Increased expression of Ca2+-sensitive K+ channels in aorta of hypertensive rats

BK Channels in Tail Artery Vascular Smooth Muscle Cells of Normotensive (WKY) and Hypertensive (SHR) Rats Possess Similar Calcium Sensitivity But Different Responses to the Vasodilator Iloprost

It has been reported that, in the spontaneously hypertensive rat (SHR) model of hypertension, different components of the G-protein/adenylate cyclase (AC)/Calcium-activated potassium channel of high conductance (BK) channel signaling pathway are altered differently. In the upstream part of the pathway (G-protein/AC), a comparatively low efficacy has been established, whereas downstream BK currents seem to be increased. Thus, the overall performance of this signaling pathway in SHR is elusive. For a better understanding, we focused on one aspect, the direct targeting of the BK channel by the G-protein/AC pathway and tested the hypothesis that the comparatively low AC pathway efficacy in SHR results in a reduced agonist-induced stimulation of BK currents. This hypothesis was investigated using freshly isolated smooth muscle cells from WKY and SHR rat tail artery and the patch-clamp technique. It was observed that: (1) single BK channels have similar current-voltage relationships, voltage-dependence and calcium sensitivity; (2) BK currents in cells with a strong buffering of the BK channel activator calcium have similar current-voltage relationships; (3) the iloprost-induced concentration-dependent increase of the BK current is larger in WKY compared to SHR; (4) the effects of activators of the PKA pathway, the catalytic subunit of PKA and the potent and selective cAMP-analogue Sp-5,6-DCl-cBIMPS on BK currents are similar. Thus, our data suggest that the lower iloprost-induced stimulation of the BK current in freshly isolated rat tail artery smooth muscle cells from SHR compared with WKY is due to the lower efficacy of upstream elements of the G-Protein/AC/BK channel pathway.

Hinokitiol produces vasodilation in aortae from normal and angiotensin II- induced hypertensive rats via endothelial-dependent and independent pathways

Hinokitiol is a natural bioactive compound with numerous pharmacological properties. Here, we aimed to examine hinokitiol's effects on vascular relaxation. Cumulative relaxation responses to hinokitiol were assessed in isolated aortae from normotensive and angiotensin II-induced hypertensive rats in the presence and absence of selective inhibitors. Hinokitiol produced vasodilation of phenylephrine preconstricted aortae using both normotensive and hypertensive rats. In normotensive rats, hinokitiol's vasodilation was reduced by endothelial denudation and nitric oxide synthase (NOS), guanylate cyclase, and cyclooxygenase inhibition. Also, hinokitiol vasodilation was attenuated by β-receptors, adenylate cyclase, Ca²⁺-activated K⁺ channels and hyperpolarization inhibition. Moreover, hinokitiol exhibited a blocking activity on Ca²⁺ mobilization through voltage dependent Ca²⁺ channels (VDCC). However, its effect was not changed by muscarinic receptor and Sarc-K⁺ ATP channels blocking but was enhanced by blocking voltage-dependent K⁺ channels. However, in angiotensin II-induced hypertension, hinokitiol vasodilating activity was attenuated by NOS inhibition and it blocked Ca²⁺ mobilization through VDCC, while its vasodilation was partially attenuated by Sarc-K⁺ ATP channels blocking. However, the vasodilating effect of hinokitiol was not attenuated by either cyclooxygenase, β-receptor, Ca²⁺-activated K⁺ channels, or voltage-dependent potassium channels inhibition, but was enhanced by blocking hyperpolarization. Hinokitiol's vasodilating effect in normotensive and hypertensive vessels is mediated through both endothelium-dependent and endothelium-independent mechanisms.

C60 fullerenes selectively inhibit BKCa but not Kv channels in pulmonary artery smooth muscle cells

Possessing unique physical and chemical properties, C₆₀ fullerenes are arising as a potential nanotechnological tool that can strongly affect various biological processes. Recent molecular modeling studies have shown that C₆₀ fullerenes can interact with ion channels, but there is lack of data about possible effects of C₆₀ molecule on ion channels expressed in smooth muscle cells (SMC). Here we show both computationally and experimentally that water-soluble pristine C₆₀ fullerene strongly inhibits the large conductance Ca²⁺-dependent K⁺ (BK_Ca), but not voltage-gated K⁺ (K_v) channels in pulmonary artery SMC. Both molecular docking simulations and analysis of single channel activity indicate that C₆₀ fullerene blocks BK_Ca channel pore in its open state. In functional tests, C₆₀ fullerene enhanced phenylephrine-induced contraction of pulmonary artery rings by about 25% and reduced endothelium-dependent acetylcholine-induced relaxation by up to 40%. These findings suggest a novel strategy for biomedical application of water-soluble pristine C₆₀ fullerene in vascular dysfunction.

Potassium channels in vascular smooth muscle: a pathophysiological and pharmacological perspective

Potassium (K⁺ ) ion channel activity is an important determinant of vascular tone by regulating cell membrane potential (MP). Activation of K⁺ channels leads to membrane hyperpolarization and subsequently vasodilatation, while inhibition of the channels causes membrane depolarization and then vasoconstriction. So far five distinct types of K⁺ channels have been identified in vascular smooth muscle cells (VSMCs): Ca⁺² -activated K⁺ channels (BK_{C a} ), voltage-dependent K⁺ channels (K_V ), ATP-sensitive K⁺ channels (K_ATP ), inward rectifier K⁺ channels (K_ir ), and tandem two-pore K⁺ channels (K₂ P). The activity and expression of vascular K⁺ channels are changed during major vascular diseases such as hypertension, pulmonary hypertension, hypercholesterolemia, atherosclerosis, and diabetes mellitus. The defective function of K⁺ channels is commonly associated with impaired vascular responses and is likely to become as a result of changes in K⁺ channels during vascular diseases. Increased K⁺ channel function and expression may also help to compensate for increased abnormal vascular tone. There are many pharmacological and genotypic studies which were carried out on the subtypes of K⁺ channels expressed in variable amounts in different vascular beds. Modulation of K⁺ channel activity by molecular approaches and selective drug development may be a novel treatment modality for vascular dysfunction in the future. This review presents the basic properties, physiological functions, pathophysiological, and pharmacological roles of the five major classes of K⁺ channels that have been determined in VSMCs.

Prenatal hypoxia plus postnatal high-fat diet exacerbated vascular dysfunction via up-regulated vascular Cav1.2 channels in offspring rats

Background

This study aimed to examine whether and how postnatal high‐fat diet had additional impact on promoting vascular dysfunction in the offspring exposed to prenatal hypoxia.

Methods and Results

Pregnant Sprague‐Dawley rats were randomly assigned to hypoxia (10.5% oxygen) or normoxia (21% O₂) groups from gestation days 5‐21. A subset of male offspring was placed on a high‐fat diet (HF, 45% fat) from 4‐16 weeks of age. Prenatal hypoxia induced a decrease in birth weight. In offspring‐fed HF diet, prenatal hypoxia was associated with increased fasting plasma triglyceride, total cholesterol, free fatty acids, and low‐density lipoprotein‐cholesterol. Compared with the other three groups, prenatal hypoxic offspring with high‐fat diet showed a significant increase in blood pressure, phenylephrine‐mediated vasoconstrictions, L‐type voltage‐gated Ca2+ (Cav1.2) channel currents, and elevated mRNA and protein expression of Cav1.2 α1 subunit in mesenteric arteries or myocytes. The large‐conductance Ca2+‐activated K+ (BK) channels currents and the BK channel units (β1, not α‐subunits) were significantly increased in mesenteric arteries or myocytes in HF offspring independent of prenatal hypoxia factor.

