Modulation of K channels in dialyzed squid axons. ATP-mediated phosphorylation

HIV-1 gp120 enhances outward potassium current via CXCR4 and cAMP-dependent protein kinase A signaling in cultured rat microglia

Microglia are critical cells in mediating the pathophysiology of neurodegenerative disorders such as HIV-associated neurocognitive disorders. We hypothesize that HIV-1 glycoprotein 120 (gp120) activates microglia by enhancing outward K(+) currents, resulting in microglia secretion of neurotoxins, consequent neuronal dysfunction, and death. To test this hypothesis, we studied the effects of gp120 on outward K(+) current in cultured rat microglia. Application of gp120 enhanced outward K(+) current in a dose-dependent manner, which was blocked by voltage-gated K(+) (K(v) ) channel blockers. Western blot analysis revealed that gp120 produced an elevated expression of K(v) channel proteins. Examination of activation and inactivation of outward K(+) currents showed that gp120 shifted membrane potentials for activation and steady-state inactivation. The gp120-associated enhancement of outward K(+) current was blocked by either a CXCR4 receptor antagonist T140 or a specific protein kinase A (PKA) inhibitor H89, suggesting the involvement of chemokine receptor CXCR4 and PKA in gp120-mediated enhancement of outward K(+) current. Biological significance of gp120-induced enhancement of microglia outward K(+) current was demonstrated by experimental results showing the neurotoxic activity of gp120-stimulated microglia, evaluated by TUNEL staining and MTT assay, significantly attenuated by K(v) channel blockers. Taken together, these results suggest that gp120 induces microglia neurotoxic activity by enhancing microglia outward K(+) current and that microglia K(v) channels may function as a potential target for the development of therapeutic strategies.

A model of the rat phrenic motor neuron

We have developed a model for the rat phrenic motor neuron (PMN) that robustly replicates many experimentally observed behaviors of PMNs in response to pharmacological, ionic, and electrical perturbations using a single set of parameters. Our model suggests that the after-depolarization (ADP) response seen in action potentials is a result of the slow deactivation of the fast sodium channel in the range of the ADP coupled with the activation of the L-type calcium channel (I(CaL)). This current and its interactions with the small and large conductance calcium-activated potassium currents (I(KCaSK) and I(KCaBK), respectively) is also important in the generation of spike frequency adaptation in the repetitive firing mode of activity. Other aspects of the model conform very well to experimental observations in both the action potential and repetitive firing mode of activity, including the role of I(KCaSK) in the medium after-hyperpolarization (AHP) and the role of I(KCaBK) in the fast AHP. We have made a number of predictions using the model, including the characterization of two putative sodium currents (fast and persistent), as well as functional roles for the N- and T-type calcium currents.

Transmitter modulation of slow, activity-dependent alterations in sodium channel availability endows neurons with a novel form of cellular plasticity

Voltage-gated Na+ channels are major targets of G protein-coupled receptor (GPCR)-initiated signaling cascades. These cascades act principally through protein kinase-mediated phosphorylation of the channel alpha subunit. Phosphorylation reduces Na+ channel availability in most instances without producing major alterations of fast channel gating. The nature of this change in availability is poorly understood. The results described here show that both GPCR- and protein kinase-dependent reductions in Na+ channel availability are mediated by a slow, voltage-dependent process with striking similarity to slow inactivation, an intrinsic gating mechanism of Na+ channels. This process is strictly associated with neuronal activity and develops over seconds, endowing neurons with a novel form of cellular plasticity shaping synaptic integration, dendritic electrogenesis, and repetitive discharge.

Outward Currents in Rat Entorhinal Cortex Stellate Cells Studied with Conventional and Perforated Patch Recordings

We have studied outward currents of neurons acutely isolated from superficial layers of the entorhinal cortex with whole-cell patch-clamp recordings. If cells were held more negative than -50 mV, depolarizing voltage commands activated a transient A-type current together with a sustained outward current. Both currents were sensitive to 4-aminopyridine, while only the sustained current was blocked by tetraethylammonium. The sustained outward current showed a considerable rundown in amplitude over prolonged recording periods. At the same time its half-maximal inactivation shifted from -74 to -114 mV. Nystatin perforated patch recordings were used to minimize these perfusion effects. Under such conditions the amplitude and the steady-state inactivation properties of the sustained outward current remained stable for more than 1 h. Pharmacological investigations revealed that only a small part of the sustained outward current could be attributed to a calcium-activated potassium current. Therefore most of the rundown has to be due to changes in the delayed rectifier outward current. These results may suggest that the delayed rectifier current is under considerable metabolic control.

