P287 K+ channel blockers and nerve excitability: Comparison with changes in motor axons in ALS

Single-channel patch-clamp techniques as well as standard and perforated-patch whole cell voltage-clamp techniques have been applied to the study of ionic channels in the corneal endothelium of several species. These studies have revealed two major K+ currents. One is due to an anion- and temperature-stimulated channel that is blocked by Cs+ but not by most other K+ channel blockers, and the other is similar to the family of A-currents found in excitable cells. The A-current is transient after a depolarizing voltage step and is blocked by both 4-aminopyridine and quinidine. These two currents are probably responsible for setting the -50 to -60 mV resting voltage reported for these cells. A Ca(2+)-activated ATP-inhibited nonselective cation channel and a tetrodotoxin-blocked Na+ channel are possible Na+ inflow pathways, but, given their gating properties, it is not certain that either channel works under physiological conditions. A large-conductance anion channel has also been identified by single-channel patch-clamp techniques. Single corneal endothelial cells have input resistances of 5-10 G omega and have steady-state K+ currents that are approximately 10 pA at the resting voltage. Pairs or monolayers of cells are electrically coupled and dye coupled through gap junctions.

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Chronic hypoxia selectively augments endothelium-dependent pulmonary arterial vasodilation

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1996.270.3.h888 ◽

1996 ◽

Vol 270 (3) ◽

pp. H888-H896 ◽

Cited By ~ 36

Author(s):

T. C. Resta ◽

B. R. Walker

Keyword(s):

Chronic Hypoxia ◽

K Channel ◽

Channel Blockers ◽

No Donors ◽

Question Mark ◽

Pulmonary Vasodilator ◽

Pulmonary Arterial ◽

Arterial Dilation ◽

Spermine Nonoate ◽

Increased Activity

We have previously demonstrated that chronic hypoxia (CH) augments pulmonary arterial dilation to the endothelium-derived nitric oxide (EDNO)-dependent pulmonary vasodilator arginine vasopressin (AVP). The present study examined 1) whether this enhanced vasoreactivity is observed with other agents that act by stimulating constitutive NO synthase (cNOS), 2) whether CH increases arterial vascular smooth muscle sensitivity to NO, and 3) whether endogenous endothelin (ET) or an endothelium-derived hyperpolarizing factor (EDHF) contributes to this altered arterial reactivity following CH. We examined responses to the receptor-mediated EDNO-dependent dilators histamine and ET-1, the nonreceptor-mediated EDNO-dependent dilator ionomycin, and the NO donors 1, 3-propanediamine, N- inverted question mark4-[1-(3-aminopropyl)-2-hydroxy-2-nitrosohydrazino] butyl inverted question mark (spermine NONOate) and S-nitroso-N-acetylpenicillamine (SNAP) in U-46619-constricted, isolated perfused lungs from control and CH rats. Additional experiments examined responses to AVP in the presence of the ET-receptor antagonist PD-145065 or the K+ channel blockers glibenclamide or tetraethylammonium (TEA) in lungs from each group. Microvascular pressure was assessed by double occlusion, allowing calculation of segmental resistances. Total and arterial vasodilatory responses to histamine, ET-1, and ionomycin were augmented in lungs from CH vs. control animals. However, CH did not alter the vasodilation to spermine NONOate or SNAP. PD-145065, glibenclamide, and TEA had no effect on responses to AVP in either group. We conclude that increased activity of arterial cNOS may be responsible for the augmented pulmonary arterial dilation to EDNO-dependent vasodilators following CH.

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Long-lasting reduction of excitability by a sodium-dependent potassium current in cat neocortical neurons

Journal of Neurophysiology ◽

10.1152/jn.1989.61.2.233 ◽

1989 ◽

Vol 61 (2) ◽

pp. 233-244 ◽

Cited By ~ 145

Author(s):

P. C. Schwindt ◽

W. J. Spain ◽

W. E. Crill

Keyword(s):

