Functional and molecular evidence for impairment of calcium-activated potassium channels in type-1 diabetic cerebral artery smooth muscle cells

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.

Activation of BKCa channel is associated with increased apoptosis of cerebrovascular smooth muscle cells in simulated microgravity rats

Cerebral arterial remodeling is one of the critical factors in the occurrence of postspaceflight orthostatic intolerance. We hypothesize that large-conductance calcium-activated K+ (BKCa) channels in vascular smooth muscle cells (VSMCs) may play an important role in regulating cerebrovascular adaptation during microgravity exposure. The aim of this work was to investigate whether activation of BKCa channels is involved in regulation of apoptotic remodeling of cerebral arteries in simulated microgravity rats. In animal studies, Sprague-Dawley rats were subjected to 1-wk hindlimb unweighting to simulate microgravity. Alterations of BKCa channels in cerebral VSMCs were investigated by patch clamp and Western blotting; apoptosis was assessed by electron microscopy and terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick-end labeling (TUNEL). To evaluate the correlation of BKCa channel and apoptosis, channel protein and cell nucleus were double-stained. In cell studies, hSloα+β1 channel was coexpressed into human embryonic kidney 293 (HEK293) cells to observe the effects of BKCa channels on apoptosis. In rats, enhanced activities and expression of BKCa channels were found to be correlated with increased apoptosis in cerebral VSMCs after simulated microgravity. In transfected HEK293 cells, activation of cloned BKCa channel induced apoptosis, whereas inhibition of cloned BKCa channel decreased apoptosis. In conclusion, activation of BKCa channels is associated with increased apoptosis in cerebral VSMCs of simulated microgravity rats.

Caveolin-1 limits the contribution of BK(Ca) channel to EDHF-mediated arteriolar dilation: implications in diet-induced obesity

AIMS

Caveolin-1 (Cav-1) interacts with large conductance Ca(2+)-activated potassium channels (BKCa) and likely exerts a negative regulatory effect on the channel activity. We investigated the role of Cav-1 in modulating BK(Ca) channel-mediated, endothelium-derived hyperpolarizing factor (EDHF)-dependent arteriolar dilation in normal condition and in an experimental model of obesity.

METHODS AND RESULTS

In isolated, pressurized (80 mmHg) gracilis muscle arterioles (approximately 100 microm) of Cav-1 knockout mice, acetylcholine (ACh)-induced, EDHF-mediated dilations were enhanced and were significantly reduced by the BK(Ca) channel inhibitor, iberiotoxin (IBTX), whereas IBTX had no effect on EDHF-mediated dilations in the wild-type mice. Dilations to the selective BK(Ca) channel opener, NS-1619 were augmented in the Cav-1 knockout mice. In high-fat diet-treated, obese rats ACh-induced coronary arteriolar dilations were preserved, whereas IBTX-sensitive, ACh-induced and also NS-1619-evoked vasodilations were augmented when compared with lean animals. In coronary arterioles of obese rats a reduced protein expression of Cav-1 was detected by western immunoblotting and immunohistochemistry. Moreover, in coronary arterioles of lean rats, disruption of caveolae with methyl-beta-cyclodextrin augmented IBTX-sensitive, ACh-induced, and also NS-1619-evoked dilations.

CONCLUSION

Thus, under normal conditions, Cav-1 limits the contribution of the BK(Ca) channel to EDHF-mediated arteriolar dilation. In obesity, a reduced expression of Cav-1 leads to greater contribution of the BK(Ca) channel to EDHF-mediated response, which seems essential for maintained coronary dilation.

Daidzein relaxes rat cerebral basilar artery via activation of large-conductance Ca2+-activated K+ channels in vascular smooth muscle cells

Daidzein, a phytoestrogen, has been reported to produce vasodilation via inhibition of Ca(2+) inflow. However, the involvement of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels in the effect of daidzein is debated. Therefore, the present study was designed to investigate the effect of daidzein on the rat cerebral basilar artery and the underlying molecular mechanisms. Isolated cerebral basilar artery rings and single vascular smooth muscle cells (VSMCs) were used for vascular reactivity and electrophysiology measurements, to investigate the effect of daidzein on BK(Ca) channels in cerebral basilar artery smooth muscle. In addition, the human BK(Ca) channel alpha-subunit gene (hslo) was transfected into HEK293 cells, to directly assess whether daidzein activates BK(Ca) channels. The results showed that daidzein produced a concentration-dependent but endothelium-independent relaxation in rat cerebral basilar arteries. Paxilline, a selective BK(Ca) channel blocker, significantly inhibited the daidzein-induced vasodilation, whereas NS1619, a selective BK(Ca) channel opener, enhanced the vasodilation. In the whole-cell configuration, daidzein increased noisy oscillation currents in cerebral basilar artery VSMCs in a concentration-dependent manner, and washout of daidzein or blockade of BK(Ca) channels with paxilline fully reversed the increase. However, daidzein did not substantially affect hSlo currents in HEK293 cells when applied to the outside of the cell membrane. In conclusion, these results indicate that the activation of BK(Ca) channels in VSMCs at least partly contributes to the daidzein-induced vasodilation of the rat cerebral basilar artery. The beta1-subunit of BK(Ca) channels plays a critical role in the activation of BK(Ca) currents by daidzein.

