, neither individual nor combined NO/PG inhibition had any effect on the vasodilatory response (P = 0.22-0.99). In protocol 2 (n = 16), we determined whether any possible contribution of both NO and PGs to ATP vasodilation was greater at low vs. high doses of ATP and whether inhibition during steady-state infusion of the respective dose of ATP impacted the dilation. FBF in this protocol was measured via Doppler ultrasound. In protocol 2, infusion of low (n = 8)- and high-dose (n = 8) ATP for 5 min evoked a significant increase in FVC above baseline (low = 198 ± 24%; high = 706 ± 79%). Infusion of L-NMMA and ketorolac together reduced steady-state FVC during both low- and high-dose ATP (P < 0.05), and in a subsequent trial with continuous NO/PG blockade, the vasodilator response from baseline to 5 min of steady-state infusion was similarly reduced for both low (ΔFVC = -31 ± 11%)- and high-dose ATP (ΔFVC -25 ± 11%; P = 0.70 low vs. high dose). Collectively, our findings indicate a potential modest role for NO and PGs in the vasodilatory response to exogenous ATP in the human forearm that does not appear to be dose or timing dependent; however, this is dependent on the method for assessing forearm vascular responses. Importantly, the majority of ATP-mediated vasodilation is independent of these putative endothelium-dependent pathways in humans.

Contributions of endothelium-derived relaxing factors to control of hindlimb blood flow in the mouse in vivo

We determined the contributions of various endothelium-derived relaxing factors to control of basal vascular tone and endothelium-dependent vasodilation in the mouse hindlimb in vivo. Under anesthesia, catheters were placed in a carotid artery, jugular vein, and femoral artery (for local hindlimb circulation injections). Hindlimb blood flow (HBF) was measured by transit-time ultrasound flowmetry. N(omega)-nitro-L-arginine methyl ester (L-NAME, 50 mg/kg plus 10 mg x kg(-1) x h(-1)), to block nitric oxide (NO) production, altered basal hemodynamics, increasing mean arterial pressure (30 +/- 3%) and reducing HBF (-30 +/- 12%). Basal hemodynamics were not significantly altered by indomethacin (10 mg x kg(-1) x h(-1)), charybdotoxin (ChTx, 3 x 10(-8) mol/l), apamin (2.5 x 10(-7) mol/l), or ChTx plus apamin (to block endothelium-derived hyperpolarizing factor; EDHF). Hyperemic responses to local injection of acetylcholine (2.4 microg/kg) were reproducible in vehicle-treated mice and were not significantly attenuated by L-NAME alone, indomethacin alone, L-NAME plus indomethacin with or without co-infusion of diethlyamine NONOate to restore resting NO levels, ChTx alone, or apamin alone. Hyperemic responses evoked by acetylcholine were reduced by 29 +/- 11% after combined treatment with apamin plus charybdotoxin, and the remainder was virtually abolished by additional treatment with L-NAME but not indomethacin. None of the treatments altered the hyperemic response to sodium nitroprusside (5 microg/kg). We conclude that endothelium-dependent vasodilation in the mouse hindlimb in vivo is mediated by both NO and EDHF. EDHF can fully compensate for the loss of NO, but this cannot be explained by tonic inhibition of EDHF by NO. Control of basal vasodilator tone in the mouse hindlimb is dominated by NO.

DCEBIO-Mediated Dilations Are Attenuated in the Female Rat Middle Cerebral Artery

BACKGROUND

Unlike in peripheral vessels, the endothelium-derived hyperpolarizing factor (EDHF)-mediated component to P2Y(2) receptor-mediated dilations is significantly attenuated in the middle cerebral artery (MCA) of female rats compared to male rats. One aspect to the EDHF phenomenon is activation of the intermediate calcium-sensitive potassium (IK(Ca)) channels located on the endothelium. In an attempt to pinpoint the site along the EDHF pathway that is compromised in females, we tested the hypothesis that direct activation of IK(Ca) channels with DCEBIO would elicit attenuated hyperpolarization in the endothelium and smooth muscle of females compared to males.

