scholarly journals Decline of electrogenic Na+/K+ pump activity in rod photoreceptors during maintained illumination.

1986 ◽  
Vol 87 (4) ◽  
pp. 633-647 ◽  
Author(s):  
H Shimazaki ◽  
B Oakley
Keyword(s):  
Membrane Resistance ◽  
Membrane Voltage ◽  
Continuous Illumination ◽  
Subretinal Space ◽  
Rod Photoreceptors ◽  
Pharmacological Agents ◽  
Pump Activity ◽  
Isolated Retina ◽  
Voltage Dependent ◽  
Toad Bufo

Light-evoked changes in membrane voltage were recorded intracellularly from rod photoreceptors in the isolated retina preparation of the toad, Bufo marinus, during superfusion with a solution containing pharmacological agents that blocked voltage-dependent conductances. Under these conditions, the amplitude of the hyperpolarizing photoresponse became much greater than under control conditions. The results of several experiments support the conclusion that this increase in photoresponse amplitude was due primarily to a voltage that was produced when the electrogenic current from the rods' Na+/K+ pump flowed across an increased membrane resistance (Torre, V. 1982. Journal of Physiology. 333:315). At the onset of a period of continuous illumination, the rod membrane first hyperpolarized and then began to repolarize, and after 180 s of illumination, the membrane voltage had recovered by 60-72% of its initial hyperpolarization. There did not appear to be any significant decrease in rod membrane resistance associated with this repolarization. Both the enhanced hyperpolarization at light onset and the slow repolarization during maintained illumination were blocked by superfusion with 10.0 microM strophanthidin. These data support the hypothesis that the activity of the rods' Na+/K+ pump declines progressively during maintained illumination. It is likely that the decline in pump activity produces significant changes in [K+]o in the subretinal space during maintained illumination.

scholarly journals Reaccumulation of [K+]o in the toad retina during maintained illumination.

1984 ◽  
Vol 84 (3) ◽  
pp. 475-504 ◽  
Author(s):  
H Shimazaki ◽  
B Oakley
Keyword(s):  
Differential Equations ◽  
Glial Cells ◽  
Time Course ◽  
Subretinal Space ◽  
Light Onset ◽  
Müller Glial Cells ◽  
Isolated Retina ◽  
Steady Level ◽  
Spatial Buffering ◽  
Toad Bufo

Using K+-selective microelectrodes, [K+]o was measured in the subretinal space of the isolated retina of the toad, Bufo marinus. During maintained illumination, [K+]o fell to a minimum and then recovered to a steady level that was approximately 0.1 mM below its dark level. Spatial buffering of [K+]o by Müller (glial) cells could contribute to this reaccumulation of K+. However, superfusion with substances that might be expected to block glial transport of K+ had no significant effect upon the reaccumulation of K+. These substances included blockers of gK (TEA+, Cs+, Rb+, 4-AP) and a gliotoxin (alpha AAA). Progressive slowing of the rods' Na+/K+ pump (perhaps caused by a light-evoked decrease in [Na+]i) also could contribute to this reaccumulation of K+ by reducing the uptake of K+ from the subretinal space. As evidence for a major contribution by this mechanism, treatments designed to prevent such slowing of the pump reversibly blocked reaccumulation. These treatments included superfusion with 2 microM ouabain, or lowering [K+]o, PO2, or temperature. It is likely that such treatments inhibit the pump, increase [Na+]i, and attenuate any light-evoked decrease in [Na+]i. The results are consistent with the following hypothesis. At light onset, the decrease in rod gNa will reduce the Na+ influx and the resulting rod hyperpolarization will reduce the K+ efflux. In combination with these reduced passive fluxes, the continuing active fluxes will lower both [K+]o and [Na+]i, which in turn will inhibit the pump. In support of this hypothesis, the solutions to a pair of coupled differential equations that model changes in both [K+]o and [Na+]i match quantitatively the time course of the observed changes in [K+]o during and after maintained illumination for all stimuli examined.

