Search PubMed⌕ Search

Biomedical subjects

A Yatani

Publications and source records attributed to A Yatani.

At least 91 records · Page 5Linked to original sources
PubMed1989

New positive inotropic agent OPC-8212 modulates single Ca2+ channels in ventricular myocytes of guinea pig.

The effects of a new positive inotropic drug, OPC-8212 (a quinolinone derivative, 3,4-dihydro-6-[4-(3,4-dimethoxybenzoyl)-1 -piperazinyl]-2(1H)-quinolinone), on whole-cell and single-channel calcium currents of guinea pig ventricular myocytes were studied by the patch-clamp method. OPC-8212 increased whole-cell calcium currents in a concentration-dependent manner. The concentration-response curve was fitted by a single binding site model that has an ED50 of 2.7 microM. At the single-channel level, OPC-8212 increased calcium currents mainly by increasing the probability of channel opening on depolarization; the open times were slightly increased, but single-channel conductance did not change. The actions of OPC-8212 on single calcium channel currents were compared with those of forskolin, isoproterenol, and caffeine, agents that cause positive inotropic effects associated with an increase in cytoplasmic cyclic AMP level, and with those of the dihydropyridine calcium channel agonist (-)Bay K 8644, an agent that has a positive inotropic effect but does not act through cyclic AMP. OPC-8212 increased calcium currents in a manner that resembled the actions of forskolin, isoproterenol, and caffeine rather than the actions of (-)Bay K 8644. We suggest that activation of calcium currents by OPC-8212 is mediated by an elevation of intracellular cyclic AMP. Since OPC-8212 produces bradycardia and antitachycardiac effects in animals and humans, unlike agents that increase cyclic AMP, additional effects on other currents are likely.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1988

The G protein-gated atrial K+ channel is stimulated by three distinct Gi alpha-subunits.

The guanine nucleotide-binding protein, Gi, which inhibits adenylyl cyclase, has recently been shown to have three subtypes of the alpha-subunit, termed Gi alpha-1, Gi alpha-2 and Gi alpha-3. They share 87-94% amino-acid sequence homology and so are difficult to separate from one another. Among other functions, purified preparations activate K+ channels but there is confusion over which of the subtypes activates the muscarinic K+ channels of the atrial muscle of the heart: Gi alpha-3, also termed Gk, has been shown to activate this channel but it is not clear whether Gi alpha-1 does or does not. To clarify this problem, we expressed the subtypes separately in Escherichia coli to eliminate contamination by other subtypes and tested the recombinant alpha- chains on atrial muscarinic K+ channels. Although we anticipated that only Gi alpha-3 would have Gk activity, to our surprise all three recombinant subtypes were active, from which we deduce that the Gi subtypes are multifunctional.

Animals↗
PubMed1988

A monoclonal antibody to the alpha subunit of Gk blocks muscarinic activation of atrial K+ channels.

The activated heterotrimeric guanine nucleotide binding (G) protein Gk, at subpicomolar concentrations, mimics muscarinic stimulation of a specific atrial potassium current. Reconstitution studies have implicated the alpha and beta gamma subunits as mediators, but subunit coupling by the endogenous G protein has not been analyzed. To study this process, a monoclonal antibody (4A) that binds to alpha k but not to beta gamma was applied to the solution bathing an inside-out patch of atrial membrane; the antibody blocked carbachol-activated currents irreversibly. The state of the endogenous Gk determined its susceptibility to block by the antibody. When agonist was absent or when activation by muscarinic stimulation was interrupted by withdrawal of guanosine triphosphate (GTP) in the presence or absence of guanosine diphosphate (GDP), the effects of the antibody did not persist. Thus, monoclonal antibody 4A blocked muscarinic activation of potassium channels by binding to the activated G protein in its holomeric form or by binding to the dissociated alpha subunit.

Acetylcholine↗
PubMed1988

The stimulatory G protein of adenylyl cyclase, Gs, also stimulates dihydropyridine-sensitive Ca2+ channels. Evidence for direct regulation independent of phosphorylation by cAMP-dependent protein kinase or stimulation by a dihydropyridine agonist.

