Activation of BKCa channels via cyclic AMP- and cyclic GMP-dependent protein kinases by eugenosedin-A in rat basilar artery myocytes

Elucidation of Cellular Mechanisms That Regulate the Sustained Contraction and Relaxation of the Mammalian Iris

Purpose

In mammals, pupil constriction and dilation form the pupillary light reflex (PLR), which is mediated by both brain-regulated (parasympathetic) and local iris-driven reflexes. To better understand the cellular mechanisms that regulate pupil physiological dynamics via central and local photoreception, we have examined the regulation of the PLR via parasympathetic and local activation, respectively.

Methods

In this study, the PLR was examined in mouse enucleated eyes ex vivo in real-time under different ionic conditions in response to acetylcholine and/or blue light (480 nm). The use of pupillometry recordings captured the relaxation, contraction, and pupil escape (redilation) processes for 10 minutes up to 1 hour.

Results

Among others, our results show that ryanodine receptor channels are the main driver for iridal stimulation-contraction coupling, in which extracellular influx of Ca2+ is required for amplification of pupil constriction. Both local and parasympathetic iridal activations are necessary, but not sufficient for sustained pupil constriction. Moreover, the degree of membrane potential repolarization in the dark is correlated with the latency and velocity of iridal constriction. Furthermore, pupil escape is driven by membrane potential hyperpolarization where voltage-gated potassium channels play a crucial role.

Conclusions

Together, this study presents new mechanisms regulating synchronized pupil dilation and contraction, sustained pupil constriction, iridal stimulation-contraction coupling, and pupil escape.

Beneficial Effect of Berberis amurensis Rupr. on Penile Erection

OBJECTIVE

To investigate whether the methanol extract of Berberis amurensis Rupr. (BAR) augments penile erection using in vitro and in vivo experiments.

METHODS

The ex vivo study used corpus cavernosum strips prepared from adult male New Zealand White rabbits. In in vivo studies for intracavernous pressure (ICP), blood pressure, mean arterial pressure (MAP), and increase of peak ICP were continuously monitored during electrical stimulation of Sprague-Dawley rats.

RESULTS

Preconstricted with phenylephrine (PE) in isolated endotheliumintact rabbit corus cavernosum, BAR relaxed penile smooth muscle in a dose-dependent manner, which was inhibited by pretreatment with NG-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, and ¹H-[1,2,4]-oxadiazole-[4,3-α]-quinoxalin-1-one, a soluble guanylyl cclase inhibitor. BAR significantly relaxed penile smooth muscles dose-dependently in ex vivo, and this was inhibited by pretreatment with L-NAME ¹H-[1,2,4]-oxadiazole-[4,3-α]-quinoxalin-1-one. BAR-induced relaxation was significantly attenuated by pretreatment with tetraethylammonium (TEA, P<0.01), a nonselective K⁺ channel blocker, 4-aminopyridine (4-AP, P<0.01), a voltage-dependent K⁺ channel blocker, and charybdotoxin (P<0.01), a large and intermediate conductance Ca²⁺ sensitive-K⁺ channel blocker, respectively. BAR induced an increase in peak ICP, ICP/MAP ratio and area under the curve dose dependently.

CONCLUSION

BAR augments penile erection via the nitric oxide/cyclic guanosine monophosphate system and Ca²⁺ sensitive-K⁺ (BK_Ca and IK_Ca) channels in the corpus cavernosum.

Ferulic acid-4-O-sulfate rather than ferulic acid relaxes arteries and lowers blood pressure in mice

Consumption of foods rich in ferulic acid (FA) such as wholegrain cereals, or FA precursors such as chlorogenic acids in coffee, is inversely correlated with risk of cardiovascular disease and type 2 diabetes. As a result of digestion and phase II metabolism in the gut and liver, FA is converted predominantly into ferulic acid-4-O-sulfate (FA-sul), an abundant plasma metabolite. Although FA-sul is the main metabolite, very little has been reported regarding its bioactivities. We have compared the ex vivo vasorelaxing effect of FA and FA-sul (10^-7-3.10^-5M) on isolated mouse arteries mounted in tissue myographs. FA-sul, but not FA, elicited a concentration-dependent vasorelaxation of saphenous and femoral arteries and aortae. The FA-sul-mediated vasorelaxation was blunted by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, a soluble guanylate cyclase (sGC) inhibitor. The role of sGC was confirmed in femoral arteries isolated from sGCα₁^(-/-) knockout mice. Furthermore, 4-aminopyridine, a specific inhibitor of voltage-dependent potassium channels, significantly decreased FA-sul-mediated effects. In anesthetized mice, intravenous injection of FA-sul decreased mean arterial pressure, whereas FA had no effect, confirming the results obtained ex vivo. FA-sul is probably one of the major metabolites accounting for the blood pressure-lowering effects associated with FA consumption.