Measurement of potassium turnover in rod photoreceptors in toad isolated retina using ion-selective microelectrodes

1987 ◽  
Vol 65 (5) ◽  
pp. 1018-1027 ◽  
Author(s):  
Burks Oakley II
Keyword(s):  
Cell Types ◽  
Bufo Marinus ◽  
Background Illumination ◽  
Rod Photoreceptors ◽  
Isolated Retina ◽  
Small Change ◽  
Toad Bufo

Ion-selective microelectrodes (ISMs) were used to measure the turnover of intracellular K+[Formula: see text] in rods in the isolated retina of the toad, Bufo marinus. The light-evoked hyperpolarization of rods decreases their passive K+ efflux, which in combination with active K+ uptake, decreases extracellular K+ concentration, [Formula: see text]. Rb+ substitutes for K+ in these processes. The turnover of [Formula: see text] was measured as Rb+ and K+ were exchanged, using ISMs that were approximately five times more sensitive to Rb+ than to K+. When [Formula: see text]was replaced by [Formula: see text], the light-evoked decrease in K+ efflux produced only a small change in ISM voltage, ΔVISM, owing to the background of [Formula: see text]. As [Formula: see text] replaced [Formula: see text], the efflux shifted from K+ to Rb+ and ΔVISM grew in amplitude. After loading the rods with [Formula: see text], [Formula: see text] was replaced by [Formula: see text]. The light-evoked decrease in Rb+ efflux lead transiently to a large ΔVISM, since the change in [Formula: see text], was superimposed upon a background of [Formula: see text]. As [Formula: see text] replaced [Formula: see text], the amplitude of ΔVISM declined. When measured using this technique, the turnover of [Formula: see text] was 95% complete in approximately 15 min. In low Ca2+ solutions, transmembrane fluxes of K+ (Rb+) increased and turnover of [Formula: see text] occurred more rapidly. During background illumination, transmembrane fluxes of K+ (Rb+) decreased and turnover of [Formula: see text] was slowed. These experiments have provided independent corroboration of earlier observations concerning rod K+ fluxes. This ISM-based technique also may be useful in measuring K+ turnover in other cell types.

scholarly journals Light-dependent ion influx into toad photoreceptors.

1982 ◽  
Vol 80 (4) ◽  
pp. 517-536 ◽  
Author(s):  
M L Woodruff ◽  
G L Fain ◽  
B L Bastian
Keyword(s):  
Membrane Potential ◽  
Incubation Period ◽  
Dark Current ◽  
Phosphodiesterase Inhibitor ◽  
Membrane Resistance ◽  
Ringer’S Solution ◽  
Gramicidin D ◽  
Isolated Retina ◽  
Ion Influx ◽  
Toad Bufo

To measure the influx of Na+ and other ions through the light-dependent permeability of photoreceptors, we superfused the isolated retina of the toad, Bufo marinus, with a low-Ca2+ (10(-8) M), low-Cl- Ringer's solution containing 0.5 mM ouabain. Under these conditions, the membrane potential of the rod is near zero and there is no light-induced potential change either in the rod or in more proximal neurons. The photoreceptors, however, continue to show a light-dependent increase in membrane resistance, which indicates that the light-sensitive channels still close with illumination. Dark-adapted retinas show a larger 22Na+ accumulation than do light-adapted retinas. The extra accumulation of 22Na+ into dark-adapted retinas can be removed if the retinas are washed in darkness with low-Ca2+ Ringer's solutions, or if the ionophore gramicidin D is present in the perfusate. The additional accumulation in dark retinas corresponds to a flux of at least 10(9) Na+ per receptor per second, which is close to the value of the photoreceptor dark current. The light-dependent uptake of 22Na+ can be prevented by exposing the retinas to Ca2+ during the incubation period, but is restored if the phosphodiesterase inhibitor IBMX is added to the perfusate. A significant light-dependent ion accumulation can be observed for the cations K+, Rb+, Cs+, and Tl+, in addition to Na+, but not for methylamine, choline, or tetraethylammonium.

scholarly journals Electrophysiological effects of basolateral [Na+] in Necturus gallbladder epithelium.

1992 ◽  
Vol 99 (2) ◽  
pp. 241-262 ◽  
Author(s):  
G A Altenberg ◽  
J S Stoddard ◽  
L Reuss
Keyword(s):  
Apical Membrane ◽  
Basolateral Membrane ◽  
Membrane Resistance ◽  
Membrane Voltage ◽  
Gallbladder Epithelium ◽  
K Channels ◽  
Necturus Gallbladder ◽  
Electrophysiological Effects ◽  
Paracellular Diffusion ◽  
Cl Conductance

