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Biomedical subjects

K Muraki

Publications and source records attributed to K Muraki.

At least 73 records · Page 4Linked to original sources
PubMed1995

Delayed rectifier K+ current in rabbit atrial myocytes.

The role of delayed rectifier K+ current(s) (IK) in rabbit left atrium was examined by applying the whole cell voltage-clamp technique to isolated single myocytes. Right-triangular waveforms, which mimic the shape of atrial action potentials (APs), and selective blockers were used to compare the contribution of IK with other K+ currents to repolarization of the APs. IK measured at 34 degrees C in atrial myocytes was very small; the maximum peak amplitude of the tail current (IK,tail) at -40 mV was approximately 50 pA. The IK,tail was almost abolished in most cells (approximately 80%) by the application of 1 microM E-4031, a class III antiarrhythmic drug. The E-4031-sensitive current recorded with the triangular command wave-form showed strong inward rectification and had a maximum amplitude of approximately 30 pA at -40 mV. Total outward current elicited by triangular command pulses depended strongly on stimulation frequency. The main frequency-dependent component was a Ca(2+)-independent transient K+ current (I(t)). I(t) elicited by triangular pulses at 1 Hz was substantially reduced by 4-aminopyridine (4-AP) at potentials positive to 0 mV but was not changed significantly by 1 microM E-4031; 100 microM E-4031 reduced I(t) by approximately 30%. The shape of the APs which were recorded from a single rabbit atrial cell strongly depended on the pulse frequency. Application of 1 microM E-4031 increased action potential duration (APD) in > 50% of cells examined but had little effect on the resting membrane potential (RMP). Application of 0.1 mM BaCl2 also lengthened APD and reduced RMP by approximately 20 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Aminopyridine↗
PubMed1995

Effects of sematilide, a novel class III antiarrhythmic agent, on action potential in guinea pig atrium.

Electrophysiological effects of sematilide, a novel class III antiarrhythmic agent, were examined and compared with those of (+/-)sotalol in guinea pig left atrium by a conventional microelectrode technique. Application of 0.1-1000 microM sematilide or 1-1000 microM (+/-)sotalol concentration-dependently prolonged the duration of action potentials (APD) that were elicited by electrical stimulation at 1 Hz. Other parameters of action potentials such as the maximum upstroke velocity of phase 0 depolarization, action potential amplitude and resting membrane potential were not affected significantly by these drugs in the concentration ranges employed. The prolongation of APD by sematilide or (+/-)sotalol was accompanied by a corresponding increase in the effective refractory period (ERP). Approximately a 30% increase in ERP was obtained by the treatment with 5 microM sematilide or 100 microM (+/-)sotalol, suggesting that sematilide as a class III antiarrhythmic agent is approximately 20 times more potent than (+/-)sotalol on a molecular basis. When the stimulation rate was increased stepwise from 0.2 to 2 Hz, the relative increase in APD at 90% repolarization by the treatment with sematilide and (+/-)sotalol was slightly larger at 2 Hz than at 0.2 Hz, indicating that "reverse rate-dependence" was not observed under these conditions. These results may suggest a possibility that sematilide effectively blocks atrial arrhythmia.

Action Potentials↗
PubMed1994

Effects of noradrenaline on membrane currents and action potential shape in smooth muscle cells from guinea-pig ureter.