Conclusion

The results demonstrated that prenatal hypoxia followed by postnatal HF caused vascular dysfunction through ion channel remodelling in myocytes.

KV channels and the regulation of vascular smooth muscle tone

VSMCs in resistance arteries and arterioles express a diverse array of K_V channels with members of the K_V 1, K_V 2 and K_V 7 families being particularly important. Members of the K_V channel family: (i) are highly expressed in VSMCs; (ii) are active at the resting membrane potential of VSMCs in vivo (-45 to -30 mV); (iii) contribute to the negative feedback regulation of VSMC membrane potential and myogenic tone; (iv) are activated by cAMP-related vasodilators, hydrogen sulfide and hydrogen peroxide; (v) are inhibited by increases in intracellular Ca²⁺ and vasoconstrictors that signal through G_q -coupled receptors; (vi) are involved in the proliferative phenotype of VSMCs; and (vii) are modulated by diseases such as hypertension, obesity, the metabolic syndrome and diabetes. Thus, K_V channels participate in every aspect of the regulation of VSMC function in both health and disease.

Aerobic Exercise of Low to Moderate Intensity Corrects Unequal Changes in BKCa Subunit Expression in the Mesenteric Arteries of Spontaneously Hypertensive Rats

Accumulating evidence indicates that hypertension is associated with “ion channel remodeling” of vascular smooth muscle cells (VSMCs). The objective of this study was to determine the effects of exercise intensity/volume on hypertension-associated changes in large-conductance Ca2+-activated K+ (BKCa) channels in mesenteric arteries (MAs) from spontaneously hypertensive rats (SHR). Male SHRs were randomly assigned to three groups: a low-intensity aerobic exercise group (SHR-L: 14 m/min), a moderate-intensity aerobic exercise group (SHR-M: 20 m/min), and a sedentary group (SHR). Age-matched Wistar-Kyoto rats (WKYs) were used as normotensive controls. Exercise groups completed an 8-week exercise program. Elevation of the α and β1 proteins was unequal in MA myocytes from SHRs, with the β1 subunit increasing more than the α subunit. BKCa contribution to vascular tone regulation was higher in the myocytes and arteries of SHRs compared to WKYs. SHR BKCa channel subunit protein expression, β1/α ratio, whole cell current density and single-channel open probability was also increased compared with WKYs. Aerobic exercise lowered systemic blood pressure and normalized hypertension-associated BKCa alterations to normotensive control levels in the SHRs. These effects were more pronounced in the moderate-intensity group than in the low-intensity group. There is a dose-effect for aerobic exercise training in the range of low to moderate-intensity and accompanying volume for the correction of the pathological adaptation of BKCa channels in myocytes of MAs from SHR.

A Role of BK Channel in Regulation of Ca2+ Channel in Ventricular Myocytes by Substrate Stiffness

Substrate stiffness is crucial for diverse cell functions, but the mechanisms conferring cells with mechanosensitivity are still elusive. By tailoring substrate stiffness with 10-fold difference, we showed that L-type voltage-gated Ca2+ channel current density was greater in chick ventricular myocytes cultured on the stiff substrate than on the soft substrate. Blockage of the BK channel increased the Ca2+ current density on the soft substrate and consequently eliminated substrate stiffness regulation of the Ca2+ channel. The expression of the BK channel, including the STREX-containing α-subunit that forms stretch-activated BK channel in myocytes and the BK channel function in myocytes (and also in HEK293 cells heterologously expressing STREX-containing α- and β1-subunits) was reduced in cells cultured on the stiff substrate. Furthermore, in HEK293 cells coexpressing the cardiac CaV1.2 channel and STREX-containing BK channel, the Ca2+ current density was greater in cells on the stiff substrate, which was not observed in cells expressing the CaV1.2 channel alone or coexpressing with the STREX-deleted BK channel. These results provide strong evidence to show that the stretch-activated BK channel plays a key role in functional regulation of cardiac voltage-gated Ca2+ channel by substrate stiffness, revealing, to our knowledge, a novel mechanosensing mechanism in ventricular myocytes.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Relaxation Induced by Atrial Natriuretic Peptide Is Impaired in Carotid but Not Renal Arteries from Spontaneously Hypertensive Rats Due to Reduced BKCa Channel Activity

Atrial natriuretic peptide (ANP) plays an important role in vascular functions such as blood pressure regulation and relaxant activity. Individual vascular beds exhibit differences in vascular reactivity to various ligands, however, the difference in responsiveness to ANP between carotid and renal arteries and the molecular mechanisms of its vasorelaxant activity in a pathophysiological state, including hypertension, remain unclear. We therefore investigated this issue by exposing carotid and renal artery rings obtained from spontaneously hypertensive rats (SHR) to ANP. In the SHR artery (vs. control WKY artery), the ANP-induced relaxations were reduced in carotid artery but not renal artery. Acetylcholine-induced relaxations were reduced in both arteries in SHR (vs. WKY). Sodium nitroprusside-induced relaxation was similar in both arteries between the groups. In carotid arteries, the ANP-induced relaxation was not affected by endothelial denudation or by treatment with inhibitors of nitric oxide synthase, cyclooxygenase, the voltage-dependent potassium channel, or ATP-sensitive potassium channel in arteries from both SHR and WKY. In the carotid artery from WKY but not SHR, the ANP-induced relaxation was significantly reduced by inhibition of the large-conductance calcium-activated potassium channel (BKCa). The BKCa activator-induced relaxation was reduced in the SHR artery (vs. WKY). These results suggest that ANP-induced relaxation is impaired in the carotid artery from SHR and this impairment may be at least in part due to the reduction of BKCa activity rather than endothelial components.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

The Human Microcirculation: Regulation of Flow and Beyond

The microcirculation is responsible for orchestrating adjustments in vascular tone to match local tissue perfusion with oxygen demand. Beyond this metabolic dilation, the microvasculature plays a critical role in modulating vascular tone by endothelial release of an unusually diverse family of compounds including nitric oxide, other reactive oxygen species, and arachidonic acid metabolites. Animal models have provided excellent insight into mechanisms of vasoregulation in health and disease. However, there are unique aspects of the human microcirculation that serve as the focus of this review. The concept is put forth that vasculoparenchymal communication is multimodal, with vascular release of nitric oxide eliciting dilation and preserving normal parenchymal function by inhibiting inflammation and proliferation. Likewise, in disease or stress, endothelial release of reactive oxygen species mediates both dilation and parenchymal inflammation leading to cellular dysfunction, thrombosis, and fibrosis. Some pathways responsible for this stress-induced shift in mediator of vasodilation are proposed. This paradigm may help explain why microvascular dysfunction is such a powerful predictor of cardiovascular events and help identify new approaches to treatment and prevention.