In Necturus gallbladder epithelium, lowering serosal [Na+] ([Na+]s) reversibly hyperpolarized the basolateral cell membrane voltage (Vcs) and reduced the fractional resistance of the apical membrane (fRa). Previous results have suggested that there is no sizable basolateral Na+ conductance and that there are apical Ca(2+)-activated K+ channels. Here, we studied the mechanisms of the electrophysiological effects of lowering [Na+]s, in particular the possibility that an elevation in intracellular free [Ca2+] hyperpolarizes Vcs by increasing gK+. When [Na+]s was reduced from 100.5 to 10.5 mM (tetramethylammonium substitution), Vcs hyperpolarized from -68 +/- 2 to a peak value of -82 +/- 2 mV (P less than 0.001), and fRa decreased from 0.84 +/- 0.02 to 0.62 +/- 0.02 (P less than 0.001). Addition of 5 mM tetraethylammonium (TEA+) to the mucosal solution reduced both the hyperpolarization of Vcs and the change in fRa, whereas serosal addition of TEA+ had no effect. Ouabain (10(-4) M, serosal side) produced a small depolarization of Vcs and reduced the hyperpolarization upon lowering [Na+]s, without affecting the decrease in fRa. The effects of mucosal TEA+ and serosal ouabain were additive. Neither amiloride (10(-5) or 10(-3) M) nor tetrodotoxin (10(-6) M) had any effects on Vcs or fRa or on their responses to lowering [Na+]s, suggesting that basolateral Na+ channels do not contribute to the control membrane voltage or to the hyperpolarization upon lowering [Na+]s. The basolateral membrane depolarization upon elevating [K+]s was increased transiently during the hyperpolarization of Vcs upon lowering [Na+]s. Since cable analysis experiments show that basolateral membrane resistance increased, a decrease in basolateral Cl- conductance (gCl-) is the main cause of the increased K+ selectivity. Lowering [Na+]s increases intracellular free [Ca2+], which may be responsible for the increase in the apical membrane TEA(+)-sensitive gK+. We conclude that the decrease in fRa by lowering [Na+]s is mainly caused by an increase in intracellular free [Ca2+], which activates TEA(+)-sensitive maxi K+ channels at the apical membrane and decreases apical membrane resistance. The hyperpolarization of Vcs is due to increase in: (a) apical membrane gK+, (b) the contribution of the Na+ pump to Vcs, (c) basolateral membrane K+ selectivity (decreased gCl-), and (d) intraepithelial current flow brought about by a paracellular diffusion potential.

Voltage-dependent modulation of GABAA receptor channel desensitization in rat hippocampal neurons

1994 ◽  
Vol 71 (6) ◽  
pp. 2151-2160 ◽  
Author(s):  
K. W. Yoon
Keyword(s):  
Membrane Potential ◽  
Hippocampal Neurons ◽  
Membrane Conductance ◽  
Cell Voltage ◽  
Chloride Conductance ◽  
Membrane Voltage ◽  
Hippocampal Cell Culture ◽  
Voltage Dependent ◽  
Agonist Concentration ◽  
Rate Of Recovery

1. The mechanism of the time-dependent decline in gamma-amino-butyric acid (GABA)-induced chloride conductance, referred to as desensitization, was studied in dissociated rat hippocampal cell culture with the use of a whole-cell voltage-clamp recording. 2. In most cells the gradual decline of membrane conductance was dependent simultaneously on the agonist concentration and membrane voltage. Even in the continued presence of GABA, desensitization could be prevented by holding the membrane potential > 0 mV in a near symmetrical chloride gradient across the cell membrane. 3. The “recovery” from desensitization occurred after removal of the agonist with a time constant of approximately 35 s. The rate of recovery from desensitization was independent of membrane voltage. 4. When the membrane potential was jumped from a negative to a positive membrane potential during steady state of desensitization, the GABA-induced chloride conductance gradually “relaxed” to the undesensitized state. This phenomenon of gradual increase in chloride conductance or “reactivation” from desensitization was both voltage and agonist dependent. 5. The process of recovery of the GABA ionophore from the desensitized state is distinct from the process of reactivation, which is dependent both on the voltage and agonist. 6. These observations suggest that the ligand-bound GABA receptor has two alternate states, i.e., permissive (activated) and desensitized. The rates of transition between these two states are voltage dependent.