METHODS

Inhibitors of nitric oxide synthase and cyclooxygenase were present throughout all experiments. Vessel diameter changes were assessed in pressurized and luminally perfused MCAs. Membrane potential changes in the endothelium and smooth muscle were measured using the perforated patch clamp method and sharp electrodes, respectively.

RESULTS AND CONCLUSIONS

The maximum vasodilation to 3 x 10(-4)M DCEBIO was significantly reduced in females (37 +/- 9%) compared to intact males (70 +/- 4%). Endothelial cell hyperpolarization to DCEBIO was similar in both males and females. Smooth muscle cell hyperpolarization was attenuated in females (2 +/- 1 mV) compared to males (15 +/- 3 mV). Taken together, our data suggest that the transfer of hyperpolarization from the endothelium to the smooth muscle is impeded in the female rat MCA.

Endothelium-dependent vasodilation of cerebral arteries is altered with simulated microgravity through nitric oxide synthase and EDHF mechanisms

Cephalic elevations in arterial pressure associated with microgravity and prolonged bed rest alter cerebrovascular autoregulation in humans. Using the head-down tail-suspended (HDT) rat to chronically induce headward fluid shifts and elevate cerebral artery pressure, previous work has likewise shown cerebral perfusion to be diminished. The purpose of this study was to test the hypothesis that 2 wk of HDT reduces cerebral artery vasodilation. To test this hypothesis, dose-response relations for endothelium-dependent (2-methylthioadenosine triphosphate and bradykinin) and endothelium-independent (nitroprusside) vasodilation were determined in vitro in middle cerebral arteries (MCAs) from HDT and control rats. All in vitro measurements were done in the presence and absence of the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (10(-5) M) and cyclooxygenase inhibitor indomethacin (10(-5) M). MCA caveolin-1 protein content was measured by immunoblot analysis. Endothelium-dependent vasodilation to 2-methylthioadenosine triphosphate and bradykinin were both lower in MCAs from HDT rats. These lower vasodilator responses were abolished with N(G)-nitro-L-arginine methyl ester but were unaffected by indomethacin. In addition, HDT was associated with lower levels of MCA caveolin-1 protein. Endothelium-independent vasodilation was not altered by HDT. These results indicate that chronic cephalic fluid shifts diminish endothelium-dependent vasodilation through alterations in the endothelial nitric oxide synthase signaling mechanism. Such decrements in endothelium-dependent vasodilation of cerebral arteries could contribute to the elevations in cerebral vascular resistance and reductions in cerebral perfusion that occur after conditions of simulated microgravity in HDT rats.

Angiotensin II-induced vasodilatation in cerebral arteries is mediated by endothelium-derived hyperpolarising factor

The angiotensin II-induced vasodilatation was evaluated in rat middle cerebral artery, especially regarding endothelium-derived hyperpolarising factor (EDHF), by use of a pressurised arteriograph. The angiotensin II dilatation was partly antagonised by inhibitors of nitric oxide synthase and cyclo-oxygenase. The remaining dilatation was inhibited by the potassium channel blockers, charybdotoxin and apamin, providing direct evidence that angiotensin II induces EDHF-mediated dilatation in cerebral arteries. The angiotensin II dilatation was blocked by the angiotensin AT1 and AT2 receptor blockers candesartan and PD 123319. Both angiotensin AT1 and AT2 receptors were detected on the endothelium by immunohistochemistry.