We demonstrated recently that purified preparations of Gs, the stimulatory G protein of adenylyl cyclase, can stabilize Ca2+ channels in inside-out cardiac ventricle membrane patches stimulated prior to excision by the beta-adrenergic agonist isoprenaline or by the dihydropyridine agonist Bay K 8644 and that such preparations of Gs can restore activity to spontaneously inactivated cardiac Ca2+ channels incorporated into planar lipid bilayers (Yatani, A., Codina, J., Reeves, J.P., Birnbaumer, L., and Brown, A.M. (1987) Science 238, 1288-1292). To test whether these effects represented true stimulation and to further identify the G protein responsible, we incorporated skeletal muscle T-tubule membranes into lipid bilayers and studied the response of their Ca2+ channels to G proteins, specifically Gs, and manipulations known to be specific for Gs. In contrast to cardiac channels, incorporated T-tubule Ca2+ channels exhibit stable average activities over prolonged periods of time (up to 20 min at room temperature), allowing assessment of possible effects of G proteins under steady-state assay conditions. We report that exogenously added human erythrocyte GTP gamma S (guanosine 5'-O-(3-thiotriphosphate]-activated Gs (Gs) or its resolved GTP gamma S-activated alpha subunit (alpha s) stimulate T-tubule Ca2+ channels by factors of 2-3 in the presence of Bay K 8644, and of 10-20 in the absence of Bay K 8644 and that they do so in a manner that is independent of concurrent or previous phosphorylation by cAMP-dependent protein kinase. Activation of purified Gs by cholera toxin increases both its adenylyl cyclase stimulatory and its Ca2+ channel stimulatory effects. Ca2+ channels previously stimulated by the combined actions of Bay K 8644 and cAMP-dependent protein kinase still respond to Gs. We conclude that the responses seen are due to Gs rather than a contaminant, that the effect on Ca2+ channel activity is that of a true stimulation, akin to that on adenylyl cyclase, and show that a given G protein may regulate more than one effector system.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1988

Amino acid sequence and physiological characterization of toxins from the venom of the scorpion Centruroides limpidus tecomanus Hoffmann.

The complete amino acid sequence of the major toxic component (II.20.3.4), named toxin 1, from the venom of the Mexican scorpion C. l. tecomanus is reported. The sequence (66 amino acids) was obtained by direct Edman degradation of reduced and alkylated toxin, followed by sequence determination of selected peptides separated after enzymatic cleavage with S. aureus V8 protease. In cultured chick dorsal root ganglion cells, 0.5 microM toxin 1 slowed down specifically the time course of Na+ current inactivation, while Ca2+ currents from the same preparation were little affected. In neonatal rat ventricular heart cells, toxin 1, at concentrations between 0.1 and 0.5 microM, reduced Na+ currents without changing the kinetics and Ca2+ currents were unaffected. Comparative analysis of the primary structure of this toxin with other scorpion toxins shows a high degree of similarity with the north American scorpion toxins. This analysis suggests that the 'fine tuning' of the molecular mechanism of action of these toxins is related to variations in the primary structure as well as to the type of membrane under study (tissue specificity).

Action Potentials↗
PubMed1988

Effects of dihydropyridine calcium channel modulators on cardiac sodium channels.

To investigate whether cardiac sodium channels have dihydropyridine (DHP) receptors we studied the effects of the optically pure (greater than 95%) enantiomers of the DHPs PN200-110 and BAY-K 8644 and the racemic DHP nitrendipine (NTD). Whole cell and single-channel sodium currents were recorded from cultured ventricular cells of neonatal rats using the patch-clamp method. NTD reduced cardiac sodium currents in a voltage-dependent manner. Inhibitory effects were due to an increase in traces without activity. The unit conductance remained unchanged. At negative holding potentials, NTD transiently increased the probability of channel opening. Both (+) and (-) PN 200-110 blocked sodium channels, although the (-) isomer was about one order of magnitude less effective. The blocking effects were voltage dependent. (+) BAY-K 8644 had similar blocking effects. (-) BAY-K 8644 produced an increase in sodium currents due to an increased frequency of channel openings and a marked prolongation of open time without any significant change in unit conductance. The DHPs have effects on cardiac sodium whole cell and single-channel currents that appear identical to and are as stereospecific as their effects on cardiac calcium currents, although the concentrations required are larger. In contrast the inwardly rectifying potassium channel (IK1) is unaffected by these DHPs. We conclude that functionally equivalent DHP receptors are present in cardiac sodium and calcium channels but not potassium channels and take this as evidence of the homology between sodium and calcium channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1988

Effects of New World scorpion toxins on single-channel and whole cell cardiac sodium currents.