Casein kinase 2 determines the voltage dependence of the Kv3.1 channel in auditory neurons and transfected cells

The Kv3.1 potassium channel can be distinguished from most other delayed rectifier channels by its very high threshold of activation and lack of use-dependent inactivation. This allows neurons that express this channel to fire at very high frequencies. We have now found that this feature of the Kv3.1 channel is strongly influenced by its constitutive phosphorylation by the enzyme casein kinase II. Using stably transfected Chinese hamster ovary cells expressing Kv3.1, we show that Kv3.1 is highly phosphorylated under basal conditions. Whole-cell patch clamp recordings were used to characterize the electrophysiological consequence of dephosphorylation using alkaline phosphatase. This enzyme produced an increase in whole-cell conductance and shifted the voltage dependence of activation to more negative potentials by >20 mV. In addition, a similar shift in the voltage dependence of inactivation was observed. These findings were also confirmed in native Kv3.1 channels expressed in medial nucleus of the trapezoid body (MNTB) neurons. Furthermore, inhibitors of casein kinase 2 mimicked the effect of phosphatase treatment on voltage-dependent activation and inactivation, whereas inhibitors of protein kinase C failed to alter these parameters. The combination of biochemical and electrophysiological evidence suggests that the biophysical characteristics of Kv3.1 that are important to its role in MNTB neurons, allowing them to follow high-frequency stimuli with fidelity, are largely determined by phosphorylation of the channel.

Casein kinase 2 determines the voltage dependence of the Kv3.1 channel in auditory neurons and transfected cells

The Kv3.1 potassium channel can be distinguished from most other delayed rectifier channels by its very high threshold of activation and lack of use-dependent inactivation. This allows neurons that express this channel to fire at very high frequencies. We have now found that this feature of the Kv3.1 channel is strongly influenced by its constitutive phosphorylation by the enzyme casein kinase II. Using stably transfected Chinese hamster ovary cells expressing Kv3.1, we show that Kv3.1 is highly phosphorylated under basal conditions. Whole-cell patch clamp recordings were used to characterize the electrophysiological consequence of dephosphorylation using alkaline phosphatase. This enzyme produced an increase in whole-cell conductance and shifted the voltage dependence of activation to more negative potentials by >20 mV. In addition, a similar shift in the voltage dependence of inactivation was observed. These findings were also confirmed in native Kv3.1 channels expressed in medial nucleus of the trapezoid body (MNTB) neurons. Furthermore, inhibitors of casein kinase 2 mimicked the effect of phosphatase treatment on voltage-dependent activation and inactivation, whereas inhibitors of protein kinase C failed to alter these parameters. The combination of biochemical and electrophysiological evidence suggests that the biophysical characteristics of Kv3.1 that are important to its role in MNTB neurons, allowing them to follow high-frequency stimuli with fidelity, are largely determined by phosphorylation of the channel.

The kinetics of a non-inactivating K(+) current in alphaT3-1 pituitary gonadotropes is not affected by holding potential

The non-inactivating K(+) currents in alphaT3-1, a gonadotroph cell line, were recorded in the presence of low intracellular free calcium concentration. The activation kinetics of the whole-cell currents and the gating charge measured from holding potential (V(HOLD)) of -10 mV, V(HOLD)=-80 mV in presence of 4-AP (4-aminopyridine), and V(HOLD)=-10 mV with a hyperpolarizing prepulse to -80 mV were similar. No difference was observed in the onset of currents elicited from the hyperpolarizing potentials, suggesting deviation from the Cole-Moore prediction of increase in the delay of current onset with increasing hyperpolarization. The data suggests that the channel opens with at least one rate-limiting voltage-dependent step, which may imply that the position of the voltage sensor is unaffected by hyperpolarization.

