Effects of soma isolation on outward currents measured under voltage clamp in spiny lobster stomatogastric motor neurons

1993 ◽  
Vol 69 (6) ◽  
pp. 2056-2071 ◽  
Author(s):  
D. K. Hartline ◽  
D. V. Gassie ◽  
B. R. Jones
Keyword(s):  
Voltage Clamp ◽  
Motor Neurons ◽  
Time Course ◽  
Spiny Lobster ◽  
Outward Current ◽  
Specific Conductance ◽  
Activation Threshold ◽  
Intact Cells ◽  
Mean Values ◽  
Outward Currents

1. Outward currents in identified cell types from the pyloric system of the stomatogastric ganglion (STG) of the spiny lobster, Panulirus marginatus, were studied under two-microelectrode voltage clamp. A comparison was made between data from intact cells and somata isolated by ligation of the primary neurite of these monopolar neurons. 2. Despite the elimination of current contributions from the extensive arborizations of STG neurons, few significant differences were found in the mean values of parameters for outward currents between populations of isolated somata and intact cells of a given type. Measurements that showed little difference included magnitude and activation threshold of a calcium-dependent outward current (IJ) and magnitude, activation threshold, voltage dependence, and inactivation time course of A current (IA). Although previous work has suggested that IJ might reside predominantly in the soma, IA is known to be distributed in poorly space-clamped neurite processes. The absence of obvious effects of isolation was thus unexpected. 3. To better understand the mechanisms involved, we used compartmental models derived from reconstructed neurons to simulate the effects of isolation. It was concluded that, for the particular conditions present in stomatogastric neurons, with a large, uniformly distributed outward current conductance activated, even though neurites and axon remain attached, most measured current flows through well-clamped soma membrane. 4. Factors contributing to this result included the outward sign of the current, the large specific conductance activated in these neurons (among the larger reported in somata), and the presence of only a single major process leaving the soma. The potential for serious errors in voltage-clamp measurements from intact cells remains if these conditions are not met.

Voltage Clamp Studies on Insect Skeletal Muscle: II. The Outward Currents

1981 ◽  
Vol 92 (1) ◽  
pp. 13-22
Author(s):  
DAISUKE YAMAMOTO ◽  
HIROSHI WASHIO
Keyword(s):  
Voltage Clamp ◽  
Time Course ◽  
Reversal Potential ◽  
Outward Current ◽  
Transient Outward Current ◽  
Muscle Fibres ◽  
Activation Process ◽  
Outward Currents ◽  
K Concentration ◽  
Insect Skeletal Muscle

Two components of outward currents were investigated under voltage clamp conditions in Tenebrio muscle fibres. The instantaneous current-voltage relation for the transient outward current showed outward rectification. The tail currents for the delayed outward currents were made up of either one or two exponential components. The activation process for the delayed current was analysed using positive tails that decayed with a simple exponential time course. The delayed current was half-activated at about + 35 mV. Two rate constants for activation are both monotonic functions of membrane potential. The reversal potential for the delayed current was only partially dependent on the external K-concentration. The role of the two outward currents in the production of the action potential was discussed.

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

1989 ◽  
Vol 61 (2) ◽  
pp. 233-244 ◽  
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)

Ca-Induced K+-Outward Current in Paramecium Tetraurelia

1980 ◽  
Vol 88 (1) ◽  
pp. 293-304 ◽  
Author(s):  
YOUKO SATOW ◽  
CHING KUNG
Keyword(s):  
Steady State ◽  
Voltage Clamp ◽  
Outward Current ◽  
Paramecium Tetraurelia ◽  
Ca Channels ◽  
Outward Currents ◽  
Ciliary Reversal ◽  
Voltage Dependent ◽  
K Current ◽  
K Outward Current