1. The effects of noradrenaline (NA) on action potential shape and underlying membrane currents were examined in single smooth muscle cells freshly isolated from the ureter of the guinea-pig. 2. The voltage-dependent Ca2+ current (ICa) elicited upon depolarization from -50 to 0 mV was reduced by 27% upon application of 10 microM NA. This reduction was inhibited or converted to potentiation by internal application of low molecular weight heparin or 5 mM EGTA, indicating that it may be mediated by Ca(2+)-dependent Ca2+ channel inactivation via inositol 1,4,5-trisphosphate production and subsequent Ca2+ release from intracellular Ca2+ storage sites. 3. In contrast, Ba2+ current (IBa) through Ca2+ channels was potentiated by 36% in the presence of 10 microM NA. Internal application of GTP gamma S made it difficult to remove potentiation of IBa by wash-out; internal application of GDP beta S abolished potentiation. 4. NA caused a greater reduction in the transient Ca(2+)-dependent K+ current (IK(Ca)) upon depolarization than it did in ICa. This reduction was inhibited by internally applied heparin, suggesting that the amount of releasable Ca2+ in the storage sites was markedly reduced in the presence of NA. The sustained component of IK(Ca) which gradually increased during depolarization was also reduced by NA. 5. Action potential duration, which was recorded in a standard solution containing Ca2+, was prolonged by the application of NA. 6. It can be concluded that Ca2+ channel activity in ureter smooth muscle cells is regulated by a dual mechanism: Ca(2+)-dependent inhibition and GTP-binding protein-mediated potentiation. Under physiological conditions, both ICa and IK(Ca) were reduced by NA but the reduction of IK(Ca) was much larger than that of ICa; this results in an increase in net inward current during the action potential plateau and prolongs the action potential.

Action Potentials↗
PubMed1994

Tetrahexylammonium ions increase Ca2+ sensitivity of contraction of guinea-pig ileal smooth muscle.

Effects of tetraalkylammonium ions, having tetraalkyl chains of increasing length from ethyl to octyl, on inositol-trisphosphate (InsP3)-induced Ca2+ release and contractile mechanics were examined in guinea-pig skinned ileal smooth muscle longitudinal strips. Although tetrahexylammonium ions (THexA) appeared to be the most potent inhibitor of Ca2+ release among the tetraalkylammonium ions examined, an additional and more prominent effect was found, i.e., the contraction induced by Ca2+ release showed a large sustained component in the presence of THexA. Potentiation of the contraction by THexA (above 30 microM) was also observed in skinned fibers in which the sarcoplasmic reticulum function was destroyed by treatment with A23187. The potentiating effect of THexA was the most potent by far among the tetraalkylammonium ions examined and was elicited by Ca(2+)-dependent and GTP-binding-protein-independent mechanisms. The potentiation was not due to activation of myosin light-chain kinase. The selective inhibitors of myosin light-chain kinase, protein kinase C and calmodulin reduced THexA-induced potentiation of contraction only at concentrations above 30 microM, at which non-specific effects are likely. Furthermore, relaxation induced by changing pCa from 4.5 to 8.5 was not affected by 1 mM THexA, suggesting that the potentiating effect is not mainly due to inhibition of myosin light-chain phosphatase. In conclusion, ThexA sensitizes guinea-pig skinned ileal smooth muscle to Ca2+ in a structure-selective manner. This sensitization appears not to be mediated mainly by a GTP-binding protein, by activation of myosin light-chain kinase or protein kinase C, by enhanced Ca2+ binding to calmodulin, or by inhibition of myosin light-chain phosphatase.

Animals↗
PubMed1994

Receptor for catecholamines responding to catechol which potentiates voltage-dependent calcium current in single cells from guinea-pig taenia caeci.

1. Single isolated cells were obtained from the taenia of the guinea-pig's caecum by enzymic digestion and held under voltage clamp. The effects of various catecholamines, sympathomimetics and related compounds were tested for their ability to potentiate the voltage-dependent calcium current (ICa) evoked in these cells by a depolarizing step. 2. ICa was potentiated by up to 60% by isoprenaline, adrenaline, and noradrenaline which were equipotent. The EC50 for isoprenaline was about 40 nM. 3. The racemic mixtures of the optical isomers of isoprenaline, adrenaline, and noradrenaline, and (+)-isoprenaline, were equipotent with the (-)-isomers of these drugs. Dopamine, L-dopa, and catechol were equipotent with these catecholamines. 4. Removal or substitution of one or more of the hydroxy groups of the catechol moiety, as in phenylephrine, salbutamol, procaterol, methoxamine, terbutaline, BRL 37344, ICI 215001 or tyramine substantially reduced efficacy and/or potency. 5. The adrenoceptor blockers propranolol, phentolamine, dihydroergotamine, atenolol, CGP 20712A and ICI 118551, or the dopamine receptor blockers, haloperidol or flupenthixol, did not block the potentiating action of catechol or the catecholamines. 6. The receptor activated by catecholamines to increase ICa we suggest should be called a C-receptor in view of its sensitivity to catechol. It may arise by enzymic modification of a conventional adrenoceptor but its transduction also involves a novel mechanism which might indicate that it is present in the muscle cells before enzyme treatment.

