Inhibition of the endothelium-dependent relaxation by 18beta-glycyrrhetinic acid in the guinea-pig aorta

Inhibitory effects of glycyrrhetinic Acid on the delayed rectifier potassium current in Guinea pig ventricular myocytes and HERG channel

Background. Licorice has long been used to treat many ailments including cardiovascular disorders in China. Recent studies have shown that the cardiac actions of licorice can be attributed to its active component, glycyrrhetinic acid (GA). However, the mechanism of action remains poorly understood. Aim. The effects of GA on the delayed rectifier potassium current (I_K), the rapidly activating (I_Kr) and slowly activating (I_Ks) components of I_K, and the HERG K⁺ channel expressed in HEK-293 cells were investigated. Materials and Methods. Single ventricular myocytes were isolated from guinea pig myocardium using enzymolysis. The wild type HERG gene was stably expressed in HEK293 cells. Whole-cell patch clamping was used to record I_K (I_Kr, I_Ks) and the HERG K⁺ current. Results. GA (1, 5, and 10 μM) inhibited I_K (I_Kr, I_Ks) and the HERG K⁺ current in a concentration-dependent manner. Conclusion. GA significantly inhibited the potassium currents in a dose- and voltage-dependent manner, suggesting that it exerts its antiarrhythmic action through the prolongation of APD and ERP owing to the inhibition of I_K (I_Kr, I_Ks) and HERG K⁺ channel.

A dietary agonist of transient receptor potential cation channel V3 elicits endothelium-dependent vasodilation

The Mediterranean diet may be responsible for lower cardiovascular disease rates in Southern versus Northern European countries. Oregano is used abundantly in Mediterranean cooking, but potential cardiovascular benefits have not been investigated. Carvacrol, present in oregano, activates the transient receptor potential (TRP) cation channels TRPA1 and TRPV3. We hypothesized that chemosensing of this dietary molecule by TRP channels in the endothelium promotes arterial relaxation. TRPA1 and TRPV3 were detected in the endothelium of intact arteries. Carvacrol causes concentration-dependent increases in the intracellular [Ca(2+)] of native cerebral artery endothelial cells and is more potent (EC(50) = 34 microM) than the TRPA1 agonist allyl isothiocyanate (EC(50) = 400 microM) or the TRPV3 agonist eugenol (EC(50) = 2.3 mM). Carvacrol also activates TRPV3-like cation currents in cerebral artery endothelial cells. Carvacrol elicits vasodilation of intact cerebral arteries (EC(50) = 4.1 microM) that is accompanied by smooth muscle hyperpolarization and a decrease in the intracellular [Ca(2+)] of arterial myocytes. Endothelium disruption inhibits carvacrol-induced vasodilation, but block of nitric-oxide synthase and cyclooxygenase activity does not alter the response. Vasodilation in response to carvacrol is inhibited when blockers of Ca(2+)-activated K(+) channels are present in the lumen or when the inwardly rectifying K(+) channel blocker BaCl(2) is present in the superfusion bath. Carvacrol-induced dilation is not diminished by a TRPA1 antagonist but is inhibited by the TRPV blocker ruthenium red. Our findings show that oregano can relax arteries by activating TRPV3 channels in the endothelium. This effect may account for some of the cardioprotective effects of the Mediterranean diet.

Endothelium-dependent cerebral artery dilation mediated by TRPA1 and Ca2+-Activated K+ channels

Although it is well established that changes in endothelial intracellular [Ca(2+)] regulate endothelium-dependent vasodilatory pathways, the molecular identities of the ion channels responsible for Ca(2+) influx in these cells are not clearly defined. The sole member of the ankyrin (A) transient receptor potential (TRP) subfamily, TRPA1, is a Ca(2+)-permeable nonselective cation channel activated by electrophilic compounds such as acrolein (tear gas), allicin (garlic), and allyl isothiocyanate (AITC) (mustard oil). The present study examines the hypothesis that Ca(2+) influx via TRPA1 causes endothelium-dependent vasodilation. The effects of TRPA1 activity on vascular tone were examined using isolated, pressurized cerebral arteries. AITC induced concentration-dependent dilation of pressurized vessels with myogenic tone that was accompanied by a corresponding decrease in smooth muscle intracellular [Ca(2+)]. AITC-induced dilation was attenuated by disruption of the endothelium and when the TRPA1 channel blocker HC-030031 was present in the arterial lumen. TRPA1 channels were found to be present in native endothelial cells, localized to endothelial cell membrane projections proximal to vascular smooth muscle cells. AITC-induced dilation was insensitive to nitric oxide synthase or cyclooxygenase inhibition but was blocked by luminal administration of the small and intermediate conductance Ca(2+)-activated K(+) channel blockers apamin and TRAM34. BaCl(2), a blocker of inwardly rectifying K(+) channels, also inhibited AITC-induced dilation. AITC-induced smooth muscle cell hyperpolarization was blocked by apamin and TRAM34. We conclude that Ca(2+) influx via endothelial TRPA1 channels elicits vasodilation of cerebral arteries by a mechanism involving endothelial cell Ca(2+)-activated K(+) channels and inwardly rectifying K(+) channels in arterial myocytes.