Functional role of canonical transient receptor potential 1 and canonical transient receptor potential 3 in normal and asthmatic airway smooth muscle cells

Canonical transient receptor potential (TRPC)-encoded nonselective cation channels (NSCCs) are crucial for many cellular responses in a variety of cells; however, their molecular expression and functional roles in airway smooth muscle cells (ASMCs) remain obscure. The objective of this study was to determine whether TRPC1 and TRPC3 molecules could be important molecular constituents of native NSCCs controlling the resting membrane potential (Vm) and [Ca(2+)](i) in freshly isolated normal and ovalbumin (OVA)-sensitized/-challenged mouse ASMCs. Western blotting, RT-PCR, single-channel recording, whole-cell current-clamp recording, and a fluorescence imaging system were used to determine TRPC expression, NSCC activity, resting Vm, and resting [Ca(2+)](i). Specific individual TRPC antibodies and siRNAs were applied to test their functional roles. TRPC1 and TRPC3 proteins and mRNAs were expressed in freshly isolated ASM tissues. TRPC3 antibodies blocked the activity of NSCCs and hyperpolarized the resting Vm in ASMCs, whereas TRPC1 antibodies had no effect. TRPC3, but not TRPC1 gene silencing, largely diminished NSCC activity, hyperpolarized the resting Vm, lowered the resting [Ca(2+)](i), and inhibited methacholine-induced increase in [Ca(2+)](i). In OVA-sensitized/-challenged ASMCs, NSCC activity was greatly augmented, resting Vm was depolarized, and TRPC3 protein expression was increased. TRPC1 and TRPC3 antibodies blocked the increased activity of NSCCs and membrane depolarization in OVA-sensitized/-challenged cells. TRPC3 is an important molecular component of native NSCCs contributing to the resting Vm and [Ca(2+)](i) in normal ASMCs, as well as membrane depolarization and hyperresponsiveness in OVA-sensitized/-challenged cells, whereas TRPC1-encoded NSCCs are only activated in OVA-sensitized/-challenged airway myocytes.

Effect of 17beta-oestradiol replacement on vascular responsiveness in ovariectomized diabetic rats

1. Women with functional ovaries exhibit a gender advantage in terms of the prevalence of cardiovascular diseases. However, whether this gender bias pertains in diabetes is unknown. 2. The aim of the present study was to examine the effects of 17beta-oestradiol (E2) on vascular responsiveness in normal and diabetic ovariectomized (OVX) rats. Aged-matched female rats were divided into four groups as follows: (i) OVX; (ii) OVX + E2 treated; (iii) diabetic OVX; and (iv) diabetic OVX + E2 treated. Bilateral ovariectomy was performed and streptozotocin was used to induce experimental diabetes. Rats were treated with 1 mg/kg per day, p.o., E2 for 8 weeks. 3. Although E2 treatment had no effect on blood glucose levels in normal and diabetic OVX rats, it significantly reduced systolic blood pressure and prevented diabetes-induced loss of bodyweight gain. 4. In segments of the thoracic aorta, concentration-dependent vasoconstrictor responses to KCl and phenylephrine were significantly attenuated following E2 treatment in both the normal and diabetic groups. The sarcoplasmic/endoplasmic reticulum calcium ATPase inhibitor thapsigargin (10(-6) mol/L) and the Ca(2+) channel blocker nifedipine (10(-6) mol/L) inhibited the transient vasoconstriction to PE in all groups. The constrictor effect of PE was increased by the nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME; 10(-6) mol/L), but was reduced by superoxide dismutase (SOD; 100 U/mL) and the cyclo-oxygenase inhibitor indomethacin (10(-6) mol/L) in all groups. Responses to acetylcholine (ACh; 10(-6) mol/L) demonstrated reduced endothelium-dependent relaxation in non-E2-treated groups. Relaxation responses to ACh were increased by 100 U/mL SOD and 10(-6) mol/L indomethacin, but were reduced by 10(-6) mol/L l-NAME in all groups. There were no differences among the four groups in terms of relaxation responses to sodium nitroprusside (10(-11) to 10(-6) mol/L). 5. In conclusion, the results of the present study suggest that oestrogen treatment has beneficial effects on vascular function in both diabetic and non-diabetic OVX rats due to Ca(2+) regulation and anti-oxidation.