K+ accumulation and K+ conductance inactivation during action potential trains in giant axons of the squid Sepioteuthis

1. During action potential trains in giant axons from the squid Sepioteuthis, decline of the peak level of the undershoot potential was observed. The time course of the decline of the undershoot could be fitted with a three-exponential function with time constants of approximately 25, approximately 400 and approximately 7,000 ms, respectively. 2. When the osmolarity of the external solution was doubled by adding glucose (1.2 M), the fast component of undershoot decline, but not the medium and slow components, was significantly reduced. 3. Under voltage clamp in high osmolarity solutions where K+ accumulation was completely removed, repeated depolarizing pulses at 40 Hz (designed to mimic a train of action potentials) elicited K+ currents whose peak value declined. The decline is consistent with inactivation of the K+ conductance (gK). The decline of gK was fitted by a two-exponential function with time constants of approximately 400 and approximately 7,000 ms, respectively. 4. Interventions designed to modify Schwann cell physiology, such as high frequency stimulation (100 Hz, 2 min), externally applied ouabain (100-500 microM), L-glutamate (100 microM), ACh (100 microM), Co2+ (5mM), Ba2+ (2mM), or removal of external Ca2+ by EGTA, had no significant effects on the fast, medium or slow components of undershoot decline. 5. The results suggest that the fast component of undershoot decline represents K+ accumulation in the space between Schwann cell and axolemma. The medium and slow components are the result of axonal gK inactivation. Schwann cells appear to be involved in K+ clearance only to the extent that they provide an efficient physical pathway for the clearance of K+ by extracellular diffusion.

Fast inactivation of delayed rectifier K conductance in squid giant axon and its cell bodies

Inactivation of delayed rectifier K conductance (gk) was studied in squid giant axons and in the somata of giant fiber lobe (GFL) neurons. Axon measurements were made with an axial wire voltage clamp by pulsing to VK (approximately -10 mV in 50-70 mM external K) for a variable time and then assaying available gK with a strong, brief test pulse. GFL cells were studied with whole-cell patch clamp using the same prepulse procedure as well as with long depolarizations. Under our experimental conditions (12-18 degrees C, 4 mM internal MgATP) a large fraction of gK inactivates within 250 ms at -10 mV in both cell bodies and axons, although inactivation tends to be more complete in cell bodies. Inactivation in both preparations shows two kinetic components. The faster component is more temperature-sensitive and becomes very prominent above 12 degrees C. Contribution of the fast component to inactivation shows a similar voltage dependence to that of gK, suggesting a strong coupling of this inactivation path to the open state. Omission of internal MgATP or application of internal protease reduces the amount of fast inactivation. High external K decreases the amount of rapidly inactivating IK but does not greatly alter inactivation kinetics. Neither external nor internal tetraethylammonium has a marked effect on inactivation kinetics. Squid delayed rectifier K channels in GFL cell bodies and giant axons thus share complex fast inactivation properties that do not closely resemble those associated with either C-type or N-type inactivation of cloned Kvl channels studied in heterologous expression systems.

Cytoplasmic ATP-dependent regulation of ion transporters and channels: mechanisms and messengers

Many ion transporters and channels appear to be regulated by ATP-dependent mechanisms when studied in planar bilayers, excised membrane patches, or with whole-cell patch clamp. Protein kinases are obvious candidates to mediate ATP effects, but other mechanisms are also implicated. They include lipid kinases with the generation of phosphatidylinositol phosphates as second messengers, allosteric effects of ATP binding, changes of actin cytoskeleton, and ATP-dependent phospholipases. Phosphatidylinositol-4,5-bisphosphate (PIP2) is a possible membrane-delimited messenger that activates cardiac sodium-calcium exchange, KATP potassium channels, and other inward rectifier potassium channels. Regulation of PIP2 by phospholipase C, lipid phosphatases, and lipid kinases would thus tie surface membrane transport to phosphatidylinositol signaling. Sodium-hydrogen exchange is activated by ATP through a phosphorylation-independent mechanism, whereas ion cotransporters are activated by several protein kinase mechanisms. Ion transport in epithelium may be particularly sensitive to changes of cytoskeleton that are regulated by ATP-dependent cell signaling mechanisms.