Purified toxins from a North American scorpion, Centruroides noxius (Cn II-10), and a South American scorpion, Tityus serrulatus (Ts-gamma), were tested on cardiac sodium channels using patch-clamp methods to record whole cell and single-channel currents. The two toxins produced similar effects on sodium currents; potassium and calcium currents were not affected. Macroscopic sodium current amplitudes, measured at test potentials greater than -20 mV where the opening probability was high, decreased in a concentration-dependent manner with a half maximum inhibitory concentration of 6 X 10(-8) M. Block was unchanged by repetitive depolarizing pulses. In the presence of scorpion toxin, the currents were rapidly blocked by tetrodotoxin (3 X 10(-5) M). Both toxins shifted the voltage dependence of sodium channel inactivation to more negative potentials. At test potentials between -50 and -70 mV, where the sodium channel opening probability is normally low, both toxins produced an increase in sodium current and slowed the rates of activation and inactivation. At intermediate potentials between -50 and -20 mV the currents in the presence of toxins crossed over the control currents. At a test potential of -20 mV, the toxins decreased single-channel activity and increased the latency to first opening. At a test potential of -60 mV, the toxins significantly prolonged channel open time. The unitary current amplitudes were unchanged at either potential. We conclude that New World scorpion toxins produce apparently complex effects on whole cell currents primarily by retarding activation gating of cardiac sodium channels.

Animals↗
PubMed1988

Alpha-subunit of Gk activates atrial K+ channels of chick, rat, and guinea pig.

A specific guanine nucleotide-binding protein, Gk, is the link by which muscarinic receptors activate atrial potassium channels (Science Wash. DC 235: 207-211, 1987). In adult guinea pigs, the alpha-subunit at picomolar concentrations mediates the holo-G protein effect (Science Wash. DC 236: 442-445, 1987), but in chick embryo it has been reported that the beta gamma-dimer at nanomolar concentrations rather than the alpha-subunit is the effective mediator (Nature Lond. 325: 321-326, 1987). This difference might have a phylogenetic or ontogenetic basis, and the present experiments tested these possibilities. Preactivated alpha k derived from human red blood cell Gk, when applied to the intracellular surface of inside-out membrane patches from the atria of embryonic chick, neonatal rat, and adult guinea pig activated single K+ channel currents. In each case, the alpha k-activated channels had the same single-channel conductance and mean open time as the muscarinic agonist-activated channels. Half-maximal activation was achieved at alpha k-concentrations of 2.4-13.8 pM. Hence, alpha k-activation of these K+ channels is independent of differences in age or species. The detergent 3-[3-cholamidopropyl)-dimethyammoniol]-1-propanesulfonate (CHAPS), which was used by Logothetis et al. (Nature Lond. 325: 321-326, 1987) at 184 microM to suspend the hydrophobic beta gamma-dimers, activated the same currents. We conclude that the effects of the beta gamma-dimer on these K+ channels is unknown and that as we had proposed earlier (Science Wash. DC 236: 442-445, 1987) it is the alpha-subunit that mediates the Gk effect.

Animals↗
PubMed1988

Alpha-subunit of Gs directly activates cardiac calcium channels in lipid bilayers.

Specific guanine nucleotide-binding proteins (G proteins) couple receptors to calcium channels indirectly via cytoplasmic agents such as protein kinases (Birnbaumer et al., Kidney Int. 32: 514-537, 1987). Direct coupling between the stimulatory G protein from adenylyl cyclase (Gs) and calcium channels has been shown recently in inside-out cell-free membrane patches excised from guinea pig myocytes, i.e., in the absence of cytoplasm (Yatani et al., Science Wash. DC 238: 1288-1292, 1987). In these patches, single calcium channel currents were unstable, and this complicated both the experiments and their interpretation. Calcium currents are more stable when calcium channels are incorporated into planar lipid bilayers (Rosenberg et al., Science Wash. DC 231: 1564-1566, 1986), and consequently, we used the latter method to examine the direct coupling hypothesis more fully. Bovine cardiac sarcolemmal vesicles were added to the cis side of a chamber divided by a preformed planar lipid bilayer. The cytoplasmic surface faced the trans chamber and was bathed by an ATP-free minimal solution. The extracellular solution in the cis chamber contained 100 mM Ba2+, and both chambers contained the dihydropyridine agonist BAY K 8644 at 1 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenylate Cyclase Toxin↗
PubMed1988

5-Hydroxytryptamine antagonist ICS 205-930 blocks cardiac potassium, sodium and calcium currents.