Late K-outward currents upon membrane depolarization were recorded in Paramecium tetraurelia under a voltage clamp. A Ca-induced K-outward component is demonstrated by subtracting the value of the outward current in a pawn A mutant lacking functional Ca-channels (pwA500). The Ca-induced K-outward current activates slowly, reaching a peak after 100 to 1000 ms. The current then remains steady or reaches the steady state after a decline of several seconds. EGTA2- injection experiments show that the Ca-induced K-outward current is dependent on the internal Ca2+ concentration. The current is shown to depend on the voltage-dependent Ca conductance, by study of the leaky pawn A mutant (pwA132), which has a lowered Ca conductance as well as a lowered Ca-induced K-current. The Ca-induced GK is thus indirectly dependent on the voltage. The maximal GK is about 40 nmho/cell at + 7 mV in 4 mM-K+. The Ca-induced K current is sustained throughout the prolonged depolarization and the prolonged ciliary reversal.

scholarly journals Depletion and accumulation of potassium in the extracellular clefts of cardiac Purkinje fibers during voltage clamp hyperpolarization and depolarization: experiments in sodium-free bathing media.

1977 ◽  
Vol 70 (2) ◽  
pp. 149-169 ◽  
Author(s):  
C M Baumgarten ◽  
G Isenberg ◽  
T F McDonald ◽  
R E Ten Eick
Keyword(s):  
Membrane Potential ◽  
Voltage Clamp ◽  
Driving Force ◽  
Time Course ◽  
Potassium Concentration ◽  
Outward Current ◽  
Voltage Step ◽  
Purkinje Fibers ◽  
Cardiac Purkinje Fibers ◽  
Voltage Dependent

Voltage clamp hyperpolarization and depolarization result in currents consistent with depletion and accumulation of potassium in the extracellular clefts o cardiac Purkinje fibers exposed to sodium-free solutions. Upon hyperpolarization, an inward current that decreased with time (id) was observed. The time course of tail currents could not be explained by a conductance exhibiting voltage-dependent kinetics. The effect of exposure to cesium, changes in bathing media potassium concentration and osmolarity, and the behavior of membrane potential after hyperpolarizing pulses are all consistent with depletion of potassium upon hyperpolarization. A declining outward current was observed upon depolarization. Increasing the bathing media potassium concentration reduced the magnitude of this current. After voltage clamp depolarizations, membrane potential transiently became more positive. These findings suggest that accumulation of potassium occurs upon depolarization. The results indicate that changes in ionic driving force may be easily and rapidly induced. Consequently, conclusions based on the assumption that driving force remains constant during the course of a voltage step may be in error.

scholarly journals Differences in calcium homeostasis between retinal rod and cone photoreceptors revealed by the effects of voltage on the cGMP-gated conductance in intact cells.

1994 ◽  
Vol 104 (5) ◽  
pp. 909-940 ◽  
Author(s):  
J L Miller ◽  
J I Korenbrot
Keyword(s):  
Guanylyl Cyclase ◽  
Outer Segment ◽  
Time Course ◽  
Reversal Potential ◽  
Outward Current ◽  
Membrane Depolarization ◽  
Maximum Enhancement ◽  
Intact Cells ◽  
Retinal Rod ◽  
Voltage Dependent