Animals↗
PubMed1993

Effect of isoprenaline on Ca2+ channel current in single smooth muscle cells isolated from taenia of the guinea-pig caecum.

1. The effects of isoprenaline (Iso) on Ca2+ channel current in enzymatically isolated single cells of the guinea-pig taenia caeci were examined using the standard whole-cell voltage-clamp method. 2. Iso potentiated the voltage-dependent Ca2+ current; the threshold and maximally effective concentration of Iso to increase Ca2+ current were 3-10 nM and 1-3 microM, respectively. The average increase in Ca2+ current produced by 3 microM Iso was 42 +/- 6% (mean +/- S.E.M.) and the response could be obtained repeatedly in the same cell. The concentration-response relationship could be fitted by a binding model with a Hill coefficient of 1 and a dissociation constant of 42 nM. 3. The effect of Iso on Ca2+ current was voltage dependent. Although potentiation of Ca2+ current by Iso was obvious between -30 and +10 mV, it was small or absent around +20 to +30 mV. Iso had little effect on the relationship between inactivation of the Ca2+ current and voltage obtained using a double-pulse protocol. 4. External application of forskolin, an adenylyl cyclase activator, or internal perfusion of cAMP or dibutyryl cAMP from the recording pipette, did not increase Ca2+ current and potentiation of Ca2+ current by Iso was observed repeatedly and was unchanged. 5. Internal perfusion of GTP gamma S or GDP beta S increased or did not affect the Ca2+ current and potentiation of Ca2+ current by Iso was unchanged and could be recorded repeatedly for about 20 min after rupture of the cell membrane. In addition, treatment of cells with the potent protein kinase C inhibitor, chelerythrine, had no effect on Ca2+ current or on potentiation of Ca2+ current by Iso. 6. These results suggest that the Ca2+ current in guinea-pig taenia caeci cells is potentiated by isoprenaline via mechanisms which do not involve either a cAMP pathway, a G-protein pathway or a protein kinase C pathway. The receptor involved appeared to be an atypical adrenoreceptor not blocked by either alpha- or beta-receptor blocking agents.

Animals↗
PubMed1993

[Physiological functions and regulation of K+ and Cl- channels in single smooth muscle cells].

The major functional roles of K channels in smooth muscle cells are as follows: (i) Keeping resting membrane potential (RMP). (ii) Induction of hyperpolarization (HP) in response to bioactive substances. (iii) Facilitation of action potential repolarization (APR). (iv) Induction of action potential after-hyperpolarization (AH). (v) Facilitation of slow wave repolarization. (vi) Inhibition of depolarization and/or action potential. Major background K channels responsible for RMP have not been identified yet, whereas it is been postulated that several K channels including Ca-dependent K channels and ATP-dependent K channels may co-contribute in part to RMP. Further activation of these channels by bioactive substances results in hyperpolarization and muscle relaxation. APR and AH in several types of smooth muscles are due to activation of mainly large conductance Ca-dependent K channels (BK) and additionally delayed rectifier K channels. Moreover, another type of delayed rectifier K channels and early inactivating K channels may also contribute to some of the functional roles shown above. Although activation of Ca-dependent Cl channels in response to agonists depolarizes the cell membrane in several types of smooth muscle cells, a single channel current has not been identified. Ca-dependent K and Cl currents can be regulated by intracellular Ca store sites, which may be exhausted after a large release induced by inositol 1,4,5-trisphosphate or caffeine. The Ca-pump activity in the store sites may also regulate indirectly but strongly the activity of the channels and consequently membrane excitability.

