Glibenclamide Inhibits Agonist-induced Vasoconstriction of Placental Chorionic Plate Arteries

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Perivascular tissue inhibits rho-kinase-dependent smooth muscle Ca(2+) sensitivity and endothelium-dependent H2 S signalling in rat coronary arteries

Interactions between perivascular tissue (PVT) and the vascular wall modify artery tone and contribute to local blood flow regulation. Using isometric myography, fluorescence microscopy, membrane potential recordings and phosphospecific immunoblotting, we investigated the cellular mechanisms by which PVT affects constriction and relaxation of rat coronary septal arteries. PVT inhibited vasoconstriction to thromboxane, serotonin and α1 -adrenergic stimulation but not to depolarization with elevated extracellular [K(+) ]. When PVT was wrapped around isolated arteries or placed at the bottom of the myograph chamber, a smaller yet significant inhibition of vasoconstriction was observed. Resting membrane potential, depolarization to serotonin or thromboxane stimulation, and resting and serotonin-stimulated vascular smooth muscle [Ca(2+) ]-levels were unaffected by PVT. Serotonin-induced vasoconstriction was almost abolished by rho-kinase inhibitor Y-27632 and modestly reduced by protein kinase C inhibitor bisindolylmaleimide X. PVT reduced phosphorylation of myosin phosphatase targeting subunit (MYPT) at Thr850 by ∼40% in serotonin-stimulated arteries but had no effect on MYPT-phosphorylation in arteries depolarized with elevated extracellular [K(+) ]. The net anti-contractile effect of PVT was accentuated after endothelial denudation. PVT also impaired vasorelaxation and endothelial Ca(2+) responses to cholinergic stimulation. Methacholine-induced vasorelaxation was mediated by NO and H2 S, and particularly the H2 S-dependent (dl-propargylglycine- and XE991-sensitive) component was attenuated by PVT. Vasorelaxation to NO- and H2 S-donors was maintained in arteries with PVT. In conclusion, cardiomyocyte-rich PVT surrounding coronary arteries releases diffusible factors that reduce rho-kinase-dependent smooth muscle Ca(2+) sensitivity and endothelial Ca(2+) responses. These mechanisms inhibit agonist-induced vasoconstriction and endothelium-dependent vasorelaxation and suggest new signalling pathways for metabolic regulation of blood flow.

Using oral agents to manage gestational diabetes: what have we learned?

Insulin has been the mainstay of treatment of diabetes during pregnancy for decades. Although glyburide and metformin are classified as category B during pregnancy, recent research has suggested that these oral agents alone or in conjunction with insulin may be safe for the treatment of gestational diabetes (GDM). This paper summarizes the data on the use of glyburide and metformin for treatment of GDM.

Oxygen sensitivity, potassium channels, and regulation of placental vascular tone

The human fetoplacental vasculature is a low-resistance circulation with deoxygenated arterial relative to venous blood. The placenta lacks neuronal innervation suggesting that local physical (e.g., oxygenation; flow rate), paracrine (e.g., endothelial cell nitric oxide), and circulating (e.g., angiotensin II) factors will contribute to blood flow regulation in small fetoplacental vessels. Oxygenation (specifically hypoxia) has received particular attention. At the macro-level, hypoxic challenge increases vascular resistance, but the data's physiological relevance remains questionable. K(+) channels are a diverse family of proteins known to play important roles in the normal physiological functions of endothelial and smooth muscle cells of a variety of vascular beds. K(+) channels are categorized by their predicted transmembrane structure or gating properties. A small number of perfused placental cotyledon and isolated blood vessels studies have assessed K(+) channel activity. Specific activator/inhibitor application suggests functional voltage-gated channels, whereas toxin inhibitor studies have documented KCa channel activity. Pharmacological KATP channel activation significantly dilates preconstricted placental arteries and veins. There is a paucity of cell subtype-specific expression studies of placental K(+) channels. This review focuses on the roles of K(+) channels and oxygenation in controlling reactivity of small fetoplacental blood vessels.

Characterisation of K+ channels in human fetoplacental vascular smooth muscle cells

Adequate blood flow through placental chorionic plate resistance arteries (CPAs) is necessary for oxygen and nutrient transfer to the fetus and a successful pregnancy. In non-placental vascular smooth muscle cells (SMCs), K(+) channels regulate contraction, vascular tone and blood flow. Previous studies showed that K(+) channel modulators alter CPA tone, but did not distinguish between effects on K(+) channels in endothelial cells and SMCs. In this study, we developed a preparation of freshly isolated CPASMCs of normal pregnancy and investigated K(+) channel expression and function. CPASMCs were isolated from normal human term placentas using enzymatic digestion. Purity and phenotype was confirmed with immunocytochemistry. Whole-cell patch clamp was used to assess K(+) channel currents, and mRNA and protein expression was determined in intact CPAs and isolated SMCs with RT-PCR and immunostaining. Isolated SMCs expressed α-actin but not CD31, a marker of endothelial cells. CPASMCs and intact CPAs expressed h-caldesmon and non-muscle myosin heavy chain-2; phenotypic markers of contractile and synthetic SMCs respectively. Whole-cell currents were inhibited by 4-AP, TEA, charybdotoxin and iberiotoxin implicating functional K(v) and BK(Ca) channels. 1-EBIO enhanced whole cell currents which were abolished by TRAM-34 and reduced by apamin indicating activation of IK(Ca) and SK(Ca) respectively. BK(Ca), IK(Ca) and SK(Ca)3 mRNA and/or protein were expressed in CPASMCs and intact CPAs. This study provides the first direct evidence for functional K(v), BK(Ca,) IK(Ca) and SK(Ca) channels in CPASMCs. These cells display a mixed phenotype implicating a dual role for CPASMCs in controlling both fetoplacental vascular resistance and vasculogenesis.

