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Y Kurachi

Publications and source records attributed to Y Kurachi.

At least 73 records · Page 4Linked to original sources
PubMed1997

An ATP-dependent inwardly rectifying potassium channel, KAB-2 (Kir4. 1), in cochlear stria vascularis of inner ear: its specific subcellular localization and correlation with the formation of endocochlear potential.

Cochlear endolymph has a highly positive potential of approximately +80 mV. This so-called endocochlear potential (EP) is essential for hearing. Although pivotal roles of K+ channels in the formation of EP have been suggested, the types and distribution of K+ channels in cochlea have not been characterized. Because EP was depressed by vascular perfusion of Ba2+, an inhibitor of inwardly rectifying K+ (Kir) channels, but not by either 4-aminopyridine or tetraethylammonium, we examined the expression of Kir channel subunits in cochlear stria vascularis, the tissue that is supposed to play the central role in the generation of positive EP. Of 11 members of the Kir channel family examined with reverse transcription-PCR, we could detect only expression of KAB-2 (Kir4.1) mRNA in stria vascularis. KAB-2 immunoreactivity was specifically localized at the basolateral membrane of marginal cells but not in either basal or intermediate cells. Developmental expression of KAB-2 in marginal cells paralleled formation of EP. Furthermore, deaf mutant mice (viable dominant spotting; WV/WV) expressed no KAB-2 in their marginal cells. These results suggest that KAB-2 in marginal cells may be critically involved in the generation of positive EP.

4-Aminopyridine↗
PubMed1997

Clustering and enhanced activity of an inwardly rectifying potassium channel, Kir4.1, by an anchoring protein, PSD-95/SAP90.

An inwardly rectifying potassium channel predominantly expressed in glial cells, Kir4.1/KAB-2, has a sequence of Ser-Asn-Val in its carboxyl-terminal end, suggesting a possible interaction with an anchoring protein of the PSD-95 family. We examined the effects of PSD-95 on the distribution and function of Kir4.1 in a mammalian cell line. When Kir4.1 was expressed alone, the channel immunoreactivity was distributed homogeneously. In contrast, when co-expressed with PSD-95, prominent clustering of Kir4.1 in the cell membrane occurred. Kir4.1 was co-immunoprecipitated with PSD-95 in the co-expressed cells. Glutathione S-transferase-fusion protein of COOH terminus of Kir4.1 bound to PSD-95. These interactions disappeared when the Ser-Asn-Val motif was deleted. The magnitude of whole-cell Kir4.1 current was increased by 2-fold in cells co-expressing Kir4.1 and PSD-95 compared with cells expressing Kir4. 1 alone. SAP97, another member of the PSD-95 family, showed similar effects on Kir4.1. Furthermore, we found that Kir4.1 as well as SAP97 distributed not diffusely but clustered in retinal glial cells. Therefore, PSD-95 family proteins may be a physiological regulator of the distribution and function of Kir4.1 in glial cells.

Adaptor Proteins, Signal Transducing↗
PubMed1997

Sulphonylurea receptor 2B and Kir6.1 form a sulphonylurea-sensitive but ATP-insensitive K+ channel.

1. We analysed the K+ channel composed of the sulphonylurea receptor 2B (SUR2B) and an inwardly rectifying K+ channel subunit Kir6.1 coexpressed in a mammalian cell line, HEK293T, with the patch clamp technique. 2. In the cell-attached configuration, K+ channel openers (pinacidil and nicorandil) activated approximately 33 pS K+ channels (approximately 145 mM external K+), which were inhibited by the sulphonylurea glibenclamide. 3. Although SUR2B forms an ATP-sensitive K+ channel with Kir6.2, whose amino acid sequence is approximately 70% homologous with that of Kir6.1, the K+ channel composed of SUR2B and Kir6.1 surprisingly did not spontaneously open on patch excision in the absence of intracellular ATP. 4. In inside-out patches, uridine diphosphate and guanosine diphosphate induced channel activity, which was inhibited by glibenclamide but not ATP. Intracellular ATP on its own activated the channels. K+ channel openers and intracellular nucleotides synergistically activated the channel. 5. Therefore, the K+ channel composed of SUR2B and Kir6.1 is not a classical ATP-sensitive K+ channel but closely resembles the nucleotide diphosphate-dependent K+ channel in vascular smooth muscle cells.

