Involvement of voltage-dependent potassium channels in the EDHF-mediated relaxation of rat hepatic artery

Altered cyclooxygenase-1 and enhanced thromboxane receptor activities underlie attenuated endothelial dilatory capacity of omental arteries in obesity

AIMS

Obesity is a risk factor for endothelial dysfunction, the severity of which is likely to vary depending on extent and impact of adiposity on the vasculature. This study investigates the roles of cyclooxygenase isoforms and thromboxane receptor activities in the differential endothelial dilatory capacities of arteries derived from omental and subcutaneous adipose tissues in obesity.

MAIN METHODS

Small arteries were isolated from omental and subcutaneous adipose tissues obtained from consented morbidly obese patients (n = 65, BMI 45 ± 6 kg m^-2 [Mean ± SD]) undergoing bariatric surgery. Relaxation to acetylcholine was studied by wire myography in the absence or presence of indomethacin (10 μM, cyclooxygenase inhibitor), FR122047 (1 μM, cyclooxygenase-1 inhibitor), Celecoxib (4 μM, cyclooxygenase-2 inhibitor), Nω-Nitro-L-arginine methyl ester (L-NAME, 100 μM, nitric oxide synthase inhibitor) or combination of apamin (0.5 μM) and charybdotoxin (0.1 μM) that together inhibit endothelium-derived hyperpolarizing factor (EDHF). Contractions to U46619 (thromboxane A₂ mimetic) were also studied.

KEY FINDINGS

Acetylcholine relaxation was significantly attenuated in omental compared with subcutaneous arteries from same patients (p < 0.01). Indomethacin (p < 0.01) and FR122047 (p < 0.001) but not Celecoxib significantly improved the omental arteriolar relaxation. Cyclooxygenase-1 mRNA and U46619 contractions were both increased in omental compared with subcutaneous arteries (p < 0.05). L-NAME comparably inhibited acetylcholine relaxation in both arteries, while apamin+charybdotoxin were less effective in omental compared with subcutaneous arteries.

SIGNIFICANCE

The results show that the depot-specific reduction in endothelial dilatory capacity of omental compared with subcutaneous arteries in obesity is in large part due to altered cyclooxygenase-1 and enhanced thromboxane receptor activities, which cause EDHF deficiency.

Nonlinear effects of potassium channel blockers on endothelium-dependent hyperpolarization

In a number of published studies on endothelium-dependent hyperpolarization and relaxation, the results of the effects of K⁺ blockers have been difficult to interpret. When the effects of two blockers have been studied, often either blocker by itself had little effect, whereas the two blockers combined tended to abolish the responses. Explanations suggested in the literature include an unusual pharmacology of the K⁺ channels, and possible blocker binding interactions. In contrast, when we applied the same blockers to segments of small blood vessels under voltage clamp, the blockers reduced the endothelium-dependent K⁺ current in a linearly additive manner. Resolution of these contrasting results is important as endothelium-derived hyperpolarization (EDH) makes its greatest contribution to vasorelaxation in arterioles and small resistance arteries, where it can exert a significant role in tissue perfusion and blood pressure regulation. Furthermore, EDH is impaired in various diseases. Here, we consider why the voltage-clamp results differ from earlier free-running membrane potential and contractility studies. We fitted voltage-clamp-derived current-voltage relationships with mathematical functions and considered theoretically the effects of partial and total block of endothelium-derived K⁺ -currents on the membrane potential of small blood vessels. When the K⁺ -conductance was partially reduced, equivalent to applying a single blocker, the effect on EDH was small. Total block of the endothelium-dependent K⁺ conductance abolished the hyperpolarization, in agreement with various published studies. We conclude that nonlinear summation of the hyperpolarizing response evoked by endothelial stimulation can explain the variable effectiveness of individual K⁺ channel blockers on endothelium-dependent hyperpolarization and resulting relaxation.

Characteristics of ACh-induced hyperpolarization and relaxation in rabbit jugular vein

BACKGROUND AND PURPOSE

The roles played by endothelium-derived NO and prostacyclin and by endothelial cell hyperpolarization in ACh-induced relaxation have been well characterized in arteries. However, the mechanisms underlying ACh-induced relaxation in veins remain to be fully clarified.

EXPERIMENTAL APPROACH

ACh-induced smooth muscle cell (SMC) hyperpolarization and relaxation were measured in endothelium-intact and -denuded preparations of rabbit jugular vein.

KEY RESULTS

In endothelium-intact preparations, ACh (≤ 10⁻⁸ M) marginally increased the intracellular concentration of Ca²⁺ ([Ca²⁺](i)) in endothelial cells but did not alter the SMC membrane potential. However, ACh (10⁻¹⁰ -10⁻⁸ M) induced a concentration-dependent relaxation during the contraction induced by PGF(2α) and this relaxation was blocked by the NO synthase inhibitor N(ω) -nitro-l-arginine. ACh (10⁻⁸ -10⁻⁶ M) concentration-dependently increased endothelial [Ca²⁺](i) and induced SMC hyperpolarization and relaxation. These SMC responses were blocked in the combined presence of apamin [blocker of small-conductance Ca²⁺-activated K⁺ (SK(Ca) , K(Ca) 2.3) channel], TRAM 34 [blocker of intermediate-conductance Ca²⁺ -activated K⁺ (IK(Ca) , K(Ca) 3.1) channel] and margatoxin [blocker of subfamily of voltage-gated K⁺ (K(V) ) channel, K(V) 1].

CONCLUSIONS AND IMPLICATIONS

In rabbit jugular vein, NO plays a primary role in endothelium-dependent relaxation at very low concentrations of ACh (10⁻¹⁰ -10⁻⁸ M). At higher concentrations, ACh (10⁻⁸ -3 × 10⁻⁶ M) induces SMC hyperpolarization through activation of endothelial IK(Ca) , K(V) 1 and (possibly) SK(Ca) channels and produces relaxation. These results imply that ACh regulates rabbit jugular vein tonus through activation of two endothelium-dependent regulatory mechanisms.

The contribution of d-tubocurarine-sensitive and apamin-sensitive K-channels to EDHF-mediated relaxation of mesenteric arteries from eNOS-/- mice

The nature of the potassium channels involved in determining endothelium-derived hyperpolarizing factor-mediated relaxation was investigated in first-order small mesenteric arteries from male endothelial nitric oxide synthase (eNOS-/-)-knockout and control (+/+) mice. Acetylcholine-induced endothelium-dependent relaxation of small mesenteric arteries of eNOS-/- was resistant to N-nitro-L-arginine and indomethacin and the guanylyl cyclase inhibitor, 1H-(1,2,4) oxadiazolo (4,3-a) quinoxalin-1-one. Apamin and the combination of apamin and iberiotoxin or apamin and charybdotoxin induced a transient endothelium-dependent contraction of small mesenteric arteries from both eNOS-/- and +/+ mice. Acetylcholine-induced relaxation in eNOS-/- mice was unaffected by charybdotoxin or apamin alone but significantly inhibited by the combination of these agents. However, the combination of scyllatoxin and iberiotoxin did not mimic the inhibitory effect of the apamin/charybdotoxin combination. Tubocurarine alone completely blocked acetylcholine-induced relaxation in eNOS-/- mice. Single channel analysis of myocytes from small mesenteric arterioles revealed a large conductance calcium-activated potassium channel that was sensitive to iberiotoxin, charybdotoxin, and tetraethylammonium. Tubocurarine blocked this channel from the cytosolic side but not when applied extracellularly. Solutions of nitric oxide (NO) gas also relaxed small mesenteric arteries that had been contracted with cirazoline in a concentration-dependent manner, and the sensitivity to NO was reduced by iberiotoxin and the combination of apamin, scyllatoxin, or tubocurarine with charybdotoxin but not by apamin, charybdotoxin, scyllatoxin, or tubocurarine alone. These data indicate that acetylcholine-induced endothelium-derived hyperpolarizing factor-mediated relaxation in small mesenteric arteries from eNOS-/- involved the activation of tubocurarine and apamin-/charybdotoxin-sensitive K-channels. In eNOS+/+ mice, the acetylcholine-induced response was primarily mediated by NO and was sensitive to iberiotoxin and the combination of apamin and charybdotoxin.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Endothelium-dependent nitric oxide and hyperpolarization-mediated venous relaxation pathways in rat inferior vena cava

INTRODUCTION

The vascular endothelium plays a major role in the control of arterial tone; however, its role in venous tissues is less clear. The purpose of this study was to determine the role of endothelium in the control of venous function and the relaxation pathways involved.

METHODS

Circular segments of inferior vena cava (IVC) from male Sprague-Dawley rats were suspended between two wires and isometric contraction to phenylephrine (Phe; 10(-5)M) and 96 mM KCl was measured. Acetylcholine (Ach; 10(-10) to 10(-5)M) was added and the percentage of venous relaxation was measured. To determine the role of nitric oxide (NO) and prostacyclin (PGI(2)), vein relaxation was measured in the presence of the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester (L-NAME; 3 × 10(-4) M) and the cyclooxygenase inhibitor indomethacin (10(-5) M). To measure the role of hyperpolarization, vein relaxation was measured in the presence of K(+) channel activator cromakalim (10(-11) to 10(-6) M), and the nonselective K(+) channel blocker tetraethylammonium (TEA; 10(-3) M). To test for the contribution of a specific K(+) channel, the effects of K(+) channel blockers: glibenclamide (adenosine triphosphate [ATP]-sensitive K(ATP), 10(-5) M), 4-aminopyridine (4-AP; voltage-dependent K(v), 10(-3) M), apamin (small conductance Ca(2+)-dependent SK(Ca), 10(-7) M), and iberiotoxin (large conductance Ca(2+)-dependent BK(Ca), 10(-8) M) on Ach-induced relaxation were tested.