Expression of an electrically silent voltage-gated potassium channel in the human placenta

Human placental expression of K(V)9.3, a voltage-gated K channel linked to tissue oxygenation responses, has been suggested at the messenger RNA level but tissue localisation has not been described. We aimed to: (1) produce an antibody to human K(V)9.3 and (2) assess channel expression and distribution in human placental tissue. We determined human placental protein expression and localisation using an antibody to K(V)9.3. Antibody specificity was confirmed by Western blotting. Staining was observed in syncytiotrophoblast microvillous membrane, endothelial cells (in intermediate, stem villi and chorionic plate blood vessels) and vascular smooth muscle cells (large diameter vessels only) by immunohistochemistry. Expression was unchanged in tissue from women with small-for-gestational age babies. It was concluded that K(V)9.3 is localised to human placental vascular tissues and syncytiotrophoblast.

In vitro assessment of mouse uterine and fetoplacental vascular function

Adequate blood flow provision through alterations in maternal vascular function is essential during pregnancy for optimal fetal development. Abnormal uterine vasculature adaptation, resulting in aberrant blood flow to the placenta, has been implicated as a possible cause of fetal growth restriction (FGR). Our study aimed to develop strategies to evaluate murine vascular function in pregnancy using wire myography. Main uterine artery loop and branch vessels isolated from near-term pregnant mice showed significant contraction to phenylephrine (PE). Endothelial-dependent relaxation was noted with acetylcholine (ACH). U46619 elicited significant contraction of umbilical arteries and veins, but relaxation was only demonstrable with the nitric oxide (NO) donor sodium nitroprusside (SNP). In conclusion, our data suggest that murine uteroplacental and fetoplacental arteries show distinct responses to vasoactive agents. Furthermore, this study indicates that wire myography represents a robust technique for the assessment of murine uteroplacental and fetoplacental vascular function, which will aid evaluation of mouse genetic models of FGR.

Oral anti-hyperglycemic agents for the management of gestational diabetes mellitus

The purpose of this review is to provide a brief overview for understanding the management guidelines of gestational diabetes. The rationale for the use of oral antidiabetic drugs is provided based on validation by appropriately conducted research trials. Concerns over teratogenicity due to possible placental transfer, neonatal and maternal outcome, and basic pharmacologic benefits are addressed.

Arginine vasopressin inhibits Kir6.1/SUR2B channel and constricts the mesenteric artery via V1a receptor and protein kinase C

Kir6.1/SUR2B channel is the major isoform of K(ATP) channels in the vascular smooth muscle. Genetic disruption of either subunit leads to dysregulation of vascular tone and regional blood flows. To test the hypothesis that the Kir6.1/SUR2B channel is a target molecule of arginine vasopressin (AVP), we performed studies on the cloned Kir6.1/SUR2B channel and cell-endogenous K(ATP) channel in rat mesenteric arteries. The Kir6.1/SUR2B channel was expressed together with V1a receptor in the HEK-293 cell line. Whole cell currents of the transfected HEK cells were activated by K(ATP) channel opener pinacidil and inhibited by K(ATP) channel inhibitor glibenclamide. AVP produced a concentration-dependent inhibition of the pinacidil-activated currents with IC(50) 2.0 nM. The current inhibition was mediated by a suppression of the open-state probability without effect on single-channel conductance. An exposure to 100 nM PMA, a potent PKC activator, inhibited the pinacidil-activated currents, and abolished the channel inhibition by AVP. Such an effect was not seen with inactive phorbol ester. A pretreatment of the cells with selective PKC blocker significantly diminished the inhibitory effect of AVP. In acutely dissociated vascular smooth myocytes, AVP strongly inhibited the cell-endogenous K(ATP) channel. In isolated mesenteric artery rings, AVP produced concentration-dependent vasoconstrictions with EC(50) 6.5 nM. At the maximum effect, pinacidil completely relaxed vasoconstriction in the continuing exposure to AVP. The magnitude of the AVP-induced vasoconstriction was significantly reduced by calphostin-C. These results therefore indicate that the Kir6.1/SUR2B channel is a target molecule of AVP, and the channel inhibition involves G(q)-coupled V1a receptor and PKC.

Relaxation of human placental arteries and veins by ATP-sensitive potassium channel openers

BACKGROUND

Adenosine triphosphate (ATP)-sensitive potassium channels (K(ATP)) are important modulators of vascular tone. Preliminary data from our laboratory suggests that K(ATP) channels are expressed in the fetoplacental vasculature where addition of pinacidil, a specific K(ATP) opener, promotes relaxation. We aimed to assess the effects of KRN2391 and KRN4884 on the fetoplacental vasculature, which are putative K(ATP) channel openers.

MATERIALS AND METHODS

Functional activity of K(ATP) channels was assessed in chorionic plate arteries and veins using wire myography. Cromakalim-, KRN2391- and KRN4884-induced relaxations were assessed in the presence and absence of agonist-induced pretone. Cromakalim, an established K(ATP) channel opener, acted as control.

RESULTS

KRN2391 evoked significantly greater relaxation of chorionic plate arteries and veins than either KRN4884 or cromakalim. KRN2391-induced relaxation of precontracted arteries and veins was reduced in the presence of inhibitors of the nitric oxide pathway (L-NNA or LY83583). With KRN4884, there was no contribution of nitric oxide to the induced relaxation.

CONCLUSIONS

We conclude that K(ATP) channels play an important role in controlling placental vascular tone. KRN2391 induces relaxation of human placental blood vessels by activation of K(ATP) channels and via activation of nitric oxide-dependent pathways.