ATP-Binding Cassette Transporters↗
PubMed1997

Phosphorylation-independent inhibition by intracellular cyclic nucleotides of brain inwardly rectifying K+ current expressed in Xenopus oocytes.

An inwardly rectifying K+ current, which was heterologously expressed in Xenopus oocytes, was inhibited by isoproterenol, a fadrenergic agonist. Poly(A)+ mRNA isolated from guinea-pig brain was injected into oocytes 2-3 days before experiments. Isoproterenol inhibition of the K+ current was time-and voltage-dependent: the inhibition became faster and more pronounced as the command voltage steps were applied to more negative potentials. This inhibition was prevented by propranolol. Dibutylyl cyclic (dB-c) AMP could mimic the effect of isoproterenol, while injection of the catalytic subunit of cAMP-dependent protein kinase into the oocytes did not affect the K+ current. Inhibitors of the protein kinases, WIPTIDE and H-8, did not prevent the inhibition by dB-cAMP. Furthermore, dB-cGMP also inhibited the K+ current in a similar time- and voltage-dependent manner. We propose that the phosphorylation-independent action of cyclic nucleotides mediates beta-adrenergic inhibition of brain inwardly rectifying K+ channels expressed in Xenopus oocytes.

Adrenergic beta-Agonists↗
PubMed1997

Osseointegration of dental implants in rabbit bone with low mineral density.

PURPOSE: The purposes of this study were to establish an experimental animal model with bone of low mineral density, and to then investigate the reaction of the bone-implant interface in these experimental animals. MATERIALS AND METHODS: To prepare the experimental model, rabbits were ovariectomized and fed a 0.15% low-calcium diet. The bone mineral density (BMD) of the tibia was periodically measured by dual-energy X-ray absorptiometry. Approximately 1 month after the introduction of the low-calcium diet, when BMD had decreased to about 12% less than that of control rabbits, a titanium implant (TI) was inserted into the tibia of each rabbit. At 2, 4, 8, and 12 weeks after the insertion of the TI, the rabbits were killed, and the reaction of the bone tissue surrounding the TI was examined both histologically and radiographically. RESULTS: The combined ovariectomy and low-calcium diet produced rabbits having bone with low mineral density, and the histologic features of the tibia in these rabbits were consistent with those in human osteoporotic bone. In control rabbits, new bone formation surrounding the TI was evident 2 weeks after its insertion, and the TI surface was almost completely covered with new bone after 8 weeks. In contrast, new bone formation in the experimental rabbits was delayed, but considerable direct bone contact with the TI was observed after 12 weeks. CONCLUSION: These results suggest that osteoporotic bone may affect the healing period of bone tissue after the insertion of dental implants, but that osseointegration of dental implants may be obtained even in osteoporotic bone.

Animals↗
PubMed1997

Function, regulation, pharmacology, and molecular structure of ATP-sensitive K+ channels in the cardiovascular system.

ATP-sensitive K+ (K[ATP]) channels are inhibited by intracellular ATP and activated by intracellular nucleoside diphosphates, and thus provide a link between cellular metabolism and excitability. K(ATP) channels are widely distributed in various tissues and may be associated with diverse cellular functions. In the heart, the K(ATP) channel appears to be activated during ischemic or hypoxic conditions and may be responsible for the increase of K+ efflux and shortening of the action potential duration. Therefore, opening of this channel may result in cardioprotective as well as proarrhythmic effects. In the vascular smooth muscle, the K(ATP) channel is believed to mediate the relaxation of vascular tone. Thus, K(ATP) channels play important regulatory roles in the cardiovascular system. Furthermore, K(ATP) channels are the targets of two important classes of drugs, i.e., the antidiabetic sulfonylureas, which block the channels, and a series of vasorelaxants called "K+ channel openers," which tend to maintain the channels in an open conformation. Recently, the molecular structure of K(ATP) channels has been clarified. The K(ATP) channel in pancreatic beta-cells is a complex composed of at least two subunits, a member of inwardly rectifying K+ channels and a sulfonylurea receptor. Subsequently, two additional homologs of the sulfonylurea receptor, which form cardiac and smooth muscle type K(ATP) channels, respectively, have been reported. Further works are now in progress to understand the molecular mechanisms of K(ATP) channel function.

