Nicotinic and muscarinic activation of motoneurons in the crayfish locomotor network

1994 ◽  
Vol 72 (4) ◽  
pp. 1622-1633 ◽  
Author(s):  
D. Cattaert ◽  
A. Araque ◽  
W. Buno ◽  
F. Clarac
Keyword(s):  
Voltage Clamp ◽  
Injection Pressure ◽  
Rhythmic Activity ◽  
Membrane Resistance ◽  
Membrane Depolarization ◽  
Muscarinic Agonist ◽  
Electrode Voltage ◽  
Voltage Dependent ◽  
Inward Currents

1. We investigated the effects of acetylcholine (Ach) on identified motoneurons (MNs) using an in vitro preparation of the crayfish thoracic nervous system. Discontinuous current-clamp and single electrode voltage-clamp recordings from 50 MNs were performed along with micropipette pressure ejection of Ach (or agonists) close to the recording electrode. 2. Localized ejections of relatively large volumes (500–2,500 pl) of Ach (10(-2) M) or of the muscarinic agonist oxotremorine (Oxo, 10(-2)M) onto the MN neuropile region, usually (90% of the cases) induced a slow, alternating rhythmic activity in antagonistic MNs. In other cases (4 experiments), with similar deliveries of Ach or Oxo, MNs developed the ability to fire rhythmically but only when depolarized by sustained current injection. Pressure ejections of smaller volumes (50–200 pl) of Ach (10(-2)M) close to the recorded MN could give rise to a fast (1–2 s) large amplitude (< or = 20 mV) membrane depolarization (12%), a long-lasting (10 s to several minutes) and small (2–5 mV) depolarization (14%), and a combination of the two (74%). These responses appeared to involve different regions of the neurite because they changed when the drug-ejection pipette was displaced in the neuropile. Moreover, fast and long-lasting depolarizing components resulted from a direct effect of Ach onto the MNs because they persisted under tetrodotoxin (TTX, 10(-6)M) and cobalt (Co2+, 5 x 10(-3) M) superfusion. 3. Whereas the membrane resistance decreased during the fast Ach-induced depolarization, it increased during the long-lasting depolarization. The increase in membrane resistance was more pronounced at depolarized potentials more than -55 mV and involve a reduction in K+ conductance. 4. Superfusion with nicotinic and muscarinic antagonists revealed that the fast Ach-induced depolarization involved nicotinic receptors, muscarinic receptors, or both, whereas the slow depolarization was exclusively muscarinic. 5. The Ach-evoked inward currents were studied under voltage clamp. The fast nicotinic component (Inic) increased with hyperpolarizing holding potentials and decreased with depolarizing potentials, reversing at between 10 and 30 mV. The fast muscarinic current (Ifmus) displayed similar characteristics and reversed at about -10 mV. Whereas both fast components were voltage independent, the long-lasting muscarinic component (Ismus) was voltage dependent. The response grew with membrane depolarization, but when the holding potential was hyperpolarized below resting level, the response declined to disappear at about -60 mV and beyond.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Membrane Dysfunction Induced by In Vitro Ischemia in Rat Hippocampal CA1 Pyramidal Neurons

1999 ◽  
Vol 81 (4) ◽  
pp. 1872-1880 ◽  
Author(s):  
E. Tanaka ◽  
S. Yamamoto ◽  
H. Inokuchi ◽  
T. Isagai ◽  
H. Higashi
Keyword(s):  
Membrane Potential ◽  
Voltage Clamp ◽  
Pyramidal Neurons ◽  
Membrane Surface ◽  
Bathing Medium ◽  
Ca1 Pyramidal Neurons ◽  
In Vitro Ischemia ◽  
Hippocampal Ca1 ◽  
Electrode Voltage