RESULTS

Ach caused concentration-dependent relaxation of Phe contraction (maximum 49.9 ± 4.9%). Removal of endothelium abolished Ach-induced relaxation. IVC treatment with L-NAME partially reduced Ach relaxation (32.8 ± 4.9%). In IVC treated with L-NAME plus indomethacin, significant Ach-induced relaxation (33.6 ± 3.2%) could still be observed, suggesting a role of endothelium-derived hyperpolarizing factor (EDHF). In IVC treated with L-NAME, indomethacin and TEA, Ach relaxation was abolished, supporting a role of EDHF. In veins stimulated with high KCl, Ach caused relaxation (maximum 59.5 ± 3.5%) that was abolished in the presence of L-NAME and indomethacin suggesting that any Ach-induced EDHF is blocked in the presence of high KCl depolarizing solution, which does not favor outward movement of K(+) ion and membrane hyperpolarization. Cromakalim, an activator of K(ATP), caused significant IVC relaxation when applied alone or on top of maximal Ach-induced relaxation, suggesting that the Ach response may not involve K(ATP). Ach-induced relaxation was not inhibited by glibenclamide, 4-AP, or apamin, suggesting little role of K(ATP), K(v) or SK(Ca), respectively. In contrast, iberiotoxin significantly inhibited Ach-induced relaxation, suggesting a role of BK(Ca).

CONCLUSIONS

Thus, endothelium-dependent venous relaxation plays a major role in the control of venous function. In addition to NO, an EDHF pathway involving BK(Ca) may play a role in endothelium-dependent venous relaxation.

Endothelium-derived hyperpolarizing factor mediated relaxations in pig coronary arteries do not involve Gi/o proteins

AIM

Endothelium-dependent relaxations to certain neurohumoral substances are mediated by pertussis toxin-sensitive Gi/o protein. Our experiments were designed to determine the role, if any, of pertussis toxin-sensitive G-proteins in relaxations attributed to endothelium-derived hyperpolarizing factor (EDHF).

METHODS

Pig coronary arterial rings with endothelia were suspended in organ chambers filled with Krebs-Ringer bicarbonate solution maintained at 37 degrees and continuously aerated with 95%O2 and 5% CO2. Isometric tension was measured during contractions to prostaglandin F2alpha in the presence of indomethacin and N(omega)- nitro-L-arginine methyl ester (L-NAME).

RESULTS

Thrombin, the thrombin receptor- activating peptide SFLLRN, bradykinin, substance P, and calcimycin produced dose-dependent relaxations. These relaxations were not inhibited by prior incubation with pertussis toxin, but were abolished upon the addition of charybdotoxin plus apamin. Relaxations to the alpha2-adrenergic agonist UK14304 and those to serotonin were abolished in the presence of indomethacin and L-NAME.

CONCLUSION

Unlike nitric oxide-mediated relaxations, EDHF-mediated relaxations of pig coronary arteries do not involve pertussis toxin-sensitive pathways and are Gi/o protein independent.

Mechanisms of relaxation by carbon monoxide-releasing molecule-2 in murine gastric fundus and jejunum

This study investigated the effects and mechanisms of action of carbon monoxide-releasing molecule-2 (CORM-2), compared to those of carbon monoxide (CO), in murine gastric fundus and jejunal circular smooth muscle. Functional in vitro experiments and cGMP measurements were conducted. In both tissues, CO and CORM-2 induced concentration-dependent relaxations. CO-induced relaxations were abolished by the soluble guanylyl cyclase (sGC) inhibitor ODQ, while CORM-2-evoked inhibitory responses were only partly prevented by ODQ. Relaxations elicited by CO (300 microM) were associated with a significant increase in cGMP levels, whereas for CORM-2 (300 microM) no significant increase in cGMP levels could be measured. The sGC sensitizer YC-1 was able to accelerate and potentiate both CO- and CORM-2-induced relaxations. Furthermore, the intermediate- and large-conductance Ca2+-activated K+ (IKCa-BKCa) channel blocker charybdotoxin significantly reduced CO- and CORM-2-induced relaxations in jejunal tissue; this same effect was observed with the BKCa channel blocker iberiotoxin. The combination of apamin plus charybdotoxin significantly reduced relaxations in gastric fundus and had synergistic inhibitory effects in jejunum. The NOS inhibitor L-NAME had no effect on the induced relaxations in gastric fundus, but significantly reduced CO- and CORM-2-evoked relaxations in jejunum. In conclusion, these results demonstrate that CO and CORM-2 produce relaxation in gastric fundus and jejunum via sGC and activation of KCa channels, and a nitric oxide (NO)-mediated amplification of CO signaling in jejunum is suggested.

Endothelium-derived hyperpolarizing factor: where are we now?

The endothelium controls vascular tone not only by releasing nitric oxide (NO) and prostacyclin but also by other pathways causing hyperpolarization of the underlying smooth muscle cells. This characteristic was at the origin of the denomination endothelium-derived hyperpolarizing factor (EDHF). We know now that this acronym includes different mechanisms. In general, EDHF-mediated responses involve an increase in the intracellular calcium concentration, the opening of calcium-activated potassium channels of small and intermediate conductance and the hyperpolarization of the endothelial cells. This results in an endothelium-dependent hyperpolarization of the smooth muscle cells, which can be evoked by direct electrical coupling through myo-endothelial junctions and/or the accumulation of potassium ions in the intercellular space. Potassium ions hyperpolarize the smooth muscle cells by activating inward rectifying potassium channels and/or Na+/K(+)-ATPase. In some blood vessels, including large and small coronary arteries, the endothelium releases arachidonic acid metabolites derived from cytochrome P450 monooxygenases. The epoxyeicosatrienoic acids (EET) generated are not only intracellular messengers but also can diffuse and hyperpolarize the smooth muscle cells by activating large conductance calcium-activated potassium channels. Additionally, the endothelium can produce other factors such as lipoxygenases derivatives or hydrogen peroxide (H2O2). These different mechanisms are not necessarily exclusive and can occur simultaneously.

Role of SK(Ca) and IK(Ca) in endothelium-dependent hyperpolarizations of the guinea-pig isolated carotid artery

1. This study was designed to determine whether the endothelium-dependent hyperpolarizations evoked by acetylcholine in guinea-pig carotid artery involve a cytochrome P450 metabolite and whether they are linked to the activation of two distinct populations of endothelial K(Ca) channels, SK(Ca) and IK(Ca.) 2. The membrane potential was recorded in the vascular smooth muscle cells of the guinea-pig isolated carotid artery. All the experiments were performed in the presence of N(omega)-L-nitro arginine (100 microM) and indomethacin (5 microM). 3. Under control conditions (Ca(2+): 2.5 mM), acetylcholine (10 nM to 10 muM) induced a concentration- and endothelium-dependent hyperpolarization of the vascular smooth muscle cells. Two structurally different specific blockers of SK(Ca), apamin (0.5 microM) or UCL 1684 (10 microM), produced a partial but significant inhibition of the hyperpolarization evoked by acetylcholine whereas charybdotoxin (0.1 microM) and TRAM-34 (10 microM), a nonpeptidic and specific blocker of IK(Ca), were ineffective. In contrast, the combinations of apamin plus charybdotoxin, apamin plus TRAM-34 (10 microM) or UCL 1684 (10 microM) plus TRAM-34 (10 microM) virtually abolished the acetylcholine-induced hyperpolarization. 4. In the presence of a combination of apamin and a subeffective dose of TRAM-34 (5 microM), the residual hyperpolarization produced by acetylcholine was not inhibited further by the addition of either an epoxyeicosatrienoic acid antagonist, 14,15-EEZE (10 microM) or the specific blocker of BK(Ca), iberiotoxin (0.1 microM). 5. In presence of 0.5 mM Ca(2+), the hyperpolarization in response to acetylcholine (1 microM) was significantly lower than in 2.5 mM Ca(2+). The EDHF-mediated responses became predominantly sensitive to charybdotoxin or TRAM-34 but resistant to apamin. 6. This investigation shows that the production of a cytochrome P450 metabolite, and the subsequent activation of BK(Ca), is unlikely to contribute to the EDHF-mediated responses in the guinea-pig carotid artery. Furthermore, the EDHF-mediated response involves the activation of both endothelial IK(Ca) and SK(Ca) channels, the activation of either one being able to produce a true hyperpolarization.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

Endothelial potassium channels, endothelium-dependent hyperpolarization and the regulation of vascular tone in health and disease

1. The elusive nature of endothelium-derived hyperpolarizing factor (EDHF) has hampered detailed study of the ionic mechanisms that underlie the EDHF hyperpolarization and relaxation. Most studies have relied on a pharmacological approach in which interpretations of results can be confounded by limited specificity of action of the drugs used. Nevertheless, small-, intermediate- and large-conductance Ca2+-activated K+ channels (SKCa, IKCa and BKCa, respectively) have been implicated, with inward rectifier K+ channels (KIR) and Na+/K+-ATPase also suggested by some studies. 2. Endothelium-dependent membrane currents recorded using single-electrode voltage-clamp from electrically short lengths of arterioles in which the smooth muscle and endothelial cells remained in their normal functional relationship have provided useful insights into the mechanisms mediating EDHF. Charybdotoxin (ChTx) or apamin reduced, whereas apamin plus ChTx abolished, the EDHF current. The ChTx- and apamin-sensitive currents both reversed near the expected K+ equilibrium potential, were weakly outwardly rectifying and displayed little, if any, time- or voltage-dependent gating, thus having the biophysical and pharmacological characteristics of IKCa and SKCa channels, respectively. 3. The IKCa and SKCa channels occur in abundance in endothelial cells and their activation results in EDHF-like hyperpolarization of these cells. There is little evidence for a significant number of these channels in healthy, contractile vascular smooth muscle cells. 4. In a number of blood vessels in which EDHF occurs, the endothelial and smooth muscle cells are coupled electrically via myoendothelial gap junctions. In contrast, in the adult rat femoral artery, in which the smooth muscle and endothelial layers are not coupled electrically, EDHF does not occur, even though acetylcholine evokes hyperpolarization in the endothelial cells. 5. In vivo studies indicate that EDHF contributes little to basal conductance of the vasculature, but it contributes appreciably to evoked increases in conductance. 6. Endothelium-derived hyperpolarizing factor responses are diminished in some diseases, including hypertension, pre-eclampsia and some models of diabetes. 7. The most economical explanation for EDHF in vitro and in vivo in small vessels is that it arises from the activation of IKCa and SKCa channels in endothelial cells. The resulting endothelial hyperpolarization spreads via myoendothelial gap junctions to result in the EDHF-attributed hyperpolarization and relaxation of the smooth muscle.