Molecular identification of SqKv1A. A candidate for the delayed rectifier K channel in squid giant axon

We have cloned the cDNA for a squid Kvl potassium channel (SqKv1A). SqKv1A mRNA is selectively expressed in giant fiber lobe (GFL) neurons, the somata of the giant axons. Western blots detect two forms of SqKv1A in both GFL neuron and giant axon samples. Functional properties of SqKv1A currents expressed in Xenopus oocytes are very similar to macroscopic currents in GFL neurons and giant axons. Macroscopic K currents in GFL neuron cell bodies, giant axons, and in Xenopus oocytes expressing SqKv1A, activate rapidly and inactivate incompletely over a time course of several hundred ms. Oocytes injected with SqKv1A cRNA express channels of two conductance classes, estimated to be 13 and 20 pS in an internal solution containing 470 mM K. SqKv1A is thus a good candidate for the "20 pS" K channel that accounts for the majority of rapidly activating K conductance in both GFL neuron cell bodies and the giant axon.

Effects of phosphorylation on ion channel function

A fundamental property of ion channels is their ability to be modulated by intracellular second messenger systems acting via covalent modifications of the channel protein itself. One such important biochemical reaction is phosphorylation on serine, threonine, and tyrosine residues. Ion channels in the kidney are no exception. Moreover, many ion channels, including many amiloride-sensitive epithelial Na+ channels, are subject to modulation by a multiplicity of inputs. For example, renal Na+ channels are not gated by voltage in their unphosphorylated state. However, upon phosphorylation by PKA plus ATP, these channels become voltage-dependent as well as having their open probability increased. Phosphorylation by PKC inhibits channel activity regardless of whether the channel was previously phosphorylated by PKA. Likewise, Na+ channel ADP-ribosylation by PTX overrides the actions of cAMP-dependent phosphorylation. Consistent with this idea is the fact that the phosphorylation sites for PKA and PKC and the ADP-ribosylation sites occur on different polypeptides comprising the channel complex. Epithelial Na+ channel activity is also regulated by methylation, arachidonic acid metabolites, and by interactions with cytoskeletal components. An exciting new age in understanding renal Na+ channel function has begun. Canessa and collaborators [103, 104] and Lingueglia et al [105] have, for the first time, identified by expression cloning an amiloride-sensitive Na+ channel from rat distal colon. The messenger RNA encoding the subunits comprising this channel are expressed in the distal tubule and cortical collecting tubule of the kidney (Rossier, unpublished observations). In addition, our laboratory has successfully cloned a mammalian homologue of this same channel from bovine renal papillary collecting ducts [106].(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin modulates the voltage dependence of delayed rectifier and Shaker potassium channels in Drosophila photoreceptors

We describe the in situ modulation of potassium channels in a semi-intact preparation of the Drosophila retina. In whole-cell recordings of photoreceptors, rapidly inactivating Shaker channels are characterized by a conspicuously negative voltage operating range; together with a delayed rectifier, these channels are specifically modulated by the putative efferent neurotransmitter serotonin. Contrary to most potassium channel modulations, serotonin induced a reversible positive shift in the voltage operating range, of +30 mV for the Shaker channels and +10-14 mV for the delayed rectifier. The maximal current amplitudes were unaffected. Modulation was not affected by the subunit-specific Shaker mutations ShE62 and T(1;Y)W32 or a null mutation of the putative modulatory subunit eag. The modulation of both channels was mimicked by intracellularly applied GTP gamma S.

Gating of the squid sodium channel at positive potentials. I. Macroscopic ionic and gating currents

Macroscopic ionic sodium currents and gating currents were studied in voltage-clamped, dialyzed giant axons of the squid Loligo pealei under conditions of regular and inverse sodium gradients. Sodium currents showed regular kinetics but inactivation was incomplete, showing a maintained current for depolarizations lasting 18 ms. The ratio of the maintained current to the peak current increased with depolarization and it did not depend on the direction of the current flow or the sodium gradient. The time constant of inactivation was not affected by the sodium gradient. Double-pulse experiments allowed the separation of a normal inactivating component and a noninactivating component of the sodium currents. In gating current experiments, the results from double-pulse protocols showed that the charge was decreased by the prepulse and that the slow component of the 'on' gating current was preferentially depressed. As expected, charge immobilization was established faster at higher depolarizations than at low depolarizations, however, the amount of immobilized charge was unaffected by the pulse amplitude. This indicates that the incomplete sodium inactivation observed at high depolarizations is not the result of decreased charge immobilization; the maintained current must be due to a conductance that appears after normal charge immobilization and fast inactivation.