Adenosine Triphosphate↗
PubMed1997

Inwardly rectifying potassium channels: their molecular heterogeneity and function.

A variety of cells including cardiac myocytes and neuronal cells possess inwardly rectifying K+ (Kir) channels through which currents flow more readily in the inward direction than outward. These K+ channels play pivotal roles in maintenance of the resting membrane potential, in regulation of the action potential duration, in receptor-dependent inhibition of cellular excitability, and in the secretion and absorption of K+ ions across cell membrane. Recent molecular biological dissection has shown that the DNAs encoding Kir channels constitute a new family of K+ channels whose subunits contain two putative transmembrane domains and a pore-forming region. So far, more than ten cDNAs of Kir channel subunits have been isolated and classified into four subfamilies: 1) IRK subfamily (IRK1-3/Kir1.1-1.3), 2) GIRK subfamily (GIRK1-4/Kir3.1-3.4), 3) ATP-dependent Kir subfamily (ROMK1/Kir1.1, K(AB)-2/Kir4.1), and 4) ATP-sensitive Kir subfamily (uKATP-1/Kir6.1, BIR/Kir6.2). Xenopus oocytes injected with the cRNAs of IRKs elicit classical Kir channel currents. GIRKs, as heteromultimers, compose the G protein-gated Kir (KG) channels, which are regulated by a variety of Gi/Go-coupled inhibitory neurotransmitter receptors such as m2-mus-carinic, serotonergic (5HT1A), GABAB, somatostatin and opioid (mu, delta, kappa) receptors. ROMK1 and KAB-2 are characterized with a Walker type-A ATP-binding motif in their carboxyl termini, and may be involved in K+ transport in renal epithelial and brain glial cells. uKATP-1 and BIR form with sulfonylurea receptors, the so-called ATP-sensitive K+ channels. Thus, it is a feature of the Kir channel family that each subfamily plays a specific physiological functional role. The (Na+)-activated Kir channels identified electrophysiologically in neurons and cardiac myocytes have not yet been cloned. In this review, we overviewed the current understandings of the features of the molecular structures and functions of the four main subfamilies of Kir channels.

Amino Acid Sequence↗
PubMed1997

[Potassium channels in glial cells].

K+ channels play pivotal roles in maintenance of the resting membrane potential, in the regulation of the action potential duration, repolarization of the membrane potential, inhibition of the cellular excitability and transport of K+ across the cell membrane. Various kinds of K+ channels have been cloned. Mammalian K+ channels can be classified into three families according to their number of putative membrane-spanning region, i.e., six-transmembrane type, four-transmembrane type and two-transmembrane type. Each family consists of many members. These K+ channels are differentially expressed in a variety of cells. The membranes of glial cells are mainly permeable to K+. This is because glial cells express an abundant number of K+ channels. Their main function has been supposed to be aspiration and transportation of extracellular K+, which is liberated from neurons by their excitation. This regulatory function of glial cells is proposed as a spatial buffering mechanism of K+. Recently, molecular level studies on K+ channels of glial cells have been initiated. In this review, we will overview the current understanding of the features and function of K+ channels in glial cells.

Animals↗
PubMed1997

[Acute necrotizing encephalopathy with horizontal gaze palsy].