Participation of intracellular Ca2+ stores in arteriolar conducted responses

We examined the role played by intracellular Ca2+ stores in conducted vasomotor responses induced by phenylephrine (PE) in isolated hamster cremasteric arterioles. When applied briefly ( approximately 1 s) to isolated, cannulated arterioles by using pressure-pulse ejection from a micropipette, PE produced a strong local vasoconstriction and a very small biphasic conducted response (a small constriction followed by a dilation) that propagated several hundred micrometers along the vessel length. The conducted vasomotion was associated with a monophasic elevation of the endothelial cell intracellular Ca2+ concentration ([Ca2+]i) at the site of stimulation, as measured with the Ca2+ indicator fura 2. The Ca2+ pump inhibitor thapsigargin was used to limit filling of Ca2+ stores in smooth muscle and endothelial cells. Thapsigargin reduced baseline diameter and elicited a strong dilator component at the local site while enhancing both the constrictor and dilator components of the PE-induced conducted response. The enhanced conducted constrictor component induced by thapsigargin was mimicked by extraluminal application of tetraethylammonium or charybdotoxin but not by iberiotoxin, apamin, glibenclamide, barium, or 4-aminopirydine. Thapsigargin increased the estimated basal endothelial cell [Ca2+]i by approximately 60 nM and converted the PE-induced change in [Ca2+]i from monotonic to biphasic with a late elevation of [Ca2+]i above baseline that coincided with the increased dilatory component of the conducted response. Luminal application of charybdotoxin plus apamin significantly reduced the dilatory component of the conducted response. These results indicate that intracellular Ca2+ stores play a dynamic role in regulating conducted vasomotor responses apparently through modulation of KCa channels in both cell types.

The vascular response to the K+ channel inhibitor 4-aminopyridine in hypertensive rats

The K+ channel inhibitor 4-aminopyridine induced an immediate increase in blood pressure and tension in spontaneously hypertensive rats (SHR). Further analysis strongly suggested this to be due to closure of vascular smooth muscle K+ channels, as previously concluded for normotensive rats (WKY). The tension response was greater in SHR than WKY, suggesting an increased channel activity in order to compensate for the high total peripheral vascular resistance in SHR. The response was enhanced after nitric oxide (NO) synthase inhibitor in both strains, probably reflecting increased channel activity after elimination of the NO-cGMP pathway. The response in SHR but not WKY was increased after alpha(1)-adrenoceptor inhibition and adrenalectomy but not sympathetic nerve transmitter depletion. It increased also after angiotensin AT(1) and endothelin ET(A) receptor antagonists and protein kinase C inhibitor. These results indicated an increased adrenal catecholamine, angiotensin AT(1) and endothelin ET(A) activation of the phospholipase C-protein kinase C pathway in SHR, inhibiting the 4-aminopyridine-sensitive K+ channels.

Vasodilator actions of abnormal-cannabidiol in rat isolated small mesenteric artery

1. The nonpsychoactive cannabinoid abnormal-cannabidiol (trans-4-[3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]-5-pentyl-1,3-benzenediol) (abn-cbd) produced concentration-dependent relaxation of methoxamine-precontracted rat small mesenteric artery. Endothelial removal reduced abn-cbd potency six-fold without affecting the maximum relaxation. 2. In endothelium-intact vessels, abn-cbd was less potent under 60 mM KCl-induced tone and inhibited by combination of L-N(G)-nitroarginine methyl ester (L-NAME) (nitric oxide synthase inhibitor; 300 micro M), apamin (small conductance Ca(2+)-activated K(+) channels inhibitor; 50 nM) and charybdotoxin (inhibitor of intermediate conductance Ca(2+)-activated K(+) channels and large conductance Ca(2+)-activated K(+) channels BK(Ca); 50 nM). L-NAME alone or in combination with either toxin alone had little effect. 3. In intact vessels, relaxations to abn-cbd were inhibited by SR 141716A (cannabinoid receptor antagonist; 1 or 3 micro M). Concomitant addition of L-NAME, apamin and charybdotoxin had no further effect. Other cannabinoid receptor antagonists either had little (SR 144528; 1 micro M and AM 251; 1 micro M) or no effect (AM 630; 10 micro M and AM 281; 1 micro M). Inhibition of gap junctions, G(i/o) protein coupling and protein kinase A also had no effect. 4. Endothelium-independent relaxation to abn-cbd was unaffected by L-NAME, apamin plus charybdotoxin or capsaicin (10 micro M). Abn-cbd inhibited CaCl(2)-induced contractions in vessels with depleted intracellular Ca(2+) stores and stimulated with methoxamine or KCl. This was insensitive to SR 141716A (3 micro M) but greatly reduced in vessels stimulated with ionomycin (Ca(2+) ionophore; 1 micro M). 5. We conclude that abn-cbd relaxes the rat small mesenteric artery by endothelium-dependent activation of K(+) channels via SR 141716A-sensitive pathways, which do not involve CB(1) and CB(2) receptors. It also causes endothelium-independent, SR 141716A-insensitive, relaxation by inhibiting Ca(2+) entry through voltage-gated Ca(2+) channels.

The Endothelium in Health and Disease-A Target for Therapeutic Intervention

In this review we discuss the contribution of NO, prostacyclin and endothelium-derived relaxing factor--endothelium-derived hyperpolarizing factor, or EDHF, to vascular function. We also explore the hypotheses (1): that tissues can store NO as nitrosothiols (RSNOs) and (2) that such RSNO stores can be modulated by physiological and pathophysiological processes. Notably in the microcirculation, EDHF appears to play an important role in the regulation of vascular tone. Leading candidates for EDHF include extracellular potassium (K+), an epoxygenase product, hydrogen peroxide and/or a contribution from myoendothelial gap junctions. Data from our laboratory indicate that in mouse vessels, different endothelium-dependent vasodilators, such as acetylcholine and protease-activated receptor (PAR) agonists, release different endothelium-derived relaxing factors. The combination of two K-channel toxins, apamin and charybdotoxin, inhibits EDHF activity in most protocols. Endothelial dysfunction is considered as the major risk factor and a very early indicator of cardiovascular disease including the cardiovascular complications of type I & types II diabetes. Impaired endothelium-dependent vasodilatation results primarily from a decreased synthesis of endothelium-derived nitric oxide (NO) and/or an increase in the production of reactive oxygen species such as superoxide. We have shown that the administration of tetrahydrobiopterin, an important co-factor for nitric oxide synthase (NOS) partially restores endothelial function (1) in leptin-deficient mice (db/db) with spontaneous type II diabetes, as well as (2) in human vascular tissue harvested for coronary artery bypass grafting (CABG). These data suggest that a deficiency in the availability of tetrahydrobiopterin plays an important role in vascular dysfunction associated with Type II diabetes. In addition, changes in the contribution of EDHF occur in vascular tissue from the db/db mice suggesting a compensatory increase in EDHF production; whether this alteration in EDHF production is physiological or pathophysiological remains controversial.

Characterization of a charybdotoxin-sensitive intermediate conductance Ca2+-activated K+ channel in porcine coronary endothelium: relevance to EDHF

1. This study characterizes the K(+) channel(s) underlying charybdotoxin-sensitive hyperpolarization of porcine coronary artery endothelium. 2. Two forms of current-voltage (I/V) relationship were evident in whole-cell patch-clamp recordings of freshly-isolated endothelial cells. In both cell types, iberiotoxin (100 nM) inhibited a current active only at potentials over +50 mV. In the presence of iberiotoxin, charybdotoxin (100 nM) produced a large inhibition in 38% of cells and altered the form of the I/V relationship. In the remaining cells, charybdotoxin also inhibited a current but did not alter the form. 3. Single-channel, outside-out patch recordings revealed a 17.1+/-0.4 pS conductance. Pipette solutions containing 100, 250 and 500 nM free Ca(2+) demonstrated that the open probability was increased by Ca(2+). This channel was blocked by charybdotoxin but not by iberiotoxin or apamin. 4. Hyperpolarizations of intact endothelium elicited by substance P (100 nM; 26.1+/-0.7 mV) were reduced by apamin (100 nM; 17.0+/-1.8 mV) whereas those to 1-ethyl-2-benzimidazolinone (1-EBIO, 600 microM, 21.0+/-0.3 mV) were unaffected (21.7+/-0.8 mV). Substance P, bradykinin (100 nM) and 1-EBIO evoked charybdotoxin-sensitive, iberiotoxin-insensitive whole-cell perforated-patch currents. 5 A porcine homologue of the intermediate-conductance Ca(2+)-activated K(+) channel (IK1) was identified in endothelial cells. 6. In conclusion, porcine coronary artery endothelial cells express an intermediate-conductance Ca(2+)-activated K(+) channel and the IK1 gene product. This channel is opened by activation of the EDHF pathway and likely mediates the charybdotoxin-sensitive component of the EDHF response.