Arachidonic acid metabolites, hydrogen peroxide, and EDHF in cerebral arteries

We tested the hypotheses that EDHF in rat middle cerebral arteries (MCAs) involves 1) metabolism of arachidonic acid through the epoxygenase pathway, 2) metabolism of arachidonic acid through the lipoxygenase pathway, or 3) reactive oxygen species. EDHF-mediated dilations were elicited in isolated and pressurized rat MCAs by activation of endothelial P2Y2receptors with either UTP or ATP. All studies were conducted after the inhibition of nitric oxide synthase and cyclooxygenase with Nω-nitro-l-arginine methyl ester (10 μM) and indomethacin (10 μM), respectively. The inhibition of epoxygenase with miconazole (30 μM) did not alter EDHF dilations to UTP, whereas the structurally different epoxygenase inhibitor N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanoic acid (20 or 40 μM) only modestly inhibited EDHF at the highest concentration of UTP. An antagonist of epoxyeicosatrienoic acids, 14,15-epoxyeicosa-5( Z)-enoic acid, had no effect on EDHF dilations to UTP. Chronic inhibition of epoxygenase in the rat with 1-aminobenzotriazol (50 mg/kg twice daily for 5 days) did not alter EDHF dilations. The inhibition of the lipoxygenase pathway with either 10 μM baicalein or 10 μM nordihydroguaiaretic acid produced no major inhibitory effects on EDHF dilations. The combination of superoxide dismutase (200 U/ml) and catalase (140 U/ml) had no effect on EDHF dilations. Neither tiron (10 mM), a cell-permeable scavenger of reactive oxygen species, nor deferoxamine (1 or 10 mM), an iron chelator that blocks the formation of hydroxyl radicals, altered EDHF dilations in rat MCAs. We conclude that EDHF dilations in the rat MCA do not involve the epoxygenase pathway, lipoxygenase pathway, or reactive oxygen species including H2O2.

Possible Role for K + in Endothelium-Derived Hyperpolarizing Factor–Linked Dilatation in Rat Middle Cerebral Artery

Background and Purpose— Endothelium-derived hyperpolarizing factor (EDHF) and K + are vasodilators in the cerebral circulation. Recently, K + has been suggested to contribute to EDHF-mediated responses in peripheral vessels. The EDHF response to the protease-activated receptor 2 ligand SLIGRL was characterized in cerebral arteries and used to assess whether K + contributes as an EDHF.

Methods— Rat middle cerebral arteries were mounted in either a wire or pressure myograph. Concentration-response curves to SLIGRL and K + were constructed in the presence and absence of a variety of blocking agents. In some experiments, changes in tension and smooth muscle cell membrane potential were recorded simultaneously.

Results— SLIGRL (0.02 to 20 μmol/L) stimulated concentration and endothelium-dependent relaxation. In the presence of N G -nitro- l -arginine methyl ester, relaxation to SLIGRL was associated with hyperpolarization and sensitivity to a specific inhibitor of IK Ca , 1-[(2-chlorophenyl)diphenylmethyl]-1 H -pyrazole (1μmol/L), reflecting activation of EDHF. Combined inhibition of K IR with Ba 2+ (30μmol/L) and Na + /K + -ATPase with ouabain (1 μmol/L) markedly attenuated the relaxation to EDHF. Raising extracellular [K + ] to 15 mmol/L also stimulated smooth muscle relaxation and hyperpolarization, which was also attenuated by combined application of Ba 2+ and ouabain.

Conclusions— SLIGRL evokes EDHF-mediated relaxation in the rat middle cerebral artery, underpinned by hyperpolarization of the smooth muscle. The profile of blockade of EDHF-mediated hyperpolarization and relaxation supports a pivotal role for IK Ca channels. Furthermore, similar inhibition of responses to EDHF and exogenous K + with Ba 2+ and ouabain suggests that K + may contribute as an EDHF in the middle cerebral artery.

Involvement of NO and EDHF in Flow-Induced Vasodilation in Isolated Hamster Cremasteric Arterioles