Acute necrotizing encephalopathy (ANE) of childhood is a newly proposed disease entity characterized by symmetrically distributed necrotic brain lesions in the thalamus, cerebral white matter, brainstem, and cerebellum. We report a 4-year-old girl with severe psychomotor delay and horizontal gaze palsy as sequelae of ANE at 17 months of age. The computed tomography showed bilaterally symmetrical low density areas in the thalamus and low density areas in the left middle cerebral artery (MCA) region and the posterior cerebral artery (PCA) region. MRI (T 1-weighted) revealed unevenly distributed small low signal intensity areas with scattered high intensity regions in the thalamus bilaterally. The T2-weighted images showed multiple small low intensity areas around high intensity areas, and low signal intensity areas in the left middle cerebral artery (MCA) region and the posterior cerebral artery (PCA) region. In addition to severe psychomotor delay, the patient exhibited a peculiar eye movements. Horizontal ocular movement was impaired, but vertical ocular movement was almost completely normal. As clearly shown by MRI of the brain, the pontine tegmentum, including bilateral abducens nucleus, paramedian pontine reticular formation (PPRF), medial longitudinal fasciculus (MLF), and the facial nerve were affected, but the thalamo-mesencephalic junction, including the rostral interstitial medial longitudinal fasciculus (riMLF) and the nucleus of Cajal, was spared. To our knowledge, this is the first case of ANE associated with this selective ocular movement disorder ever reported. Because of the multiple symmetrical lesions and pons and the asymmetrical lesions of the MCA and PCA regions in the present case, occlusion of a single vessel could not account for the pathology. The pathophysiological mechanism of ANE is unknown. We postulate that some toxic or vasoactive agent caused vasospasm and subsequent breakdown of the blood-brain barrier, especially in the thalamus and pons, resulting in the unique distribution of the lesions and the rare eye movement disorder observed in the present case.

Brain↗
PubMed1996

Cloning and functional expression of a novel isoform of ROMK inwardly rectifying ATP-dependent K+ channel, ROMK6 (Kir1.1f).

We have identified from rat kidney a novel isoform of ROMK/Kir1.1, designated ROMK6/Kir1.1f. ROMK6 was nearly identical to ROMK1, but possessed an 122-bp insertion in the 5' region. Its deduced amino acid sequence was shorter by 19 amino acids than that of ROMK1 in the amino-terminus. Unlike other previously reported ROMK isoforms, ROMK6 mRNA was ubiquitously expressed in various tissues, including kidney, brain, heart, liver, pancreas and skeletal muscle. Xenopus oocytes injected with ROMK6 cRNA expressed a Ba2+-sensitive weakly inwardly rectifying K+ current. These results indicate that ROMK6 is a novel functional K+ channel and might be involved in K+ secretion in various tissue.

Adenosine Triphosphate↗
PubMed1996

A novel sulfonylurea receptor forms with BIR (Kir6.2) a smooth muscle type ATP-sensitive K+ channel.

We have isolated a cDNA encoding a novel isoform of the sulfonylurea receptor from a mouse heart cDNA library. Coexpression of this isoform and BIR (Kir6.2) in a mammalian cell line elicited ATP-sensitive K+ (KATP) channel currents. The channel was effectively activated by both diazoxide and pinacidil, which is the feature of smooth muscle KATP channels. Sequence analysis indicated that this clone is a variant of cardiac type sulfonylurea receptor (SUR2). The 42 amino acid residues located in the carboxyl-terminal end of this novel sulfonylurea receptor is, however, divergent from that of SUR2 but highly homologous to that of the pancreatic one (SUR1). Therefore, this short part of the carboxyl terminus may be important for diazoxide activation of KATP channels. The reverse transcription-polymerase chain reaction analysis showed that mRNA of this clone was ubiquitously expressed in diverse tissues, including brain, heart, liver, urinary bladder, and skeletal muscle. These results suggest that this novel isoform of sulfonylurea receptor is a subunit reconstituting the smooth muscle KATP channel.

ATP-Binding Cassette Transporters↗
PubMed1996

Immunolocalization of an inwardly rectifying K+ channel, K(AB)-2 (Kir4.1), in the basolateral membrane of renal distal tubular epithelia.

Immunolocalization of K(AB)-2 (Kir4.1), an inwardly rectifying K+ channel with a putative ATP-binding domain, was examined in rat kidney where expression of K(AB)-2 mRNA was previously shown. Anti-K(AB)-2 antibody was raised in rabbit and then affinity-purified. An immunohistochemical study revealed that K(AB)-2 immunoreactivity was detected specifically in the basolateral membrane of distal tubular epithelia. Therefore, K(AB)-2 is the first K+ channel shown to be localized in the basolateral membrane of renal epithelia. The finding suggests that K(AB)-2 may contribute to supplying K+ to the Na(+)-K+ pump, which is abundant in the basolateral membrane of distal tubular epithelia, as well as to maintenance of the deep negative membrane potential of these cells.