We measured currents under voltage clamp in intact retinal rod photoreceptors with tight seal electrodes in the perforated patch mode. In the dark, membrane depolarization to voltages > or = +20 mV activates a time- and voltage-dependent outward current in the outer segment. This dark voltage-activated current (DVAC) increases in amplitude with a sigmoidal time course that is voltage dependent. DVAC reaches its maximum enhancement of approximately 30% in 4-6 s at +60 mV. DVAC is entirely suppressed by light and its current-voltage curve and reversal potential are the same as those of the photocurrent. Therefore, DVAC arises from the opening in darkness of the cGMP-gated channels of the outer segment. DVAC is blocked by BAPTA loaded into the cell's cytoplasm and is enhanced by lowering extracellular Ca2+ concentration. Because the cGMP-gated channels are not directly gated by voltage and because BAPTA blocks DVAC, we suggest this signal arises from a voltage-dependent decrease in cytoplasmic Ca2+ concentration that, in turn, activates guanylyl cyclase and causes cGMP synthesis. In rods loaded with high cytoplasmic Na+, membrane depolarization in darkness to voltages > or = +20 mV inactivates the outward current in the outer segment with an exponential time course. We call this DVIC (dark, voltage-inactivated current). DVIC reflects voltage-dependent closing of the cGMP-gated channel in the dark. DVIC, too, is blocked by cytoplasmic BAPTA, and it arises from a voltage-dependent rise in cytoplasmic Ca2+ in darkness, which occurs only if cytoplasmic Na is high. We develop a quantitative model to calculate the rate and extent of the voltage-dependent change in cytoplasmic Ca2+ concentration in a normal rod. We assume that this concentration is controlled by the balance between Ca2+ influx through the cGMP-gated channels and its efflux through a Na+/Ca2+, K+ exchanger. Lowered cytoplasmic Ca2+ is linked to guanylyl cyclase activation with characteristics determined from biochemical studies. The model considers the cytoplasmic buffering of both Ca2+ and cGMP. Simulated data generated by the model fit well DVAC measured in rods and also DVAC previously measured in cones. DVAC in cones is larger in magnitude and faster in time course than that in rods. The successful fit of DVAC by the model leads us to suggest that the activity and Ca2+ dependence of the enzymes of transduction are not different in rods and cones, but the quantitative features of Ca2+ homeostasis in the outer segment of the two receptor types differ profoundly.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca(2+)-dependent outward currents in myocytes from epicardial border zone of 5-day infarcted canine heart

1997 ◽  
Vol 273 (3) ◽  
pp. H1386-H1394 ◽  
Author(s):  
R. Aggarwal ◽  
J. Pu ◽  
P. A. Boyden
Keyword(s):  
Action Potentials ◽  
Time Course ◽  
Outward Current ◽  
Border Zone ◽  
Disulfonic Acid ◽  
Transient Outward Current ◽  
Canine Heart ◽  
Whole Cell Patch Clamp ◽  
Outward Currents ◽  
Transmembrane Action Potentials

Myocytes from the epicardial border zone (EBZ) of the 5-day infarcted canine heart (IZ) have abnormal transmembrane action potentials, reduced L-type Ca2+ currents (ICa,L) and altered intracellular Ca2+ (Cai) transients compared with those of normal epicardial myocytes (NZ). We hypothesized that altered Cai cycling might be reflected in differences in Cai-dependent outward currents (Ito2). We recorded Ito2 in NZ and IZ using whole cell patch-clamp techniques. Ito2 was defined as the amplitude of the 4-aminopyridine-resistant transient outward current that was blocked by 200 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or DIDS+ ryanodine (2 microM). Ito2 were present in both NZ and IZ, but peak density was significantly reduced in IZ, particularly at positive plateau voltages. Time course of decay of Ito2 was biexponential and similar in NZ and IZ. A given peak ICa,L was usually associated with a smaller peak Ito2 in IZ. These differences were exaggerated when Ito2 and Cai transients were determined in rapidly paced cells. In summary, myocytes surviving in the EBZ of the infarcted heart have Ito2, yet they are reduced in density and can vary, particularly at fast pacing rates.

Properties of a persistent inward current in normal and TEA-injected motoneurons

1980 ◽  
Vol 43 (6) ◽  
pp. 1700-1724 ◽  
Author(s):  
P. C. Schwindt ◽  
W. E. Crill
Keyword(s):  
Voltage Clamp ◽  
Outward Current ◽  
Membrane Currents ◽  
Negative Slope ◽  
Activation Threshold ◽  
Inward Current ◽  
Plateau Potential ◽  
Current Voltage ◽  
Persistent Inward Current ◽  
Fast Transient