Developmental acceleration of bradykinin-dependent relaxation by prenatal chronic hypoxia impedes normal development after birth

Bradykinin-induced activation of the pulmonary endothelium triggers nitric oxide production and other signals that cause vasorelaxation, including stimulation of large-conductance Ca(2+)-activated K(+) (BKCa) channels in myocytes that hyperpolarize the plasma membrane and decrease intracellular Ca(2+). Intrauterine chronic hypoxia (CH) may reduce vasorelaxation in the fetal-to-newborn transition and contribute to pulmonary hypertension of the newborn. Thus we examined the effects of maturation and CH on the role of BKCa channels during bradykinin-induced vasorelaxation by examining endothelial Ca(2+) signals, wire myography, and Western immunoblots on pulmonary arteries isolated from near-term fetal (∼ 140 days gestation) and newborn, 10- to 20-day-old, sheep that lived in normoxia at 700 m or in CH at high altitude (3,801 m) for >100 days. CH enhanced bradykinin-induced relaxation of fetal vessels but decreased relaxation in newborns. Endothelial Ca(2+) responses decreased with maturation but increased with CH. Bradykinin-dependent relaxation was sensitive to 100 μM nitro-L-arginine methyl ester or 10 μM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, supporting roles for endothelial nitric oxide synthase and soluble guanylate cyclase activation. Indomethacin blocked relaxation in CH vessels, suggesting upregulation of PLA2 pathways. BKCa channel inhibition with 1 mM tetraethylammonium reduced bradykinin-induced vasorelaxation in the normoxic newborn and fetal CH vessels. Maturation reduced whole cell BKCa channel α1-subunit expression but increased β1-subunit expression. These results suggest that CH amplifies the contribution of BKCa channels to bradykinin-induced vasorelaxation in fetal sheep but stunts further development of this vasodilatory pathway in newborns. This involves complex changes in multiple components of the bradykinin-signaling axes.

A Combination of Intravenous Genistein Plus Mg2+ Enhances Antihypertensive Effects in SHR by Endothelial Protection and BKCa Channel Inhibition

BACKGROUND

The effects of combining genistein (GST) plus magnesium (Mg) upon the development of hypertension were examined in 28 twelve-week-old male spontaneous hypertension rats (SHRs). Four experimental groups were tested: SHR (0.9% NaCl and DMSO), SHR + GST (0.9% NaCl and GST 5mg/kg/day), SHR + Mg (Mg(2+) 0.75 mmol/kg/day and DMSO), and SHR + GST + Mg (Mg(2+) 0.75 mmol/kg/day and GST 5mg/kg/day). A group of normotensive genetic control, Wistar-Kyoto (WKY) rats were also included for comparison. Drugs were administrated intravenously daily for 30 days.

METHODS

Systolic blood pressure (SBP) and heart rate were measured by tail-cuff plethysmography every five days. Vascular tone of mesenteric arteries was examined by an isometric force transducer. Big-conductance calcium-activated potassium channel (BKCa) currents were detected by whole-cell patch-clamp techniques.

RESULTS

SBP in SHRs was significantly elevated vs. that in WKY rats. GST or Mg lowered SBP of SHRs. Their combination enhanced antihypertensive effects, as indicated by significantly lowered SBP and shorter onset times. GST or Mg individually improved endothelial dysfunction of SHRs. However, again their combination enhanced endothelial protection, nearly restoring maximal relaxation responses to those seen in WKY. BKCa currents in SHRs were increased compared with WKY rats. GST, Mg, and their combination restored BKCa currents to those of WKY rats.

CONCLUSIONS

The combination of GST and Mg produces antihypertensive effects and improvement of endothelial dysfunction, which are substantially greater than that when either is used individually. These results suggest a novel and feasible protocol for the prevention and treatment of hypertension and related cardio- and cerebro-vascular diseases.

Exercise training reverses alterations in Kv and BKCa channel molecular expression in thoracic aorta smooth muscle cells from spontaneously hypertensive rats

Previous studies have shown that exercise training influences potassium channel protein expression in arteries. The purpose of this study was to investigate the effect of exercise training on alterations in voltage-gated potassium channels (Kv) and large-conductance, calcium-activated potassium channels (BKCa) in thoracic aorta smooth muscle cells from spontaneously hypertensive rats (SHRs). Male SHRs were randomly assigned to a sedentary group (SHR-SED) and exercise training group (SHR-EX). Age-matched Wistar-Kyoto rats (WKYs) were used as controls. After 8 weeks of aerobic exercise training, blood pressure was significantly lower in the SHR-EX group than in the SHR-SED group. Exercise training increased the contribution of the Kv1.2 and Kv1.5 channels and decreased the contribution of BKCa channel to resting tone in the SHR-EX group compared to the SHR-SED group as indicated by vessel contractility experiments. Immunohistochemistry and Western blotting showed that Kv1.2 and Kv1.5 channel expression was significantly lower in the SHR-SED group than in the WKY group and exercise training attenuated this reduction. BKCa α-subunit expression was statistically unchanged between the groups; however, β1-subunit expression was reduced significantly by exercise training in the SHR-EX group compared to the SHR-SED group. These data suggest that exercise training reverses the pathological expression of the Kv1.2, Kv1.5, and BKCa channels in aortic myocytes from SHRs. This is one of the favorable effects of exercise training on large conduit arteries.

Enhanced large conductance K+ channel activity contributes to the impaired myogenic response in the cerebral vasculature of Fawn Hooded Hypertensive rats

Recent studies have indicated that the myogenic response (MR) in cerebral arteries is impaired in Fawn Hooded Hypertensive (FHH) rats and that transfer of a 2.4 megabase pair region of chromosome 1 (RNO1) containing 15 genes from the Brown Norway rat into the FHH genetic background restores MR in a FHH.1(BN) congenic strain. However, the mechanisms involved remain to be determined. The present study examined the role of the large conductance calcium-activated potassium (BK) channel in impairing the MR in FHH rats. Whole-cell patch-clamp studies of cerebral vascular smooth muscle cells (VSMCs) revealed that iberiotoxin (IBTX; BK inhibitor)-sensitive outward potassium (K+) channel current densities are four- to fivefold greater in FHH than in FHH.1(BN) congenic strain. Inside-out patches indicated that the BK channel open probability (NPo) is 10-fold higher and IBTX reduced NPo to a greater extent in VSMCs isolated from FHH than in FHH.1(BN) rats. Voltage sensitivity of the BK channel is enhanced in FHH as compared with FHH.1(BN) rats. The frequency and amplitude of spontaneous transient outward currents are significantly greater in VSMCs isolated from FHH than in FHH.1(BN) rats. However, the expression of the BK-α and -β-subunit proteins in cerebral vessels as determined by Western blot is similar between the two groups. Middle cerebral arteries (MCAs) isolated from FHH rats exhibited an impaired MR, and administration of IBTX restored this response. These results indicate that there is a gene on RNO1 that impairs MR in the MCAs of FHH rats by enhancing BK channel activity.

Involvement of BKCa and KV potassium channels in cAMP-induced vasodilatation: their insufficient function in genetic hypertension

Spontaneously hypertensive rats (SHR) are characterized by enhanced sympathetic vasoconstriction, whereas their vasodilator mechanisms are relatively attenuated compared to their high BP. The objective of our in vivo study was to evaluate whether the impaired function of BKCa and/or KV channels is responsible for abnormal cAMP-induced vasodilatation in genetic hypertension. Using conscious SHR and normotensive WKY rats we have shown that under the basal conditions cAMP overproduction elicited by the infusion of beta-adrenoceptor agonist (isoprenaline) caused a more pronounced decrease of baseline blood pressure (BP) in SHR compared to WKY rats. Isoprenaline infusion prevented BP rises induced by acute NO synthase blockade in both strains and it also completely abolished the fully developed BP response to NO synthase blockade. These cAMP-induced vasodilator effects were diminished by the inhibition of either BKCa or KV channels in SHR but simultaneous blockade of both K(+) channel types was necessary in WKY rats. Under basal conditions, the vasodilator action of both K(+) channels was enhanced in SHR compared to WKY rats. However, the overall contribution of K(+) channels to cAMP-induced vasodilator mechanisms is insufficient in genetic hypertension since a concurrent activation of both K(+) channels by cAMP overproduction is necessary for the prevention of BP rise elicited by acute NO/cGMP deficiency in SHR. This might be caused by less effective activation of these K(+) channels by cAMP in SHR. In conclusion, K(+) channels seem to have higher activity in SHR, but their vasodilator action cannot match sufficiently the augmented vasoconstriction in this hypertensive strain.