Flow-induced vasodilation in hamster cremasteric arterioles was investigated with special reference to the roles of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). Arterioles (approximately 60 microm resting diameter) were cannulated, and suffused with MOPS solution at 37 degrees C (mean intraluminal pressure: 80 cm H(2)O). Step increases in the perfusate flow elicited a dose-dependent vasodilation, which was almost proportional to the increases in calculated wall shear stress (WSS). N(omega)-nitro L-arginine methyl ester (L-NAME, 100 microM) reduced the flow-induced vasodilation by approximately 50%, whereas indomethacin (10 microM) produced no significant effect. In the presence of L-NAME, the residual vasodilation was eliminated by treatment with the cytochrome P-450 monooxygenase inhibitor 17-octadecynoic acid (17-ODYA, 50 microM), sulfaphenazol (10 microM), tetraethylammonium (TEA, 3 mM; a nonselective Ca(2+)-activated K(+) channel inhibitor), or charybdotoxin (ChTX, 0.1 microM; intermediate or large conductance Ca(2+)-activated K(+) channel inhibitor). In the absence of L-NAME, the dilation was also reduced by approximately 50% by treatment with 17-ODYA, TEA, or ChTX. The residual vasodilation was eliminated by additional treatment with L-NAME. The inhibitor of ATP-sensitive K(+) channels (K(ATP)), glibenclamide, also caused a significant, but partial, reduction of the flow-induced vasodilation. The residual vasodilation was completely reduced by additional treatment with 17-ODYA, but not L-NAME. These findings suggest that in hamster cremaster, higher flow rate produces NO, K(ATP), and EDHF vasodilation of the arterioles under physiological conditions.

Simulated microgravity enhances cerebral artery vasoconstriction and vascular resistance through endothelial nitric oxide mechanism

Elevations in arterial pressure associated with hypertension, microgravity, and prolonged bed rest alter cerebrovascular autoregulation in humans. Using head-down tail suspension (HDT) in rats to induce cephalic fluid shifts and elevate arterial pressure, this study tested the hypothesis that 2-wk HDT enhances cerebral artery vasoconstriction and that an enhanced vasoconstriction described in vitro will alter regional cerebral blood flow (CBF) and vascular resistance (CVR) during standing and head-up tilt. To test this hypothesis, basal tone and vasoconstrictor responses to increases in transmural pressure, shear stress, and K+ were determined in vitro in middle cerebral arteries (MCAs) from HDT and control rats. All in vitro measurements were done in the presence and absence of the nitric oxide synthase (NOS) inhibitor NG-nitro-l-arginine methyl ester (l-NAME; 10−5 M) and with endothelium removal. Endothelial NOS (eNOS) mRNA and protein expression levels were measured by RT-PCR and immunoblot, respectively. Regional CBF and CVR were determined with a radiolabeled tracer technique and quantitative autoradiography. Basal tone and all vasoconstrictor responses were greater in MCAs from HDT rats. l-NAME and endothelium removal abolished these differences between groups, and HDT was associated with lower levels of MCA eNOS protein. CBF in select regions was lower and CVR higher during standing and head-up tilt in HDT rats. These results indicate that chronic cephalic fluid shifts enhanced basal tone and vasoconstriction through alterations in the eNOS signaling mechanism. The functional consequence of these vascular alterations with HDT is regional elevations in CVR and corresponding reductions in cerebral perfusion.

Responses of cerebral arterioles to ADP: eNOS-dependent and eNOS-independent mechanisms

ADP mediates platelet-induced relaxation of blood vessels and may function as an important intercellular signaling molecule in the brain. We used pharmacological and genetic approaches to examine mechanisms that mediate responses of cerebral arterioles to ADP, including the role of endothelial nitric oxide synthase (eNOS). We examined responses of cerebral arterioles (control diameter approximately 30 microm) in anesthetized wild-type (WT, eNOS+/+) and eNOS-deficient (eNOS-/-) mice using a cranial window. In WT mice, local application of ADP produced vasodilation that was not altered by indomethacin but was reduced by approximately 50% by NG-nitro-L-arginine (L-NNA) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (inhibitors of NOS and soluble guanylate cyclase, respectively). In eNOS-/- mice, responses to ADP were largely preserved, and a significant component of the response was resistant to L-NNA (a finding similar to that in WT mice treated with L-NNA). In the absence of L-NNA, responses to ADP were markedly reduced by charybdotoxin plus apamin [inhibitors of Ca2+-dependent K+ channels and responses mediated by endothelium-derived hyperpolarizing factor (EDHF)] in both WT and eNOS-/- mice. Thus pharmacological and genetic evidence suggests that a significant portion of the response to ADP in cerebral microvessels is mediated by a mechanism independent of eNOS. The eNOS-independent mechanism is functional in the absence of inhibited eNOS and most likely is mediated by an EDHF.

