Endothelium-dependent hyperpolarization: age, gender and blood pressure, do they matter?

Under physiological conditions, the endothelium generates vasodilator signals [prostacyclin, nitric oxide NO and endothelium-dependent hyperpolarization (EDH)], for the regulation of vascular tone. The relative importance of these two signals depends on the diameter of the blood vessels: as the diameter of the arteries decreases, the contribution of EDH to the regulation of vascular tone increases. The mechanism involved in EDH varies with species and blood vessel types; nevertheless, activation of endothelial intermediate- and small-conductance calcium-activated potassium channels (IKCa and SKCa , respectively) is characteristic of the EDH pathway. IKCa - and SKCa -mediated EDH are reduced with endothelial dysfunction, which develops with ageing and hypertension, and is less pronounced in female than in age-matched male until after menopause. Impaired EDH-mediated relaxation is related to a reduced involvement of SKCa , so that the response becomes more dependent on IKCa . The latter depends on the activation of adenosine monophosphate-activated protein kinase (AMPK) and silent information regulator T1 (SIRT1), proteins associated with the process of cellular senescence and vascular signalling in response to the female hormone. An understanding of the role of AMPK and/or SIRT1 in EDH-like responses may help identifying effective pharmacological strategies to prevent the development of vascular complications of different aetiologies.

Transient Receptor Potential Channel Opening Releases Endogenous Acetylcholine, which Contributes to Endothelium-Dependent Relaxation Induced by Mild Hypothermia in Spontaneously Hypertensive Rat but Not Wistar-Kyoto Rat Arteries.

Mild hypothermia causes endothelium-dependent relaxations, which are reduced by the muscarinic receptor antagonist atropine. The present study investigated whether endothelial endogenous acetylcholine contributes to these relaxations. Aortic rings of spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto (WKY) rats were contracted with prostaglandin F2 α and exposed to progressive mild hypothermia (from 37 to 31°C). Hypothermia induced endothelium-dependent, Nω-nitro-l-arginine methyl ester-sensitive relaxations, which were reduced by atropine, but not by mecamylamine, in SHR but not in WKY rat aortae. The responses in SHR aortae were also reduced by acetylcholinesterase (the enzyme responsible for acetylcholine degradation), bromoacetylcholine (inhibitor of acetylcholine synthesis), hemicholinium-3 (inhibitor of choline uptake), and vesamicol (inhibitor of acetylcholine release). The mild hypothermia-induced relaxations in both SHR and WKY rat aortae were inhibited by AMTB [N-(3-aminopropyl)-2-[(3-methylphenyl)methoxy]-N-(2-thienylmethyl)-benzamide; the transient receptor potential (TRP) M8 inhibitor]; only those in SHR aortae were inhibited by HC-067047 [2-methyl-1-[3-(4-morpholinyl)propyl]-5-phenyl-N-[3-(trifluoromethyl)phenyl]-1H-pyrrole-3-carboxamide; TRPV4 antagonist] while those in WKY rat aortae were reduced by HC-030031 [2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)-N-(4-isopropylphenyl)acetamide; TRPA1 antagonist]. The endothelial uptake of extracellular choline and release of cyclic guanosine monophosphate was enhanced by mild hypothermia and inhibited by HC-067047 in SHR but not in WKY rat aortae. Compared with WKY rats, the SHR preparations expressed similar levels of acetylcholinesterase and choline acetyltransferase, but a lesser amount of vesicular acetylcholine transporter, located mainly in the endothelium. Thus, mild hypothermia causes nitric oxide-dependent relaxations by opening TRPA1 channels in WKY rat aortae. By contrast, in SHR aortae, TRPV4 channels are opened, resulting in endothelial production of acetylcholine, which, in an autocrine manner, activates muscarinic receptors on neighboring cells to elicit endothelium-dependent relaxations in response to mild hypothermia.

Kaempferol enhances endothelium-dependent relaxation in the porcine coronary artery through activation of large-conductance Ca(2+) -activated K(+) channels.

BACKGROUND AND PURPOSE: Kaempferol, a plant flavonoid present in normal human diet, can modulate vasomotor tone. The present study aimed to elucidate the signalling pathway through which this flavonoid enhanced relaxation of vascular smooth muscle. EXPERIMENTAL APPROACH: The effect of kaempferol on the relaxation of porcine coronary arteries to endothelium-dependent (bradykinin) and -independent (sodium nitroprusside) relaxing agents was studied in an in vitro organ chamber setup. The whole-cell patch-clamp technique was used to determine the effect of kaempferol on potassium channels in porcine coronary artery smooth muscle cells (PCASMCs). KEY RESULTS: At a concentration without direct effect on vascular tone, kaempferol (3 × 10(-6) M) enhanced relaxations produced by bradykinin and sodium nitroprusside. The potentiation by kaempferol of the bradykinin-induced relaxation was not affected by N(ω)-nitro-L-arginine methyl ester, an inhibitor of NO synthase (10(-4) M) or TRAM-34 plus UCL 1684, inhibitors of intermediate- and small-conductance calcium-activated potassium channels, respectively (10(-6) M each), but was abolished by tetraethylammonium chloride, a non-selective inhibitor of calcium-activated potassium channels (10(-3) M), and iberiotoxin, a selective inhibitor of large-conductance calcium-activated potassium channel (KCa 1.1; 10(-7) M). Iberiotoxin also inhibited the potentiation by kaempferol of sodium nitroprusside-induced relaxations. Kaempferol stimulated an outward-rectifying current in PCASMCs, which was abolished by iberiotoxin. CONCLUSIONS AND IMPLICATIONS: The present results suggest that, in smooth muscle cells of the porcine coronary artery, kaempferol enhanced relaxations caused by endothelium-derived and exogenous NO as well as those due to endothelium-dependent hyperpolarization. This vascular effect of kaempferol involved the activation of KCa 1.1 channels.

