Endothelin-1 inhibition of cardiac ATP-sensitive K+ channels via pertussis-toxin-sensitive G-proteins

Analysis on mechanism of ATP-sensitive K+ channel opener natakalim improving congestive heart failure after myocardial infarction

The action mechanism of natakalim, a novel ATP-sensitive potassium channel opener, was studied in ameliorating the congestive heart failure (CHF) after myocardial infarction. A total of 25 healthy Wistar male rats (age, 10 weeks; average weight, 300 g) were selected, and the CHF models after acute myocardial infarction (AMI) were prepared by ligation of left anterior descending branch. They were randomly divided into the sham operation group, the model group and the groups of 1, 3 and 9 mg/kg/day natakalims. Each group had 5 mice that were sacrificed after 8 weeks. We compared left ventricular end-diastolic diameter (LVEDD), left ventricular ejection fraction (LVEF), N-terminal prohormone of brain natriuretic peptide (NT-proBNP), left ventricular mass index, myocardial cell cross-sectional area, myocardial collagen content, plasma endothelin-1 (ET-1) and endothelial nitric oxide synthase (eNOS) levels. Compared with the sham operation, the LVEDD and NT-proBNP in the model group and each natakalim group were elevated. LVEF decreased significantly, while the left ventricular mass index, myocardial cell cross-sectional area, myocardial collagen content, plasma ET-1 and eNOS levels increased. Natakalim intervention improved the above changes and the improvement effect of 3 mg/kg/day group was the highest. The mechanism of natakalim against the endothelin system can be explained by the fact that inhibiting ET-1 synthesis can reduce the ET-1 levels in circulation leading to the release of NO and PGI2. Inhibition of the vasoconstriction effect of ET-1 can improve the hemodynamics of high-load status and ameliorate the cardiac systolic and diastolic functions. In conclusion, natakalim can improve the ventricular remodeling of CHF after AMI, and 3 mg/kg/day was the most effective dose.

Exercise training restores coronary arteriolar dilation to NOS activation distal to coronary artery occlusion: role of hydrogen peroxide

OBJECTIVE

Exercise training has been shown to restore vasodilation to nitric oxide synthase (NOS) activation in arterioles distal to coronary artery occlusion. Because reactive oxygen species are generated during NOS uncoupling and the production of vasodilator H2O2 is increased during exercise in patients with coronary disease, we proposed that H2O2 may contribute to the restoration of vasodilation in porcine coronary occlusion model.

METHODS AND RESULTS

Left circumflex (LCX) coronary artery of miniature swine was progressively occluded for 8 weeks followed by exercise training (EX; 5 days/wk treadmill) or sedentary (SED) protocols for 12 weeks. Arterioles were isolated from distal LCX and nonoccluded left anterior descending (LAD) artery for in vitro study. Vasodilation to NOS activators adenosine and ionomycin was impaired in SED LCX, but not LAD, arterioles. This impairment was restored by L-arginine. NO production induced by adenosine was also reduced in SED LCX arterioles. EX had no effect on LAD arterioles but improved NO production and restored dilation of LCX arterioles. NOS blockade (L-NAME) inhibited vasodilation to NOS activators in LAD (SED & EX) arterioles but was ineffective in SED LCX arterioles. In EX LCX arterioles, vasodilation to NOS activators was slightly inhibited by L-NAME but abolished by catalase. H2O2 production was markedly increased by adenosine in EX LCX arterioles.

CONCLUSIONS

This study demonstrates that endothelium-dependent NO-mediated dilation is impaired in SED LCX arterioles and that EX training restores the impaired function. It appears that H2O2, in addition to NO, contributes significantly to EX-induced restoration of endothelium-dependent dilation of coronary arterioles distal to occlusion.