Chronic nitric oxide exposure alters the balance between endothelium-derived relaxing factors released from rat renal arteries: prevention by treatment with NOX-100, a NO scavenger

We investigated whether nitric oxide (NO) exposure alters the balance between NO and endothelium-derived hyperpolarizing factor (EDHF) released from rat renal arteries. To produce states of acutely or chronically excessive NO, lipopolysaccharide (LPS) was administered intraperitoneally to rats in a single dose of 4 mg/kg (LPS-single group) or in stepwise doses of 0.5, 1.0 and 2.0 mg/kg every other day (LPS-repeated group). On the day after LPS treatment, the protein levels of inducible NO synthase (iNOS) and endothelial NOS (eNOS) were measured, and the relaxation responses were determined in the renal arteries. The protein levels of iNOS markedly increased in both LPS-treated groups, while those of eNOS significantly increased in the LPS-repeated group compared with those in the respective control groups. In both LPS-treated groups, the relaxations in response to acetylcholine (ACh) and sodium nitroprusside remained unchanged. The ACh-induced relaxations in the presence of N(G)-nitro-L-arginine methyl ester, a NOS inhibitor, or by 1H-[1, 2, 4-] oxadiazole [4, 3-a] quinoxalin-1-one, a soluble guanylyl cyclase inhibitor, i.e. EDHF-mediated relaxations were significantly impaired in the LPS-repeated group but not in the LPS-single group, indicating increase in NO-mediated relaxation in the LPS-repeated group. These changes in the protein levels and EDHF-mediated relaxations induced by ACh observed in the LPS-repeated group were restored by treatment with NOX-100, a NO scavenger. These results suggest that persistent but not acute excessive NO exposure in rats impairs EDHF-mediated relaxation in renal arteries, leading to a compensatory upregulation of the eNOS/NO pathway.

Nitric oxide-epoxygenase interactions and arachidonate-induced dilation of rat renal microvessels

Nitric oxide (NO) is an inhibitor of hemoproteins including cytochrome P-450 enzymes. This study tested the hypothesis that NO inhibits cytochrome P-450 epoxygenase-dependent vascular responses in kidneys. In rat renal pressurized microvessels, arachidonic acid (AA, 0.03-1 microM) or bradykinin (BK, 0.1-3 microM) elicited NO- and prostanoid-independent vasodilation. Miconazole (1.5 microM) or 6-(2-propargyloxyphenyl)hexanoic acid (30 microM), both of which are inhibitors of epoxygenase enzymes, or the fixing of epoxide levels with 11,12-epoxyeicosatrienoic acid (11,12-EET; 1 and 3 microM) inhibited these responses. Apamin (1 microM), which is a large-conductance Ca2+-activated K+ (BKCa) channel inhibitor, or 18alpha-glycyrrhetinic acid (30 microM), which is an inhibitor of myoendothelial gap junctional electromechanical coupling, also inhibited these responses. NO donors spermine NONOate (1 and 3 microM) or sodium nitroprusside (0.3 and 3 microM) but not 8-bromo-cGMP (100 microM), which is an analog of cGMP (the second messenger of NO), blunted the dilation produced by AA or BK in a reversible manner without affecting that produced by hydralazine. However, the non-NO donor hydralazine did not affect the dilatory effect of AA or BK. Spermine NONOate did not affect the dilation produced by 11,12-EET, NS-1619 (a BKCa channel opener), or cromakalim (an ATP-sensitive K+ channel opener). AA and BK stimulated EET production, whereas hydralazine had no effect. On the other hand, spermine NONOate (3 microM) attenuated basal (19 +/- 7%; P < 0.05) and AA stimulation (1 microM, 29 +/- 9%; P < 0.05) of renal preglomerular vascular production of all regioisomeric EETs: 5,6-; 8,9-; 11,12-; and 14,15-EET. These results suggest that NO directly and reversibly inhibits epoxygenase-dependent dilation of rat renal microvessels without affecting the actions of epoxides on K+ channels.