Anandamide-induced vasorelaxation in rabbit aortic rings has two components: G protein dependent and independent

The endogenous cannabinoid anandamide (arachidonylethanolamide) produces vasorelaxation in different vascular beds. In the present study, we found that anandamide and a metabolically stable analog, methanandamide, produced dose-dependent (10 nM-10 microM) vasorelaxation of approximately 80% in a rabbit aortic ring preparation in an endothelium-dependent manner. Non-endothelium-dependent vasorelaxation was observed to be a maximum of 20-22% at >10 microM methanandamide. The efficacious CB(1) receptor analogs desacetyllevonantradol (10 microM) and WIN55212-2 (10 microM) failed to produce vasorelaxation; however, the endothelium-dependent vasorelaxation evoked by methanandamide was partially (75%) blocked by the CB(1) receptor antagonist SR141716A. The VR(1) vanilloid receptor antagonist capsazepine or the calcitonin gene-related peptide (CGRP) antagonist CGRP-(8-37) partially attenuated (25%) the vasorelaxation in endothelium-intact preparations and greatly reduced the response in endothelium-denuded preparations. Pretreatment of aortic rings with N(G)-nitro-L-arginine methyl ester completely blocked the methanandamide-, capsaicin-, and CGRP-induced vasorelaxation. Pretreatment of aortic rings with pertussis toxin attenuated the methanandamide-induced vasorelaxation in endothelium-intact aortic rings, indicating the involvement of G(i/o) proteins in the vasorelaxation; however, pertussis toxin treatment failed to block the endothelium-independent response. Thus, in the rabbit aorta, methanandamide-induced vasorelaxation exhibits two components: 1) in endothelium-intact rings, an SR141716A-sensitive, non-CB(1) receptor component that requires pertussis toxin-sensitive G proteins and nitric oxide (NO) production; and 2) in endothelium-denuded rings, a component that is mediated by VR(1) vanilloid receptors and possibly by the subsequent release of CGRP that requires NO production but is independent of pertussis toxin-sensitive G proteins.

Effects of different tetra-n-alkylammonium ions on acetylcholine-induced endothelium-dependent hyperpolarization in rat mesenteric artery

The effects of a series of symmetric tetra-n-alkylammonium (TAA) compounds with alkyl side chains of one to six carbons in length on acetylcholine-induced endothelium-dependent hyperpolarization were examined in rat mesenteric artery. All TAA compounds caused a concentration-dependent inhibition of the hyperpolarizing response to acetylcholine. The potency of TAAs showed a general trend to increase with the lengths of the alkyl side chains. The inhibitory effects of TAAs, excepting the smallest compound, on the acetylcholine response were reversible. However, TAAs with long alkyl side chains may act as antagonists at muscarinic receptors, because the suppressive effect on A23187-induced endothelium-dependent hyperpolarization was more marked with TAAs having smaller alkyl side chains. Conversely, the hyperpolarizing response to pinacidil, an ATP-sensitive K+ channel opener, was significantly prevented only by TAA compounds with long alkyl side chains. TAA compounds with one-to three-carbon alkyl side chains caused a modest and reversible depolarization of the membrane, whereas the depolarizing effects of the compounds with four-to six-carbon alkyl side chains were marked and irreversible. These results suggest that TAAs could gain access to the target K+ channels for endothelium-derived hyperpolarizing factor, the ATP-sensitive K+ channels, and the K+ channels responsible for the regulation of the resting membrane potential in different ways.

Increases in endothelial Ca(2+) activate K(Ca) channels and elicit EDHF-type arteriolar dilation via gap junctions

In skeletal muscle arterioles, the pathway leading to non-nitric oxide (NO), non-prostaglandin-mediated endothelium-derived hyperpolarizing factor (EDHF)-type dilations is not well characterized. To elucidate some of the steps in this process, simultaneous changes in endothelial intracellular Ca(2+) concentration ([Ca(2+)](i)) and the diameter of rat gracilis muscle arterioles (approximately 60 microm) to acetylcholine (ACh) were measured by fura 2 microfluorimetry (in the absence of NO and prostaglandins). ACh elicited rapid increases in endothelial [Ca(2+)](i) (101 +/- 7%), followed by substantial dilations (73 +/- 2%, coupling time: 1.3 +/- 0.2 s) that were prevented by endothelial loading of an intracellular Ca(2+) chelator [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid]. Arteriolar dilations to ACh were also inhibited by intraluminal administration of the Ca(2+)-activated K(+) (K(Ca)) channel blockers charybdotoxin plus apamin or by palmitoleic acid, an uncoupler of myoendothelial gap junctions without affecting changes in endothelial [Ca(2+)](i). The presence of large conductance K(Ca) channels on arteriolar endothelial cells was demonstrated with immunohistochemisty. We propose that in skeletal muscle arterioles, EDHF-type mediation is evoked by an increase in endothelial [Ca(2+)](i), which by activating endothelial K(Ca) channels elicits hyperpolarization that is conducted via myoendothelial gap junctions to the smooth muscle resulting in decreases in [Ca(2+)](i) and consequently dilation.

Determinants of renal microvascular response to ACh: afferent and efferent arteriolar actions of EDHF

The renal microvascular actions of ACh were investigated using the in vitro perfused hydronephrotic rat kidney. ACh reversed ANG II-induced vasoconstriction in the afferent and efferent arteriole by 106 +/- 2 and 75 +/- 5%, respectively. Inhibition of nitric oxide synthase [NOS; 100 micromol/l N(G)-nitro-L-arginine methyl ester (L-NAME)] and cyclooxygenase (COX; 10 micromol/l ibuprofen) prevented the sustained response of the afferent arteriole but did not reduce the magnitude of the initial dilation (97 +/- 7%). However, NOS/COX inhibition abolished the response of the efferent arteriole. The underlying mechanisms mediating this endothelium-derived hyperpolarizing factor (EDHF)-like response were characterized using K channel blockers. Ba (100 micromol/l), tetraethylammonium (1 mmol/l), and ouabain (3 mmol/l) had no effect, arguing against a role of an inward rectifier K channel, large-conductance Ca-activated K channel, or Na,K-ATPase. Charybdotoxin (10 nmol/l) and apamin (1.0micromol/l) attenuated the response when administered alone (63 +/- 7% and 37 +/- 5%, respectively) and abolished the response when coadministered (0.1 +/- 1.0%). These findings indicate that, as in other vascular beds, the renal EDHF-like response to ACh involves K channels that are sensitive to a combination of apamin and charybdotoxin. Our finding that EDHF modulates preglomerular, but not postglomerular, tone is consistent with the evolving concept that vasomotor mechanisms in cortical efferent arterioles do not involve voltage-gated Ca entry.

Epoxyeicosatrienoic acids (EET(11,12)) may partially restore endothelium-derived hyperpolarizing factor-mediated function in coronary microarteries

BACKGROUND

Endothelial cells derive nitric oxide, prostacyclin, and endothelium-derived hyperpolarizing factor (EDHF). The cytochrome P-450-monooxygenase metabolites of arachidonic acid (epoxyeicosatrienoic acids [EETs]) have been suggested to be EDHF. This study was designed to examine the effect of EET(11,12) with regard to the possibility of restoring EDHF function when added into hyperkalemic cardioplegic solution.

METHODS

Porcine coronary microartery rings were studied in a myograph. In groups 1 and 2, paired arteries were incubated in either hyperkalemic solution (K+ 20 mmol/L) or Krebs' solution (control). In group 3, the paired arteries were incubated in hyperkalemia plus EET(11,12) (1 x 10(-6.5) mol/L) or hyperkalemia alone (control) at 37 degrees C for 1 hour, followed by Krebs' washout and then precontracted with 1 x 10(-8.5) mol/L U46619. The EDHF-mediated relaxation to EET(11,12) (group 1) or bradykinin (groups 2 and 3) was studied in the presence of N(G)-nitro-L-arginine, indomethacin, and oxyhemoglobin.

RESULTS

After exposure to hyperkalemia, the EDHF-mediated maximal relaxation by bradykinin (72.5% +/- 7.8% versus 41.6% +/- 10.6%; p < 0.05), but not by EET(11,12) (18.4% +/- 3.3% versus 25.1% +/- 4.9%; p > 0.05) was significantly reduced. Incubation with EET(11,12) partially restored EDHF function (33.3% +/- 9.5% versus 62.0% +/- 8.5%; p < 0.05).

CONCLUSIONS

In coronary microarteries, hyperkalemia impairs EDHF-mediated relaxation, and EET(11,12) may partially mimic the EDHF function. Addition of EET(11,12) into cardioplegic solution may partially restore EDHF-mediated function reduced by exposure to hyperkalemia.