Effects of sulfonylureas on left ventricular mass in type 2 diabetic patients

Myocardial ATP-sensitive potassium (KATP) channels have been implicated in attenuating cardiac hypertrophy by modulating endothelin-1 concentrations. Sulfonylureas differ in their affinity for cardiac KATPchannels and therefore may vary in their effects on left ventricular (LV) mass. We sought to determine the differential effects of sulfonylureas on LV mass in type 2 diabetic patients. All patients had been taking glibenclamide for more than 3 mo before being randomized to either switch to an equipotent dose of gliclazide or continue glibenclamide. A total of consecutive 240 diabetic patients were randomized into glibenclamide, gliclazide, a combination of glibenclamide and nicorandil, or gliclazide and nicorandil for 6 mo. In the gliclazide-treated group, the LV mass index was significantly decreased compared with that in the glibenclamide-treated groups. Nicorandil administration significantly reduced LV mass in glibenclamide-treated patients compared with patients treated with glibenclamide alone. Measurements of endothelin-1 concentrations mirrored the functional status of KATPchannel. Multivariate analysis revealed that regression of LV mass was significantly correlated only with the changes in endothelin-1 ( P < 0.0001). Our results show that KATPchannels may play a pathogenetic role, probably through an endothelin-1-dependent pathway, in diabetes mellitus-related ventricular hypertrophy. Patients treated with gliclazide may have a beneficial effect in attenuating ventricular mass.

Effect of pravastatin on left ventricular mass in the two-kidney, one-clip hypertensive rats

We have demonstrated that myocardial ATP-sensitive potassium (KATP) channels are implicated in the development of cardiac hypertrophy in hyperlipidemic rabbits. We investigated the effect of pravastatin on development of ventricular hypertrophy in male normolipidemic Wistar rats with two-kidney, one-clip (2K1C) hypertension and whether the attenuated hypertrophic effect was via activation of KATPchannels. Twenty-four hours after the left renal artery was clipped, rats were treated with one of the following therapies for 8 wk: vehicle, nicorandil (an agonist of KATPchannels), pravastatin, glibenclamide (an antagonist of KATPchannels), hydralazine, nicorandil plus glibenclamide, or pravastatin plus glibenclamide. Systolic blood pressure, relative left ventricular (LV) weight, and cardiomyocyte sizes significantly increased in vehicle-treated 2K1C rats compared with those in sham-operated rats. Treatment with either nicorandil or pravastatin significantly attenuated LV hypertrophy/body weight compared with the vehicle, which was further confirmed by downregulation of LV atrial natriuretic peptide mRNA. Nicorandil-induced effects were abolished by administering glibenclamide. Similarly, pravastatin-induced beneficial effects were reversed by the addition of glibenclamide, implicating KATPchannels as the relevant target. A dissociation between the effects of blood pressure and cardiac structure was noted because pravastatin and hydralazine reduced arterial pressure similarly. These results suggest a crucial role of cardiac KATPchannel system in the development of ventricular hypertrophy in the 2K1C hypertensive rats. Pravastatin is endowed with cardiac antihypertrophic properties probably through activation of KATPchannels, independent of lipid and hemodynamic changes.

Expression and function of potassium channels in the human placental vasculature

In the placental vasculature, where oxygenation may be an important regulator of vascular reactivity, there is a paucity of data on the expression of potassium (K) channels, which are important mediators of vascular smooth muscle tone. We therefore addressed the expression and function of several K channel subtypes in human placentas. The expression of voltage-gated (Kv)2.1, KV9.3, large-conductance Ca2+-activated K channel (BKCa), inward-rectified K+ channel (KIR)6.1, and two-pore domain inwardly rectifying potassium channel-related acid-sensitive K channels (TASK)1 in chorionic plate arteries, veins, and placental homogenate was assessed by RT-PCR and Western blot analysis. Functional activity of K channels was assessed pharmacologically in small chorionic plate arteries and veins by wire myography using 4-aminopyridine, iberiotoxin, pinacidil, and anandamide. Experiments were performed at 20, 7, and 2% oxygen to assess the effect of oxygenation on the efficacy of K channel modulators. KV2.1, KV9.3, BKCa, KIR6.1, and TASK1 channels were all demonstrated to be expressed at the message level. KV2.1, BKCa, KIR6.1, and TASK1 were all demonstrated at the protein level. Pharmacological manipulation of voltage-gated and ATP-sensitive channels produced the most marked modifications in vascular tone, in both arteries and veins. We conclude that K channels play an important role in controlling placental vascular function.