Oestrogen upregulates the sarcoplasmic reticulum Ca(2+) ATPase pump in coronary arteries

The presence of circulating plasma 17β-oestradiol (E2) is beneficial in women against abnormal vascular tone development, such as coronary arterial vasospasms. Several vascular diseases have demonstrated that increased expression of the sarcoplasmic reticulum Ca(2+) -ATPase pump (SERCA2b) serves to limit the excessive accumulation of intracellular Ca(2+) . Therefore, the hypothesis of the present study was that E2 would increase SERCA2b expression in the coronary vasculature. Coronary arteries were dissected from hearts obtained from mature female pigs. Artery segments were cultured for 24 h in E2 (1 pmol/L or 1 nmol/L) and homogenized for western blot analysis. At 1 nmol/L, E2 induced an approximate 50% increase in immunoreactivity for SERCA2b. In addition, E2 increased the protein expression of the known SERCA regulatory proteins, protein kinase A (PKA) and protein kinase G (PKG). The E2-induced increase in SERCA2b was attenuated when the culture medium was supplemented with the oestrogen receptor (ER) α/β antagonist ICI 182,780 and the PKG antagonist KT5823 (10 μmol/L, 24 h for both). The PKA antagonist (KT5720; 10 μmol/L, 24 h) had no effect on SERCA2b expression. Removal of the endothelium (using a wooden toothpick) from artery segments prior to culture decreased the E2-mediated increase in SERCA2b and PKG expression by 45% and 47%, respectively. Overall, the findings suggest that one of the potential cardiovascular benefits of E2 in women is upregulation of SERCA2b, via activation of the classic ERα and ERβ pathway.

Differential effects of central and peripheral fat tissues on the delayed rectifier K(+) outward currents in cardiac myocytes

OBJECTIVES

The amount of fat tissue is associated with an increasing incidence of cardiac arrhythmias. The purpose of this study was to investigate effects of adipocytokines from different body fat on delayed rectifier K(+) outward currents (IK).

METHODS

H9c2 cells were treated with adipocytokine-free medium (the Adipo-free group) and with adipocytokines from epicardial (central fat group) and limb (peripheral fat group) rat fat tissues. IK, as well as expressions of Kv2.1 and Kv2.1 mRNA in H9c2 cells, were measured and compared between different groups.

RESULTS

IK measured in H9c2 cells immediately after treatment with adipocytokines were not significantly different from those treated with adipocytokine-free medium. After H9c2 cells were treated with adipocytokines for 18 h, IK were significantly decreased in the peripheral and central fat groups in comparison with the Adipo-free group. Compared with the peripheral fat group, IK were more significantly decreased in the central fat group. Expressions of Kv2.1 and Kv2.1 mRNA in H9c2 cells were not significantly different among the three groups.

CONCLUSIONS

Adipocytokines significantly decreased IK in H9c2 cells, and IK was more prominently decreased by adipocytokines from epicardial fat than from limb fat tissues. The decrease in IK by adipocytokines may partially contribute to the mechanisms of arrhythmogenesis by fat tissues.

Cyclic nucleotide-dependent relaxation pathways in vascular smooth muscle

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (vasoconstriction) and release of force (vasodilation). The initiation of force is associated with increases in intracellular calcium concentrations, activation of myosin light-chain kinase, increases in the phosphorylation of the regulatory myosin light chains, and actin-myosin crossbridge cycling. There are, however, several signaling pathways modulating Ca(2+) mobilization and Ca(2+) sensitivity of the contractile machinery that secondarily regulate the contractile response of vascular smooth muscle to receptor agonists. Among these regulatory mechanisms involved in the physiological regulation of vascular tone are the cyclic nucleotides (cAMP and cGMP), which are considered the main messengers that mediate vasodilation under physiological conditions. At least four distinct mechanisms are currently thought to be involved in the vasodilator effect of cyclic nucleotides and their dependent protein kinases: (1) the decrease in cytosolic calcium concentration ([Ca(2+)]c), (2) the hyperpolarization of the smooth muscle cell membrane potential, (3) the reduction in the sensitivity of the contractile machinery by decreasing the [Ca(2+)]c sensitivity of myosin light-chain phosphorylation, and (4) the reduction in the sensitivity of the contractile machinery by uncoupling contraction from myosin light-chain phosphorylation. This review focuses on each of these mechanisms involved in cyclic nucleotide-dependent relaxation of vascular smooth muscle under physiological conditions.

The guanylyl cyclase activator YC-1 directly inhibits the voltage-dependent K+ channels in rabbit coronary arterial smooth muscle cells

We investigated the effects of YC-1, an activator of soluble guanylyl cyclase (sGC), on voltage-dependent K+ (Kv) channels in smooth muscle cells from freshly isolated rabbit coronary arteries by using the whole-cell patch clamp technique. YC-1 inhibited the Kv current in a dose-dependent fashion with an apparent K(d) of 9.67 microM. It accelerated the decay rate of Kv channel inactivation without altering the kinetics of current activation. The rate constants of association and dissociation for YC-1 were 0.36 +/- 0.01 microM(-1) x s(-1) and 3.44 +/- 0.22 s(-1), respectively. YC-1 did not have a significant effect on the steady-state activation and inactivation curves. The recovery time constant from inactivation was decreased in the presence of YC-1, and application of train pulses (1 or 2 Hz) caused a progressive increase in the YC-1 blockade, indicating that YC-1-induced inhibition of Kv currents is use-dependent. Pretreatment with Bay 41-2272 (also a sGC activator), ODQ (a sGC inhibitor), or Rp-8-Br-PET-cGMPs (a protein kinase G inhibitor) did not affect the basal Kv current and also did not significantly alter the inhibitory effect of YC-1. From these results, we suggest that YC-1 directly inhibits the Kv current independently of sGC activation and in a state-, time-, and use-dependent fashion.