Functional expression of smooth muscle-specific ion channels in TGF-β(1)-treated human adipose-derived mesenchymal stem cells

Human adipose tissue-derived mesenchymal stem cells (hASCs) have the power to differentiate into various cell types including chondrocytes, osteocytes, adipocytes, neurons, cardiomyocytes, and smooth muscle cells. We characterized the functional expression of ion channels after transforming growth factor-β1 (TGF-β1)-induced differentiation of hASCs, providing insights into the differentiation of vascular smooth muscle cells. The treatment of hASCs with TGF-β1 dramatically increased the contraction of a collagen-gel lattice and the expression levels of specific genes for smooth muscle including α-smooth muscle actin, calponin, smooth mucle-myosin heavy chain, smoothelin-B, myocardin, and h-caldesmon. We observed Ca(2+), big-conductance Ca(2+)-activated K(+) (BKCa), and voltage-dependent K(+) (Kv) currents in TGF-β1-induced, differentiated hASCs and not in undifferentiated hASCs. The currents share the characteristics of vascular smooth muscle cells (SMCs). RT-PCR and Western blotting revealed that the L-type (Cav1.2) and T-type (Cav3.1, 3.2, and 3.3), known to be expressed in vascular SMCs, dramatically increased along with the Cavβ1 and Cavβ3 subtypes in TGF-β1-induced, differentiated hASCs. Although the expression-level changes of the β-subtype BKCa channels varied, the major α-subtype BKCa channel (KCa1.1) clearly increased in the TGF-β1-induced, differentiated hASCs. Most of the Kv subtypes, also known to be expressed in vascular SMCs, dramatically increased in the TGF-β1-induced, differentiated hASCs. Our results suggest that TGF-β1 induces the increased expression of vascular SMC-like ion channels and the differentiation of hASCs into contractile vascular SMCs.

Ion channel remodeling in vascular smooth muscle during hypertension: Implications for novel therapeutic approaches

Ion channels are multimeric, transmembrane proteins that selectively mediate ion flux across the plasma membrane in a variety of cells including vascular smooth muscle cells (VSMCs). The dynamic interplay of Ca(2+) and K(+) channels on the plasma membrane of VSMCs plays a pivotal role in modulating the vascular tone of small arteries and arterioles. The abnormally-elevated arterial tone observed in hypertension thus points to an aberrant expression and function of Ca(2+) and K(+) channels in the VSMCs. In this short review, we focus on the three well-studied ion channels in VSMCs, namely the L-type Ca(2+) (CaV1.2) channels, the voltage-gated K(+) (KV) channels, and the large-conductance Ca(2+)-activated K(+) (BK) channels. First, we provide a brief overview on the physiological role of vascular CaV1.2, KV and BK channels in regulating arterial tone. Second, we discuss the current understanding of the expression changes and regulation of CaV1.2, KV and BK channels in the vasculature during hypertension. Third, based on available proof-of-concept studies, we describe the potential therapeutic approaches targeting these vascular ion channels in order to restore blood pressure to normotensive levels.

Function and regulation of large conductance Ca(2+)-activated K+ channel in vascular smooth muscle cells

Large conductance Ca(2+)-activated K(+) (BK(Ca)) channels are abundantly expressed in vascular smooth muscle cells. Activation of BK(Ca) channels leads to hyperpolarization of cell membrane, which in turn counteracts vasoconstriction. Therefore, BK(Ca) channels have an important role in regulation of vascular tone and blood pressure. The activity of BK(Ca) channels is subject to modulation by various factors. Furthermore, the function of BK(Ca) channels are altered in both physiological and pathophysiological conditions, such as pregnancy, hypertension and diabetes, which has dramatic impacts on vascular tone and hemodynamics. Consequently, compounds and genetic manipulation that alter activity and expression of the channel might be of therapeutic interest.

Impaired β-adrenoceptor-induced relaxation in small mesenteric arteries from DOCA-salt hypertensive rats is due to reduced K(Ca) channel activity

β-Adrenoceptor (β-AR)-mediated relaxation plays an important role in the regulation of vascular tone. β-AR-mediated vascular relaxation is reduced in various disease states and aging. We hypothesized that β-AR-mediated vasodilatation is impaired in DOCA-salt hypertension due to alterations in the cAMP pathway. β-AR-mediated relaxation was determined in small mesenteric arteries from DOCA-salt hypertensive and control uninephrectomized (Uni) rats. To exclude nitric oxide (NO) and cyclooxygenase (COX) pathways, relaxation responses were determined in the presence of l-NNA and indomethacin, NO synthase inhibitor and COX inhibitors, respectively. Isoprenaline (ISO)-induced relaxation was reduced in arteries from DOCA-salt compared to Uni rats. Protein kinase A (PKA) inhibitors (H89 or Rp-cAMPS) or adenylyl cyclase inhibitor (SQ22536) did not abolish the difference in ISO-induced relaxation between the groups. Forskolin (adenylyl cyclase activator)-induced relaxation was similar between the groups. The inhibition of IK(Ca)/SK(Ca) channels (TRAM-34 plus UCL1684) or BK(Ca) channels (iberiotoxin) reduced ISO-induced relaxation only in Uni rats and abolished the relaxation differences between the groups. The expression of SK(Ca) channel was decreased in DOCA-salt arteries. The expression of BK(Ca) channel α subunit was increased whereas the expression of BK(Ca) channel β subunit was decreased in DOCA-salt arteries. The expression of receptor for activated C kinase 1 (RACK1), which is a binding protein for BK(Ca) channel and negatively modulates its activity, was increased in DOCA-salt arteries. These results suggest that the impairment of β-AR-mediated relaxation in DOCA-salt mesenteric arteries may be attributable to altered IK(Ca)/SK(Ca) and/or BK(Ca) channels activities rather than cAMP/PKA pathway. Impaired β-AR-stimulated BK(Ca) channel activity may be due to the imbalance between its subunit expressions and RACK1 upregulation.

Altered neural and vascular mechanisms in hypertension

Essential hypertension is a multifactorial disorder which belongs to the main risk factors responsible for renal and cardiovascular complications. This review is focused on the experimental research of neural and vascular mechanisms involved in the high blood pressure control. The attention is paid to the abnormalities in the regulation of sympathetic nervous system activity and adrenoceptor alterations as well as the changes of membrane and intracellular processes in the vascular smooth muscle cells of spontaneously hypertensive rats. These abnormalities lead to increased vascular tone arising from altered regulation of calcium influx through L-VDCC channels, which has a crucial role for excitation-contraction coupling, as well as for so-called "calcium sensitization" mediated by the RhoA/Rho-kinase pathway. Regulation of both pathways is dependent on the complex interplay of various vasodilator and vasoconstrictor stimuli. Two major antagonistic players in the regulation of blood pressure, i.e. sympathetic nervous system (by stimulation of adrenoceptors coupled to stimulatory and inhibitory G proteins) and nitric oxide (by cGMP signaling pathway), elicit their actions via the control of calcium influx through L-VDCC. However, L-type calcium current can also be regulated by the changes in membrane potential elicited by the activation of potassium channels, the impaired function of which was detected in hypertensive animals. The dominant role of enhanced calcium influx in the pathogenesis of high blood pressure of genetically hypertensive animals is confirmed not only by therapeutic efficacy of calcium antagonists but especially by the absence of hypertension in animals in which L-type calcium current was diminished by pertussis toxin-induced inactivation of inhibitory G proteins. Although there is considerable information on the complex neural and vascular alterations in rats with established hypertension, the detailed description of their appearance during the induction of hypertension is still missing.