Voltage Clamp ◽

Time Course ◽

Outward Current ◽

Repetitive Firing ◽

K Channel ◽

Channel Blockers ◽

Dantrolene Sodium ◽

Current Time ◽

Tail Current ◽

Neocortical Neurons

1. The function and ionic mechanism of a slow outward current were studied in large layer V neurons of cat sensorimotor cortex using an in vitro slice preparation and single microelectrode voltage clamp. 2. With Ca2+ influx blocked, a slow relaxation ("tail") of outward current followed either (1) repetitive firing evoked for 1 s or (2) a small 1-s depolarizing voltage clamp step that activated the persistent Na+ current of neocortical neurons, INaP. When a depolarization that activated INaP was maintained, an outward current gradually developed and increased in amplitude over a period of tens of seconds to several minutes. An outward tail current of similar duration followed repolarization. The slow outward current was abolished by TTX, indicating it depended on Na+ influx. 3. With Ca2+ influx blocked, the onset of the slow Na+-dependent outward current caused spike frequency adaptation during current-evoked repetitive firing. Following the firing, the decay of the Na+-dependent current caused a slow afterhyperpolarization (sAHP) and a long-lasting reduction of excitability. It also was responsible for habituation of the response to repeated identical current pulses. 4. The Na+-dependent tail current had properties expected of a K+ current. Membrane chord conductance increased during the tail, and tail amplitude was reduced or reversed by membrane potential hyperpolarization and raised extracellular K+ concentration [( K+]0). 5. The current tail was reduced reversibly by the K+ channel blockers TEA (5-10 mM), muscarine (5-20 microM), and norepinephrine (100 microM). These agents also resulted in a larger, more sustained inward current during the preceding step depolarization. Comparison of current time course before and after the application of blocking agents suggested that, in spite of its capability for slow buildup and decay, the onset of the Na+-dependent outward current occurs within 100 ms of an adequate step depolarization. 6. With Ca2+ influx blocked, extracellular application of dantrolene sodium (30 microM) had no clear effect on the current tail or the corresponding sAHP.(ABSTRACT TRUNCATED AT 400 WORDS)

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Effect of Na+ and K+ channel blockade on baseline and anoxia-induced catecholamine release from rat carotid body

Journal of Applied Physiology ◽

10.1152/jappl.1994.77.6.2606 ◽

1994 ◽

Vol 77 (6) ◽

pp. 2606-2611 ◽

Cited By ~ 25

Author(s):

T. P. Doyle ◽

D. F. Donnelly

Keyword(s):

Carotid Body ◽

K Channel ◽

Channel Blockers ◽

Nerve Activity ◽

Sham Treatment ◽

K Channels ◽

Nerve Response ◽

Carotid Bodies ◽

Carotid Body Glomus Cells

Ionic membrane currents are hypothesized to play a major role in determining secretion from carotid body glomus cells, and increased secretion likely mediates the increase in nerve activity in response to hypoxia. The hypothesis that Na+ and K+ channels play an important role in determining secretion and nerve activity was tested by measuring single-fiber afferent nerve activity along with an estimate of free tissue catecholamine using Nafion-covered carbon-fiber micro-electrodes placed in rat carotid bodies in vitro. Baseline and anoxia-stimulated (1 min duration; PO2 of approximately 0 Torr at nadir) levels were quantified. Sham treatment had no significant effect. Tetrodotoxin (2 microns) ablated the nerve activity and reduced peak catecholamine (19.5 +/- 3.1 to 14.5 +/- 3.4 microM; P < 0.05). Cesium (10 microns) had no effect on catecholamine but reduced the nerve response (19.8 +/- 2.7 to 7.8 +/- 2.0 Hz; P < 0.05). 4-Aminopyridine (4 mM) significantly reduced the nerve response (17.2 +/- 3.7 to 4.9 +/- 1.9 Hz; P < 0.05) and increased the baseline (0.9 +/- 0.2 to 3.1 +/- 0.8 microM; P < 0.05) and reduced the peak catecholamine (10.0 to 4.3 +/- 0.8 microM; P < 0.05) levels. These results demonstrate that Na+ and K+ channels play an important role in modulating the secretory and nerve responses. However, channel blockers do not emulate severe hypoxia, suggesting that hypoxia transduction procedes, at least in part, through an alternate pathway.

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Rat Prl and TSH secretion are regulated differently by K(+)-channel blockers

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.1994.266.1.e39 ◽

1994 ◽

Vol 266 (1) ◽

pp. E39-E43 ◽

Cited By ~ 2

Author(s):

X. Wang ◽

T. Inukai ◽

M. A. Greer ◽

S. E. Greer

Keyword(s):

Channel Blocker ◽

Cell Types ◽

Inhibitory Effect ◽

Thyroid Stimulating Hormone ◽

K Channel ◽

Channel Blockers ◽

Pituitary Cells ◽

Dependent Manner ◽

K Channels ◽

Tsh Secretion

All four different K(+)-channel blockers [tetraethylammonium (TEA), a nonselective K(+)-channel blocker; tolbutamide, an ATP-sensitive K(+)-channel blocker; quinine and 4-aminopyridine, both primarily voltage-dependent K(+)-channel blockers] stimulated prolactin (Prl) secretion by acutely dispersed anterior pituitary cells but had no effect on thyroid-stimulating hormone (TSH) secretion. TEA stimulated Prl secretion in a dose-dependent manner between 1 microM and 20 mM, but even as high as 20 mM, TEA did not induce TSH secretion. Valinomycin (2 microM), a K+ ionophore, inhibited both basal and TEA-induced Prl secretion. TEA-stimulated Prl secretion was abolished by using a Ca(2+)-depleted medium or adding 10 microM dopamine. TEA did not reverse the inhibitory effect of dopamine on thyrotropin-releasing hormone-induced Prl secretion. Our data indicate that K+ channels may play a role in the secretion of adenohypophysial hormones that is idiosyncratic for each hormone. Differences in the role of K+ channels may reflect differences between the various pituitary cell types in plasma membrane ion channel composition, membrane potential, or the mechanism of exocytosis.

