ATP-sensitive potassium channels in capillaries isolated from guinea-pig heart

Potassium Channels and Membrane Potential in the Modulation of Intracellular Calcium in Vascular Endothelial Cells

The endothelium plays a vital role in the control of vascular functions, including modulation of tone; permeability and barrier properties; platelet adhesion and aggregation; and secretion of paracrine factors. Critical signaling events in many of these functions involve an increase in intracellular free Ca(2+) concentration ([Ca(2+)](i)). This rise in [Ca(2+)](i) occurs via an interplay between several mechanisms, including release from intracellular stores, entry from the extracellular space through store depletion and second messenger-mediated processes, and the establishment of a favorable electrochemical gradient. The focus of this review centers on the role of potassium channels and membrane potential in the creation of a favorable electrochemical gradient for Ca(2+) entry. In addition, evidence is examined for the existence of various classes of potassium channels and the possible influence of regional variation in expression and experimental conditions.

[125I]A-312110, a novel high-affinity 1,4-dihydropyridine ATP-sensitive K+ channel opener: characterization and pharmacology of binding

Although ATP-sensitive K+ channels continue to be explored for their therapeutic potential, developments in high-affinity radioligands to investigate native and recombinant KATP channels have been less forthcoming. This study reports the identification and pharmacological characterization of a novel iodinated 1,4-dihydropyridine KATP channel opener, [125I]A-312110 [(9R)-9-(4-fluoro-3-125iodophenyl)-2,3,5,9-tetrahydro-4H-pyrano[3,4-b]thieno[2,3-e]pyridin-8(7H)-one-1,1-dioxide]. Binding of [125I]A-312110 to guinea pig cardiac (KD = 5.8 nM) and urinary bladder (KD = 4.9 nM) membranes were of high affinity, saturable, and to a single set of binding sites. Displacement of [125I]A-312110 by structurally diverse potassium channel openers (KCOs) indicated a similar rank order of potency in both guinea pig cardiac and bladder membranes (Ki, heart): A-312110 (4.3 nM) > N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine (P1075) > (-)-N-(2-ethoxyphenyl)-N'-(1,2,3-trimethylpropyl)-2-nitroethene-1,1-diamine (Bay X 9228) > pinacidil > (-)-cromakalim > N-(4-benzoyl phenyl)-3,3,3-trifluro-2-hydroxy-2-methylpropionamine (ZD6169) > 9-(3-cyanophenyl)-3,4,6,7,9,10-hexahydro-1,8-(2H,5H)-acridinedione (ZM244085) >> diazoxide (16.7 microM). Displacement by KATP channel blockers, the sulfonylurea glyburide, and the cyanoguanidine N-[1-(3-chlorophenyl)cyclobutyl]-N'-cyano-N"-3-pyridinyl-guanidine (PNU-99963) were biphasic in the heart but monophasic in bladder with about a 100- to 500-fold difference in Ki values between high- and low-affinity sites. Good correlations were observed between cardiac or bladder-binding affinities of KCOs with functional activation as assessed by their respective potencies to either suppress action potential duration (APD) in Purkinje fibers or to relax electrical field-stimulated bladder contractions. Collectively, these results demonstrate that [125I]A-312110 binds with high affinity and has an improved activity profile compared with other radiolabeled KCOs. [125I]A-312110 is a useful tool for investigation of the molecular and functional properties of the KATP channel complex and for the identification, in a high throughput manner, of both novel channel blockers and openers that interact with cardiac/smooth muscle-type KATP channels.

Diabetes mellitus impairs vasodilation to hypoxia in human coronary arterioles: reduced activity of ATP-sensitive potassium channels

ATP-sensitive K+ channels (K(ATP)) contribute to vasomotor regulation in some species. It is not fully understood the extent to which K(ATP) participate in regulating vasomotor tone under physiological and pathophysiological conditions in the human heart. Arterioles dissected from right atrial appendage were studied with video microscopy, membrane potential recordings, reverse transcription-polymerase chain reaction, and immunohistochemistry. Hypoxia produced endothelium-independent vasodilation and membrane hyperpolarization of vascular smooth muscle cells, both of which were attenuated by glibenclamide. Aprikalim, a selective K(ATP) opener, also induced a potent endothelium-independent and glibenclamide-sensitive vasodilation with membrane hyperpolarization. Reverse transcription-polymerase chain reaction detected mRNA expression for K(ATP) subunits, and immunohistochemistry confirmed the localization of the inwardly rectifying Kir6.1 protein in the vasculature. In patients with type 1 or type 2 diabetes mellitus (DM), vasodilation was reduced to both aprikalim (maximum dilation, DM(+) 90+/-2% versus DM(-) 96+/-1%, P<0.05) and hypoxia (maximum dilation, DM(+) 56+/-8% versus DM(-) 85+/-5%, P<0.01) but was not altered to sodium nitroprusside or bradykinin. Baseline myogenic tone and resting membrane potential were not affected by DM. We conclude that DM impairs human coronary arteriolar dilation to K(ATP) opening, leading to reduced dilation to hypoxia. This reduction in K(ATP) function could contribute to the greater cardiovascular mortality and morbidity in DM.