Differential effects of diet-induced obesity on BKCa {beta}1-subunit expression and function in rat skeletal muscle arterioles and small cerebral arteries

Mechanisms underlying obesity-related vascular dysfunction are unclear. This study examined the effect of diet-induced obesity on expression and function of large conductance Ca(2+)-activated potassium channel (BK(Ca)) in rat pressurized small resistance vessels with myogenic tone. Male Sprague-Dawley rats fed a cafeteria-style high fat diet (HFD; ∼30% energy from fat) for 16-20 wk were ∼30% heavier than controls fed standard chow (∼13% fat). Obesity did not alter BK(Ca) α-subunit function or α-subunit protein or mRNA expression in vessels isolated from the cremaster muscle or middle-cerebral circulations. In contrast, BK(Ca) β(1)-subunit protein expression and function were significantly reduced in cremaster muscle arterioles but increased in middle-cerebral arteries from obese animals. Immunohistochemistry showed α- and β(1)-subunits were present exclusively in the smooth muscle of both vessels. Cremaster muscle arterioles from obese animals showed significantly increased medial thickness, and media-to-lumen ratio and pressurized arterioles showed increased myogenic tone at 30 mmHg, but not at 50-120 mmHg. Myogenic tone was not affected by obesity in middle-cerebral arteries. The BK(Ca) antagonist iberiotoxin constricted both cremaster muscle and middle-cerebral arterioles from control rats; this effect of iberiotoxin was abolished in cremaster muscle arteries only from obese rats. Diet-induced obesity has contrasting effects on BK(Ca) function in different vascular beds, through differential effects on β(1)-subunit expression. However, these alterations in BK(Ca) function had little effect on overall myogenic tone, suggesting that the mechanisms controlling myogenic tone can be altered and compensate for altered BK(Ca) expression and function.

Mitochondrial monoamine oxidase-A-mediated hydrogen peroxide generation enhances 5-hydroxytryptamine-induced contraction of rat basilar artery

BACKGROUND AND PURPOSE

We evaluated the role(s) of monoamine oxidase (MAO)-mediated H₂O₂ generation on 5-hydroxytryptamine (5-HT)-induced tension development of isolated basilar artery of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats.

EXPERIMENTAL APPROACH

Basilar artery (endothelium-denuded) was isolated for tension measurement and Western blots. Enzymically dissociated single myocytes from basilar arteries were used for patch-clamp electrophysiological and confocal microscopic studies.

KEY RESULTS

Under resting tension, 5-HT elicited a concentration-dependent tension development with a greater sensitivity (with unchanged maximum tension development) in SHR compared with WKY (EC(50) : 28.4 ± 4.1 nM vs. 98.2 ± 9.4 nM). The exaggerated component of 5-HT-induced tension development in SHR was eradicated by polyethylene glycol-catalase, clorgyline and citalopram whereas exogenously applied H₂O₂ enhanced the 5-HT-elicited tension development in WKY. A greater protein expression of MAO-A was detected in basilar arteries from SHR than in those from WKY. In single myocytes and the entire basilar artery, 5-HT generated (clorgyline-sensitive) a greater amount of H₂O₂ in SHR compared with WKY. Whole-cell iberiotoxin-sensitive Ca(2+) -activated K(+) (BK(Ca) ) amplitude measured in myocytes of SHR was approximately threefold greater than that in WKY (at +60 mV: 7.61 ± 0.89 pA·pF(-1) vs. 2.61 ± 0.66 pA·pF(-1) ). In SHR myocytes, 5-HT caused a greater inhibition (clorgyline-, polyethylene glycol-catalase- and reduced glutathione-sensitive) of BK(Ca) amplitude than in those from WKY.

CONCLUSIONS AND IMPLICATIONS

5-HT caused an increased generation of mitochondrial H₂O₂ via MAO-A-mediated 5-HT metabolism, which caused a greater inhibition of BK(Ca) gating in basilar artery myocytes, leading to exaggerated basilar artery tension development in SHR.

Effects of bladder outlet obstruction on properties of Ca2+-activated K+ channels in rat bladder

In this study, we investigated the effects of bladder outlet obstruction (BOO) on the expression and function of large conductance (BK) and small conductance (SK) Ca(2+)-activated K(+) channels in detrusor smooth muscle. The bladder from adult female Sprague-Dawley rats with 6-wk BOO were used. The mRNA expression of the BK channel alpha-subunit, beta1-, beta2-, and beta4-subunits and SK1, SK2, and SK3 channels were investigated using real-time RT-PCR. All subunits except for the BK-beta2, SK2, and SK3 channels were predominantly expressed in the detrusor smooth muscle rather than in the mucosa. The mRNA expression of the BK channel alpha-subunit was not significantly changed in obstructed bladders. However, the expression of the BK channel beta1-subunit and the SK3 channel was remarkably increased in obstructed bladders. On the other hand, the expression of the BK channel beta4-subunit was decreased as the severity of BOO-induced bladder overactivity progressed. In detrusor smooth muscle strips from obstructed bladders, blockade of BK channels by iberiotoxin (IbTx) or charybdotoxin (CTx) and blockade of SK channels by apamin increased the amplitude of spontaneous contractions. These blockers also increased the contractility and affinity of these strips for carbachol during cumulative applications. The facilitatory effects elicited by these K(+) channel blockers were larger in the strips from obstructed bladders compared with control bladders. These results suggest that long-term exposure to BOO for 6 wk enhances the function of both BK and SK types of Ca(2+)-activated K(+) channels in the detrusor smooth muscle to induce an inhibition of bladder contractility, which might be a compensatory mechanism to reduce BOO-induced bladder overactivity.

Large conductance, Ca2+-activated K+ channels (BKCa) and arteriolar myogenic signaling

Myogenic, or pressure-induced, vasoconstriction is critical for local blood flow autoregulation. Underlying this vascular smooth muscle (VSM) response are events including membrane depolarization, Ca(2+) entry and mobilization, and activation of contractile proteins. Large conductance, Ca(2+)-activated K(+) channel (BK(Ca)) has been implicated in several of these steps including, (1) channel closure causing membrane depolarization, and (2) channel opening causing hyperpolarization to oppose excessive pressure-induced vasoconstriction. As multiple mechanisms regulate BK(Ca) activity (subunit composition, membrane potential (Em) and Ca(2+) levels, post-translational modification) tissue level diversity is predicted. Importantly, heterogeneity in BK(Ca) channel activity may contribute to tissue-specific differences in regulation of myogenic vasoconstriction, allowing local hemodynamics to be matched to metabolic requirements. Knowledge of such variability will be important to exploiting the BK(Ca) channel as a therapeutic target and understanding systemic effects of its pharmacological manipulation.