Membrane dysfunction induced by in vitro ischemia in rat hippocampal CA1 pyramidal neurons. Intracellular and single-electrode voltage-clamp recordings were made to investigate the process of membrane dysfunction induced by superfusion with oxygen and glucose-deprived (ischemia-simulating) medium in hippocampal CA1 pyramidal neurons of rat tissue slices. To assess correlation between potential change and membrane dysfunction, the recorded neurons were stained intracellularly with biocytin. A rapid depolarization was produced ∼6 min after starting superfusion with ischemia-simulating medium. When oxygen and glucose were reintroduced to the bathing medium immediately after generating the rapid depolarization, the membrane did not repolarize but depolarized further, the potential reaching 0 mV ∼5 min after the reintroduction. In single-electrode voltage-clamp recording, a corresponding rapid inward current was observed when the membrane potential was held at −70 mV. After the reintroduction of oxygen and glucose, the current induced by ischemia-simulating medium partially returned to preexposure levels. These results suggest that the membrane depolarization is involved with the membrane dysfunction. The morphological aspects of biocytin-stained neurons during ischemic exposure were not significantly different from control neurons before the rapid depolarization. On the other hand, small blebs were observed on the surface of the neuron within 0.5 min of generating the rapid depolarization, and blebs increased in size after 1 min. After 3 min, neurons became larger and swollen. The long and transverse axes and area of the cross-sectional cell body were increased significantly 1 and 3 min after the rapid depolarization. When Ca2+-free (0 mM) with Co2+ (2.5 mM)-containing medium including oxygen and glucose was applied within 1 min after the rapid depolarization, the membrane potential was restored completely to the preexposure level in the majority of neurons. In these neurons, the long axis was lengthened without any blebs being apparent on the membrane surface. These results suggest that the membrane dysfunction induced by in vitro ischemia may be due to a Ca2+-dependent process that commences ∼1.5 min after and is completed 3 min after the onset of the rapid depolarization. Because small blebs occurred immediately after the rapid depolarization and large blebs appeared 1.5–3 min after, it is likely that the transformation from small to large blebs may result in the observed irreversible membrane dysfunction.

Voltage-clamp analysis of a Ca2+- and voltage-dependent chloride conductance in cultured mouse spinal neurons

1986 ◽  
Vol 55 (6) ◽  
pp. 1115-1135 ◽  
Author(s):  
D. G. Owen ◽  
M. Segal ◽  
J. L. Barker
Keyword(s):  
Voltage Clamp ◽  
Reversal Potential ◽  
Gamma Aminobutyric Acid ◽  
Spinal Neurons ◽  
Exponential Process ◽  
Voltage Clamp Analysis ◽  
Voltage Dependent ◽  
Inward Currents ◽  
Ca 50 ◽  
O 10

Current and voltage-clamp recordings were made at room temperature from cultured mouse spinal neurons using conventional two-electrode voltage-clamp techniques and electrodes filled with either 3 M KCl, 3 M CsCl, or 3 M Cs2SO4. In the presence of tetraethylammonium and tetrodotoxin, “fast” (rapidly rising and falling) action potentials (FAP) of variable duration were recorded in most neurons. “Slow” (slowly rising and falling) depolarizing potentials (SDP) occurred in 23% of the cells, when using KCl-filled electrodes, and in 82% of the cells with CsCl-filled electrodes. The SDP was frequently preceded by an FAP, although in some cells activation of the SDP occurred before the FAP threshold was reached and in a graded fashion. Both the FAP and SDP were abolished by Cd2+ and other Ca2+ antagonists. In cells exhibiting SDPs, voltage-clamp analysis revealed a sustained (noninactivating) inward current (Isin) during depolarizing steps to potentials more positive than -45 mV. Repolarizing steps resulted in slowly decaying inward tail currents (Itail). Both Isin and Itail were abolished in solutions nominally free of Cao2+, or containing Ca2+-channel antagonists. Bao2+ did not support Isin. The data indicated a U-shaped activation curve for Isin, peaking at about -10 mV. Activation of Isin occurred exponentially with a time constant of approximately 140 ms at -23 mV, becoming faster at more depolarized potentials (ca. 50 ms at -2 mV). Deactivation was slow, giving rise to tail currents lasting seconds. In some cases deactivation could be described by a single exponential process, although frequently the kinetics were more complex. Deactivation was faster at hyperpolarized potentials and sensitive to extracellular ([Ca2+]o), duration of activating voltage steps, and the degree of activation of Isin. Using CsCl-filled electrodes, the reversal potential (Erev) for Isin was -1.7 mV (SEM 3.5 mV, n = 20). Erev always corresponded to the reversal potential for gamma-aminobutyric acid-evoked currents in the same cell. In experiments in which Cs2SO4-filled electrodes were used, Erev was estimated to be -44 mV (SEM 2.3 mV, n = 9). Neither complete substitution of Nao+ with choline ions nor elevation of [K+]o 10-fold significantly affected the estimated Erev. However, substitution of Cl0- with isethionate or methanesulphonate increased the amplitude of inward currents (recorded with CsCl-filled electrodes) and shifted Erev to more depolarized potentials. The results indicate that Cl- are the primary charge carriers for this current and that Cai2+ is required for its activation, leading us to identify it as ICl(Ca).(ABSTRACT TRUNCATED AT 400 WORDS)