TREK-1 regulation by nitric oxide and cGMP-dependent protein kinase. An essential role in smooth muscle inhibitory neurotransmission

Potassium channels activated by membrane stretch may contribute to maintenance of relaxation of smooth muscle cells in visceral hollow organs. Previous work has identified K(+) channels in murine colon that are activated by stretch and further regulated by NO-dependent mechanisms. We have screened murine gastrointestinal, vascular, bladder, and uterine smooth muscles for the expression of TREK and TRAAK mRNA. Although TREK-1 was expressed in many of these smooth muscles, TREK-2 was expressed only in murine antrum and pulmonary artery. TRAAK was not expressed in any smooth muscle cells tested. Whole cell currents from TREK-1 expressed in mammalian COS cells were activated by stretch, and single channel recordings showed that the stretch-dependent conductance was due to 90 pS channels. Sodium nitroprusside (10(-6) or 10(-5) m) and 8-Br-cGMP (10(-4) or 10(-3) m) increased TREK-1 currents in perforated whole cell and single channel recordings. Mutation of the PKG consensus sequence at serine 351 blocked the stimulatory effects of sodium nitroprusside and 8-Br-cGMP on open probability without affecting the inhibitory effects of 8-Br-cAMP. TREK-1 encodes a component of the stretch-activated K(+) conductance in smooth muscles and may contribute to nitrergic inhibition of gastrointestinal muscles.

Cellular target of voltage and calcium-dependent K(+) channel blockers involved in EDHF-mediated responses in rat superior mesenteric artery

1. We have investigated the cellular target of K(+) channel blockers responsible for the inhibition of the EDHF-mediated relaxation in the rat mesenteric artery by studying their effects on tension, smooth muscle cell (SMC) membrane potential and endothelial cell Ca(2+) signal ([Ca(2+)](endo)). 2. In arteries contracted with prostaglandin F(2 alpha) (2.5 - 10 microM), relaxation evoked by ACh (0.01 - 3 microM) was abolished by a combination of charybdotoxin (ChTX, 0.1 microM) plus apamin (Apa, 0.1 microM) and was inhibited by 68+/-6% (n=6) by 4-aminopyridine (4-AP, 5 mM). 3. ACh(0.001 - 3 microM) increased [Ca(2+)](endo) and hyperpolarized SMCs with the same potency, the pD(2) values were equal to 7.2+/-0.08 (n=4) and 7.2+/-0.07 (n=9), respectively. SMCs hyperpolarization to ACh (1 microM) was abolished by high K(+) solution or by ChTX/Apa. It was decreased by 66+/-5% (n=6) by 4-AP. 4. The increase in [Ca(2+)](endo) evoked by ACh (1 microM) was insensitive to ChTX/Apa but was depressed by 58+/-16% (n=6) and 27+/-4% (n=7) by raising external K(+) concentration and by 4-AP, respectively. 5. The effect of 4-AP on [Ca(2+)](endo) was not affected by increasing external K(+) concentration. In Ca-free/EGTA solution, the transient increase in [Ca(2+)](endo) evoked by ACh (1 microM) was abolished by thapsigargin (1 microM) and was decreased by 75+/-7% (n=5) by 4-AP. 6. These results show that inhibition of EDHF-evoked responses by 4-AP may be attributed to a decrease in the Ca(2+) release activated by ACh in endothelial cells. The abolition of SMCs hyperpolarization to ACh by ChTX/Apa is not related to an interaction with the [Ca(2+)](endo).

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.

Endothelium-derived hyperpolarizing factor : identification and mechanisms of action in human subcutaneous resistance arteries

BACKGROUND

Both a vascular endothelial cytochrome P450 (CYP450) product of arachidonic acid metabolism and the potassium ion (K(+)) have been identified as endothelium-derived hyperpolarizing factors (EDHFs) in animal vascular tissues. We studied the relative importance of EDHF, nitric oxide (NO), and prostacyclin (PGI(2)) as vasodilators in human subcutaneous arteries. We also examined the mechanisms underlying the vasodilator action of EDHF to elucidate its identity.

METHODS AND RESULTS

Subcutaneous resistance arteries were obtained from 41 healthy volunteers. The contribution of EDHF to the vasodilation induced by acetylcholine was assessed by inhibiting production of NO, PGI(2), and membrane hyperpolarization. The mechanisms underlying the relaxation evoked by K(+) and EDHF were elucidated. EDHF was found to account for approximately 80% of acetylcholine-mediated vasorelaxation. Its action was insensitive to the combination of barium and ouabain, whereas barium and ouabain reversed K(+)-mediated vasorelaxation. EDHF-mediated vasorelaxation, however, was sensitive to the phospholipase A(2) inhibitor oleyloxyethyl phosphorylcholine and the CYP450 inhibitor ketoconazole.

CONCLUSIONS

EDHF is the major contributor to endothelium-dependent vasorelaxation in human subcutaneous resistance arteries. A product of phospholipase A(2)/CYP450-dependent metabolism of arachidonic acid and not K(+) is the likely identity of EDHF in human subcutaneous resistance arteries.

K+ currents underlying the action of endothelium-derived hyperpolarizing factor in guinea-pig, rat and human blood vessels

Membrane currents attributed to endothelium-derived hyperpolarizing factor (EDHF) were recorded in short segments of submucosal arterioles of guinea-pigs using single microelectrode voltage clamp. The functional responses of arterioles and human subcutaneous, rat hepatic and guinea-pig coronary arteries were also assessed as changes in membrane potential recorded simultaneously with contractile activity. The current-voltage (I-V) relationship for the conductance due to EDHF displayed outward rectification with little voltage dependence. Components of the current were blocked by charybdotoxin (30-60 nM) and apamin (0.25-0.50 microM), which also blocked hyperpolarization and prevented EDHF-induced relaxation. The EDHF-induced current was insensitive to Ba2+ (20-100 microM) and/or ouabain (1 microM to 1 mM). In human subcutaneous arteries and guinea-pig coronary arteries and submucosal arterioles, the EDHF-induced responses were insensitive to Ba2+ and/or ouabain. Increasing [K+]o to 11-21 mM evoked depolarization under conditions in which EDHF evoked hyperpolarization. Responses to ACh, sympathetic nerve stimulation and action potentials were indistinguishable between dye-labelled smooth muscle and endothelial cells in arterioles. Action potentials in identified endothelial cells were always associated with constriction of the arterioles. 18beta-Glycyrrhetinic acid (30 microM) and carbenoxolone (100 microM) depolarized endothelial cells by 31 +/- 6 mV (n = 7 animals) and 33 +/- 4 mV (n = 5), respectively, inhibited action potentials in smooth muscle and endothelial cells and reduced the ACh-induced hyperpolarization of endothelial cells by 56 and 58 %, respectively. Thus, activation of outwardly rectifying K+ channels underlies the hyperpolarization and relaxation due to EDHF. These channels have properties similar to those of intermediate conductance (IKCa) and small conductance (SKCa) Ca2+-activated K+ channels. Strong electrical coupling between endothelial and smooth muscle cells implies that these two layers function as a single electrical syncytium. The non-specific effects of glycyrrhetinic acid precludes its use as an indicator of the involvement of gap junctions in EDHF-attributed responses. These conclusions are likely to apply to a variety of blood vessels including those of humans.

Bradykinin attenuates the [Ca(2+)](i) response to angiotensin II of renal juxtamedullary efferent arterioles via an EDHF

1. Bradykinin (BK) effect on the [Ca(2+)](i) response to 1 nM angiotensin II was examined in muscular juxtamedullary efferent arterioles (EA) of rat kidney. 2. BK (10 nM) applied during the angiotensin II-stimulated [Ca(2+)](i) increase, induced a [Ca(2+)](i) drop (73+/-2%). This drop was prevented by de-endothelialization and suppressed by HOE 140, a B2 receptor antagonist. It was neither affected by L-NAME or indomethacin, nor mimicked by sodium nitroprusside, 8-bromo-cyclic GMP or PGI(2). The BK effect did not occur when the [Ca(2+)](i) increase was caused by 100 mM KCl-induced membrane depolarization and was abolished by 0.1 microM charybdotoxin, a K(+) channel blocker. 3. Although proadifen prevented the BK-caused [Ca(2+)](i) fall, more selective cytochrome P450 inhibitors, 17-octadecynoic acid (50 microM) and 7-ethoxyresorufin (10 microM) were without effect. 4. Increasing extracellular potassium from 5 to 15 mM during angiotensin II stimulation caused a [Ca(2+)](i) decrease (26+/-4%) smaller than BK which was charybdotoxin-insensitive. Inhibition of inward rectifying K(+) channels by 30 microM BaCl(2) and/or of Na(+)/K(+) ATPase by 1 mM ouabain abolished the [Ca(2+)](i) decrease elicited by potassium but not by BK. 5. A voltage-operated calcium channel blocker, nifedipine (1 microM) did not prevent the BK effect but reduced the [Ca(2+)](i) drop. 6. These results indicate that the BK-induced [Ca(2+)](i) decrease in angiotensin II-stimulated muscular EA is mediated by an EDHF which activates charybdotoxin-sensitive K(+) channels. In these vessels, EDHF seems to be neither a cytochrome P450-derived arachidonic acid metabolite nor K(+) itself. The closure of voltage-operated calcium channels is not the only cellular mechanism involved in this EDHF-mediated [Ca(2+)](i) decrease.