Baicalin, a flavonoid from Scutellaria baicalensis Georgi, activates large-conductance Ca2+-activated K+ channels via cyclic nucleotide-dependent protein kinases in mesenteric artery

Baicalin isolated from Scutellaria baicalensis is a traditional Chinese herbal medicine used for cardiovascular dysfunction. The ionic mechanism of the vasorelaxant effects of baicalin remains unclear. We investigated whether baicalin relaxes mesenteric arteries (MAs) via large-conductance Ca2+-activated K+ (BK(Ca)) channel activation and voltage-dependent Ca2+ channel (VDCC) inhibition. The contractility of MA was determined by dual wire myograph. BK(Ca) channels and VDCCs were measured using whole-cell recordings in single myocytes, enzymatically dispersed from rat MAs. Baicalin (10-100 microM) attenuated 80 mM KCl-contracted MA in a concentration-related manner. L-NAME (30 microM) and indomethacin (10 microM) little affected baicalin (100 microM)-induced vasorelaxations. Contractions induced by iberiotoxin (IbTX, 0.1 microM), Bay K8644 (0.1 microM) or PMA (10 microM) were abolished by baicalin 100 microM. In MA myocytes, baicalin (0.3-30 microM) enhanced BK(Ca) channel activity in a concentration-dependent manner. Increased BK(Ca) currents were abolished by IbTX (0.1 microM). Baicalin-mediated (30 microM) BK(Ca) current activation was significantly attenuated by an adenylate cyclase inhibitor (SQ 22536, 10 microM), a soluble guanylate cyclase inhibitor (ODQ, 10 microM), competitive antagonists of cAMP and cGMP (Rp-cAMP, 100 microM and Rp-cGMP, 100 microM), and cAMP- and cGMP-dependent protein kinase inhibitors (KT5720, 0.3 microM and KT5823, 0.3 microM). Perfusate with PMA (0.1 microM) abolished baicalin-enhanced BK(Ca) currents. Additionally, baicalin (0.3-30 microM) reduced the amplitude of VDCC currents in a concentration-dependent manner and abolished VDCC activator Bay K8644-enhanced (0.1 microM) currents. Baicalin produced MA relaxation by activating BK(Ca) and inhibiting VDCC channels by endothelium-independent mechanisms and by stimulating the cGMP/PKG and cAMP/PKA pathways.

Hypotensive and antiaggregative effects of eugenosedin-B with serotonin and alpha/beta-adrenoceptor antagonistic activities in rats and human platelets

Eugenosedin-B is able to block serotonin (5-HT) and alpha/beta receptors and to inhibit platelet aggregation. In Wistar rats, intravenous injections of eugenosedin-B (2.4, 7.2, 12 micromoL/kg) caused a dose-dependent decrease in blood pressure and heart rate. In contrast, intracisternal injection of eugenosedin-B (0.3, 0.03 micromoL) and an alpha2-antagonist yohimbine (0.03 micromoL) increased blood pressure and heart rate. Eugenosedin-B and yohimbine prevented hypotension induced by intracisternal injection of an alpha2-agonist clonidine (38 pmol). In in vitro experiments, eugenosedin-B (10, 10, 10 M) competitively antagonized norepinephrine-, clonidine-, and 5-HT (10 to 10 M)-induced vasocontractions in isolated rat aorta. It also competitively antagonized the isoproterenol (10 to 10 M)-induced positive inotropic effects in isolated rat atrium. These findings clearly suggest that eugenosedin-B possesses alpha1, alpha2, beta1, and 5-HT2A receptor blocking activities. In isolated rabbit ear artery sensitized with 16 mM K, eugenosedin-B antagonized 5-nonyloxytryptamine- and 5-HT-induced vasocontractions, indicating it also blocked 5-HT1B and 5-HT2A receptors. In radioligand-binding experiments, eugenosedin-B had significant binding affinities on alpha1, alpha2, beta1, 5-HT1B, and 5-HT2A receptors. In human platelets, eugenosedin-B inhibited epinephrine and 5-HT-induced aggregations. It also had competitive binding effects in human platelet with [H]yohimbine (alpha2), [H]ketanserin (5-HT2A). We conclude that hypotensive and vasorelaxant effects of eugenosedin-B can be attributed to its multiple actions on the blockade of 5-HT1B, 5-HT2A, alpha1/2 and beta1 receptors, and its ability to reduce platelet aggregation attributed to its blockade of alpha2 and 5-HT2A receptors.