Diverse current and voltage responses to baclofen in an identified molluscan photoreceptor

1995 ◽  
Vol 74 (2) ◽  
pp. 506-518 ◽  
Author(s):  
L. D. Matzel ◽  
I. A. Muzzio ◽  
R. F. Rogers
Keyword(s):  
Voltage Clamp ◽  
Action Potentials ◽  
Gabab Receptor ◽  
Outward Current ◽  
Gabab Receptors ◽  
Equilibrium Potential ◽  
K Channel ◽  
Electrode Voltage ◽  
Voltage Dependent ◽  
K Current

1. gamma-Aminobuturic acid-B (GABAB) receptors play a role in the mediation of slow inhibitory postsynaptic potentials in mammalian as well as some nonmammalian species. In identified photoreceptors from the marine mollusc Hermissenda, recent evidence has suggested that GABA, as well as the GABAB receptor agonist baclofen, might simultaneously modulate multiple conductances on the postsynaptic membrane. Here, using intracellular current-clamp and single-electrode voltage-clamp techniques, we have characterized responses to baclofen in the B photoreceptors of the Hermissenda eye. 2. Microapplication of baclofen (12.5–62.5 microM) to the terminal branches of the B photoreceptors induced a slow, concentration-dependent hyperpolarization (approximately 3–8 mV) that was accompanied by a cessation of spontaneous action potentials and a positive shift in firing threshold. Both the hyperpolarization and the shift in spike threshold in response to baclofen were attenuated largely by the K+ channel blocker tetraethylammonium chloride (TEA; 50 mM). 3. Bath application of baclofen (100 microM) decreased the amplitude, duration, and the afterhyperpolarization (AHP) of evoked action potentials. Although baclofen's effect on spike duration and amplitude persisted in the absence of extracellular Ca2+, the reduction of the AHP by baclofen was eliminated, suggesting that multiple conductances mediated the baclofen-induced modification of the action potential. 4. Using a single-electrode voltage-clamp technique, microapplication of baclofen to the terminal branches of the B photoreceptor produced a slow, net outward current (< 0.5 nA) that reversed near the equilibrium potential for K+ and shifted to more positive potentials when extracellular K+ was increased, in approximate agreement with the Nernst equation for K+. 5. Baclofen induced an increase in amplitude of the nonvoltage dependent leak conductance (IL), and the increase was blocked by TEA. The baclofen-induced increase of IL was accompanied by an increase in amplitude and a negative shift in the voltage dependence of a slow, steeply voltage-dependent K+ current (IK), which displays selective sensitivity to TEA but does not normally contribute to leak conductance. The amplitude and steady-state inactivation of a fast, transient K+ current, as well as the amplitude of an inwardly rectifying K+ current were unaffected by baclofen. 6. Both the rate of activation as well as the amplitude of a voltage-dependent Ca2+ current (ICa) were reduced by baclofen. The reduction of ICa resulted in a concomitant suppression of a Ca(2+)-dependent K+ current (IK-Ca) that was sufficient to account for the reduction of the AHP after evoked action potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

Rhythmic patterns evoked in locust leg motor neurons by the muscarinic agonist pilocarpine