Structural properties of lipid reconstructs and lipid composition of normotensive and hypertensive rat vascular smooth muscle cell membranes

Multiple cell membrane alterations have been reported to be the cause of various forms of hypertension. The present study focuses on the lipid portion of the membranes, characterizing the microviscosity of membranes reconstituted with lipids extracted from the aorta and mesenteric arteries of spontaneously hypertensive (SHR) and normotensive control rat strains (WKY and NWR). Membrane-incorporated phospholipid spin labels were used to monitor the bilayer structure at different depths. The packing of lipids extracted from both aorta and mesenteric arteries of normotensive and hypertensive rats was similar. Lipid extract analysis showed similar phospholipid composition for all membranes. However, cholesterol content was lower in SHR arteries than in normotensive animal arteries. These findings contrast with the fact that the SHR aorta is hyporeactive while the SHR mesenteric artery is hyperreactive to vasopressor agents when compared to the vessels of normotensive animal strains. Hence, factors other than microviscosity of bulk lipids contribute to the vascular smooth muscle reactivity and hypertension of SHR. The excess cholesterol in the arteries of normotensive animal strains apparently is not dissolved in bulk lipids and is not directly related to vascular reactivity since it is present in both the aorta and mesenteric arteries. The lower cholesterol concentrations in SHR arteries may in fact result from metabolic differences due to the hypertensive state or to genes that co-segregate with those that determine hypertension during the process of strain selection.

Vascular large conductance calcium-activated potassium channels: functional role and therapeutic potential

Large-conductance Ca2+-activated K+ channels (BK Ca or maxiK channels) are expressed in different cell types. They play an essential role in the regulation of various cell functions. In particular, BK Ca channels have been extensively studied in vascular smooth muscle cells, where they contribute to the control of vascular tone. They facilitate the feedback regulation against the rise of intracellular Ca2+, membrane depolarization and vasoconstriction. BK Ca channels promote a K+ outward current and lead to membrane hyperpolarization. In endothelial cells expression and function of BK Ca channels play an important role in the regulation of the vascular smooth muscle activity. Endothelial BK Ca channels modulate the biosyntheses and release of various vasoactive modulators and regulate the membrane potential. Because of their regulatory role in vascular tone, endothelial BK Ca channels have been suggested as therapeutic targets for the treatment of cardiovascular diseases. Hypertension, atherosclerosis, and diabetes are associated with altered current amplitude, open probability, and Ca2+-sensing of BK Ca channels. The properties of BK Ca channels and their role in endothelial and vascular smooth muscle cells would address them as potential therapeutic targets. Further studies are necessary to identify the detailed molecular mechanisms of action and to investigate selective BK Ca channels openers as possible therapeutic agents for clinical use.

Ebselen reduces nitration and restores voltage-gated potassium channel function in small coronary arteries of diabetic rats

Small coronary arteries (SCA) from diabetic rats exhibit enhanced peroxynitrite (ONOO(-)) formation and concurrent impairment of voltage-dependent potassium (K(v)) channel function. However, it is unclear whether ONOO(-) plays a causative role in this impairment. We hypothesized that functional loss of K(v) channels in coronary smooth muscle cells (SMC) in diabetes is due to ONOO(-) with subsequent tyrosine nitration of K(v) channel proteins. Diabetic rats and nondiabetic controls were treated with or without ebselen (Eb) for 4 wk. SCA were prepared for immunohistochemistry (IHC), immunoprecipitation (IP) followed by Western blot (WB), videomicroscopy, and patch-clamp analysis. IHC revealed excess ONOO(-) in SCA from diabetic rats. IP and WB revealed elevated nitration of the K(v)1.2 alpha-subunit and reduced K(v)1.2 protein expression in diabetic rats. Each of these changes was improved in Eb-treated rats. Protein nitration and K(v)1.5 expression were unchanged in SCA from diabetic rats. Forskolin, a direct cAMP activator that induces K(v)1 channel activity, dilated SCA from nondiabetic rats in a correolide (Cor; a selective K(v)1 channel blocker)-sensitive fashion. Cor did not alter the reduced dilation to forskolin in diabetic rats; however, Eb partially restored the Cor-sensitive component of dilation. Basal K(v) current density and response to forskolin were improved in smooth muscle cells from Eb-treated DM rats. We conclude that enhanced nitrosative stress in diabetes mellitus contributes to K(v)1 channel dysfunction in the coronary microcirculation. Eb may be beneficial for the therapeutic treatment of vascular complications in diabetes mellitus.

Protective effects of nebivolol and reversal of endothelial dysfunction in diabetes associated with hypertension

This study aims to decipher the potential effects of nebivolol in prevention and/or regression of renal artery dysfunction in diabetes associated with hypertension. Renal arteries were isolated from 80 male mice divided into four experimental groups: (i) group D: diabetics, at 2 months since streptozotocin injection; (ii) group Din: mice that at the initiation of streptozotocin diabetes were treated with 10 mg/kg b.w./day nebivolol for 2 months, to test for the potential prevention of vascular dysfunction; (iii) group Dfin: mice that after 2 months of diabetes were treated daily with 10 mg/kg b.w./day nebivolol for additional 2 months, in order to follow the possible regression of the dysfunction, and (iv) controls (C), age-matched healthy animals. The following measurements were performed: arterial blood pressure, plasma glucose concentration, and the vascular reactivity of the renal arteries in response to noradrenaline (10(-4) M), acetylcholine (10(-4) M) and sodium nitroprusside (10(-4) M). To assess the molecular mechanisms involved in the reactivity of the renal artery, the contribution of mitogen-activated protein kinase (MAP kinase) pathway and of L-type voltage gated Ca(2+) channels (in the contractile response to noradrenaline), of nitric oxide (NO) and Ca(2+) activated K(+) channels (in the endothelium-dependent vasodilator response), and of cGMP (in the endothelium-independent vasodilator response) was examined by exposing the arteries to corresponding inhibitors, and by using myograph and patch-clamp techniques, immunoblotting and NO assays. Results showed that, group D was characterized by hyperglycemia (blood glucose concentration: 136.66 +/- 4.96 mg/dl, a value approximately 65% increased compared to group C) and hypertension (systolic blood pressure: 145.66 +/- 5.96 mm Hg, a value approximately 34% increased compared to group C). Compared to group D, group Din was characterized by diminished blood glucose concentration ( approximately 1.6 fold), reduced systolic and diastolic blood pressure ( approximately 1.3 fold) and heart rate ( approximately 1.6 fold), as well as by increased contractile response of the renal artery to noradrenaline ( approximately 1.84 fold) and of the impeded vasodilator response to acetylcholine ( approximately 1.81 fold) and sodium nitroprusside ( approximately 1.42 fold). Together, these effects demonstrate that administration of 10 mg/kg b.w./day nebivolol at the moment of diabetes induction has preventive effects, ameliorating diabetes dysfunctions. Compared to group D, group Dfin was characterized by diminished glucose concentration ( approximately 1.3 fold), reduced systolic and diastolic blood pressure and heart rate (both approximately 1.2 fold), and by augmentation of contractile response of the renal artery to noradrenaline ( approximately 1.62 fold) and of vasodilator response to acetylcholine ( approximately 1.13 fold) and sodium nitroprusside ( approximately 1.19 fold). These effects assess that administration of 10 mg/kg b.w./day nebivolol after 2 months of diabetes contributes to regression of diabetes-associated dysfunctionalies. Nebivolol influenced the molecular mechanisms involved in renal artery reactivity in diabetic and hypertensive mice: it increased the NO production and endothelial NO synthase (eNOS) protein expression, decreased the expression of proportional, variant protein in L-type calcium channels and Ca(2+) activated K(+) channels, and diminished the MAP kinase activity. The reported data suggest that nebivolol may offer additional vascular protection for treating diabetes associated with hypertension.