Amino Acid Sequence↗
PubMed1996

Competition between Mg2+ and spermine for a cloned IRK2 channel expressed in a human cell line.

1. A cloned inwardly rectifying K+ channel, IRK2, was expressed in a human cell line, human embryonic kidney (HEK) 293T. Its electrophysiological properties were examined using the patch clamp technique in the whole-cell, cell-attached and inside-out patch configurations. 2. The cells transfected with IRK2 cDNA exhibited a K+ current which showed classical properties of inwardly rectifying K+ channels at both whole-cell and single-channel levels. 3. In the inside-out patch configuration, intracellular Mg2+ (Mg2+i blocked the outward currents in a voltage-dependent and virtually time-independent manner. Mg2+i (1-100 microM) caused a decrease in the unitary current amplitude of the IRK2 channel by inducing subconducting levels. 4. In the absence of Mg2+i, intracellular spermine blocked the outwardly flowing IRK2 currents in a voltage- and time-dependent manner. Spermine (1-100 nM) did not affect the unitary channel current amplitude but reduced the channel open probability. The spermine block showed a slower time and steeper voltage dependence than the Mg2+i++ block. 5. When both these blockers were present, Mg2+i apparently attenuated the inhibitory effect of spermine on the outwardly flowing IRK2 currents. This interaction was voltage and time dependent, and could be well explained by a model in which Mg2+i and spermine competitively bind to the channel with their individual first-order kinetics. This competition would induce time-dependent transits of the channel between the Mg2+i -and spermine-blocked states via a single open state, thereby preserving a certain size of persistent outward currents at depolarized potentials.

Barium↗
PubMed1996

Neurokinin A and Ca2+ current induce Ca(2+)-activated Cl(-) currents in guinea-pig tracheal myocytes.

1. Membrane currents were recorded by a patch clamp technique in guinea-pig tracheal myocytes, using the whole cell mode with Cs(+) internal solution. 2. Both neurokinin A (NKA, 1 mu M) and caffeine (10 mM) evoked Ca(2+)-activated Cl- currents (I[Cl(Ca)]) transiently. In Ca(2+)-free bathing solution, the first application of NKA or caffeine elicited I[Cl(Ca)] but the second application of these substances failed to activate it. In addition, pretreatment with ryanodine in the presence of caffeine abolished the response to both NKA and caffeine whilst heparin (200 mu g ml(-1)) only blocked the NKA-induced response. I[Cl(Ca)] was also elicited by inositol 1,4,5-trisphosphate (IP(3)). 3. Command voltage pulses positive to 0 mV from a holding potential of -60 mV activated the voltage-dependent L-type Ca2+ current (I(Ca,L)) and late outward current. Upon repolarization to the holding potential, slowly decaying inward tail currents were recorded. The outward current during the depolarizing pulses and the inward tail current were enhanced by Bay K 8644, but completely blocked by Cd2+ or nifedipine. Replacement of external Ca2+ with Ba2+, removal of Ca2+ from the bath solution, or inclusion of EGTA (5 mM) in the patch pipette, also led to abolition of these currents, indicating that they were Ca2+ dependent, and that Ca2+ influx due to I(Ca,L) activated the currents. 4. When [Cl(-)](O) or [Cl(-)](i) was changed, the reversal potential (E(rev)) of the Ca2+-activated currents shifted, thus behaving like a Cl(-)-selective ion channel as predicted by the Nernst equation. DIDS (1 mM) completely abolished the currents, also suggesting that they were I[Cl(Ca)]. 5. NKA (1 mu M) and caffeine (30 mM) transiently activated I[Cl(Ca)], and after that both agents markedly reduced I[Cl(Ca)] induced by I(Ca,L). This is probably due to sarcoplasmic reticulum (SR) Ca2+ release induced by NKA or caffeine, followed by inhibition of the Ca(2+)-induced Ca2+ release from the SR. 6. The present results indicate that I[Cl(Ca)] can be activated by SR Ca2+ release due to NKA or caffeine (through IP(3) or ryanodine receptors) as well as by Ca2+ influx due to I(Ca,L). It also suggests that activation of I[Cl(Ca)] by NKA may be mediated by the production of IP(3), which releases Ca2+ from the SR.

Action Potentials↗