KATP channel conductance of descending vasa recta pericytes

Using nystatin-perforated patch-clamp and whole cell recording, we tested the hypothesis that K(ATP) channels contribute to resting conductance of rat descending vasa recta (DVR) pericytes and are modulated by vasoconstrictors. The K(ATP) blocker glybenclamide (Glb; 10 microM) depolarized pericytes and inhibited outward currents of cells held at -40 mV. K(ATP) openers pinacidil (Pnc; 10 microM) and P-1075 (1 microM) hyperpolarized pericytes and transiently augmented outward currents. All effects of Pnc and P-1075 were fully reversed by Glb. Inward currents of pericytes held at -60 mV in symmetrical 140 mM K(+) were markedly augmented by Pnc and fully reversed by Glb. Ramp depolarizations in symmetrical K(+), performed in Pnc and Pnc + Glb, yielded a Pnc-induced, Glb-sensitive K(ATP) difference current that lacked rectification and reversed at 0 mV. Immunostaining identified both K(IR)6.1, K(IR)6.2 inward rectifier subunits and sulfonurea receptor subtype 2B. ANG II (1 and 10 nM) and endothelin-1 (10 nM) but not vasopressin (100 nM) significantly lowered holding current at -40 mV and abolished Pnc-stimulated outward currents. We conclude that DVR pericytes express K(ATP) channels that make a significant contribution to basal K(+) conductance and are inhibited by ANG II and endothelin-1.

Developmental expression of vasoactive and growth factors in human lung. Role in pulmonary vascular resistance adaptation at birth

The factors that mediate the postnatal fall in pulmonary vascular resistance, which is crucial for normal gas exchange, are not fully understood. The endothelium has been implicated in this phenomenon, through the release of vasorelaxant factors such as nitric oxide (NO). Human pulmonary expression of endothelial NO synthase increases up to 31 wk of gestation, together with vascular endothelial growth factor (VEGF), and both factors potently mediate pulmonary angiogenesis and vasorelaxation. During the perinatal period, when pulmonary vasodilatation is maximal, endothelial NO synthase and VEGF are weakly expressed. This raises the involvement of vasorelaxant factors other than NO at birth. One candidate is endothelial-derived hyperpolarizing factor, which induces smooth muscle cell hyperpolarization by activating K(ATP) channels. The marked vasorelaxation induced by activation of these channels in newborn animals, and their strong perinatal expression in the human lung, suggest their involvement during this phase. Another candidate is endothelin (ET)-1, together with its receptors ET-A and ET-B. ET-A receptors are located exclusively on smooth muscle cells and mediate vasoconstriction, whereas ET-B receptors mediate vasoconstriction when located on smooth muscle cells and vasodilatation when located on endothelial cells. ET-B receptors, which are strongly expressed in the human fetal lung both at the end of gestation and after birth, may be involved in perinatal pulmonary vasodilatation. Thus, in human fetal lung, K(ATP) channels and ET-B receptors could be important in mediating the perinatal pulmonary vasodilatation crucial for adapting the pulmonary circulation to extrauterine life.

Sarafotoxin 6c (S6c) Reduces Infarct Size and Preserves mRNA for the ETB Receptor in the Ischemic/Reperfused Myocardium of Anesthetized Rats

The aims of this study were to determine if the ETB receptor agonist, sarafotoxin 6c (S6c) reduces myocardial infarct size following myocardial ischemia and reperfusion and to investigate whether any changes in mRNA for endothelin receptors in the injured myocardium were modified by S6c pretreatment. Hypnorm/Hypnovel anesthetized rats were subjected to occlusion of the left main coronary artery for 30 minutes, followed by 120 minutes reperfusion. Animals were administered a bolus dose of S6c (0.24 nmol kg-1 i.v., n = 10) or saline (n = 15) 5 minutes prior to occlusion. At the end of reperfusion, hearts were stained with Evan's Blue dye to delineate area at risk. A 1.5- to 2.0-mm thick slice was cut transmurally 1 mm below the site of ligation for assessment of infarct size by triphenyltetrazolium chloride. A further transmural slice (2.5-3-mm thick) was cut for assessment of receptor mRNA levels by RTPCR. Administration of S6c caused a transient fall in mean arterial blood pressure (MABP) prior to occlusion and attenuated the fall in MABP induced by coronary occlusion. S6c significantly reduced infarct size (13 +/- 4% of area of slice at risk) compared with control hearts (35 +/- 5%; P < 0.05). In control hearts, there was a marked reduction in mRNA content for both ETA (50% reduction) and ETB (70% reduction) receptors in the ischemic zone, compared with non-ischemic tissue. In hearts pre-treated with S6c there was a reduction in ETA, but not ETB receptor mRNA in the ischemic zone. This study has shown that S6c reduces myocardial infarct size and results in preservation of ETB receptor mRNA in ischemic/reperfused tissue.