Low-dose desmopressin improves hypothermia-induced impairment of primary haemostasis in healthy volunteers.

Mild hypothermia (34-35 °C) increases peri-operative blood loss. We have previously demonstrated the beneficial effect of in vitro desmopressin on impairment of primary haemostasis associated with hypothermia. This study evaluated subcutaneous desmopressin in 52 healthy volunteers, randomly assigned to receive either normal saline or desmopressin 1.5, 5 or 15 µg (with 13 in each group). Blood samples were collected before and 2 h after drug administration and incubated at 32 and 37 °C. Platelet function analyser PFA-100(®) closure times were measured. Hypothermia at 32 °C prolonged mean (95% CI) closure times (for adenosine diphosphate/collagen by 11.3% (7.5-15.2%) and for adrenaline/collagen by 16.2% (11.3-21.2%); these changes were reversed by desmopressin. A very small dose was found to be effective (1.5 µg); this dose did not significantly change closure times at 37 °C, but fully prevented its prolongation at 32 °C. Subcutaneous desmopressin prevents the development of hypothermia-induced impairment of primary haemostasis.

Effects of the aqueous extract of Salvia Miltiorrhiza (danshen) and its magnesium tanshinoate B-enriched form on blood pressure.

Danshen is the dried root and rhizome of the Chinese medicinal plant Salvia miltiorrhiza Bunge (Labiatae), which has been used to treat hypertension and myocardial infarction. One of its water-soluble active components is magnesium tanshinoate B (MTB). The present study examined and compared the cardiovascular effects of the water-soluble extract of danshen (SME) and its MTB-enriched form (containing 70% of MTB (MTB70)). Anaesthetized rats were infused intravenously with saline or phenylephrine to achieve a normal or elevated blood pressure, respectively. Different doses of SME, MTB70 or vehicle were then injected intravenously and their effect on blood pressure was monitored. The results indicate that SME and MTB70 reduce blood pressure dose-dependently. Independently of the initial blood pressure, SME caused a smaller reduction in blood pressure than MTB70. In rats infused with phenylephrine, MTB70 caused greater decreases in blood pressure than in rats infused with saline, while the responses to SME did not differ between the two groups. From these findings, it appears that MTB is one of the major components responsible for the cardiovascular effects of danshen, and that the beneficial cardiovascular effect of the extract is more prominent under conditions of elevated blood pressure.

Structure-activity relationships of flavonoids for vascular relaxation in porcine coronary artery.

Flavonoids are polyphenolic compounds that are widespread in the plant kingdom, and structure-activity relationships (SAR) for vascular relaxation effects were examined for 17 of them using porcine coronary arteries. Density functional theory was employed to calculate the chemical parameters of these compounds. The order of potency for vascular relaxation was as follows: flavones (apigenin and luteolin) >or= flavonols (kaempferol and quercetin)>isoflavones (genistein and daidzein)>flavanon(ol)es (naringenin)>chalcones (phloretin)>anthocyanidins (pelargonidin)>flavan(ol)es ((+)-catechin and (-)-epicatechin). SAR analysis revealed that for good relaxation activity, the 5-OH, 7-OH, 4'-OH, C2=C3 and C4=O functionalities were essential. Comparison of rutin with quercetin, genistin with genistein, and puerarin with daidzein demonstrated that the presence of a glycosylation group greatly reduced relaxation effect. Total energy and molecular volume were also predictive of their relaxation activities. Our findings indicated that the most effective relaxing agents are apigenin, luteolin, kaempferol and genistein. These flavonoids possess the key chemical structures demonstrated in our SAR analysis.

Kaempferol enhances endothelium-independent and dependent relaxation in the porcine coronary artery.

The vascular effects of kaempferol were investigated in isolated porcine coronary artery rings. U46619 (9,11-dideoxy-9alpha, 11alpha-methanoepoxy prostaglandin F2alpha, 30 nM) was used to contract porcine coronary arterial rings. Concentration relaxation curve of kaempferol (1 nM - 100 microM) was constructed and kaempferol demonstrated significant relaxation at high concentrations. At low concentration with no significant effect on relaxation, kaempferol (10 microM) enhanced relaxation produced by bradykinin, the calcium ionophore A23187, isoproterenol and sodium nitroprusside in endothelium-intact porcine coronary arteries. In endothelium-disrupt rings, kaempferol (10 microM) also enhanced the relaxation caused by isoproterenol, sodium nitroprusside, levcromakalim and nifedipine. On the other hand, antioxidant agents did not affect bradykinin-induced relaxation or the enhancement effect of kaempferol. In summary, a low concentration of kaempferol (10 microM), devoid of significant vascular effect, has the ability to enhance endothelium-dependent and endothelium-independent relaxations. This action of kaempferol is unrelated to its antioxidant property.