ATP-sensitive K+ channels of vascular smooth muscle cells

ATP-sensitive potassium channels (K(ATP)) of vascular smooth muscle cells represent potential therapeutic targets for control of abnormal vascular contractility. The biophysical properties, regulation and pharmacology of these channels have received intense scrutiny during the past twenty years, however, the molecular basis of vascular K(ATP) channels remains ill-defined. This review summarizes the recent advancements made in our understanding of the molecular composition of vascular K(ATP) channels with a focus on the evidence that hetero-octameric complexes of Kir6.1 and SUR2B subunits constitute the vascular K(ATP) subtype responsible for control of arterial diameter by vasoactive agonists.

Molecular cloning of a pore-forming subunit (Kir6.2 gene) of the ATP-sensitive potassium channel in the bullfrog, Rana catesbeiana Shaw

A cDNA sequence encoding a pore-forming subunit of ATP-sensitive potassium channel (Kir6.2 gene) of the bullfrog, Rana catesbeiana Shaw, termed RcKir6.2, was isolated from a liver cDNA library. The cDNA contained a single open reading frame of 1,173 bp encoding 391 amino acids with a calculated molecular mass of 42.9 kDa, which has a structural motif (a GFG motif) of the putative pore-forming loop of Kir6.2. Analysis of its phlyogenetic position revealed that the RcKir6.2 is close to Kir6.2 of rabbits. The predicted amino acid sequence shared sequence identity with Kir6.2 of Homo sapiens, Cavia porcellus, Mus musculus, Rattus norvegicus, and Oryctolagus cuniculus by 95.9, 95.6, 96.7, 96.7 and 99.7%, respectively. Expression of RcKir6.2 was detected in various tissues, including heart, kidney, liver, lung, spleen, and stomach of the bullfrog.

Four novel splice variants of sulfonylurea receptor 1

ATP-sensitive K(+) (K(ATP)) channels are composed of pore-forming Kir6.x subunits and regulatory sulfonylurea receptor (SUR) subunits. SURs are ATP-binding cassette proteins with two nucleotide-binding folds (NBFs) and binding sites for sulfonylureas, like glibenclamide, and for channel openers. Here we report the identification and functional characterization of four novel splice forms of guinea pig SUR1. Three splice forms originate from alternative splicing of the region coding for NBF1 and lack exons 17 (SUR1Delta17), 19 (SUR1Delta19), or both (SUR1Delta17Delta19). The fourth (SUR1C) is a COOH-terminal SUR1-fragment formed by exons 31-39 containing the last two transmembrane segments and the COOH terminus of SUR1. RT-PCR analysis showed that these splice forms are expressed in several tissues with strong expression of SUR1C in cardiomyocytes. Confocal microscopy using enhanced green fluorescent protein-tagged SUR or Kir6.x did not provide any evidence for involvement of these splice forms in the mitochondrial K(ATP) channel. Only SUR1 and SUR1Delta17 showed high-affinity binding of glibenclamide (K(d) approximately 2 nM in the presence of 1 mM ATP) and formed functional K(ATP) channels upon coexpression with Kir6.2.

ATP-sensitive K(+) channels composed of Kir6.1 and SUR2B subunits in guinea pig gastric myocytes

This study was designed to identify the single-channel properties and molecular entity of ATP-sensitive K(+) (K(ATP)) channels in guinea pig gastric myocytes with patch-clamp recording and RT-PCR. Pinacidil and diazoxide activated K(ATP) currents in a glibenclamide-sensitive manner. The open probability of channels was enhanced by the application of 10 microM pinacidil from 0.085 +/- 0.04 to 0.20 +/- 0.05 (n = 7) and was completely blocked by 10 microM glibenclamide. Single-channel conductance was 37.3 +/- 2.5 pS (n = 4) between -80 and -20 mV in symmetrical K(+) gradient conditions. In inside-out mode, K(ATP) channels showed no spontaneous openings and were activated by the application of nucleotide diphosphates to the cytoplasmic side. These single-channel properties are similar to those of the nucleotide diphosphate-dependent K(+) channels in vascular smooth muscle, which are composed of Kir6.1 and sulfonylurea receptor (SUR)2B. RT-PCR demonstrated the presence of Kir6.1, Kir6.2, and SUR2B in guinea pig stomach smooth muscle cells. These results suggest that K(ATP) channels in smooth muscle cells of the guinea pig stomach are composed of Kir6.1 and SUR2B.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Basic electrical properties of in situ endothelial cells of small pulmonary arteries during postnatal development