Involvement of CPI-17 downregulation in the dysmotility of the colon from dextran sodium sulphate-induced experimental colitis in a mouse model

The mechanism of gastrointestinal dysmotility in inflammatory bowel disease has not been clarified. In this study, we examined the mechanism involved in the inflamed distal colon isolated from a mouse model of dextran sodium sulphate-induced ulcerative colitis (DSS-treated mouse). Although substance P-induced contraction was not changed, carbachol-induced contraction was reduced in the DSS-treated mouse colon. Pre-incubation with the NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA) or the cyclooxygenase inhibitor indomethacin did not reverse the carbachol-induced contraction in the DSS-treated mouse colon. In semi-quantitative reverse transcription-polymerase chain reaction experiments and Western blot analysis, muscarinic M3 receptor expressions were not changed. The Ca2+ -sensitization of contractile elements induced by carbachol with GTP or GTPgammaS was reduced in the beta-escin-permeabilized DSS-treated mouse colon. Although the expression of proteins such as rhoA, ROCK1, ROCK2 or MYPT1 in smooth muscles was not changed, the expression of CPI-17, the functional protein involved in smooth muscle Ca2+ -sensitization, was significantly decreased in the DSS-treated mouse colon. These results suggest that the suppression of carbachol-induced contraction in mice with colitis is attributable at least partially to the increased activity of myosin phosphatase following the downregulation of CPI-17.

Differential regulation of L-type Ca2+ channels in cerebral and mesenteric arteries after simulated microgravity in rats and its intervention by standing

This study was designed to clarify whether simulated microgravity can induce differential changes in the current and protein expression of the L-type Ca(2+) channel (Ca(L)) in cerebral and mesenteric arteries and whether these changes can be prevented by daily short-duration -G(x) exposure. Tail suspension [hindlimb unloading (HU)] for 3 and 28 days was used to simulate short- and medium-term microgravity-induced deconditioning effects. Standing (STD) for 1 h/day was used to provide -G(x) as a countermeasure. Whole cell patch-clamp experiments revealed an increase in current density of Ca(L) of vascular smooth muscle cells (VSMCs) isolated from cerebral arteries of rats subjected to HU and a decrease in VSMCs from mesenteric arteries. Western blot analysis revealed a significant increase and decrease of Ca(L) channel protein expression in cerebral and small mesenteric arterial VSMCs, respectively, only after 28 days of HU. STD for 1 h/day did not prevent the increase of Ca(L) current density in cerebral arterial VSMCs, but it prevented completely (within 3 days) and partially (28 days) the decrease of Ca(L) current density in small mesenteric arterial VSMCs. Consistent with the changes in Ca(L) current, STD for 1 h/day did not prevent the increase of Ca(L) expression in cerebrovascular myocytes but did prevent the reduction of Ca(L) expression in mesenteric arterial VSMCs subjected to 28 days of HU. These data indicate that simulated microgravity up- and downregulates the current and expression of Ca(L) in cerebral and hindquarter VSMCs, respectively. STD for 1 h/day differentially counteracted the changes of Ca(L) function and expression in cerebral and hindquarter arterial VSMCs of HU rats, suggesting the complexity of the underlying mechanisms in the effectiveness of intermittent artificial gravity for prevention of postflight cardiovascular deconditioning, which needs further clarification.

Carbon monoxide-mediated activation of large-conductance calcium-activated potassium channels contributes to mesenteric vasodilatation in cirrhotic rats

Large-conductance calcium-activated potassium channels (BK(Ca)s) are important regulators of arterial tone and represent a mediator of the endogenous vasodilator carbon monoxide (CO). Because an up-regulation of the heme oxygenase (HO)/CO system has been associated with mesenteric vasodilatation of cirrhosis, we analyzed the interactions of BK(Ca) and of HO/CO in the endothelium-dependent dilatation of mesenteric arteries in ascitic cirrhotic rats. In pressurized mesenteric arteries (diameter, 170-350 microm) of ascitic cirrhotic rats, we evaluated the effect of inhibition of BK(Ca), HO, and guanylyl-cyclase on dilatation induced by acetylcholine and by exogenous CO; and HO-1 and BK(Ca) subunit protein expression. Inhibition of HO and of BK(Ca) reduced acetylcholine-induced vasodilatation more in cirrhotic rats than in control rats, whereas inhibition of guanylyl-cyclase had a similar effect in the two groups. CO was more effective in cirrhotic rats than in control rats, and the effect was hindered by BK(Ca) inhibition. The expression of HO-1 and of BK(Ca) alpha-subunit was higher in mesenteric arteries of cirrhotic rats compared with that of control animals, whereas the expression of the BK(Ca) beta1-subunit was lower. In conclusion, an overexpression of BK(Ca) alpha-subunits, possibly due to HO up-regulation with increased CO production, participates in the endothelium-dependent alterations and mesenteric arterial vasodilatation of ascitic cirrhotic rats.

Calcium-channel modulators for cardiovascular disease

It is generally accepted that hypertension doubles the risk of cardiovascular disease, of which coronary heart disease is the most common and lethal. Hypertension is a predisposing factor for the development of stroke, peripheral arterial disease, heart failure and end-state renal disease. Atherosclerosis-causing coronary heart disease is related to the severity of hypertension. Inhibition of calcium entry reduces the active tone of vascular smooth muscle and produces vasodilatation. This pharmacological action has been the basis for the use of calcium-channel blockers (CCBs) for the management of hypertension. Other drug families may achieve this: diuretics, beta-blockers, angiotensin-converting enzyme inhibitors, angiotensin-receptor antagonists. Cardiovascular hypertrophy and atherosclerosis are major complications related to high blood pressure. Cardiac hypertrophy is considered as an independent risk factor associated with abnormalities of diastolic function and can result in heart failure. Atherosclerosis is associated with activation of innate immunity. Atherosclerosis is expressing itself not only as coronary heart disease, but as a cerebrovascular and peripheral arterial disease. By impairing physiological vasomotor function, atherosclerosis includes ultimately necrosis of myocardium. CCBs reduce blood pressure. Do they prevent the progress of the main complications of hypertension? This major question is the matter of the present paper.

Molecular and electrophysiological characteristics of K+ conductance sensitive to acidic pH in aortic smooth muscle cells of WKY and SHR

Changes in K(+) conductances and their contribution to membrane depolarization in the setting of an acidic pH environment have been studied in myocytes from aortic smooth muscle cells of spontaneously hypertensive rats (SHR) compared with those from Wistar-Kyoto (WKY) rats. The resting membrane potential (RMP) of aortic smooth muscle at extracellular pH (pH(o)) of 7.4 was significantly more depolarized in SHR than in WKY rats. Acidification to pH(o) 6.5 made this difference in RMP between SHR and WKY rats more significant by further depolarizing the SHR myocytes. Large-conductance Ca(2+)-activated K(+) (BK) currents, which were markedly suppressed by acidification, were larger in aortic myocytes of SHR than in those of WKY rats. In contrast, acid-sensitive, non-BK currents were smaller in SHR. Western blot analyses showed that expression of BK-alpha- and -beta(1) subunits in SHR aortas was upregulated and comparable with those in WKY rats, respectively. Additional electrophysiological and molecular studies showed that pH- and halothane-sensitive two-pore domain weakly inward rectifying K(+) channel (TWIK)-like acid-sensitive K(+) (TASK) channel subtypes were functionally expressed in aortas, and TASK1 expression was significantly higher in WKY than in SHR. Although the background current through TASK channels at normal pH(o) (7.4) was small and may not contribute significantly to the regulation of RMP, TASK channel activation by halothane or alkalization (pH(o) 8.0) induced significant hyperpolarization in WKY but not in SHR. In conclusion, the larger depolarization and subsequent abnormal contractions after acidification in aortic myocytes in the setting of SHR hypertension are mainly attributable to the larger contribution of BK current to the total membrane conductance than in WKY aortas.