Lipopolysaccharide reduces electrical coupling in microvascular endothelial cells by targeting connexin40 in a tyrosine-, ERK1/2-, PKA-, and PKC-dependent manner

Electrical coupling along the endothelium is central in the arteriolar conducted response and in control of vascular resistance. It has been shown that exposure of endothelium to lipopolysaccharide (LPS, an initiating factor in sepsis) reduces intercellular communication in vitro and in vivo. The molecular basis for this reduction is not known. We examined the effect of LPS on electrical coupling in monolayers of cultured mouse microvascular endothelial cells (MMEC) derived from the mouse hindlimb skeletal muscle. To assess coupling, we measured the spread of electrical current injected into the monolayer and computed the monolayer intercellular resistance (inverse measure of coupling). LPS (10 microg/ml, 1 h) reduced coupling (i.e., increased resistance) in MMEC isolated from wild-type, connexin37 (Cx37) null and Cx43(G60S) (nonfunctional mutant) mice, but not in MMEC derived from Cx40 null mice. LPS also activated JNK1/2, p38 and ERK1/2 MAP kinases. Pretreatment of WT monolayers with ERK1/2 inhibitor U0126 (20 microM, 1 h) prevented the LPS-induced decrease in coupling, while inhibition of JNK1/2 with SP600125 (20 microM, 1 h) and p38 with a p38 inhibitor (10 nM, 1 h) had no effect. Furthermore, inhibition of tyrosine kinases with PP-2 (10 nM, 1 h), activation of PKA by 8-bromo-cAMP (1 mM, 5 min), and activation of PKC by bryostatin-2 (10 nM, 1 h) also prevented the reduction in coupling. We propose that LPS reduces inter-endothelial electrical coupling via tyrosine-, ERK1/2-, PKA-, and PKC-dependent signaling that targets Cx40. We suggest that this mechanism contributes to compromised arteriolar function following LPS exposure.

[Recent advances in the study of endothelium-dependent hyperpolarizing factor (EDHF)]

Recent advances in the properties and physiological functions of endothelium-dependent hyperpolarizing factor (EDHF) in vascular tissues were reviewed briefly. The EDHF-induced hyperpolarization is inhibited by charybdotoxin, indicating that the potential is produced mainly by activation of intermediate conductance Ca-sensitive K-channels. During generation of EDHF responses, endothelial Ca2+ concentration was elevated, suggesting that the activated K-channels were distributed on the endothelial membrane. This was confirmed by direct recording of membrane potentials from endothelial and smooth muscle cells using double patch electrodes. Measurement of the propagation of potentials applied to endothelial or smooth muscle cells to surrounding cells revealed that there were tight electrical connections between endothelial cells much more than between endothelial and smooth muscle cells or between smooth muscle cells, and these observations yielded a possible spread of electrical signal along the endothelial layer first, and then the signals would be conducted to smooth muscle cell layers. These properties of vascular tissues allow speculating that EDHF is an electrical signal propagated from endothelial cells electrotonically through myoendothelial gap junctions. Several candidates have been proposed as EDHF, and possibilities of individual substances for EDHF were discussed. The cellular mechanism of the hyperpolarization-induced vasodilatation remains unclear, and this should be clarified in the future for further understanding of the EDHF-induced vasodilatation.

Do gap junctions couple interstitial cells of Cajal pacing and neurotransmission to gastrointestinal smooth muscle?

Interstitial cells of Cajal (ICC) pace gastrointestinal phasic activity and transmit nerve activity. Gap junctions may couple these cells to smooth muscle, but no functional evidence exists. The objective of this study was to use uncouplers of gap junctions, 18 alpha-glycyrrhetenic acid and its water-soluble analogue carbenoxolone, to evaluate if gap junctions function in pacing and neurotransmission. After inhibition of nerve function with tetrodotoxin (TTX) and N(G)-nitro-L-arginine (L-NOARG), ionomycin- or carbachol-initiated regular phasic activities of circular muscle strips from canine colon and ileum. In some cases, the primary ICC network responsible for pacing was removed. The effects of inhibitors of gap junction conductance (10(-5)-10(-4) mol L(-1)) on frequencies and amplitudes of contraction were compared to appropriate time controls. Lower oesophageal sphincter (LOS) relaxations to nerve stimulation were studied before and after inhibition of gap junction functions. No major changes in LOS relaxations or frequencies of colonic or ileal contractions occurred, but amplitudes of contractions decreased from these agents. Similar results were obtained when the myenteric plexus-ICC network of ileum was removed. Regular phasic activity was not obtained after removal of the colon submuscular plexus ICC. These findings suggest that mechanisms other than gap junctions couple gut pacemaking activity and nerve transmission.