1993 ◽  
Vol 69 (5) ◽  
pp. 1583-1595 ◽  
Author(s):  
S. Ryckebusch ◽  
G. Laurent
Keyword(s):  
Motor Neurons ◽  
Rhythmic Activity ◽  
Cycle Period ◽  
Muscarinic Agonist ◽  
Strongly Coupled ◽  
Weakly Coupled ◽  
Metathoracic Ganglion ◽  
Common Drive ◽  
Muscarinic Cholinergic

1. When an isolated metathoracic ganglion of the locust was superfused with the muscarinic cholinergic agonist pilocarpine, rhythmic activity was induced in leg motor neurons. The frequency of this induced rhythm increased approximately linearly from 0 to 0.2 Hz with concentrations of pilocarpine from 10(-5) to 10(-4) M. Rhythmic activity evoked by pilocarpine could be completely and reversibly blocked by 3 x 10(-5) M atropine, but was unaffected by 10(-4) M d-tubocurarine. 2. For each hemiganglion, the observed rhythm was characterized by two main phases: a levator phase, during which the anterior coxal rotator, levators of the trochanter, flexors of the tibia, and common inhibitory motor neurons were active; and a depressor phase, during which depressors of the trochanter, extensors of the tibia, and depressors of the tarsus were active. Activity in depressors of the trochanter followed the activity of the levators of the trochanter with a short, constant interburst latency. Activity in the levator of the tarsus spanned both phases. 3. The levator phase was short compared with the period (0.5-2 s, or 10-20% of the period) and did not depend on the period. The interval between the end of a levator burst and the beginning of the following one thus increased with cycle period. The depressor phase was more variable, and was usually shorter than the interval between successive levator bursts. 4. Motor neurons in a same pool often received common discrete synaptic potentials (e.g., levators of trochanter or extensors of tibia), suggesting common drive during the rhythm. Coactive motor neurons on opposite sides (such as left trochanteral depressors and right trochanteral levators), however, did not share obvious common postsynaptic potentials. Depolarization of a pool of motor neurons during its phase of activity was generally accompanied by hyperpolarization of its antagonist(s) on the same side. 5. Rhythmic activity was generally evoked in both hemiganglia of the metathoracic ganglion, but the intrinsic frequencies of the rhythms on the left and right were usually different. The activity of the levators of the trochanter on one side, however, was strongly coupled to that of the depressors of the trochanter on the other side. 6. The locomotory rhythm was weakly coupled to the ventilatory rhythm such that trochanteral levator activity on either side never occurred during the phase of spiracle opener activity corresponding to inspiration. 7. The rhythmic activity observed in vitro bears many similarities to patterns of neural and myographic activity recorded during walking. The similarities and differences are discussed.

Neuromedin U Depolarizes Rat Hypothalamic Paraventricular Nucleus Neurons In Vitro by Enhancing IH Channel Activity

2003 ◽  
Vol 90 (2) ◽  
pp. 843-850 ◽  
Author(s):  
De-Lai Qiu ◽  
Chun-Ping Chu ◽  
Tetsuro Shirasaka ◽  
Takashi Nabekura ◽  
Takato Kunitake ◽  
...  
Keyword(s):  
Voltage Clamp ◽  
Paraventricular Nucleus ◽  
Dependent Manner ◽  
Inward Current ◽  
Whole Cell Patch Clamp ◽  
Voltage Dependent ◽  
Ih Channel ◽  
Parvocellular Neurons ◽  
Zd 7288

The effect of neuromedin U (NMU) on rat paraventricular nucleus (PVN) neurons was examined using whole cell patch-clamp recordings. Under current-clamp, 31% of PVN parvocellular neurons ( n = 243) were depolarized by 100 nM NMU, but magnocellular neurons were not affected. NMU (10 nM to 1 μM) resulted in increased basal firing rate and depolarization in a dose-dependent manner with an EC50 of 70 nM. NMU-induced depolarization was unaffected by co-perfusion with 0.5 μM TTX + 10 μM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) + 10 μM bicuculline. Extracellular application of 70 μM ZD 7288 completely inhibited NMU-induced depolarization. Under voltage-clamp, 1 μM NMU produced negligible inward current but did increase the hyperpolarization-activated current ( IH) at step potentials less than –80 mV. The effects of NMU on IH were voltage-dependent, and NMU shifted the IH conductance-voltage relationship ( V1/2) by about 10.8 mV and enhanced IH kinetics without changing the slope constant ( k). Extracellular application of 70 μM ZD 7288 or 3 mM Cs+ blocked IH and the effects of NMU in voltage-clamp. These results suggest that NMU selectively depolarizes the subpopulation of PVN parvocellular neurons via enhancement of the hyperpolarization-activated inward current.