Rotenone partially reverses decreased BK Ca currents in cerebral artery smooth muscle cells from streptozotocin-induced diabetic mice

1. Reactive oxygen species (ROS) cause vascular complications and impair vasodilation in diabetes mellitus. Large-conductance Ca(2+)-activated potassium channels (BK(Ca)) modulate vascular tone and play an important negative feedback role in vasoconstriction. In the present study, we tested the hypothesis that ROS regulate the function of BK(Ca) in diabetic cerebral artery smooth muscle cells. 2. Diabetes was induced in male BALB/c mice by injection of streptozotocin (STZ; 180 mg/kg, i.p., dissolved in sterile saline). Control and diabetic mice were treated with 12.7 micromol/L rotenone, an inhibitor of the mitochondrial electron transport chain complex I, or placebo every other day for 5 weeks. The whole-cell patch clamp-technique and functional vasomotor methods were used to record BK(Ca) currents and myogenic tone of cerebral artery smooth muscle cells. 3. In the diabetic group, there was a significant decrease in spontaneous transient outward currents in cerebral artery smooth muscle cells compared with control. Although the currents were only moderately increased in rotenone-treated diabetic mice, they remained significantly lower than in the control group. Furthermore, the macroscopic BK(Ca) currents that were decreased in diabetic mice were partially recovered in rotenone-treated diabetic mice (P < 0.05 vs untreated diabetic group). 4. The posterior cerebral artery from diabetic mice had a significantly higher myogenic tone than the control group, but this impaired contraction was partially reversed in the rotenone-treated diabetic group (P < 0.05 vs untreated diabetic group). 5. The H(2)O(2) concentration was significantly increased in cerebral arteries from diabetic mice compared with control. This increase in H(2)O(2) was significantly blunted by rotenone treatment. 6. In conclusion, rotenone partially reverses the decreased macroscopic BK(Ca) currents in STZ-induced Type 1 diabetic mice and this reversal of BK(Ca) currents may be related to the inhibitory effects of rotenone on H(2)O(2) production. Reactive oxygen species, particularly H(2)O(2), are important regulators of BK(Ca) channels and myogenic tone in diabetic cerebral artery.

Calcium-activated potassium channels and endothelial dysfunction: therapeutic options?

The three subtypes of calcium-activated potassium channels (K(Ca)) of large, intermediate and small conductance (BK(Ca), IK(Ca) and SK(Ca)) are present in the vascular wall. In healthy arteries, BK(Ca) channels are preferentially expressed in vascular smooth muscle cells, while IK(Ca) and SK(Ca) are preferentially located in endothelial cells. The activation of endothelial IK(Ca) and SK(Ca) contributes to nitric oxide (NO) generation and is required to elicit endothelium-dependent hyperpolarizations. In the latter responses, the hyperpolarization of the smooth muscle cells is evoked either via electrical coupling through myo-endothelial gap junctions or by potassium ions, which by accumulating in the intercellular space activate the inwardly rectifying potassium channel Kir2.1 and/or the Na(+)/K(+)-ATPase. Additionally, endothelium-derived factors such as cytochrome P450-derived epoxyeicosatrienoic acids and under some circumstances NO, prostacyclin, lipoxygenase products and hydrogen peroxide (H(2)O(2)) hyperpolarize and relax the underlying smooth muscle cells by activating BK(Ca). In contrast, cytochrome P450-derived 20-hydroxyeicosatetraenoic acid and various endothelium-derived contracting factors inhibit BK(Ca). Aging and cardiovascular diseases are associated with endothelial dysfunctions that can involve a decrease in NO bioavailability, alterations of EDHF-mediated responses and/or enhanced production of endothelium-derived contracting factors. Because potassium channels are involved in these endothelium-dependent responses, activation of endothelial and/or smooth muscle K(Ca) could prevent the occurrence of endothelial dysfunction. Therefore, direct activators of these potassium channels or compounds that regulate their activity or their expression may be of some therapeutic interest. Conversely, blockers of IK(Ca) may prevent restenosis and that of BK(Ca) channels sepsis-dependent hypotension.