Effects of the 5-hydroxytryptamine receptor antagonist ICS 205-930 [(3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester] on cardiac membrane currents were investigated in single isolated ventricular cells using the whole-cell patch clamp method. Ca++ and K+ currents were studied in guinea pig ventricular cells and Na+ currents were studied in ventricular cells from cultured neonatal rat; these cells are more suitable for Na+ current measurements than are ventricular cells from guinea pig. Under current clamp conditions, ICS 205-930 at 3 x 10(-5) M prolonged the action potential plateau and increased its amplitude of guinea pig cell. The effect was reversible. Increasing the concentration to 3 x 10(-4) M shortened the plateau, reduced its amplitude and depolarized the resting membrane potential. The effects between 10(-7) and 10(-3) M were examined under voltage-clamp conditions. ICS 205-930 produced a concentration-dependent suppression of inwardly rectifying K+ currents with an IC50 of 1.95 x 10(-5) M at a test potential of -40 mV. The effects were time-and voltage-dependent and the IC50 increased to 1.16 x 10(-4) M at -100 mV. The time-dependent outward current and the time-dependent outward tail currents upon repolarization to between -10 and -30 mV also were blocked by the drug in a concentration-dependent manner with IC50 of 3.7 x 10(-5) M. Ca++ currents and Na+ currents also were inhibited in the presence of higher concentration of ICS 205-930 (greater than 10(-4) M), although potency was stronger on Na+ currents. The results show that ICS 205-930 exerts mixed class III and class I antiarrhythmic properties in ventricular myocytes.

Action Potentials↗
PubMed1987

A G protein directly regulates mammalian cardiac calcium channels.

A possible direct effect of guanine nucleotide binding (G) proteins on calcium channels was examined in membrane patches excised from guinea pig cardiac myocytes and bovine cardiac sarcolemmal vesicles incorporated into planar lipid bilayers. The guanosine triphosphate analog, GTP gamma S, prolonged the survival of excised calcium channels independently of the presence of adenosine 3',5'-monophosphate (cAMP), adenosine triphosphate, cAMP-activated protein kinase, and the protein kinase C activator tetradecanoyl phorbol acetate. A specific G protein, activated Gs, or its alpha subunit, purified from the plasma membranes of human erythrocytes, prolonged the survival of excised channels and stimulated the activity of incorporated channels. Thus, in addition to regulating calcium channels indirectly through activation of cytoplasmic kinases, G proteins can regulate calcium channels directly. Since they also directly regulate a subset of potassium channels, G proteins are now known to directly gate two classes of membrane ion channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1987

Hormonal regulation of pituitary GH3 cell K+ channels by Gk is mediated by its alpha-subunit.

The resolved alpha-GTP gamma S (alpha*) and beta gamma-subunits of human erythrocyte Gk, the stimulatory regulatory component of hormone-responsive K+ channels, were tested for their potential stimulatory activities on the K+ channel of the endocrine GH3 cell. Concentrations as low as 0.5 pM alpha k* consistently activated K+ channels in isolated membrane patches, and saturating effects were obtained with 50 pM alpha k*. In contrast 2000-4000 pM beta gamma was without effect. We conclude that Gk acts on K+ channels through its alpha-subunit in a manner akin to that of Gs acting on adenylyl cyclase and transducin acting on cGMP-specific phosphodiesterase of photo-receptor cells.

Animals↗
PubMed1987

The alpha subunit of the GTP binding protein Gk opens atrial potassium channels.

Guanine nucleotide binding (G) proteins (subunit composition alpha beta gamma) dissociate on activation with guanosine triphosphate (GTP) analogs and magnesium to give alpha-guanine nucleotide complexes and free beta gamma subunits. Whether the opening of potassium channels by the recently described Gk in isolated membrane patches from mammalian atrial myocytes was mediated by the alpha k subunit or beta gamma dimer was tested. The alpha k subunit was found to be active, while the beta gamma dimer was inactive in stimulating potassium channel activity. Thus, Gk resembles Gs, the stimulatory regulatory component of adenylyl cyclase, and transducin, the regulatory component of the visual system, in that it regulates its effector function--the activity of the ligand-gated potassium channel--through its guanine nucleotide binding subunit.

Animals↗