Small pulmonary arteries are the major determinants of pulmonary artery pressure and vascular resistance. Their endothelium modulates pulmonary resistance, remodeling, and blood fluidity. We developed a method that provides access to the luminal surface of small pulmonary arteries of rat and allows the patch-clamp study of electrical properties of in situ endothelium. At birth, the membrane was predominantly permeable for K(+), showing a resting potential of -70 mV. This conductance was not voltage-dependent and was insensitive to standard blockers of K(+) channels such as tetraethylammonium, charybdotoxin, and 4-aminopyridine. The first 22 d of development were accompanied by an additional expression of a Cl(-) conductance, increasing membrane potential to -45 mV. Acidosis reduced K(+) conductance and depolarized the membrane, whereas alkalosis resulted in hyperpolarization. Two-electrode recordings revealed tight electrical coupling (83%) between neighboring cells in the circumferential direction of the artery. The electrotonic length constant for endothelium was 13.3 microm, indicating that most cells in one cross section of a small artery are well coupled. Thus, the resting membrane conductances in small pulmonary artery endothelial cells change with postnatal development and are modulated by pH.

Is the molecular composition of K(ATP) channels more complex than originally thought?

ATP-sensitive K+ (K(ATP)) channels are abundantly expressed in the heart and may be involved in the pathogenesis of myocardial ischemia. These channels are heteromultimeric, consisting of four pore-forming subunits (Kir6.1, Kir6.2) and four sulfonylurea receptor (SUR) subunits in an octameric assembly. Conventionally, the molecular composition of K(ATP) channels in cardiomyocytes and pancreatic beta -cells is thought to include the Kir6.2 subunit and either the SUR2A or SUR1 subunits, respectively. However, Kir6.1 mRNA is abundantly expressed in the heart, suggesting that Kir6.1 and Kir6.2 subunits may co-assemble to form functional heteromeric channel complexes. Here we provide two independent lines of evidence that heteromultimerization between Kir6.1 and Kir6.2 subunits is possible in the presence of SUR2A. We generated dominant negative Kir6 subunits by mutating the GFG residues in the channel pore to a series of alanine residues. The Kir6.1-AAA pore mutant subunit suppressed both wt-Kir6.1/SUR2A and wt-Kir6.2/SUR2A currents in transfected HEK293 cells. Similarly, the dominant negative action of Kir6.2-AAA does not discriminate between either of the wild-type subunits, suggesting an interaction between Kir6.1 and Kir6.2 subunits within the same channel complex. Biochemical data support this concept: immunoprecipitation with Kir6.1 antibodies also co-precipitates Kir6.2 subunits and conversely, immunoprecipitation with Kir6.2 antibodies co-precipitates Kir6.1 subunits. Collectively, our data provide direct electrophysiological and biochemical evidence for heteromultimeric assembly between Kir6.1 and Kir6.2. This paradigm has profound implications for understanding the properties of native K(ATP)channels in the heart and other tissues.

Fluorescence-based functional assay for sarcolemmal ATP-sensitive potassium channel activation in cultured neonatal rat ventricular myocytes

INTRODUCTION

Activation of ATP-sensitive K+ channels (K(ATP)) has been shown to induce ischemic preconditioning that serves as a protective mechanism in the heart. A high throughput assay for identifying K(ATP) channel openers would therefore be desirable.

METHODS

We describe a cell-based 96-well format fluorescence assay using bis-(1,3-dibutylbarbituric acid)trimethine oxonol (DiBAC4(3)) to evaluate membrane potential changes evoked by K(ATP) channel openers and blockers in cultured neonatal rat ventricular myocytes.

RESULTS

Pinacidil and its analog P1075 (N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine), ZD6169 (N-(4-benzoylphenyl)-3,3,3,-trifluoro-2-hydroxy-2-methyl propionamide), and the enantiomers of cromakalim evoked concentration-dependent decreases in DiBAC4(3) fluorescence responses. Pretreatment with the K(ATP) channel blocker, glyburide attenuated opener-evoked decreases in fluorescence responses in a concentration-dependent manner. The rank order potency of openers in cardiac myocytes correlated well, but showed 6-10-fold higher potency in activating vascular smooth muscle K(ATP) channels in A10 cells.