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Complexity of the outward K+ current of the rat megakaryocyte

AJP Cell Physiology ◽

10.1152/ajpcell.1997.272.5.c1525 ◽

1997 ◽

Vol 272 (5) ◽

pp. C1525-C1531 ◽

Cited By ~ 5

Author(s):

E. Romero ◽

R. Sullivan

Keyword(s):

K Channel ◽

Delayed Rectifier ◽

Channel Blockers ◽

Excitable Cells ◽

Electrophysiological Properties ◽

Relative Contribution ◽

Delayed Rectifier Current ◽

Dependent Inactivation ◽

Voltage Dependent ◽

K Current

Megakaryocytes isolated from rat bone marrow express a voltage-dependent, outward K+ current with complex kinetics of activation and inactivation. We found that this current could be separated into at least two components based on differential responses to K+ channel blockers. One component, which exhibited features of the "transient" or "A-type" K+ current of excitable cells, was more strongly blocked by 4-aminopyridine (4-AP) than by tetrabutylammonium (TBA). This current, which we designated as "4-AP-sensitive" current, activated rapidly at potentials more positive than -40 mV and subsequently underwent rapid voltage-dependent inactivation. A separate current that activated slowly was blocked much more effectively by TBA than by 4-AP. This "TBA-sensitive" component, which resembled a typical delayed rectifier current, was much more resistant to voltage-dependent inactivation. The relative contribution of each of these components varied from cell to cell. The effect of charybdotoxin was similar to that of 4-AP. Our data indicate that the voltage-dependent K+ current of resting megakaryocytes is more complex than heretofore believed and support the emerging concept that megakaryocytes possess intricate electrophysiological properties.

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K+-channel blockers do not decrease acetylcholine depolarizations in canine trachealis

Canadian Journal of Physiology and Pharmacology ◽

10.1139/y92-007 ◽

1992 ◽

Vol 70 (1) ◽

pp. 43-52 ◽

Cited By ~ 2

Author(s):

E. E. Daniel ◽

J. Jury ◽

R. Serio ◽

L. P. Jager

Keyword(s):

Chloride Channels ◽

Reversal Potential ◽

Membrane Conductance ◽

Membrane Resistance ◽

Resting Potential ◽

K Channel ◽

Channel Blockers ◽

Membrane Effects ◽

Sucrose Gap ◽

Time Courses

Using the double sucrose gap, we have examined the role of K+ channels in the cholinergic depolarizations in response to field stimulation and acetylcholine (Ach) in canine trachealis. Acetylcholine-like depolarization per se decreased electrotonic potentials from hyperpolarizing currents. The net effect of acetylcholine (10−6 M) depolarization on membrane conductance was a small increase after the depolarization was compensated by current clamp. Reversal potentials for acetylcholine depolarization and for the excitatory junction potential (EJP) were determined by extrapolation to be 20–30 mV positive to the resting potential, previously shown to be approximately −55 mV. They were shifted positively by tetraethylammonium ion (TEA) at 20 mM or Ba2+ at 1 mM. TEA or Ba2+ initially depolarized the membrane and increased membrane resistance. Repolarization of the membrane restored any reductions in EJP amplitudes associated with depolarization. After 15 min, the membrane potential partially repolarized, and acetylcholine-induced depolarization and contractions were then increased by TEA. 4-Aminopyridine depolarized the membrane but decreased membrane resistance. Apamin (10−6 M), charybdotoxin (10−7 M), and glybenclamide (10−5 M) each failed to significantly depolarize membranes, increase membrane resistance, or reduce EJP amplitudes or depolarization to 10−6 M Ach. Glybenclamide reduced depolarizations to added acetylcholine slightly. TEA occasionally reduced the EJP markedly, but this was shown to be most likely a prejunctional effect mediated by norepinephrine release. TEA alone among K+-channel blockers slowed the onset and the time courses of the EJP as well as the acetylcholine-induced depolarization. K+-channel closure cannot be a complete explanation of acetylcholine-induced membrane effects on this tissue. Acetylcholine must have increased the conductance of an ion with a reversal potential positive to the resting potential in addition to any effect to close K+ channels.Key words: acetylcholine, tracheal smooth muscle, trachea, chloride channels, sucrose gap, potassium channels, tetraethylammonium, Ba2+.

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