Alpha1-adrenoceptor-mediated breakdown of phosphatidylinositol 4,5-bisphosphate inhibits pinacidil-activated ATP-sensitive K+ currents in rat ventricular myocytes

Phosphatidylinositol 4,5-bisphosphate (PIP2) stimulates ATP-sensitive K+ (K(ATP)) channel activity. Because phospholipase C (PLC) hydrolyzes membrane-bound PIP2, which in turn may potentially decrease K(ATP) channel activity, we investigated the effects of the alpha1-adrenoceptor-G(q)-PLC signal transduction axis on pinacidil-activated K(ATP) channel activity in adult rat and neonatal mouse ventricular myocytes. The alpha1-adrenoceptor agonist methoxamine (MTX) reversibly inhibited the pinacidil-activated K(ATP) current in a concentration-dependent manner (IC50 20.9+/-6.6 micromol/L). This inhibition did not occur when the specific alpha1-adrenoceptor antagonist, prazosin, was present. An involvement of G proteins is suggested by the ability of GDPbetaS to prevent this response. Blockade of PLC by U-73122 (2 micromol/L) or neomycin (2 mmol/L) attenuated the MTX-induced inhibition of K(ATP) channel activity. In contrast, the MTX response was unaffected by protein kinase C inhibition or stimulation by H-7 (100 micro mol/L) or phorbol 12,13-didecanoate. The MTX-induced inhibition became irreversible in the presence of wortmannin (20 micro mol/L), an inhibitor of phosphatidylinositol-4 kinase, which is expected to prevent membrane PIP2 replenishment. In excised inside-out patch membranes, pinacidil induced a significantly rightward shift of ATP sensitivity of the channel. This phenomenon was reversed by pretreatment of myocytes with MTX. Direct visualization of PIP2 subcellular distribution using a PLCdelta pleckstrin homology domain-green fluorescent protein fusion constructs revealed reversible translocation of green fluorescent protein fluorescence from the membrane to the cytosol after alpha1-adrenoceptor stimulation. Our data demonstrate that alpha1-adrenoceptor stimulation reduces the membrane PIP2 level, which in turn inhibits pinacidil-activated K(ATP) channels.

Coronary metabolic adaptation restricted by endothelin in the dog heart

Endothelin elicits long-lasting vasoconstriction in the coronary bed. This remarkable spastic response raises the question whether or not the metabolic adaptive mechanisms of the coronaries are activated under endothelin effect. The role of the compensatory mediators adenosine and inosine was investigated before and after intracoronary (i.c.) administration of endothelin-1 (ET-1, 1.0 nmol) using 1-min reactive hyperemia (RH) tests on in situ dog hearts (n=15) with or without blocking the ATP-sensitive potassium (K+(ATP)) channels by glibenclamide (GLIB, 1.0 micromol min(-1), i.c.). The release of adenosine and inosine via the coronary sinus was measured by HPLC during the first minute of RH. Endothelin-1 reduced baseline coronary blood flow (CBF) and RH response (hyperemic excess flow (EF) control vs. ET-1: 81.7+/-13.6 vs. 43.4+/-10.9 ml, P<0.01), while it increased the net nucleoside release (adenosine, control vs. ET-1: 58.9+/-20.4 vs. 113.7+/-39.4 nmol, P<0.05; inosine: 242.1+/-81.8 vs. 786.9+/-190.8 nmol, P<0.05). GLIB treatment alone did not change baseline CBF but also reduced RH significantly and increased nucleoside release (EF control vs. GLIB: 72.1+/-11.7 vs. 31.9+/-5.5 ml, P<0.01; adenosine: 18.8+/-4.6 vs. 63.0+/-24.8 nmol, P<0.05; inosine: 113.0+/-37.2 vs. 328.2+/-127.5 nmol, P<0.05). Endothelin-1 on GLIB-treated coronaries further diminished RH and increased nucleoside release (EF: 21.5+/-8.0 ml, P<0.05 vs. GLIB; adenosine: 75.3+/-28.1 nmol, NS; inosine: 801.9+/-196.6 nmol, P<0.05 vs. GLIB). The data show that ET-1 reduces metabolic adaptive capacity of the coronaries, and this phenomenon is due to decreased vascular responsiveness and not to the blockade of ischemic mediator release from the myocardium. The coronary effect of ET-1 may partially be dependent on K+(ATP) channels.