1. Membrane currents of normal and TEA-injected cat lumbar motoneurons were investigated using the technique of somatic voltage clamp. 2. The current-voltage (I-V) relation of healthy motoneurons contains a region of negative slope conductance caused by a persistent inward current component (Ii). In the most striking examples, Ii is net inward at some potentials between 10 and 30 mV positive to resting potential. 3. Near its activation threshold (greater than or equal to 10 mV positive to rest), Ii does not decrement during prolonged voltage steps and, in most cells, activates very slowly. Ii amplitude increases and time to peak Ii decreases with further small increments of depolarization, and Ii decrements during sustained voltage steps. Maximum Ii amplitude occurs 20--30 mV positive to rest in most cells. Ii is not visible at sufficiently large depolarizations. 4. Ii appears to be mixed with potassium current components at nearly every potential where it is visible. These include a slow outward current first activated near Ii activation threshold, a fast outward current additonally activated at larger depolarizing potentials, and a fast, transient outward current that obscures the true onset of Ii at nearly every potential. 5. Ii is not carried by sodium entering via the fast, transient channels and is present after pharmacological blockage of sodium currents. It is proposed that Ii is predominantly carried by calcium ions. 6. The presence of inward tail currents after repolarization from potentials that activate a steady outward current suggest that Ii remains present but hidden at large depolarizations. Ii inactivation was further investigated in TEA-injected motoneurons since Ii and the tail currents are more prominent in these cells. 7. Conventional recordings from TEA-injected motoneurons suggest that a prolonged, postspike plateau potential is maintained by a persistent inward current. Voltage-clamp data can account for the principal features of the plateau potential. 8. Voltage-clamp results in TEA-injected motoneurons suggest that Ii is subject to little or no inactivation at potentials less than or equal to 30 mV positive to rest and to partial inactivation, at most, at higher potentials during steps lasting less than or equal to 100 ms. The apparent decay of Ii during sustained depolarization is caused by the development of a larger outward current. 9. Ii is similar in several ways to a persistent calcium current observed in some molluscan neurons. Theoretical and experimental results suggest that Ii is generated predominantly in a local region under voltage control and that the observed membrane currents govern somatic membrane potential and cell behavior.

Slowly inactivating outward currents in a cuticular mechanoreceptor neuron of the cockroach (Periplaneta americana)

1995 ◽  
Vol 74 (3) ◽  
pp. 1200-1211 ◽  
Author(s):  
P. H. Torkkeli ◽  
A. S. French
Keyword(s):  
Voltage Clamp ◽  
Time Constant ◽  
Action Potentials ◽  
Periplaneta Americana ◽  
Outward Current ◽  
Equilibrium Potential ◽  
Channel Blockers ◽  
Outward Currents ◽  
Frequency Of Action Potentials ◽  
K Outward Current

1. Although rapid adaptation is a widespread feature of sensory receptors, its ionic basis has not been clearly established in any touch receptor, because their small sizes have severely restricted the range of experiments tat can be performed. In the cockroach tactile spine, intracellular voltage-clamp recordings are now possible. 2. The basic electrophysiological properties of the cockroach femoral tactile spine neuron were studied using discontinuous (switching) single-electrode current- and voltage-clamp recordings. A slowly inactivating voltage-sensitive K+ outward current was detected after the major inward currents were blocked with tetrodotoxin. 3. The total outward current activated in < 1 ms at voltages above 0 mV. At moderate depolarizations it did not inactivate, but at higher depolarizations an inactivation time constant of approximately 260 ms was measured. Some recordings also revealed an additional, slower inactivation time constant of approximately 2.5 s. 4. More than half of the voltage-sensitive K+ outward current could be blocked with the Ca2+ channel blockers Co2+ and Cd2+. Tetraethylammonium chloride (TEA) also reduced the amplitude of the outward current to about half of its original amplitude. The actions of both blockers were reversible and probably reflect overlapping blockades of two separate outward currents. 5. The reversal potentials of the currents that remained after block with Co2+ (-91.7 mV) or TEA (-86.8 mV) were both near the K+ equilibrium potential expected for the tactile spine neuron. The voltage dependencies of activation of the Co(2+)- and TEA-resistant currents were both well fitted by Boltzmann distributions, giving values of half maximal activation (V50) equal to -34.5 mV for the Co(2+)-resistant current and -51.3 mV for the TEA-resistant current. 6. Current-clamp recordings revealed that the TEA-sensitive K+ current was the major component of action potential repolarization but that it did not effect the frequency of action potentials evoked by steady depolarization. On the other hand, blockers of Ca(2+)-sensitive K+ currents (Cd2+, Co2+, or charybdotoxin) reduced adaptation and increased the frequency of action potentials significantly but did not effect the duration or amplitude of individual action potentials.