Action Potentials↗
PubMed1992

Ca-dependent K channels in smooth muscle cells permeabilized by beta-escin recorded using the cell-attached patch-clamp technique.

Using the cell-attached patch-clamp technique, the activity of single, Ca-dependent K channels was recorded in single smooth muscle cells permeabilized by beta-escin. The conductance and the relationship between the open probability of the channels and pCa recorded in permeabilized cells were very similar to those obtained in excised inside-out patches. At pCa 7, application of 30 microM acetylcholine (ACh) or 0.1 microM substance P (SP) together with 1 mM guanosine 5'-trisphosphate to permeabilized cells elicited transient bursts of channel openings similar to those which occur in intact cells. Transient activation was also observed when 2-30 microM inositol trisphosphate (IP3) was applied to permeabilized cells. This single channel activity was inhibited by pretreatment with low-molecular-weight heparin at 50-100 micrograms/ml. Channel activity at pCa 7.0 was greatly enhanced by 200 microM cyclic adenosine monophosphate. These results provide direct evidence that single Ca-dependent K channel activity is regulated by the transmitters ACh and SP, as well as a second messenger, IP3, via the release of intracellular Ca from intracellular sites which are blocked by heparin. This novel approach is valuable in elucidating second messenger mechanisms involved in the regulation of single channel activity by transmitters and autocoids, since permeabilization by beta-escin preserves the entire system of receptor-operated signal transduction and allows intracellular application of second messengers at fixed concentrations.

Acetylcholine↗
PubMed1992

Cyclopiazonic acid, an inhibitor of the sarcoplasmic reticulum Ca(2+)-pump, reduces Ca(2+)-dependent K+ currents in guinea-pig smooth muscle cells.

1. Effects of cyclopiazonic acid (CPA), a specific inhibitor of the Ca(2+)-ATPase in sarcoplasmic reticulum (SR), on membrane ionic currents were examined in single smooth muscle cells freshly isolated from ileal longitudinal strips and urinary bladder of the guinea-pig. 2. Under whole-cell clamp, CPA (1-10 microM) reduced peak outward current elicited by depolarization in a concentration-dependent manner. The concentration of CPA required for 50% decrease in the peak outward current was approximately 3 microM in ileal cells under these conditions. The current reduced by CPA recovered by more than 70% after washout. 3. The transient outward current elicited by application of 5 mM caffeine at a holding potential of -50 mV in Ca2+ free solution was almost abolished, when the preceding Ca(2+)-loading of the cell in a solution containing 2.2 mM Ca2+ was performed in the presence of 3 microM CPA. 4. When the Ca(2+)-dependent K+ current (IK-Ca) and Ca2+ current (ICa) were inhibited by addition of Ca2+, the remaining delayed rectifier type K+ current was not affected by 10 microM CPA. When outward currents were blocked by replacement of K+ by Cs+ in the pipette solution, the remaining ICa was not affected by 10 microM CPA. 5. CPA (10 microM) did not affect the conductance of single maxi Ca(2+)-dependent K+ channels or the Cd(2+)-dependence of their open probability in both inside- and outside-out configurations. 6. These results indicate that IK-Ca is selectively and strongly suppressed by CPA.Its effects may be attributed to a decrease in Ca2"-uptake into SR, resulting in a decrease in Ca2"-induced Ca24 release which is triggered by Ca24 entering through voltage-dependent Ca24 channels and therefore less activation of these K channels.7. CPA may be extremely valuable pharmacological tool for investigating intracellular Ca24 mobilization and ionic currents regulated by intracellular Ca24.

Animals↗
PubMed1991

Sodium currents in smooth muscle cells freshly isolated from stomach fundus of the rat and ureter of the guinea-pig.