Effects of Octopamine on Calcium Action Potentials in Insect Oocytes

1989 ◽  
Vol 142 (1) ◽  
pp. 115-124
Author(s):  
M. J. O'DONNELL ◽  
B. SINGH
Keyword(s):  
Action Potentials ◽  
Rank Order ◽  
Potassium Conductance ◽  
Membrane Resistance ◽  
Threshold Concentration ◽  
Resting Potential ◽  
Calcium Dependent ◽  
Octopamine Receptors ◽  
Voltage Dependent ◽  
Maximum Rate Of Rise

Our experiments show that octopamine receptors are present on the developing follicles of an insect, Rhodnius prolixus. Application of D,L-octopamine decreased the duration and overshoot of calcium-dependent action potentials (APs), and increased the intrafollicular concentration of cyclic AMP. The threshold concentration of D,L-octopamine for the reduction in electrical excitability was between 1 and 5×10−7moll−1, and maximal effects of a 40–50% reduction in AP overshoot and duration were apparent at 10−4moll−1. At concentrations above 10−5moll−1, a small (<10%) hyperpolarization of the resting potential was also apparent. Effects of D,L-octopamine on oocyte excitability were independent of these small shifts in resting potential. Current injection experiments, in which calcium entry was blocked by cobalt, demonstrated that D,L-octopamine reduced membrane resistance at both hyperpolarizing and depolarizing potentials. Octopamine did not affect the maximum rate of rise of the AP, dV/dtmax, which is an indicator of inward calcium current. It is suggested that octopamine may mediate its effects on excitability through an increase in a voltage-dependent potassium conductance. Application of other phenolamines indicated a rank order of potency of D, Loctopamine > D,L-synephrine > tyramine. The α-adrenergic agonists clonidine, naphazoline and tolazoline were without significant effect at 10−5-10−3moll−1. Reduction of excitability by D,L-octopamine was effectively blocked by phentolamine and metoclopramide. Yohimbine and gramine were less effective as antagonists. Possible functions of octopamine receptors in insect follicles are discussed.

Ba2+, TEA+, and quinine effects on apical membrane K+ conductance and maxi K+ channels in gallbladder epithelium

1990 ◽  
Vol 259 (1) ◽  
pp. C56-C68 ◽  
Author(s):  
Y. Segal ◽  
L. Reuss
Keyword(s):  
Apical Membrane ◽  
Membrane Conductance ◽  
Membrane Resistance ◽  
K Channel ◽  
Dependent Manner ◽  
Gallbladder Epithelium ◽  
Concentration Dependent Manner ◽  
K Channels ◽  
Voltage Dependent ◽  
Channel Currents

The apical membrane of Necturus gallbladder epithelium contains a voltage-activated K+ conductance [Ga(V)]. Large-conductance (maxi) K+ channels underlie Ga(V) and account for 17% of the membrane conductance (Ga) under control conditions. We examined the Ba2+, tetraethylammonium (TEA+), and quinine sensitivities of Ga and single maxi K+ channels. Mucosal Ba2+ addition decreased resting Ga in a concentration-dependent manner (65% block at 5 mM) and decreased Ga(V) in a concentration- and voltage-dependent manner. Mucosal TEA+ addition also decreased control Ga (60% reduction at 5 mM). TEA+ block of Ga(V) was more potent and less voltage dependent that Ba2+ block. Maxi K+ channels were blocked by external Ba2+ at millimolar levels and by external TEA+ at submillimolar levels. At 0.3 mM, quinine (mucosal addition) hyperpolarized the cell membranes by 6 mV and reduced the fractional apical membrane resistance by 50%, suggesting activation of an apical membrane K+ conductance. At 1 mM, quinine both activated and blocked K(+)-conductive pathways. Quinine blocked maxi K+ channel currents at submillimolar concentrations. We conclude that 1) Ba2+ and TEA+ block maxi K+ channels and other K+ channels underlying resting Ga; 2) parallels between the Ba2+ and TEA+ sensitivities of Ga(V) and maxi K+ channels support a role for these channels in Ga(V); and 3) quinine has multiple effects on K(+)-conductive pathways in gallbladder epithelium, which are only partially explained by block of apical membrane maxi K+ channels.