Modulation of Ca2+ transients by photorelease of caged nucleotides in frog skeletal muscle fibers

Action potentials and intracellular Ca2+ transients were monitored in current-clamped segments of frog skeletal muscle fibers using the triple vaseline-gap technique. Calcium signals were measured with the fluorescent indicator rhod 2. Action potentials produced a transient increase in intracellular Ca2+ that was estimated, by deconvolution of the fluorescence signals, to range between 3 and 12 microM. The comparative effects of flash photolysis of caged adenosine 3',5'-cyclic monophosphate (cAMP) and caged ATP on action potentials and Ca signals in muscle were investigated. The photorelease of both nucleotides produced a reduction in the amplitude of the afterpotential that follows the spike. Photorelease of cAMP and ATP prolonged the rate of decay of the Ca signals. No changes in either the rate of rise or in the latent period between stimulation and onset of the Ca signal were observed. Release of cAMP reduced the amplitude of Ca signals, whereas release of ATP had the opposite effect. Our results show that cAMP and ATP, released above their endogenous levels, modulate intracellular Ca2+ release. The cAMP modulation is more significant and may be of physiological importance.

Analysis of the modulation by serotonin of a voltage-dependent potassium current in sensory neurons of Aplysia

Potassium currents in pleural sensory neurons of Aplysia were studied under control conditions and in the presence of serotonin (5-HT). Using pharmacological techniques we isolated a current that we refer to as IK,V. Although it is not known whether IK,V represents a distinct type of membrane channel, we described its properties using a Hodgkin-Huxley type model. The effects of 5-HT on IK,V were complex. 5-HT decreased by 50% the steady-state magnitude (Iss) of IK,V in response to a voltage-clamp pulse from -50 mV to +20 mV. In addition, 5-HT significantly slowed both activation kinetics (the time constant of activation was increased by 29% at +20 mV) and inactivation kinetics (the time constant of inactivation was increased by 518% at +20 mV). Mathematical descriptions of IK,V in control conditions and in the presence of 5-HT were used to estimate the relative contribution of serotonergic modulation of IK,V to the total 5-HT-induced modulation of membrane currents. Effects of 5-HT on IK,V account for more than 87% of the 5-HT-induced reduction in outward current during the first 20 ms of a voltage-clamp pulse to +20 mV. This result implies that 5-HT exerts many of its effects on spike width in sensory neurons via modulation of IK,V. Effects of 5-HT on IK,V are consistent with a model in which the maximal conductance underlying the current is decreased by 50%, and the rate constants between open and closed states of both the activation and inactivation processes are diminished in magnitude across all membrane potentials.

Modulation of voltage-dependent sodium and potassium currents by charged amphiphiles in cardiac ventricular myocytes. Effects via modification of surface potential

Modulation of voltage-dependent sodium and potassium currents by charged amphiphiles was investigated in cardiac ventricular myocytes using the patch-clamp technique. Negatively charged sodium dodecylsulfate (SDS) increased amplitude of INa, whereas positively charged dodecyltrimethylammonium (DDTMA) decreased INa. Furthermore, SDS shifted the steady-state activation and inactivation of INa in the negative direction, whereas DDTMA shifted the curves in the opposite direction. These shifts provided an explanation for the changes in current amplitude. Activation and inactivation kinetics of INa were accelerated by SDS but slowed by DDTMA. These changes in both steady-state gating and kinetics of INa are consistent with a decrease of the intramembrane field by SDS and an increase of the field by DDTMA due to an alteration of surface potential after their insertion into the outer monolayer of the sarcolemma. The effect of SDS on the steady-state inactivation of INa was concentration dependent and partially reversed by screening surface charges with increased extracellular [Ca2+]. These amphiphiles also altered the activation of the delayed rectifier K+ current (IK,del), producing a shift in the negative direction by SDS but in the positive direction by DDTMA. These results suggest that the insertion of charged amphiphiles into the cell membrane alters the behavior of voltage-dependent INa and IK,del by changing the surface charge density, and consequently the surface potential and implies, although indirectly, that the lipid surface charges are important to the voltage-dependent gating of these channels.