Mechanism of hypoxic pulmonary vasoconstriction involves ET(A) receptor-mediated inhibition of K(ATP) channel

There is controversy on the role of endothelin (ET)-1 in the mechanism of hypoxic pulmonary vasoconstriction (HPV). Although HPV is inhibited by ET-1 subtype A (ET(A))-receptor antagonists in animals, it has been reported that ET(A)-receptor blockade does not affect HPV in isolated lungs. Thus we reassessed the role of ET-1 in HPV in both rats and isolated blood- and physiological salt solution (PSS)-perfused rat lungs. In rats, the ET(A)-receptor antagonist BQ-123 and the nonselective ET(A)- and ET(B)-receptor antagonist PD-145065, but not the ET(B)-receptor antagonist BQ-788, inhibited HPV. Similarly, BQ-123, but not BQ-788, attenuated HPV in blood-perfused lungs. In PSS-perfused lungs, either BQ-123, BQ-788, or the combination of both attenuated HPV equally. Inhibition of HPV by combined BQ-123 and BQ-788 in PSS-perfused lungs was prevented by costimulation with angiotensin II. The ATP-sensitive K(+) (K(ATP))-channel blocker glibenclamide also prevented inhibition of HPV by BQ-123 in both lungs and rats. These results suggest that ET-1 contributes to HPV in both isolated lungs and intact animals through ET(A) receptor-mediated suppression of K(ATP)-channel activity.

Effects of low doses of endothelin-1 on basal vascular tone and autoregulatory vasodilation in canine coronary microcirculation in vivo

The plasma level of endothelin-1 (ET-1) increases in several cardiovascular disorders. The present study examined whether threshold doses of ET-1 affect vascular tone and autoregulatory vasodilation during a reduction in perfusion pressure in the coronary microcirculation in vivo. In anesthetized open-chest dogs, arterial microvessels in the epimyocardium were observed through a microscope equipped with a floating objective. In 6 dogs, ET-1 (10(-13) to 10(-8)mol/L) was superfused onto the epimyocardium in a cumulative fashion. In another set of dogs (n= 16), the perfusion pressure of the observed vascular bed was reduced to 60 mmHg (mild stenosis) and to 40 mmHg (severe stenosis) by a hydraulic occluder, and the microvascular responses were observed in the presence (n=9) or absence (n=7) of ET-1 (10(-12) or 10(-11) mol/L). ET-1 > or =10(-11) mol/L constricted coronary arterioles (< or =100 microm in diameter) and small arteries (>100 microm in diameter) in a dose-dependent fashion. ET-1 of 10(-12) mol/L affected neither the basal diameters nor the dilation of vessels during the pressure reduction. ET-1 of 10(-11) mol/L decreased the diameters of arterioles and small arteries before and during the mild and severe stenosis. However, ET-1 did not attenuate the percentage dilation of arterioles from the baseline in response to the mild and severe stenosis. The data indicates the following: (1) ET-1 at doses > or =10(-11) mol/L similarly constricts coronary arterioles and small arteries; (2) ET-1 at 10(-11) mol/L, which is slightly higher than the pathophysiological plasma level, increases the basal vascular tone, but does not attenuate the autoregulatory vasodilation of the coronary microcirculation.

Compensation of endothelin-1-induced coronary vasoconstriction

The vasodilator capacity of the coronaries was determined by the reactive hyperemia (RH) test in open-chest anesthetized dogs. The myocardial release of adenine nucleosides (adenosine and inosine) was measured by the HPLC-UV method. In group I (n = 9) after the control RH test, a bolus injection of endothelin-1 (ET-1; 1.0 nmol i.c.) was administered and was followed by a second RH test. In group II (n = 9), glibenclamide (GLIB) was infused continuously (1.0 mumol/min i.c.) and RH tests were performed during the control period and then before and after bolus injection of ET-1. In contrast to the significant reduction of the RH response after ET-1 in group I and after GLIB in group II, the nucleoside release into the coronary sinus during the first minute of the RH test was significantly higher (adenosine release 0.05 +/- 0.02 vs. 0.10 +/- 0.04 mumol, and 0.02 +/- 0.00 vs. 0.08 +/- 0.02 mumol; p < 0.05). Injection of ET-1 did not result in further RH reduction in GLIB-pretreated dogs (group II) but significantly increased nucleoside release. High doses of ET-1 activated the metabolic compensatory mechanisms of the myocardium and thereby increased the release of adenine nucleosides into the venous blood of the heart. However, whether these metabolites can exert any significant compensatory vasodilator effects appears doubtful.