K+ Currents Generated by NMDA Receptor Activation in Rat Hippocampal Pyramidal Neurons

2002 ◽  
Vol 87 (6) ◽  
pp. 2983-2989 ◽  
Author(s):  
Mala M. Shah ◽  
Dennis G. Haylett
Keyword(s):  
Nmda Receptor ◽  
Time Course ◽  
Pyramidal Neurons ◽  
Outward Current ◽  
Receptor Activation ◽  
Bk Channel ◽  
Hippocampal Pyramidal Neurons ◽  
Sk Channel ◽  
Outward Currents ◽  
Nmda Receptor Activation

Long lasting outward currents mediated by Ca2+-activated K+ channels can be induced by Ca2+ influx through N-methyl-d-aspartate (NMDA)-receptor channels in voltage-clamped hippocampal pyramidal neurons. Using specific inhibitors, we have attempted to identify the channels that underlie these outward currents. At a holding potential of −50 mV, applications of 1 mM NMDA to the soma of cultured hippocampal pyramidal neurons induced the expected inward currents. In 44% of cells tested, these were followed by outward currents (average amplitude 60 ± 7 pA) that peaked 2.5 s after the initiation of the inward NMDA currents and decayed with a time constant of 1.4 s. In 43% of those cells exhibiting an outward current, SK channel inhibitors, UCL 1848 (100 nM) and apamin (100 nM) abolished the outward current. In the remainder of the cells, the outward currents were either insensitive or only partly inhibited (44 ± 4%) by 100 nM UCL 1848. In these cells, the outward currents were reduced by the slow afterhyperpolarization (sAHP) inhibitors, muscarine (3 μM; 43 ± 9%), UCL 1880 (3 μM; 34 ± 10%), and UCL 2027 (3 μM; 57 ± 6%). Neither the BK channel inhibitor, charybdotoxin (100 nM), nor the Na+/K+ ATPase inhibitor, ouabain (100 μM), reduced these outward currents. Irrespective of the pharmacology, the time course of the outward current did not differ. Interestingly, no correlation was observed between the presence of a slow apamin-insensitive afterhyperpolarization and an outward current insensitive to SK channel blockers following NMDA-receptor activation. It is concluded that an NMDA-mediated rise in [Ca2+]i can result in the activation of apamin-sensitive SK channels and of the channels that underlie the sAHP. The activation of these channels may, however, depend on their location relative to NMDA receptors as well as on the spatial Ca2+ buffering within individual neurons.

Effects of manganese chloride on the outward currents in frog atrial trabeculae

1985 ◽  
Vol 63 (9) ◽  
pp. 1065-1069 ◽  
Author(s):  
Julio L. Alvarez ◽  
Miguel Garcia ◽  
Francisco R. Dorticós ◽  
Jesús A. Morlans
Keyword(s):  
Voltage Clamp ◽  
Outward Current ◽  
Manganese Chloride ◽  
Exchange Current ◽  
Time Dependent ◽  
Slow Inward Current ◽  
Background Current ◽  
Inward Current ◽  
Atrial Muscle ◽  
Outward Currents

The effects of MnCl2 on outward currents in frog atrial muscle were investigated under voltage-clamp conditions. MnCl2 (3 mmol/L), which completely abolished the slow inward current, produced a decrease in the outward background current (Ib) at potentials positive to −50 mV. The delayed outward current (Ix, time dependent) was not altered by Mn. "Isochronic activation curves" for Ix and decay of current tails at −40 mV remained unaffected after Mn. Effects on Ib probably reflect a decrease in [Formula: see text] related to the decrease in Ca influx as well as a reduction in the Na–Ca exchange current.

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