Hyperpolarization-induced dilatation of submucosal arterioles in the guinea-pig ileum

1. The effects of inhibition of acetylcholine (ACh)-induced hyperpolarization on dilatation of submucosal arterioles were investigated in the guinea-pig ileum. 2. In smooth muscles of the arterioles depolarized by Ba(2+) (0.5 mM) to about -40 mV, ACh (3 microM) repolarized the membrane to about -65 mV (hyperpolarization), irrespective of the absence or presence of L-N(omega)-nitroarginine (L-NOARG, 0.1 mM) and diclofenac (1 microM), and increased the diameter (dilatation). 3. Combined application of charybdotoxin (CTX, 50 nM) and apamin (0.1 microM), inhibitors of some types of K(+)-channels, abolished the ACh-induced hyperpolarization and dilatation. 4. 18 beta-Glycerrhetinic acid (18 beta-GA, 30 microM), a known inhibitor of gap junctions, depolarized the membrane to about -36 mV, either in the absence or in the presence of Ba(2+), with no associated contraction of the arterioles. In the presence of 18 beta-GA, ACh-induced hyperpolarization was abolished, however the dilatation was inhibited only partially, with associated inhibition of constriction produced by Ba(2+) and NA. 5. 18 beta-GA inhibited the dilatation produced by sodium nitroprusside, an NO donor. 6. The ACh-induced hyperpolarization and dilatation were abolished in the presence of 2-aminoethoxydiphenyl borate (30 microM), an inhibitory modulator of inositol trisphosphate receptor-mediated Ca(2+) release from intracellular stores. 7. It is concluded that in submucosal arterioles, hyperpolarizations produced by ACh have causal relationship to the arteriolar dilatation. 18 beta-GA did not induce parallel relationship between hyperpolarization and dilatation produced by ACh. 18 beta-GA may have unidentified inhibitory effects on agonist-mediated actions, in addition to the inhibition of gap junctions.

Role of ET(A) receptors in the regulation of vascular reactivity in rats with congestive heart failure

Endothelium-derived nitric oxide (NO) and endothelin (ET)-1 interact to regulate vascular tone. In congestive heart failure (CHF), the release and/or the activity of both factors is affected. We hypothesized that the increased ET-1 production associated with CHF may result in a reduced smooth muscle sensitivity to NO. The aim of this study was to evaluate the effects of a chronic treatment with the ET(A)-receptor (ET receptor A) antagonist LU-135252 (LU) on cerebrovascular reactivity to sodium nitroprusside (SNP) in the rat infarct model of CHF. Rats were subjected to coronary artery ligation and were treated for 4 wk with placebo (n = 24) or LU (50 mg. kg(-1). day(-1), n = 29). CHF was associated with a decreased (P < 0.05) efficacy of SNP to induce relaxation of isolated middle cerebral arteries. Furthermore, neither NO synthase inhibition with N(omega)-nitro-L-arginine (L-NNA) nor endothelial denudation affected the efficacy of SNP. Thus the endothelium no longer influences smooth muscle sensitivity to SNP. LU treatment, however, normalized (P < 0.05) smooth muscle sensitivity to SNP. Sensitivity of ET-1-induced contraction was increased in CHF only in the presence of L-NNA, whereas contraction induced by ET(B) receptor (receptor B) stimulation was increased (P < 0.05) in endothelium-denuded vessels. LU treatment restored these changes in reactivity and revealed a significant endothelium-dependent ET(B)-mediated relaxation after NO synthase inhibition. In conclusion, CHF decreases and uncouples cerebrovascular smooth muscle sensitivity to SNP from endothelial regulation. The observation that chronic ET(A) blockade restored most of the changes associated with CHF suggests that activation of the ET-1 system importantly contributes to the alteration in vascular reactivity observed in experimental CHF.

Endothelial factors involved in the bradykinin-induced relaxation of the guinea-pig aorta

Endothelial factors involved in the bradykinin (BK)-induced relaxation of the guinea-pig aorta were investigated using isolated aortic rings. In intact aortic rings, higher concentrations of BK (> or = 10(-7) M) produced contraction, possibly as a direct action on smooth muscle. This BK-induced contraction was enhanced either by Nw-nitro-L-arginine (NOLA), an inhibitor of the production of nitric oxide or by indomethacin (IND), an inhibitor of cyclooxygenase, but not by carbenoxolone (CX), a known inhibitor of gap junctions. In aortic rings contracted with noradrenaline, BK elicited a relaxation with two components; an initial fast relaxation followed by a gradually diminishing slow relaxation, both in an endothelium-dependent manner. The BK-induced relaxation was inhibited in a drug specific manner by either NOLA, IND or CX. NOLA either abolished the fast relaxation, or sometimes converted it into a contractile response. IND reduced the amplitude and duration of the relaxation, by inhibiting the fast relaxation and abolishing the following slow relaxation. CX reduced both components of the relaxation. In the presence of both NOLA and CX, the BK-induced relaxation was converted to a contractile response followed by an IND-sensitive slow relaxation. In the presence of NOLA and IND together, BK stimulation caused a contraction with no following relaxation. These results indicate that in aortic rings of the guinea-pig, BK stimulates endothelial cells to release nitric oxide and prostanoids that produce the fast and slow components of the relaxation respectively. The effects of CX suggest that the contribution of EDHF to the BK-induced relaxation is weak.