Functional communication between cardiac ATP-sensitive K+ channel and Na/K ATPase

INTRODUCTION

Functional interaction between K(ATP) channel and Na/K ATPase was studied in single guinea pig ventricular myocytes because both membrane molecules are known to be involved in ischemic episodes.

METHODS AND RESULTS

K(ATP) channel currents were recorded at 36 degrees C by using whole cell, cell-attached, inside-out, and open cell-attached modes of patch clamp techniques on enzymatically isolated ventricular myocytes. In the whole cell mode, ouabain (1 microM) reversibly inhibited the K(ATP) currents induced by metabolic stress (ATP-free pipette solution and 1 mM NaCN), but not those activated by cromakalim (100 microM), a K(ATP) channel opener. In the cell-attached mode, ouabain concentration dependently inhibited K(ATP) channel opening induced by metabolic suppression (5.5 mM 2-deoxyglucose and 1 mM CN-). Half-inhibition concentration for ouabain was 21.0 +/- 5.5 nM and the Hill coefficient was 0.8 +/- 0.1 (n = 26). However, ouabain did not have an effect on the channel activity induced by cromakalim (100 microM). In the inside-out mode, ouabain applied to the internal side of membrane did not affect the channel. In the open cell-attached mode made by preincubation with streptolysin-O (0.08 U/mL), the K(ATP) channels were not activated by the metabolic inhibitors but were by reducing extracellular ATP concentrations, because subsarcolemmal ATP concentration could be controlled through tiny membrane holes. The channels thus activated were not suppressed by ouabain.

CONCLUSION

The inhibition of Na/K ATPase by ouabain appeared to block the K(ATP) channels by accumulating subsarcolemmal ATP caused by a decrease of the transition from ATP to ADP. In the presence of ischemic episodes, the administration of digitalis compounds may affect the opening of K(ATP) channels, which is primarily protective against the development of irreversible myocardial damage.

Endothelin impairs ATP-sensitive K+ channel function after brain injury

In piglets, pial arteries constrict, ATP-sensitive K+ (KATP) channel function is impaired, and cerebrospinal fluid endothelin-1 (ET-1) increases to 10(-10) M after brain injury [fluid percussion injury (FPI)]. Nitric oxide (NO) elicits dilation via guanosine 3',5'-cyclic monophosphate (cGMP) and KATP channel activation. This study was designed to characterize the relationship between ET-1 and impaired function of KATP channels after FPI. Injury was produced via the lateral FPI technique in piglets equipped with a closed cranial window. Cromakalim, a KATP agonist, produced dilation that was attenuated by FPI and partially restored by BQ-123, an ET-1 antagonist (11 +/- 1 and 23 +/- 2 vs. 2 +/- 1 and 4 +/- 1 vs. 8 +/- 1 and 17 +/- 2% for responses to 10(-8) and 10(-6) M cromakalim before FPI, after FPI, and after FPI with BQ-123, respectively). Because ET-1 constriction may antagonize dilation, separate experiments were conducted under conditions of equivalent baseline diameter in the absence and presence of ET-1 (10(-10) M). Cromakalim dilation was attenuated by ET-1 and partially restored by the protein kinase C (PKC) inhibitor staurosporine (12 +/- 1 and 28 +/- 1 vs. 2 +/- 1 and 21 +/- 3 vs. 9 +/- 1 and 29 +/- 2% for 10(-8) and 10(-6) M cromakalim, cromakalim with ET-1, and cromakalim with ET-1 + staurosporine, respectively). Similar interactions were observed with calcitonin gene-related peptide, 8-bromoguanosine 3',5'-cyclic monophosphate, and the NO releasers sodium nitroprusside and S-nitroso-N-acetylpenicillamine. These data show that ET-1 blunts KATP channel-, NO-, and cGMP-mediated dilation. These data suggest that ET-1 contributes to altered cerebral hemodynamics after FPI through impairment of KATP channel function via PKC activation.