Blockade of chloride channels reveals relaxations of rat small mesenteric arteries to raised potassium

1. Raised extracellular K(+) relaxes some arteries, and has been proposed as Endothelium-Derived Hyperpolarizing Factor (EDHF). However, relaxation of rat small mesenteric arteries to K(+) is highly variable. We have investigated the mechanism of K(+)-induced dilatation and relaxation of pressurized arteries and arteries mounted for measurement of isometric force. 2. Raising [K(+)](o) from 5.88 - 10.58 mM did not dilate or relax pressurized or isometric arteries. Relaxation to raised [K(+)](o) was revealed in the presence of 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB); this effect of NPPB was concentration-dependent (IC(50): 1.16 microM). 3. Relaxations to raised [K(+)](o) in the presence of NPPB, were abolished by 30 microM Ba(2+) or endothelial-denudation. Acetycholine (10 microM) relaxed endothelium-intact arteries in presence of raised [K(+)](o) NPPB and Ba(2+). 4. Relaxations to raised [K(+)](o) were revealed in hyperosmotic superfusate (+60 mM sucrose). These relaxations were abolished by 30 microM Ba(2+). In the presence of raised [K(+)](o), 60 mM sucrose and 30 microM Ba(2+), 10 microM acetycholine still relaxed all arteries. 5. Fifty microM 18 alpha-glycyrrhetinic acid (18 alpha-GA), a gap junction inhibitor, depressed relaxations to both 10 microM acetylcholine and raised [K(+)](o), in the presence of 10 microM NPPB. 6. In summary, blockade of a volume-sensitive Cl(-) conductance in small rat mesenteric arteries, using NPPB or hyperosmotic superfusion, reveals a endothelium-dependent, Ba(2+) sensitive dilatation or relaxation of rat mesenteric arteries to raised [K(+)](o). We conclude that inwardly rectifying potassium channels on the endothelium underlie relaxations to raised [K(+)](o) in rat small mesenteric arteries.

Mechanisms of NO-resistant relaxation induced by acetylcholine in rabbit renal arteries

The effects of K+ channel blockers and P2Y receptor agonist/antagonist on the vasorelaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) were investigated in the rabbit renal artery. Acetylcholine (ACh, 1 nM-10 microM) induced endothelium-dependent relaxation of arterial rings precontracted with norepinephrine (NE, 1 microM) in a concentration-dependent manner. NG-nitro-L-arginine (L-NAME. 0.1 mM), an inhibitor of NO synthase, partially inhibited the ACh-induced endothelium-dependent relaxation. The ACh-induced relaxation was only partially inhibited by L-NAME whereas combined addition of L-NAME and 30 mM KCl completely inhibited the relaxation. The ACh-induced relaxation observed in the presence of L-NAME was significantly reduced by a combination of iberiotoxin (0.1 microM) and apamin (1 microM), and almost completely blocked by 4-aminopyridine (5 mM). The ACh-induced relaxation was antagonized by P2Y receptor antagonist, cibacron blue (10 and 100 microM) in a concentration-dependent manner. Furthermore, ADPbetaS, a potent P2Y agonist, induced the endothelium-dependent relaxation, and this relaxation was markedly reduced by either the combination of iberiotoxin and apamin or by cibacron blue alone. In conclusion, ACh may activate the release of ATP from endothelial cells which in turn activates a P2Y receptor on the endothelial cells followed by a release of EDHF, resulting in a vasorelaxation via a mechanism that involves activation of both the voltage-gated K+ channels and the Ca2+-activated K+ channels. EY WORDS: ATP, K+ channel, rabbit renal artery.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Role of EDHF in the vasodilatory effect of loop diuretics in guinea-pig mesenteric resistance arteries

1. Relaxing effect of loop diuretics, piretanide and furosemide in comparison with acetylcholine (ACh) was investigated in guinea-pig isolated mesenteric resistance arteries. 2. Concentration-response curves to ACh (0.001 - 10 microM) and diuretics (0.0001 - 1 microM) were constructed in noradrenaline (10 - 30 microM)-precontracted arteries incubated either in normal physiological salt solution (PSS) or in 30 mM KCl PSS (K-PSS). 3. In PSS, maximal relaxations (R(max)) and pD(2) to ACh were 87+/-2% and 7.1+/-0.1 (n=10). L-N(G)-nitro-arginine methyl ester (L-NAME, 100 microM) reduced R(max) by 20% (P<0.01, n=7) and pD(2) by 10% (P<0.01). In contrast, indomethacin (10 microM) increased R(max) by 19% (P<0.01, n=8) and pD(2) by 10% (P<0.05). Combination of L-NAME+indomethacin reversed the effect observed with either of these inhibitors used alone. In K-PSS, R(max) was attenuated by 40% (P<0.001, n=6) compared to PSS. L-NAME reduced R(max) by 65% (P<0.01, n=5) and increased pD(2) by 15 fold. L-NAME+indomethacin suppressed the resistant relaxation. 4. In PSS+L-NAME+indomethacin, inhibitors of small (SK(Ca); apamin, 0.1 microM) and large (BK(Ca); iberiotoxin and charybdotoxin, 0.1 microM) conductance Ca(2+)-sensitive K(-)-channels used alone had little effect on the ACh-response. Combination of apamin+iberiotoxin reduced R(max) by 40% (P<0.05, n=7) while apamin+charybdotoxin fully abolished the resistant relaxation. 5. In PSS, piretanide and furosemide induced relaxation with R(max): 89+/-3% vs 84+/-5% and pD(2): 8.5+/-0.1 vs 7.7+/-0.2 (P<0.01) for piretanide (n=11) and furosemide (n=10), respectively. Endothelial abrasion suppressed relaxation to diuretics. L-NAME and indomethacin used alone or in combination did not significantly modify the response to diuretics. 6. In K-PSS, piretanide-induced relaxation was abolished whereas that to furosemide was reduced by 70% (P<0.001, n=9) compared to PSS and was suppressed by L-NAME+indomethacin. In PSS+L-NAME+indomethacin, apamin slightly reduced relaxation to diuretics whereas charybdotoxin or iberiotoxin abolished the response. 7. These results indicate that ACh-evoked relaxation is mediated by both NO/PGl(2)-dependent and -independent mechanisms. The EDHF-dependent component relies on activation of Ca(2+)-activated K(+) channels, is sensitive to a combination of apamin+charybdotoxin and to a smaller degree to a combination of apamin+iberiotoxin. Loop diuretic-induced relaxation is endothelium-dependent, appears to be mediated by NO, PGl(2) and EDHF for furosemide and EDHF only for piretanide. For the two diuretics, opening of BK(Ca) channels may be involved in the relaxation.

The effects of potassium channel blockers on progesterone-induced suppression of rat portal vein contractility

The suppression of contractility of rat portal vein caused by progesterone appears to be due to the potassium (K+) channel opening effect of this hormone. The identity of the specific K+ channels involved has been investigated using a variety of K+ channel blockers. Incubation with 100 nM iberiotoxin antagonised the progesterone-induced inhibition of spontaneous and 20 mM K+-induced phasic activity of the portal vein such that the contractions resembled those of the non-progesterone, non-iberiotoxin control tissues treated with the corresponding solvent vehicles. Incubation with barium chloride (20 and 100 microM), 4-aminopyridine (1 mM), tetraethylammonium chloride (1 mM), glibenclamide (1 microM) or apamin (1 microM) did not, however, have the same antagonistic effect. These results suggest that progesterone's selective suppression of rat portal vein contractility is mediated by the opening of BKCa channels.

Potassium does not mimic EDHF in rat mesenteric arteries

1. K(+) has been proposed to be EDHF in small arteries. We compared ACh-stimulated, EDHF-mediated dilatation/relaxation with raised [K(+)](o) in rat mesenteric arteries. 2. In pressurized arteries, ACh (10 microM) dilated all arteries. Raising [K(+)](o) from 5.88 to 10. 58 mM only dilated 30% of arteries. Ba(2+) (30 microM) did not affect dilatation to ACh, but abolished 40% of dilatations to raised [K(+)](o). 3. If [K(+)](o) was lowered to 1.18 mM, restoring [K(+)](o) to 5.88 mM produced dilatation which was depressed by Ba(2+) or ouabain (1 mM). Combined application of Ba(2+) and ouabain abolished dilatation. In 1.18 mM K(+), dilatation to ACh was depressed by ouabain, but not by Ba(2+). Combined application of Ba(2+) and ouabain depressed dilatation further. Gap junction inhibitors (Gap-27; 300 microM and 18-alpha-glycyrrhetinic acid; 100 microM) also depressed dilatation to ACh. 4. In arteries mounted isometrically, ACh (1 microM) relaxed endothelium intact (+E), but not endothelium denuded (-E) arteries. Raising [K(+)](o) from 5.9 - 10.9 mM failed to relax all arteries. When [K(+)](o) was lowered to 1 mM, raising [K(+)](o) to 6 mM produced relaxation. In -E arteries, relaxation was unaffected by Ba(2+) but abolished by ouabain. In +E arteries, Ba(2+) depressed and ouabain abolished relaxation. In +E arteries, with 1 mM K(+), ACh relaxation was depressed by ouabain but not Ba(2+). The combined application of Ba(2+) and ouabain further depressed relaxation. 5. In summary, both EDHF and raised [K(+)](o) dilate/relax rat mesenteric arteries, though sensitivities to barium and ouabain differ. K(+) may be a relaxing factor in this tissue, but its characteristics differ from EDHF. Gap junction inhibitors depress EDHF, implying an important role for myo-endothelial gap junctions.

Mechanisms of nitric oxide-independent relaxations induced by carbachol and acetylcholine in rat isolated renal arteries

1. In rat isolated renal artery segments contracted with 0.1 microM phenylephrine and in the presence of the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME), carbachol and acetylcholine produced endothelium-dependent relaxations. The mechanisms underlying these relaxations were studied. 2. These relaxations were not affected by ODQ (1H-[1,2,4]oxadiazolo[4,3, -a]quinoxalin-1-one) or indomethacin. In arteries contracted with 20 - 30 mM K(+), L-NAME-resistant relaxations induced by carbachol and acetylcholine were virtually absent. 3. The Na(+)-K(+) ATPase inhibitor ouabain reduced these relaxations in a concentration-dependent manner. 4. In K(+)-free media, addition of K(+) (5 mM) produced 90. 5+/-3.9% (n=3) relaxation of phenylephrine-induced tone. This relaxation was endothelium-independent and ouabain-sensitive. 5. Tetraethylammonium (TEA), charybdotoxin (ChTX) and iberiotoxin (IbTX) reduced the sensitivity of carbachol-induced relaxations, but did not change the maximal response. These relaxations were not altered by 4-aminopyridine (4-AP), glibenclamide or apamin. Acetylcholine (1 microM)-induced relaxation was reduced by ChTX, but not by TEA or IbTX. 6. The cytochrome P450 inhibitor miconazole, but not 17-octadecynoic acid, reduced the sensitivity of carbachol-induced relaxations, without changing the maximal response. 7. In conclusion, in rat isolated renal arteries, acetylcholine and carbachol produced a non-NO/non-PGI(2) relaxation which is mediated by an endothelium-derived hyperpolarizing factor (EDHF). This factor does not appear to be a cytochrome P450 metabolite. The inhibition by ouabain of these relaxations suggests the possible involvement of Na(+)-K(+) ATPase activation in EDHF responses, although other mechanisms cannot be totally ruled out.