Cable properties of layer V neurons from cat sensorimotor cortex in vitro

1984 ◽  
Vol 52 (2) ◽  
pp. 278-289 ◽  
Author(s):  
C. E. Stafstrom ◽  
P. C. Schwindt ◽  
W. E. Crill
Keyword(s):  
Voltage Clamp ◽  
Time Constant ◽  
Neuron Model ◽  
Membrane Resistance ◽  
Resting Potential ◽  
Voltage Range ◽  
Cable Properties ◽  
Layer V ◽  
Specific Membrane

The passive cable properties of neurons from layer V of cat neocortex were studied in an in vitro slice preparation using current-clamp techniques and a single-microelectrode voltage clamp. Neurons were examined in the presence and absence of several agents that block time- and voltage-dependent conductances. The charging response to an injected current pulse was well fitted by a single exponential in 12 of 17 cells examined. By itself, this result would suggest that most of the neurons are isopotential. However, the existence of a nonisopotential region was demonstrated in all neurons examined using two alternative, independent methods: application of voltage-clamp steps and current impulses. The decay of the capacitive charging transient following a voltage-clamp step reflects charge redistribution solely in the nonisopotential region and had a mean time constant about 17% of the membrane time constant, tau m. The voltage decay following a current impulse was always fitted by (at least) two exponentials, the shorter of which was about 9% of tau m. These results suggest that a nonisopotential region exists but is electrotonically short, of relatively low-input conductance, or both, independent of a particular neuron model. Adopting Rall's (23, 24) idealized neuron model (isopotential compartment attached to a finite-length uniform cable) resulted in a mean value for the equivalent electrotonic length (L) of the nonisopotential compartment of 0.72 space constants from voltage-clamp data and 1.21 space constants from impulse-response data. A dendrite-to-soma conductance ratio (p) of 2-4 was obtained from either procedure. There were no significant differences in the cable parameters between normal cells and those where conductance-blocking agents were present. A specific membrane resistance (Rm) ranging from 2,300 to 11,700 omega X cm2 was estimated by assuming values of specific membrane capacitance reported in the literature. We conclude that large layer V neocortical neurons in vitro are electrotonically compact in the voltage range near resting potential and in the absence of significant tonic synaptic input. In this respect, their electrotonic cable properties resemble those of other mammalian neurons in vitro.

Single-electrode voltage-clamp analysis of the N-methyl-D-aspartate component of synaptic responses in neocortical slices from children with intractable epilepsy

1992 ◽  
Vol 67 (1) ◽  
pp. 84-93 ◽  
Author(s):  
J. P. Wuarin ◽  
W. J. Peacock ◽  
F. E. Dudek
Keyword(s):  
Electrical Stimulation ◽  
Membrane Potential ◽  
Nmda Receptor ◽  
Voltage Clamp ◽  
Intractable Epilepsy ◽  
Perfusion Solution ◽  
Electrode Voltage ◽  
Voltage Dependent ◽  
Abnormal Tissue ◽  
Synaptic Responses