Sodium-potassium pump inhibitors increase neuronal excitability in the rat hippocampal slice: role of a Ca2+-dependent conductance

1987 ◽  
Vol 57 (2) ◽  
pp. 496-509 ◽  
Author(s):  
M. McCarren ◽  
B. E. Alger
Keyword(s):  
Hippocampal Slice ◽  
Neuronal Excitability ◽  
Pyramidal Cells ◽  
Input Resistance ◽  
Extracellular Potassium ◽  
Gamma Aminobutyric Acid ◽  
Extracellular Potassium Concentration ◽  
Spike Threshold ◽  
Pump Activity ◽  
Voltage Dependent

We have used the rat hippocampal slice preparation as a model system for studying the epileptogenic consequences of a reduction in neuronal Na+-K+ pump activity. The cardiac glycosides (CGs) strophanthidin and dihydroouabain were used to inhibit the pump. These drugs had readily reversible effects, provided they were not applied for longer than 15-20 min. Hippocampal CA1 pyramidal cells were studied with intracellular recordings; population spike responses and changes in extracellular potassium concentration ([K+]o) were also measured in some experiments. This investigation focused on the possibility that intrinsic neuronal properties are affected by Na+-K+ pump inhibitors. The CGs altered the CA1 population response evoked by an orthodromic stimulus from a single spike to an epileptiform burst. Measurements of [K+]o showed that doses of CGs sufficient to cause bursting were associated with only minor (less than 1 mM) changes in resting [K+]o. However, the rate of K+ clearance from the extracellular space was moderately slowed, confirming that a decrease in pump activity had occurred. Intracellular recording indicated that CG application resulted in a small depolarization and apparent increase in resting input resistance of CA1 neurons. Although CGs caused a decrease in fast gamma-aminobutyric acid mediated inhibitory postsynaptic potentials (IPSPs), CGs could also enhance the latter part of the epileptiform burst induced by picrotoxin, an antagonist of these IPSPs. Since intrinsic Ca2+ conductances comprise a significant part of the burst, this suggested the possibility that Na+-K+ pump inhibitors affected an intrinsic neuronal conductance. CGs decreased the threshold for activation of Ca2+ spikes (recorded in TTX and TEA) without enhancing the spikes themselves, indicating that a voltage-dependent subthreshold conductance might be involved. The action of CGs on Ca2+ spike threshold could not be mimicked by increasing [K+]o up to 10 mM. A variety of K+ conductance antagonists, including TEA, 4-AP, Ba2+ (in zero Ca2+), and carbachol were ineffective in preventing the CG-induced threshold shift of the Ca2+ spike. The shift was also seen in the presence of a choline-substituted low Na+ saline. Enhancement of a slow inward Ca2+ current is a possible mechanism for the decrease in Ca2+ spike threshold; however, it is impossible to use the Ca2+ spike as an assay when testing the effects of blocking Ca2+ conductances. Therefore, we studied the influence of CGs on the membrane current-voltage (I-V) curve, since persistent voltage-dependent conductances appear as nonlinearities in the I-V plot obtained under current clamp.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological characterization of upper portion of descending limb of long-looped nephron

1991 ◽  
Vol 260 (3) ◽  
pp. F311-F316 ◽  
Author(s):  
K. Yoshitomi ◽  
M. Imai
Keyword(s):  
Apical Membrane ◽  
Basolateral Membrane ◽  
High Water ◽  
Membrane Resistance ◽  
Membrane Voltage ◽  
Major Pathway ◽  
K Concentration ◽  
Electrophysiological Characterization

The upper portion of the descending limb of long-looped nephron (LDLu) of the hamster is characterized by high water and ion permeabilities. Although the paracellular route is considered to be the major pathway representing cation permselectivity of this segment, ion transport mechanisms through the transcellular pathway are unknown. To study this issue; we applied cable analysis and conventional microelectrode technique to the hamster LDLu perfused in vitro. The transmural voltage (VT) was not different from zero, and transmural resistance (RT) was very low, 18.3 +/- 2.0 omega.cm2 (n = 12). The basolateral membrane voltage was -80 +/- 2 mV (n = 55), and fractional apical membrane resistance was 0.92 +/- 0.23 (n = 5). Ouabain (0.1 mM) in the bath decreased basolateral membrane voltage (VB) by 23 +/- 3 mV (n = 6, P less than 0.001). Increase in K+ concentration in bath and in lumen from 5 to 50 mM decreased VB by 39 +/- 2 (n = 7, P less than 0.01) and apical membrane voltage (VA) by 10 +/- 1 mV (n = 7, P less than 0.001), respectively. Addition of 2 mM Ba2+ to bath and to lumen decreased VB by -47 +/- 2 (n = 11, P less than 0.001) and decreased VA by 8 +/- 1 mV, respectively. Reduction of HCO3- in bath from 25 to 2.5 mM decreased VB by 4 +/- 1 mV (n = 7, P less than 0.005). Reduction of bath Cl- did not cause any rapid deflection of VB. No appreciable Na+ conductance was detected in the apical membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

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