Effects of inhibitors of small- and intermediate-conductance calcium-activated potassium channels, inwardly-rectifying potassium channels and Na(+)/K(+) ATPase on EDHF relaxations in the rat hepatic artery

1. In the rat hepatic artery, the SK(Ca) inhibitors UCL 1684 (300 nM) completely blocked, and scyllatoxin (1 microM) and d-tubocurarine (100 microM) partially inhibited EDHF relaxations when each of them was combined with charybdotoxin (300 nM). 2. The IK(Ca) inhibitors clotrimazole (3 microM) and 2-chlorophenyl-bisphenyl-methanol (3 microM) strongly depressed EDHF relaxations when each of them was combined with apamin (300 nM). The cytochrome P450 mono-oxygenase inhibitor ketoconazole (10 microM) had no effect in the presence of apamin. 3. Ciclazindol (10 microM), which abolishes EDHF relaxations in the presence of apamin, almost completely prevented the calcium ionophore (A23187) stimulated (86)Rb(+) influx via the Gardos channel (IK(Ca)) in human erythrocytes. 4. The Na(+)/K(+) ATPase inhibitor ouabain (500 microM) and the K(IR) blocker Ba(2+) (30 microM) neither alone nor in combination inhibited EDHF relaxations. Ba(2+) was also without effect in the presence of either apamin or charybdotoxin. 5. In contrast to EDHF, an increase in extracellular [K(+)] from 4.6 mM to 9.6, 14.6 and 19.6 mM inconsistently relaxed arteries. In K(+)-free physiological salt solution, re-admission of K(+) always caused complete and sustained relaxations which were abolished by ouabain but unaffected by Ba(2+). 6. The present study provides pharmacological evidence for the involvement of SK(Ca) and IK(Ca) in the action of EDHF in the rat hepatic artery. Our results are not consistent with the idea that EDHF is K(+) activating Na(+)/K(+) ATPase and K(IR) in this blood vessel.

Role of endothelial cell hyperpolarization in EDHF-mediated responses in the guinea-pig carotid artery

1. Experiments were performed to identify the potassium channels involved in the acetylcholine-induced endothelium-dependent hyperpolarization of the guinea-pig internal carotid artery. Smooth muscle and endothelial cell membrane potentials were recorded in isolated arteries with intracellular microelectrodes. Potassium currents were recorded in freshly-dissociated smooth muscle cells using patch clamp techniques. 2. In single myocytes, iberiotoxin (0.1 microM)-, charybdotoxin (0.1 microM)-, apamin (0.5 microM)- and 4-aminopyridine (5 mM)-sensitive potassium currents were identified indicating the presence of large- and small-conductance calcium-sensitive potassium channels (BK(Ca) and SK(Ca)) as well as voltage-dependent potassium channels (K(V)). Charybdotoxin and iberiotoxin inhibited the same population of BK(Ca) but a conductance specifically sensitive to the combination of charybdotoxin plus apamin could not be detected. 4-aminopyridine (0. 1 - 25 mM) induced a concentration-dependent inhibition of K(V) without affecting the iberiotoxin- or the apamin-sensitive currents. 3. In isolated arteries, both the endothelium-dependent hyperpolarization of smooth muscle and the hyperpolarization of endothelial cells induced by acetylcholine or by substance P were inhibited by 5 mM 4-aminopyridine. 4. These results indicate that in the vascular smooth muscle cells of the guinea-pig carotid artery, a conductance specifically sensitive to the combination of charybdotoxin plus apamin could not be detected, comforting the hypothesis that the combination of these two toxins should act on the endothelial cells. Furthermore, the inhibition by 4-aminopyridine of both smooth muscle and endothelial hyperpolarizations, suggests that in order to observe an endothelium-dependent hyperpolarization of the vascular smooth muscle cells, the activation of endothelial potassium channels is likely to be required.

An indirect influence of phenylephrine on the release of endothelium-derived vasodilators in rat small mesenteric artery

1. The possibility that stimulation of smooth muscle alpha(1)-adrenoceptors modulates contraction via the endothelium was examined in rat small mesenteric arteries. 2. N(omega)-nitro-L-arginine methyl ester, (L-NAME, 100 microM to inhibit NO synthase) increased contraction to single concentrations of phenylephrine (1 - 3 microM) by approximately 2 fold (from a control level of 14.2+/-3.0 to 34. 1+/-4.2% of the maximum contraction of the artery, n=20). The action of L-NAME was abolished by disrupting the endothelium. 3. The subsequent addition of apamin (to inhibit small conductance Ca(2+)-activated K(+) channels, 50 nM) further augmented phenylephrine contractions, in an endothelium-dependent manner, to more than 3 fold above control (50.4+/-5.3% of the maximum contraction, n=11). 4.Charybdotoxin (non-selective inhibitor of large conductance Ca(2+)-activated K(+) channels, BK(Ca), 50 nM) plus L-NAME augmented the level of phenylephrine contraction to 4 - 5-fold above control (64.1+/-3.1%, n=5), but this effect was independent of the endothelium. The potentiation of contraction by charybdotoxin could be mimicked with the selective BK(Ca) inhibitor, iberiotoxin,. 5. Apamin together with L-NAME and charybdotoxin further significantly increased the phenylephrine contraction by 5 - 6-fold, to 79.9+/-3.5% of the maximum contraction of the artery (n=13). 6. Phenylephrine failed directly to increase the intracellular Ca(2+) concentration in endothelial cells freshly isolated from the small mesenteric artery. 7. Stimulation of smooth muscle alpha(1)-adrenoceptors in the mesenteric artery induces contraction that is markedly suppressed by the endothelium. The attenuation of contraction appears to reflect both the release of NO from the endothelium and the efflux of K(+) from both endothelial and smooth muscle cells. This suggests that the release of NO and endothelium-derived hyperpolarizing factor can be evoked indirectly by agents which act only on the smooth muscle cells.

K(+) channel blockers and cytochrome P450 inhibitors on acetylcholine-induced, endothelium-dependent relaxation in rabbit mesenteric artery

Acetylcholine caused an endothelium-dependent relaxation in isolated rabbit mesenteric small artery in the presence of nitro L-arginine and indomethacin. The acetylcholine-induced relaxation was attenuated by high K(+) solution, suggesting that the response is mediated by a membrane potential-sensitive mechanism, presumably an endothelium-derived hyperpolarizing factor. The acetylcholine-induced relaxation was also inhibited with tetraethylammonium, 4-aminopyridine and charybdotoxin, but not with Ba(2+), apamin, iberiotoxin nor glibenclamide. The relaxation was abolished by a combination of apamin and charybdotoxin, but iberiotoxin could not replace charybdotoxin in this combination. The responses to charybdotoxin and 4-aminopyridine were synergistic but neither apamin nor iberiotoxin increased the effect of 4-aminopyridine. Clotrimazole and proadifen inhibited the acetylcholine-induced relaxation, but these drugs also inhibited the cromakalim-induced relaxation, while protoporphyrin IX inhibited the acetylcholine- but not cromakalim-induced relaxation. 17-Octadecynoic acid and 1-aminobenzotriazole did not affect the response to acetylcholine. Four regioisomers of epoxyeicosatrienoic acids did not relax endothelium-denuded artery. A gap junction inhibitor 18alpha-glycyrrhetinic acid attenuated the relaxation to acetylcholine. It is suggested that in rabbit mesenteric artery, the acetylcholine-induced, nitric oxide- and prostacyclin-independent relaxation is mainly mediated by 4-aminopyridine- and charybdotoxin-sensitive K(+) channels and that the relaxation is not mediated through cytochrome P450 enzyme metabolites. The contribution of heterocellular gap junctional communication to the relaxation is discussed.

Further investigation of endothelium-derived hyperpolarizing factor (EDHF) in rat hepatic artery: studies using 1-EBIO and ouabain

1. The characteristics of endothelium-dependent hyperpolarization in rat hepatic artery have been further investigated in the presence of inhibitors of cyclo-oxygenase and nitric oxide synthase. 2. Using sharp micro-electrodes, the smooth muscle hyperpolarization induced by acetylcholine, KCl or 1-ethyl-2-benzimidazolinone (1-EBIO) in intact hepatic arteries was abolished by 30 micronM barium plus 500 nM ouabain. 3. In vessels without endothelium, the smooth muscle hyperpolarization induced by KCl was not reduced by 30 micronM barium alone. However, in the presence of barium the effects of KCl were partially inhibited by 100 nM ouabain and essentially abolished by 500 nM ouabain. 4. Using sharp micro-electrodes, the hyperpolarization of both the smooth muscle and the endothelium induced by 1-EBIO or by acetylcholine was unaffected by 100 nM iberiotoxin. However, in the presence of 100 nM charybdotoxin, the effects of 1-EBIO were abolished whereas those of acetylcholine were only partially reduced. The hyperpolarization induced by levcromakalim was unaffected by either charybdotoxin or iberiotoxin. 5 Under whole-cell patch-clamp recording conditions, 1-EBIO induced a voltage-insensitive, charybdotoxin-sensitive K+ current in cultured endothelial cells but was without effect on K+ currents in smooth muscle cells isolated from hepatic arteries. 6 It is concluded that the endothelium-dependent hyperpolarization of smooth muscle induced by either acetylcholine or by 1-EBIO in rat hepatic artery is initially associated with the opening of endothelial calcium-sensitive K+-channels insensitive to iberiotoxin. The resulting accumulation of K+ in the myoendothelial space activates an isoform of Na+/K+-ATPase which is sensitive to low concentrations of ouabain.