1. Synaptic transmission mediated by the N-methyl-D-aspartate (NMDA)-receptor type was studied in neocortex from children undergoing surgical treatment for intractable epilepsy. Intracellular recordings from pyramidal cells were obtained in slices of neocortical tissue by use of microelectrodes. Synaptic responses were induced by electrical stimulation and studied with current-clamp and single-electrode voltage-clamp techniques. The NMDA-receptor-mediated component of the synaptic responses was isolated by addition of 10 microM bicuculline and 30 microM 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (CNQX) in the perfusion solution. 2. In the presence of bicuculline and CNQX, electrical stimulation evoked an excitatory postsynaptic potential (EPSP) in every recorded cell. The amplitude of this EPSP increased when membrane potential was depolarized with injected current. 3. All cells studied in voltage clamp were recorded with microelectrodes containing Cs+ and QX 314. To avoid contamination of the responses from voltage-dependent Ca2+ conductances, membrane potential was held at depolarized potentials until Ca2+ spiking inactivated completely. The evoked excitatory postsynaptic currents (EPSCs) measured at resting membrane potential ranged from 100 to 400 pA. The NMDA receptor-selective antagonist DL-2-amino-5-phosphonopentanoic acid (AP-5) reversibly decreased the current amplitude by 60% for 10 microM and 80% for 30 microM. 4. The current-voltage (I-V) relation showed a region of negative slope conductance between -100 and -20 mV. The largest currents (-250 to -900 pA) were recorded in the range of -45 to -20 mV and reversed between -10 and +10 mV. Removing Mg2+ from the perfusion solution decreased the negativity of the slope, which is consistent with a reduction in the voltage-dependent Mg2+ block of the NMDA-receptor channel. 5. The I-V plots obtained from cells recorded in the most abnormal tissue were averaged and compared with those from the least abnormal tissue. No significant difference was found between these two groups. The averaged plots from the youngest patients (8 and 10 mo old) and those from the oldest (5-15 yr old) patients were also compared, and the results from these two groups were not significantly different.(ABSTRACT TRUNCATED AT 400 WORDS)

Anomalous rectification in neurons from cat sensorimotor cortex in vitro

1987 ◽  
Vol 57 (5) ◽  
pp. 1555-1576 ◽  
Author(s):  
W. J. Spain ◽  
P. C. Schwindt ◽  
W. E. Crill
Keyword(s):  
Voltage Clamp ◽  
Time Constant ◽  
Sensorimotor Cortex ◽  
Brain Slices ◽  
Resting Potential ◽  
Extracellular Potassium ◽  
Membrane Hyperpolarization ◽  
Anomalous Rectification ◽  
Voltage Dependent

The ionic mechanisms underlying anomalous rectification in large neurons from layer V of cat sensorimotor cortex were studied in an in vitro brain slice. The anomalous rectification was apparent as an increase of slope conductance during membrane hyperpolarization, and the development of anomalous rectification during a hyperpolarizing current pulse was signaled by a depolarizing sag of membrane potential toward resting potential (RP). Voltage-clamp analysis revealed the time- and voltage-dependent inward current (IAR) that produced anomalous rectification. IAR reversal potential (EAR) was estimated to be approximately -50 mV from extrapolation of linear, instantaneous, current-voltage relations. The conductance underlying IAR (GAR) had a sigmoidal steady-state activation characteristic. GAR increased with hyperpolarization from -55 to -105 mV with half-activation at approximately -82 mV. The time course of both GAR and IAR during a voltage step was described by two exponentials. The faster exponential had a time constant (tau F) of approximately 40 ms; the slow time constant (tau S) was approximately 300 ms. Neither tau F nor tau S changed with voltage in the range -60 mV to -110 mV. The fast component constituted approximately 80% of IAR at each potential. Both IAR and GAR increased in raised extracellular potassium [( K+]o) and EAR shifted positive, but the GAR activation curve did not shift along the voltage axis. Solutions containing an impermeable Na+ substitute caused an initial transient decrease in IAR followed by a slower increase of IAR. Brain slices bathed in Na+-substituted solution developed a gradual increase in [K+]o as measured with K+-sensitive microelectrodes. We conclude that GAR is permeable to both Na+ and K+, but the full contribution of Na+ was masked by the slow increase of [K+]o that occurred in Na+ substituted solutions. Chloride did not appear to contribute significantly to IAR since estimates of EAR were similar in neurons impaled with microelectrodes filled with potassium chloride or methylsulfate, whereas, ECl (estimated from reversal of a GABA-induced ionic current) was approximately 30 mV more positive with the KCl-filled microelectrodes. Extracellular Cs+ caused a reversible dose- and voltage-dependent reduction of GAR, whereas intracellular Cs+ was ineffective. The parameters measured during voltage clamp were used to formulate a quantitative empirical model of IAR.(ABSTRACT TRUNCATED AT 400 WORDS)