Antioxidant effects and the therapeutic mode of action of calcium channel blockers in hypertension and atherosclerosis

Drugs currently known as calcium channel blockers (CCB) were initially called calcium antagonists because of their ability to inhibit calcium-evoked contractions in depolarized smooth muscles. Blocking the entry of calcium reduces the active tone of vascular smooth muscle and produces vasodilatation. This pharmacological property has been the basis for the use of CCBs in the management of hypertension and coronary heart disease. A major question is whether drugs reducing blood pressure have other effects that help prevent the main complications of hypertension, such as atherosclerosis, stroke, peripheral arterial disease, heart failure and end-state renal disease. Experimental studies that focus on this question are reviewed in the present paper.

Exercise and Vasorelaxing Effects of CO-Releasing Molecules in Hypertensive Rats

PURPOSE

Because carbon monoxide (CO) has been reported able to induce relaxation, we aimed to investigate the effects of exercise training on the rat thoracic aorta responsiveness to a CO-releasing molecule (CORM), tricarbonyldichlororuthenium ([Ru(CO)3Cl2]2).

METHODS

Male Wistar rats (N = 32) were divided in hypertensive and normotensive groups using the two-kidney, one-clip model of Goldblatt or SHAM surgery. Hypertensive and normotensive groups were assigned to an exercise training protocol on a level treadmill over a 10-wk period or were assigned to remain sedentary. After the exercise training protocol, blood pressure and cardiac tissue weight were assessed. The responsiveness of endothelium-denuded thoracic aortic rings to [Ru(CO)3Cl2]2 was evaluated by isometric contractions recordings.

RESULTS

Systolic, diastolic, and mean blood pressures were significantly increased in hypertensive rats compared with control rats. Exercise training did not significantly alter blood pressure but decreased pulse pressure in hypertensive animals compared with sedentary hypertensive rats. In all groups, application of [Ru(CO)3Cl2]2 induced relaxation in precontracted aortic rings. Compared with normotensive rats, CO-induced relaxation was significantly decreased in hypertensive rats. Nevertheless, training exercise increased relaxation markedly in response to [Ru(CO)3Cl2]2 application in hypertensive rats, whereas it remained without effect in control rings. Pretreatment with TEA, a nonselective K+ channel inhibitor, decreased [Ru(CO)3Cl2]2-induced relaxation in all groups that became similar. In trained hypertensive rats, iberiotoxin had effects similar to those of TEA.

CONCLUSIONS

This finding supports the concept that the CORM [Ru(CO)3Cl2]2 can induce relaxation in both normotensive and hypertensive rats with an impairment of the CO-induced relaxation during hypertension. However, exercise training improves the aorta's ability to relax in response to [Ru(CO)3Cl2]2 during hypertension, probably by increasing K+ channel activity.

Iptakalim, opener of K(ATP), reverses the enhanced expression of genes encoding K(ATP) subunits in spontaneously hypertensive rats

ATP-sensitive potassium channels (K(ATP)) are thought to be targets for antihypertensive drugs that are potassium channel openers. In this study, the expression of genes encoding the K(ATP) subunits, SUR2, Kir6.1 and Kir6.2, was detected in tissues from Wistar-Kyoto rats (WKY), spontaneously hypertensive rats (SHR), and SHR undergoing long-term treatment with iptakalim, a novel antihypertensive drug that acts via K(ATP). The transcript levels for SUR2, Kir6.1 and Kir6.2 in the heart, aortic smooth muscle, and tail artery smooth muscle were determined by reverse transcription-polymerase chain reaction (RT-PCR). In general, Kir6.2 and SUR2 were more highly represented in all SHR tissues compared with those of WKY, and transcripts of Kir6.2 were significantly higher in tail artery smooth muscle from SHR. Following long-term treatment with iptakalim, mRNA levels of Kir6.2 and SUR2 were reduced significantly in all tissues compared with those of untreated SHR. Kir6.1 expression was not significantly different between SHR and WKY, and was unaffected by iptakalim treatment. These results indicate that the expression of the K(ATP) subunits genes, SUR2 and Kir6.2, are closely associated with hypertensive pathological states, and the effect of iptakalim on K(ATP) mRNA levels may explain, in part, the effects of iptakalim in reversing vascular and cardiac remodeling. Furthermore, changes in Kir6.2 mRNA levels suggest that Kir6.x, as well as SUR, is responsible for drug binding.

Insulin resistance does not impair contractile responses of cerebral arteries

Insulin resistance (IR) impairs endothelium-mediated vasodilation in cerebral arteries as well as K+ channel function in vascular smooth muscle. Peripheral arteries also show an impaired endothelium-dependent vasodilation in IR and concomitantly show an enhanced contractile response to endothelin-1 (ET-1). However, the contractile responses of the cerebral arteries in IR have not been examined systematically. This study examined the contractile responses of pressurized isolated middle cerebral arteries (MCAs) in fructose-fed IR and control rats. IR MCAs showed no difference in pressure-mediated (80 mmHg) vasoconstriction compared to controls, either in time to develop spontaneous tone (control: 61+/-3 min, n=30; IR: 63+/-2 min, n=26) or in the degree of that tone (control: 60 min: 33+/-2%, n=22 vs. IR 60 min: 34+/-3%, n=17). MCAs treated with ET-1 (10(-8.5) M) constrict similarly in control (53+/-3%, n=14) and IR (53+/-3%, n=14) arteries. Constrictor responses to U46619 (10(-6) M) are also similar in control (48+/-9%, n=8) and IR (42+/-5%, n=6) MCAs as are responses to extraluminal uridine 5'-triphosphate (UTP; 10(-4.5) M) (control: 35+/-7%, n=11 vs. IR: 38+/-3%, n=10). These findings demonstrate that constrictor responses remain intact in IR despite a selective impairment of dilator responses and endothelial and vascular smooth muscle K+ channel function in cerebral arteries. Thus, it appears that the increased susceptibility to cerebrovascular abnormalities associated with IR and diabetes (including cerebral ischemia, stroke, vertebrobasilar transient ischemic attacks) is not due to an enhanced vasoreactivity to constrictor agents.

Ca2+-dependent K+ channels are targets for bradykinin B1 receptor ligands and for lipopolysaccharide in the rat aorta

Although rat aorta smooth muscle cells in culture constitutively express bradykinin B1 receptors, the normotensive rat aorta does not respond to the bradykinin B1 receptor agonist des-Arg9-bradykinin, whereas vessels from the spontaneously hypertensive rat (SHR) respond to bradykinin B1 receptor agonists with cell membrane hyperpolarization and relaxation. Bacterial lipopolysaccharide also is inactive on the normotensive rat but hyperpolarizes the SHR aorta. To determine whether this could be due to the increased intracellular Ca2+ concentration ([Ca2+]i) in the SHR, we raised [Ca2+]i in normotensive rats by treatment with thapsigargin. In the thapsigargin-treated aorta, both lipopolysaccharide and des-Arg9-bradykinin induced hyperpolarization, which was reversed by the Ca2+-dependent K+ channel inhibitor iberiotoxin and by the bradykinin B1 receptor antagonists Lys-[Leu8]-des-Arg9-bradykinin and [Leu8]-des-Arg9-bradykinin. Thus the bradykinin B1 receptor, as well as lipopolysaccharide, needs activated Ca2+-dependent K+ channels for functional expression. The two bradykinin B1 receptor inhibitors, however, have effects on Ca2+-dependent K+ channels which are not mediated by bradykinin B1 receptors.