DISCUSSION

Our studies demonstrate that the pharmacological modulation of sarcolemmal K(ATP) channels can be readily assessed in a high throughput manner by measuring glyburide-sensitive fluorescence changes in cardiac ventricular myocytes.

Comparison of cloned Kir2 channels with native inward rectifier K+ channels from guinea-pig cardiomyocytes

The aim of the study was to compare the properties of cloned Kir2 channels with the properties of native rectifier channels in guinea-pig (gp) cardiac muscle. The cDNAs of gpKir2.1, gpKir2.2, gpKir2.3 and gpKir2.4 were obtained by screening a cDNA library from guinea-pig cardiac ventricle. A partial genomic structure of all gpKir2 genes was deduced by comparison of the cDNAs with the nucleotide sequences derived from a guinea-pig genomic library. The cell-specific expression of Kir2 channel subunits was studied in isolated cardiomyocytes using a multi-cell RT-PCR approach. It was found that gpKir2.1, gpKir2.2 and gpKir2.3, but not gpKir2.4, are expressed in cardiomyocytes. Immunocytochemical analysis with polyclonal antibodies showed that expression of Kir2.4 is restricted to neuronal cells in the heart. After transfection in human embryonic kidney cells (HEK293) the mean single-channel conductance with symmetrical K+ was found to be 30.6 pS for gpKir2.1, 40.0 pS for gpKir2.2 and 14.2 pS for Kir2.3. Cell-attached measurements in isolated guinea-pig cardiomyocytes (n = 351) revealed three populations of inwardly rectifying K+ channels with mean conductances of 34.0, 23.8 and 10.7 pS. Expression of the gpKir2 subunits in Xenopus oocytes showed inwardly rectifying currents. The Ba2+ concentrations required for half-maximum block at -100 mV were 3.24 M for gpKir2.1, 0.51 M for gpKir2.2, 10.26 M for gpKir2.3 and 235 M for gpKir2.4. Ba2+ block of inward rectifier channels of cardiomyocytes was studied in cell-attached recordings. The concentration and voltage dependence of Ba2+ block of the large-conductance inward rectifier channels was virtually identical to that of gpKir2.2 expressed in Xenopus oocytes. Our results suggest that the large-conductance inward rectifier channels found in guinea-pig cardiomyocytes (34.0 pS) correspond to gpKir2.2. The intermediate-conductance (23.8 pS) and low-conductance (10.7 pS) channels described here may correspond to gpKir2.1 and gpKir2.3, respectively.

Investigation of mechanisms that mediate reactive hyperaemia in guinea-pig hearts: role of K(ATP) channels, adenosine, nitric oxide and prostaglandins

1. Reactive hyperaemia is a transient vasodilatation following a brief ischaemic period. ATP-dependent K(+) (K(ATP)) channels may be important in mediating this response, however it is unclear whether mitochondrial K(ATP) channels contribute to this in the heart. 2. We examined the involvement of K(ATP) channels and the relative role of mitochondrial channels as mediators of coronary reactive hyperaemia and compared them to mechanisms involving NO, prostaglandins and adenosine in the guinea-pig isolated heart. 3. Reactive hyperaemic vasodilatation (peak vasodilator response and flow debt repayment) were assessed after global zero-flow ischaemia (5 -- 120 s) in the presence of nitro-L-arginine methyl ester (L-NAME, 10(-5) M, n=9), 8-phenyltheophylline (8-PT, 10(-6) M, n=12) and indomethacin (10(-5) M, n=12). 4. Glibenclamide (10(-6) M, n=12) a non-selective K(ATP) channel inhibitor and 5-hydroxy-decanoic acid (5-HD, 10(-4) M, n=10) a selective mitochondrial K(ATP) channel inhibitor were also used. The specificity of the effects of glibenclamide and 5-HD (n=6 each) were confirmed using pinacidil (38 nmol -- 10 micromol) and diazoxide (42 nmol -- 2 micromol). Glibenclamide was most effective in blocking the hyperaemic response (by 87%, P<0.001) although 5-HD and 8-PT also had a marked effect (40% inhibition, P<0.001 and 32%, P<0.001, respectively). L-NAME and indomethacin had little effect. 5. Perfusion with L-NAME and glibenclamide significantly reduced baseline coronary flow (22%, P<0.01 and 33%, P<0.01) while 8-PT, indomethacin and 5-HD had no effect. 6. K(ATP) channels are the major mediators of the coronary reactive hyperaemic response in the guinea-pig. Although mitochondrial K(ATP) channels contribute, they appear less important than sarcolemmal channels.