1. Inward currents elicited by depolarization from holding potentials of -80 to -10 mV in single smooth muscle cells isolated from stomach fundus of the rat and ureter of the guinea-pig had two components. The initial fast component (Ifi) was activated and mostly inactivated within 1-2 and 10 ms, respectively, at 21 degrees C. The following sustained component (Isi) lasted over 50 and 500 ms in fundus and ureter cells, respectively. Ifi was blocked by tetrodotoxin but not affected by 0.5 microM-mu-conotoxin in both types of cells. Isi was abolished by the substitution of extracellular Ca2+ with Mn2+. 2. The sensitivity of Ifis to TTX was markedly different in fundus and ureter cells. The half-inhibition was obtained at 870 and 11 nM, respectively. The amplitude of Ifi was highly dependent on extracellular Na+ concentration in a solution containing 2.2 mM-Mn2+ and 0 mM-Ca2+ in both cells. It is concluded that Ifis in these cells are TTX-sensitive and mu-conotoxin-insensitive Na+ currents. 3. Some of the kinetics of INa measured at 10 degrees C were markedly different in fundus and ureter cells. The current-voltage relationships for Ifi in fundus and ureter cells had peaks at about -10 and 0 mV, respectively. The voltage dependence of the steady-state inactivation of Ifi was also significantly different in these cell types. The half-inactivation voltages were about -74 and -45 mV, respectively. The recovery time course from inactivation in fundus cells was about 10 times slower than that in ureter at -80 mV, where it was 25 ms. 4. The contribution of Ifi to the rising phase of an action potential was examined using TTX under current clamp mode at 21 degrees C. A fast notch-like potential elicited by a subthreshold stimulus for action potential generation was blocked by TTX in both types of cells. Action potentials elicited by a stimulus around threshold were occasionally suppressed by TTX, whereas an action potential was never observed when extracellular Ca2+ was replaced with Mn2+. 5. In conclusion, the existence of at least two types of Na+ channel currents, which were distinguished by their TTX sensitivity and kinetics, was strongly suggested in smooth muscle cells from the rat fundus and the guinea-pig ureter. INa in these cells may have a physiological role to accelerate the generation of an action potential by triggering a rapid activation of ICa, while not being essential for activation of action potentials.

Animals↗
PubMed1991

Mechanisms of NE-induced reduction of Ca current in single smooth muscle cells from guinea pig vas deferens.

Effects of norepinephrine (NE) on voltage-dependent Ca channel current (ICa) were examined applying whole cell patch-clamp technique to single smooth muscle cells freshly isolated from vas deferens of the guinea pig. K currents and contraction of the cell were abolished by Cs and EGTA in the pipette solution, respectively. The peak ICa and Ba current (IBa) elicited by depolarization from -60 mV in a solution containing 2.2 mM Ca or Ba were reduced by 10-60% in voltage- and dose-dependent manners by the application of NE or phenylephrine. This effect was greatly attenuated in the presence of prazosin. The decrease in IBa was always smaller than that in ICa at any potential. Even after simultaneous application of 5 mM caffeine and 10 microM NE to the cells in a Ba-containing solution, the second challenge with NE again reduced IBa in a similar manner. The decrease in IBa by 10 microM NE could not be explained well by a small shift (-5 mV) of the voltage dependence of the steady-state inactivation. The effect of NE on IBa was irreversibly enhanced by 0.1 mM guanosine 5'-O-(3-thiotriphosphate) and almost abolished by 1 mM guanosine 5'-O-(2-thiodiphosphate) added to the pipette solution but appeared not to be affected by the treatment with pertussis toxin. It can be concluded that, under these experimental conditions, the activation of alpha 1-adrenoceptor in vas deferens smooth muscle cells reduces Ca channel activity possibly via a mechanism involving GTP-binding protein in addition to Ca-mediated Ca channel inactivation mechanism.

Animals↗
PubMed1990

Isolation and structures of xenopsin-related peptides from rat stomach, liver and brain.