Phosphorylation of K+ channels in the squid giant axon. A mechanistic analysis

Protein phosphorylation is an important mechanism in the modulation of voltage-dependent ionic channels. In squid giant axons, the potassium delayed rectifier channel is modulated by an ATP-mediated phosphorylation mechanism, producing important changes in amplitude and kinetics of the outward current. The characteristics and biophysical basis for the phosphorylation effects have been extensively studied in this preparation using macroscopic, single-channel and gating current experiments. Phosphorylation produces a shift in the voltage dependence of all voltage-dependent parameters including open probability, slow inactivation, first latency, and gating charge transferred. The locus of the effect seems to be located in a fast 20 pS channel, with characteristics of delayed rectifier, but at least another channel is phosphorylated under our experimental conditions. These results are interpreted quantitatively with a mechanistic model that explains all the data. In this model the shift in voltage dependence is produced by electrostatic interactions between the transferred phosphate and the voltage sensor of the channel.

Ion channels in transit: voltage-gated Na and K channels in axoplasmic organelles of the squid Loligo pealei

Ion channels that give rise to the excitable properties of the neuronal plasma membrane are synthesized, transported, and degraded in cytoplasmic organelles. To determine whether plasma membrane ion channels from these organelles could be physiologically activated, we extruded axoplasm from squid giant axons, dissociated organelles from the cytoskeletal matrix, and fused the free organelles with planar lipid bilayers. Three classes of ion channels normally associated with the plasma membrane were identified based on conductance, selectivity, and gating properties determined from steady-state single-channel recordings: (i) voltage-dependent Na channels, (ii) voltage-dependent delayed rectifier K channels, and (iii) large, voltage-independent K channels. The identity of the delayed rectifier channels was confirmed by reconstructing the time course of activation from single-channel responses to depolarizing voltage steps applied across the bilayer. These observations suggest that several classes of plasma membrane ion channels are transported in cytoplasmic organelles in physiologically active forms.

Mechanism of anoxic conduction block in mammalian nerve

The mechanism by which anoxia blocks impulse conduction was studied in isolated sciatic nerves from the rat. The desheathed nerve was mounted in a recording chamber, and the compound action potential (CAP) was measured at controlled temperature (23 and 37 degrees C). When the nerve was irrigated with nitrogenated Ringer's solution compound action potential decreased to 50% in 10 min at 37 degrees C and in 35 min at 23 degrees C, whereas in oxygenated solution compound action potential decreased less than 5% in 60 min. A Na-free nitrogenated solution similarly caused anoxic block, that is the effect was independent of impulse activity. Ouabain (1 mM) decreased compound action potential by only ca. 4% in 30 min, and the effect of anoxia was delayed in presence of ouabain. Dinitrophenol (0.05 mM) reduced compound action potential to 50% in 5 min. These findings indicated that the anoxic block was not related to changes in axonal concentration of Na or K following impulse activity or inhibition of Na-K-ATPase. Instead the findings imply that the anoxic block is due to inactivation of Na-channels as a consequence of inhibition of another ATP-dependent process in the axon.

Regulation of transmitter release at the squid giant synapse by presynaptic delayed rectifier potassium current