Nascent EDHF-mediated cerebral vasodilation in ovariectomized rats is not induced by eNOS dysfunction

In estrogen-depleted [i.e., ovariectomized (Ovx)] animals, an endothelium-derived hyperpolarizing factor (EDHF)-like mechanism may arise to, at least partially, replace endothelial nitric oxide (NO) synthase (eNOS)-derived NO in modulating cerebral arteriolar tone. Additional findings show that eNOS expression and function is restored in estrogen-treated Ovx female rats, while the nascent EDHF-like activity disappears. Because NO has been linked to repression of EDHF activity in the periphery, the current study was undertaken to examine whether the nascent EDHF role in cerebral vessels of Ovx females relates to a chronically repressed eNOS-derived NO-generating function. We compared the effects of chronic NOS inhibition with Nomega-nitro-L-arginine-methyl ester (L-NAME; 100 mg. kg-1. day-1 for 3 wk) on EDHF-mediated pial arteriolar vasodilation in anesthetized intact, Ovx, and 17beta-estradiol-treated (0.1 mg. kg-1. day-1 ip, 1 wk) Ovx (OVE) female rats as well as in male rats that were prepared with closed cranial windows. In the chronic NOS inhibition groups, pial arteriolar responses were monitored in the absence (all groups) and presence (females only) of indomethacin (Indo; 10 mg/kg iv). Finally, the gap junction inhibitory peptide Gap 27 (300 muM) was applied to block EDHF-related vasodilation. NO donor (S-nitroso-N-acetyl-penicillamine) responses were similar in all rats studied. Acetylcholine (ACh) reactivity was virtually absent in control Ovx rats and chronically NOS-inhibited intact female, OVE, and male rats. However, a partial recovery of ACh reactivity was seen in L-NAME-treated Ovx females. In addition, in the presence of L-NAME, a normal CO2 reactivity was observed in all females, whereas a 50% reduction in CO2 reactivity was seen in males. In intact and OVE rats, both chronic and acute (NG-nitro-L-arginine suffusion) NOS inhibition, combined with Indo, depressed ADP-induced dilation by > or =50%, and subsequent application of Gap 27 had no further effect on ADP-induced vasodilation. ADP reactivity was retained in Ovx rats after combined chronic NOS inhibition and acute Indo, but was attenuated significantly by Gap 27. In males, Gap 27 had no effect on arteriolar reactivity. Taken together, our data demonstrate that in the cerebral microcirculation, NO does not have an inhibitory effect on EDHF production or action. The increased EDHF-like function in chronic estrogen-depleted animals is not due to eNOS deficiency, suggesting a more direct effect of estrogen in modulating EDHF-mediated cerebral vasodilation.

Altered expression of small-conductance Ca2+-activated K+ (SK3) channels modulates arterial tone and blood pressure

The endothelium is a critical regulator of vascular tone, and dysfunction of the endothelium contributes to numerous cardiovascular pathologies. Recent studies suggest that apamin-sensitive, small-conductance, Ca2+-activated K+ channels may play an important role in active endothelium-dependent vasodilations, and expression of these channels may be altered in disease states characterized by vascular dysfunction. Here, we used a transgenic mouse (SK3T/T) in which SK3 expression levels can be manipulated with dietary doxycycline (DOX) to test the hypothesis that the level of expression of the SK subunit, SK3, in endothelial cells alters arterial function and blood pressure. SK3 protein was elevated in small mesenteric arteries from SK3T/T mice compared with wild-type mice and was greatly suppressed by dietary DOX. SK3 was detected in the endothelium and not in the smooth muscle by immunohistochemistry. In whole-cell patch-clamp experiments, SK currents in endothelial cells from SK3T/T mice were almost completely suppressed by dietary DOX. In intact arteries, SK3 channels contributed to sustained hyperpolarization of the endothelial membrane potential, which was communicated to the arterial smooth muscle. Pressure- and phenylephrine-induced constrictions of SK3T/T arteries were substantially enhanced by treatment with apamin, suppression of SK3 expression with DOX, or removal of the endothelium. In addition, suppression of SK3 expression caused a pronounced and reversible elevation of blood pressure. These results indicate that endothelial SK3 channels exert a profound, tonic, hyperpolarizing influence in resistance arteries and suggest that the level of SK3 channel expression in endothelial cells is a fundamental determinant of vascular tone and blood pressure.