Using radioimmunoassay for detection, a mammalian counterpart to amphibian xenopsin (XP) was isolated and sequenced from pepsin-treated extracts of three different rat tissues and shown to be H-Phe-His-Pro-Lys-Arg-Pro-Trp-Ile-Leu-OH. This peptide, which shares six of the eight residues in XP, existed primarily in large molecular form(s) in the rat from which it could be liberated by the enzyme, pepsin. The XP-related sequence was differentially distributed through tissues, with concentrations ranging from ca. 80 pmol/g in diaphragm and skeletal muscle to ca. 800 pmol/g in stomach, liver and intestine. Like XP, the mammalian peptide potently crossreacted in a radioreceptor assay for neurotensin. These results prove the existence of radioreceptor-active XP-related sequences in multiple tissues of the rat.

Amino Acid Sequence↗
PubMed1990

Canine neurotensin, neurotensin6-13 and neuromedin N: primary structures and receptor activity.

Canine neurotensin (NT) and neuromedin N (NMN) were isolated from extracts of ileal mucosa using radioimmunoassay for detection. The structures determined were consistent with those predicted by earlier cDNA work. The molar ratio of NT to NMN was ca. 7, suggesting that the NT/NMN precursor, which contains one copy of each peptide, undergoes complex posttranslational processing or that other NT-precursors lacking NMN exist. In addition to NT, small quantities of NT6-13 and NT2-13 were obtained. Native and synthetic preparations of these peptides were indistinguishable in a radioreceptor assay employing rat brain membranes and 125I-labeled NT; NT6-13 was ca. 8-times more potent than NT and NMN was about one-sixth as potent as NT. NT6-13 was also ca. 10 times more potent than NT in inhibiting spontaneous contractile activity in longitudinally-oriented smooth muscle strips of porcine jejunum. Preparations of intestinal N-cells as well as N-cell vesicles also appeared to contain NT2-13 and NT6-13; however, it is not yet clear whether these peptides are utilized physiologically or simply represent metabolites of NT. These results suggest that further work on the processing of NT precursor and on biologic abilities of partial sequences of NT could be fruitful.

Amino Acid Sequence↗
PubMed1990

Effects of tetraethylammonium and 4-aminopyridine on outward currents and excitability in canine tracheal smooth muscle cells.

1. The effects of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on membrane currents and on single channel K currents in smooth muscle cells isolated from canine trachea were examined by use of tight seal whole cell- and patch-clamp techniques. 2. Depolarizing current applied through a recording pipette did not elicit an action potential under current clamp. A strong outward rectification was observed. 3. In most cells under voltage-clamp, only an outward current was observed upon depolarization from -60 mV when a pipette solution contained mainly KCl. The outward current consisted of three components; a large initial transient, a following sustained component and an additional component of irregular small transients on the sustained one. The two transient components were almost abolished when extracellular and pipette solutions contained 2.2 mM Cd2+ (0 mM Ca2+) and 10 mM EGTA, respectively. The sustained component was well maintained under these conditions. 4. TEA at low concentrations (less than 1 mM) effectively decreased the transient components and made the outward current smooth; it also suppressed the sustained component at higher concentrations. In outside-out patches, external 1 mM TEA reduced the single channel conductance of Ca-activated K channels by about 87% whereas 3 mM 4-AP did not. 4-AP at low concentrations (less than 3 mM) selectively reduced the sustained component of the outward current. 5. A Ca current recorded after the suppression of outward current by internal Cs+ had a peak of approximately 200 pA at +10 mV (holding potential: -60 mV). The half inactivation voltage in the steady-state was approximately -30 mV. 6. Simultaneous application of 1 mM TEA and 4-AP reduced the outward current and unmasked a Ca current. Under these conditions, an action potential with overshoot was easily elicited under current clamp. 7. It is concluded that the low excitability of canine tracheal smooth muscle cell upon depolarization is due to a large outward K current which consists of Ca-dependent and Ca-independent components. The peak amplitude of the Ca current is similar to that in highly excitable smooth muscle cells such as those of the ureter.

4-Aminopyridine↗