1. The three-microelectrode voltage clamp technique and pharmacological agents were used to examine the properties and functions of potassium currents in squid giant presynaptic terminals. 2. Outward currents consisted of two components: a slow component which activated over hundreds of milliseconds and was blocked by extracellular application of tetraethylammonium (TEA) ions and a more rapidly activating component which was relatively insensitive to extracellular TEA. 3. The more rapid component was studied in isolation by treating presynaptic terminals with extracellular TEA, as well as tetrodotoxin (to block sodium channel currents) and manganese (to block calcium channel currents). The magnitude of this current component was 1-2 mA cm-2 at 0 mV. Rates of activation and deactivation were voltage dependent and little evidence of inactivation was seen for depolarizations less than several seconds in duration. 4. The reversal potential of the current was -70 to -80 mV in normal saline and became more positive with elevated extracellular potassium concentrations, suggesting that potassium is the primary permeant ion. Accumulation of extracellular potassium appeared to be marked during depolarizations that produced significant activation of the current. 5. Extracellular application of 3,4-diaminopyridine (DAP) blocked the current with an apparent dissociation constant of 7 microM at 0 mV. Intracellular applications of DAP and TEA also were effective in reducing this current. These treatments, but not extracellular TEA application, broadened presynaptic action potentials and increased the magnitude and time-to-peak of postsynaptic currents elicited by the broadened presynaptic action potentials. Postsynaptic currents were a sensitive and linear function of action potential duration; a 30% increase in action potential duration increased postsynaptic current amplitude by 190%. 6. Estimation of the magnitude and time course of the presynaptic calcium current, based on previous measurements of calcium channel gating, indicated that action potential broadening produces a large increase in calcium current magnitude. These calculations predict that a 30% increase in presynaptic action potential duration will increase the peak amplitude of the calcium current by approximately 170% and the total amount of calcium entry by approximately 230%. This implies a linear relationship between transmitter release and calcium entry during an action potential and can be explained by assuming that calcium co-operatively triggers release within intracellular domains that do not overlap.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of extracellular ATP-induced depolarization in rat isolated ventricular cardiomyocytes

Adenosine triphosphate (ATP) is released during neural stimulation and cardiac hypoxia and several mechanisms of its action have been reported in different tissues. ATP stimulates P1 and P2 purinergic receptors; it also activates receptor-operated channels and increases membrane permeability to small ions. In single rat ventricular cells under whole-cell patch-clamp, a stepwise application of ATP in the micromolar range affects the resting potential and membrane currents through an entirely novel mechanism of action which involves several steps. Extracellular ATP induces an inward current and depolarization of the cell, leading to automaticity. The inward current is non-specific for cations, its reversal potential is around -5 mV. The conductance change evoked by ATP is suppressed by 4,4-diisothiocyanostilbene 2,2-disulphonic acid (DIDS) and low-chloride media and is prolonged by adding intracellular bicarbonate. These effects are specific for ATP in the presence of magnesium and are not evoked by a non-hydrolysable analogue of ATP or in the presence of vanadate. Other nucleotides are ineffective. We propose that ATP hydrolysis activates the chloride/bicarbonate (Cl-/HCO3-) exchanger. The induced local acidification could then increase intracellular free calcium and as a consequence, increases the sarcolemmal conductance. Thus, a sudden release of ATP in pathological conditions would induce a depolarization which could generate ventricular arrhythmias.

Phosphorylation modulates potassium conductance and gating current of perfused giant axons of squid

The presence of internal Mg-ATP produced a number of changes in the K conductance of perfused giant axons of squid. For holding potentials between -40 and -50 mV, steady-state K conductance increased for depolarizations to potentials more positive than approximately -15 mV and decreased for smaller depolarizations. The voltage dependencies of both steady-state activation and inactivation also appears shifted toward more positive potentials. Gating kinetics were affected by internal ATP, with the activation time constant slowed and the characteristic delay in K conductance markedly enhanced. The rate of deactivation also was hastened during perfusion with ATP. Internal ATP affected potassium channel gating currents in similar ways. The voltage dependence of gating charge movement was shifted toward more positive potentials and the time constants of ON and OFF gating current also were slowed and hastened, respectively, in the presence of ATP. These effects of ATP on the K conductance occurred when no exogenous protein kinases were added to the internal solution and persisted even after removing ATP from the internal perfusate. Perfusion with a solution containing exogenous alkaline phosphatase reversed the effects of ATP. These results provide further evidence that the effects of ATP on the K conductance are a consequence of a phosphorylation reaction mediated by a kinase present and active in perfused axons. Phosphorylation appears to alter the K conductance of squid giant axons via a minimum of two mechanisms. First, the voltage dependence of gating parameters are shifted toward positive potentials. Second, there is an increase in the number of functional closed states and/or a decrease in the rates of transition between these states of the K channels.