The endothelial component of cannabinoid-induced relaxation in rabbit mesenteric artery depends on gap junctional communication

1. We have shown that the endocannabinoid anandamide and its stable analogue methanandamide relax rings of rabbit superior mesenteric artery through endothelium-dependent and -independent mechanisms that are unaffected by blockade of NO synthase and cyclooxygenase. 2. The endothelium-dependent component of the responses was attenuated by the gap junction inhibitor 18alpha-glycyrrhetinic acid (18alpha-GA; 50 microM), and a synthetic connexin-mimetic peptide homologous to the extracellular Gap 27 sequence of connexin 43 (43Gap 27, SRPTEKTIFII; 300 microM). By contrast, the corresponding connexin 40 peptide (40Gap 27, SRPTEKNVFIV) was inactive. 3. The cannabinoid CB1 receptor antagonist SR141716A (10 microM) also attenuated endothelium-dependent relaxations but this inhibition was not observed with the CB1 receptor antagonist LY320135 (10 microM). Furthermore, SR141716A mimicked the effects of 43Gap 27 peptide in blocking Lucifer Yellow dye transfer between coupled COS-7 cells (a monkey fibroblast cell line), whereas LY320135 was without effect, thus suggesting that the action of SR141716A was directly attributable to effects on gap junctions. 4. The endothelium-dependent component of cannabinoid-induced relaxation was also attenuated by AM404 (10 microM), an inhibitor of the high-affinity anandamide transporter, which was without effect on dye transfer. 5. Taken together, the findings suggest that cannabinoids derived from arachidonic acid gain access to the endothelial cytosol via a transporter mechanism and subsequently stimulate relaxation by promoting diffusion of an to adjacent smooth muscle cells via gap junctions. 6. Relaxations of endothelium-denuded preparations to anandamide and methanandamide were unaffected by 43Gap 27 peptide, 18alpha-GA, SR141716A, AM404 and indomethacin and their genesis remains to be established.

Involvement of CYP3A-derived arachidonic acid metabolite(s) in responses to endothelium-derived K+ channel opening substance in monkey lingual artery

1. In monkey lingual artery strips partially contracted with prostaglandin F2alpha, acetylcholine-induced, concentration-related relaxations were abolished by removal of the endothelium. The response was not significantly influenced by indomethacin but attenuated by NG-nitro-L-arginine (L-NOARG); the effect of the nitric oxide (NO) synthase inhibitor was reversed by L-arginine. 2. The response to acetylcholine resistant to L-NOARG was suppressed in the strips exposed to high K+ media. Charybdotoxin partially inhibited the relaxation, and the remaining relaxation was abolished by additional treatment with apamin, whereas glibenclamide, iberiotoxin or apamin alone was without effect. Relaxations induced by sodium nitroprusside were not influenced by charybdotoxin. 3. The L-NOARG-resistant acetylcholine-induced relaxation was inhibited by metyrapone, proadifen and 17-octadecynoic acid, non-selective cytochrome P450 mono-oxygenase (CYP) inhibitors, and progesterone and ketoconazole, inhibitors selective to CYP3A. The inhibitors did not affect the nitroprusside-induced relaxation. Selective inhibitors of other CYP isoforms, such as debrisoquine and lauric acid, did not reduce the response to acetylcholine. 4. Reaction mixture containing human liver microsome rich in CYPs, arachidonic acid and NADPH incubated at 37 degrees C and filtrated relaxed endothelium-denuded monkey lingual artery strips, used as bioassay tissues. This response was abolished in the strips exposed to high K+ media. The response was also suppressed by combined treatment of the assay tissue with charybdotoxin plus apamin, but was not affected by treatment with iberiotoxin. The reaction mixture co-incubated with ketoconazole failed to relax the strips. 5. It is concluded that the monkey lingual arterial relaxation dependent on the endothelium is mediated by NO and also by a charybdotoxin plus apamin-sensitive but iberiotoxin-insensitive Ca2+-activated K+ channel opening substance(s) that may be a CYP3A-derived arachidonic acid metabolite(s).

Cyclic AMP mediates EDHF-type relaxations of rabbit jugular vein

Isolated rings of rabbit jugular vein have been used to test the hypothesis that formation of cAMP within the endothelial cell contributes to relaxations that are attributable to the endothelium-derived hyperpolarizing factor, EDHF. Relaxations induced by acetylcholine under conditions of combined NO synthase and cyclooxygenase blockade were almost abolished by inhibition of adenylate cyclase with the selective P-site agonist 2', 3'-dideoxyadenosine (2',3'-DDA). They were similarly attenuated by the gap junction inhibitors 18alpha-glycyrrhetinic acid (18alpha-GA) and Gap 27 peptide which interrupt direct endothelium-smooth muscle communication without themselves affecting smooth muscle tone. By contrast, stimulation of adenylate cyclase with forskolin promoted gap junction-dependent relaxations, with concentration-relaxation curves to this agent exhibiting an equivalent rightward shift in the presence of 18alpha-GA and following endothelial denudation. The findings suggest that cAMP may cross from the endothelium to smooth muscle via gap junction channels and/or enhance the endothelial hyperpolarization normally associated with agonist stimulation. Both mechanisms may contribute to EDHF/gap junction-dependent relaxations.

Altered endothelium-derived hyperpolarizing factor-mediated endothelial function in coronary microarteries by St Thomas' Hospital solution

OBJECTIVES

We examined the effect of St Thomas' Hospital solution on endothelium-derived hyperpolarizing factor-mediated function in the porcine coronary microarteries with emphasis on the effect of temperature and washout time.

METHODS

Microartery rings (diameter, 200-450 micrometers) were studied in myograph. The arteries were incubated in St Thomas' Hospital or Krebs solution (control) at 4 degrees C for 4 hours followed by 45 minutes (group Ia) or 90 minutes washout (group Ib) or at 22 degrees C for 1 hour followed by 45 minutes (group IIa) or 90 minutes washout (group IIb) and precontracted with -8.5 log M U 46619. The endothelium-derived hyperpolarizing factor-mediated relaxation to bradykinin was studied when endothelium-derived nitric oxide and prostaglandin I2 were inhibited with the presence of 7 micromol/L indomethacin and 300 micromol/L NG-nitro-L -arginine.

RESULTS

After exposure to St Thomas' Hospital solution, the maximal endothelium-derived hyperpolarizing factor-mediated relaxation (percentage of the precontraction) was significantly reduced at either temperature after washout for 45 minutes (group Ia, 42.7% +/- 3.5% vs 69.0% +/- 5.3%; n = 9; P =.000; and group IIa, 12.3% +/- 1.6% vs 56.1% +/- 4. 4%; n = 8; P =.000) but fully recovered after washout for 90 minutes. The U46619-induced contraction force was also significantly reduced after washout for 45 minutes (P <.001) but fully recovered at 90 minutes.

CONCLUSIONS

Under profound and moderate hypothermia, St Thomas' Hospital solution impairs endothelium-derived hyperpolarizing factor-mediated relaxation and smooth muscle contraction in the coronary microarteries. These effects exist during the reperfusion period for at least 45 minutes after exposure to St Thomas' Hospital solution and may account for the possible myocardial dysfunction during reperfusion.

Potassium ions and endothelium-derived hyperpolarizing factor in guinea-pig carotid and porcine coronary arteries

Experiments were designed to determine in two arteries (the guinea-pig carotid and the porcine coronary arteries) whether or not the endothelium-derived hyperpolarizing factor (EDHF) can be identified as potassium ions, and to determine whether or not the inwardly rectifying potassium current and the Na+/K+ pump are involved in the hyperpolarization mediated by EDHF. The membrane potential of vascular smooth muscle cells was recorded with intracellular microelectrodes in the presence of N(omega)-L-nitro-arginine (L-NA) and indomethacin. In vascular smooth muscle cells of guinea-pig carotid and porcine coronary arteries, acetylcholine and bradykinin induced endothelium-dependent hyperpolarizations (-18+/-1 mV, n = 39 and -19+/-1 mV, n = 7, respectively). The hyperpolarizations were not affected significantly by ouabain (1 microM), barium chloride (up to 100 microM) or the combination of ouabain plus barium. In both arteries, increasing extracellular potassium concentration by 5 or 10 mM induced either depolarization or in a very few cases small hyperpolarizations which never exceeded 2 mV. In isolated smooth muscle cells of the guinea-pig carotid artery, patch-clamp experiments shows that only 20% of the vascular smooth muscle cells expressed inwardly rectifying potassium channels. The current density recorded was low (0.5+/-0.1 pA pF(-1), n = 8). These results indicate that, in two different vascular preparations, barium sensitive-inwardly rectifying potassium conductance and the ouabain sensitive-Na+/K+ pump are not involved in the EDHF-mediated hyperpolarization. Furthermore, potassium did not mimic the effect of EDHF pointing out that potassium and EDHF are not the same entity in those arteries.