Magnesium affects excitation, conduction, and contraction of isolated mammalian cardiac muscle

1992 ◽  
Vol 263 (2) ◽  
pp. H622-H633 ◽  
Author(s):  
S. K. Hall ◽  
C. H. Fry
Keyword(s):  
Ventricular Myocytes ◽  
Mammalian Species ◽  
Negative Inotropic Effect ◽  
Membrane Resistance ◽  
Resting Membrane Potential ◽  
Activation Curve ◽  
Voltage Dependent ◽  
Inward Currents ◽  
Specific Membrane ◽  
Action Potential Configuration

An increase of extracellular Mg concentration, [Mg]o, reduced myocardial excitability and conduction without affecting the resting membrane potential or action potential configuration in ventricular myocytes and papillary muscles from a number of mammalian species. Although there was a small increase of specific membrane resistance and no change to intracellular resistivity, the threshold voltage was shifted to depolarized potentials. Thus loss of excitability can be explained by a shift of the activation of inward currents to depolarized potentials, and reduced conduction velocity is due solely to a diminution of local circuit currents. Mgo also was negatively inotropic, the magnitude of this effect being species dependent. Raised [Mg]o caused a small increase of intracellular [Mg] with a small decrease of intracellular [Na+], did not affect intracellular pH, and attenuated the intracellular Ca2+ transient associated with cell shortening in rat (but not rabbit) myocytes. An increase of [Mg]o reduced the magnitude of the voltage-dependent inward Ca2+ current, ICa, in rat and rabbit myocytes, and the activation curve of ICa was shifted to more depolarized potentials. A scheme to account for the negative inotropic effect of Mg is presented.

Inward currents underlying action potentials in rat adrenal chromaffin cells

1996 ◽  
Vol 76 (2) ◽  
pp. 1195-1211 ◽  
Author(s):  
B. Hollins ◽  
S. R. Ikeda
Keyword(s):  
Ion Channels ◽  
Voltage Clamp ◽  
Action Potentials ◽  
Chromaffin Cells ◽  
Na Channels ◽  
Adrenal Chromaffin Cells ◽  
Voltage Dependent ◽  
Slope Factor ◽  
Inward Currents ◽  
Adrenal Chromaffin

1. Current- and voltage-clamp studies were conducted on isolated rat adrenal chromaffin cells to identify the voltage-dependent ion channels mediating inward currents. 2. Mean resting membrane potential of the isolated cells was -62 +/- 3 (SE) mV. Evoked action potentials were both Na+ and Ca2+ based, and whole cell voltage-clamp studies in normal saline revealed an inward-rectifier-type current. 3. Na+ channels were studied in isolation and showed a half-inactivation of -60 +/- 2 mV with a slope factor of -6 mV and a half-activation of -26.8 +/- 2 mV with a slope factor of 6.5 +/- 0.7 mV. 4. Isolated Ca2+ currents, elicited in 10 mM external Ca2+, revealed a T-type current in a subset of cells. Ca2+ currents were sensitive to both N- and L-type channel antagonists, and blockade of the current by the L-type channel antagonist nimodipine and the N-type channel antagonist omega-conotoxin GVIA revealed a third Ca2+-current component that was unaffected by the P-type channel antagonist omega-agatoxin IVA. 5. Ca2+ currents were facilitated 5-20% by a depolarizing prepulse, and facilitation was completely blocked by nimodipine. The effects of the dihydropyridine L-type channel agonist, (+)202-791 and depolarizing prepulses on the currents were additive. 6. The results of this study show that the properties of voltage-dependent ion channels in rat chromaffin cells differ from those reported in their counterparts in bovine chromaffin cells. Na+ channels differ in activation and inactivation properties and Ca2+ channels differ in activation, sensitivity to antagonists, and the magnitude of voltage-dependent facilitation.

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