Role of cytoplasmic phospholipase A2 in endothelium-derived hyperpolarizing factor dilations of rat middle cerebral arteries

Very little is known regarding the mechanism of action for the endothelium-derived hyperpolarizing factor (EDHF) response in cerebral vessels. The authors tested two hypotheses: (1) activation of the cytoplasmic form of phospholipase A (cPLA ) is involved with EDHF-mediated dilations in rat middle cerebral arteries; and (2) activation of the cPLA involves an increase in endothelial Ca through activation of phospholipase C. Middle cerebral arteries were isolated from the rat, pressurized to 85 mm Hg, and luminally perfused. The EDHF response was elicited by luminal application of uridine triphosphate (UTP) after NO synthase and cyclooxygenase inhibition (10 mol/L -nitro-l-arginine methyl ester and 10 mol/L indomethacin, respectively). AACOCF and PACOCF, inhibitors of cPLA (Ca -sensitive) and Ca -insensitive PLA (iPLA ), dose dependently attenuated the EDHF response. A selective inhibitor for iPLA2, haloenol lactone suicide substrate, had no effect on the EDHF response. The EDHF response elicited by UTP was accompanied by an increase in endothelial Ca (144 to 468 nmol/L), and the EDHF dilation was attenuated with U73122, a phospholipase C inhibitor. The authors conclude that the EDHF response elicited by luminal UTP in rat middle cerebral arteries involved activation of phospholipase C, an increase in endothelial Ca, and activation of cPLA.

ADP-induced pial arteriolar dilation in ovariectomized rats involves gap junctional communication

It was previously shown that, despite the loss of nitric oxide (NO) dependence, ADP-induced pial arteriolar dilation was not attenuated in estrogen-depleted [i.e., ovariectomized (Ovx)] rats. Additional evidence suggested that the NO was replaced by an endothelium-dependent hyperpolarizing factor (EDHF)-like mechanism. To further characterize the nascent EDHF role in Ovx females, the current study was undertaken to test whether, in Ovx rats, ADP-induced pial arteriolar dilation retained its endothelial dependence and whether gap junctions are involved in that response. A closed cranial window and intravital microscopy system was used to monitor pial arteriolar diameter changes in anesthetized rats. The endothelial portion of the ADP-induced dilation was evaluated using light dye endothelial injury (L/D). The study was organized around three experimental approaches. First, the responses of pial arterioles to ADP before and after L/D exposure in intact and Ovx female rats were tested. L/D reduced the ADP response by 50-70% in both groups, thereby indicating that the endothelium dependence of ADP-induced vasodilation is not altered by chronic estrogen depletion. Second, the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA) and the prostanoid synthesis inhibitor indomethacin (Indo) were coapplied. In intact females, L-NNA-Indo attenuated the response to ADP by 50%, with no further changes upon the addition of L/D. On the other hand, L-NNA-Indo did not affect ADP reactivity in Ovx rats, but subsequent L/D exposure reduced the ADP response by >50%. The NO-prostanoid-independent, but endothelium-dependent, nature of the response in Ovx females is a hallmark of EDHF participation. Third, gap junctional inhibition strategies were applied. A selective inhibitor of gap junctional function, Gap 27, did not affect ADP reactivity in intact females but reduced the the ADP response by 50% in Ovx females. A similar result was obtained following application of a connexin43 antisense oligonucleotide. These findings suggest that the nascent EDHF dependency of ADP-induced pial arteriolar dilation in Ovx females involves connexin43-related gap junctional communication.