Inhibition of a slowly inactivating high-voltage-activated calcium current by the neuropeptide FMRFa in molluscan neuroendocrine cells

Using the whole-cell voltage-clamp technique, the effects of the neuropeptide Phe-Met-Arg-Phe-NH₂ (FMRFa) on two types of dihydropyridine-sensitive, high-voltage-activated calcium currents were investigated in isolated neuroendocrine caudo-dorsal cells (CDCs), which control egg-laying in the molluscLymnaea stagnalis. These currents are: (1) a transient current (Τ_inact = ∼10–25 ms) with an activation threshold of −40 mV and maximal amplitude at +10 mV and (2) a sustained current (Τ_inact = ∼ 100–300 ms) with a threshold of −10 mV and apeak at +30 mV. FMRFa caused a partial block of the calcium current that was rapid, reversible and dose-dependent (ED₅₀ = 4.3 nM). The FMRFa-sensitive and insensitive currents differed in voltage-dependence of activation and inactivation, steady-state inactivation characteristics and time course of recovery from inactivation, all indicating that FMRFa selectively suppressed the sustained calcium current. Internal perfusion of CDCs with GTP-γ-S or GDP-Β-S depressed the FMRFa response, suggesting the involvement of G-proteins. Experiments aimed at elucidation of the signal transduction pathway between the FMRFa receptor and the calcium channel revealed no involvement of second messengers and protein kinases. The FMRFa-induced inhibition of the sustained calcium current probably results from a direct interaction between a G-protein, activated by the FMRFa receptor, and the calcium channel. The selective inhibition of this calcium current is likely to decrease the influx of calcium during the action potential, which will reduce the release of autoexcitatory CDC-peptides and contribute to a suppression of excitability.     

A slowly inactivating potassium current in native oocytes of Xenopus laevis

Membrane currents were recorded in voltage-clamped oocytes of Xenopus laevis in response to voltage steps. We describe results obtained in oocytes obtained from one donor frog, which showed an unusually large outward current upon depolarization. Measurements of reversal potentials of tail currents in solutions of different K+ concentration indicated that this current is carried largely by K+ ions. It was strongly reduced by extracellular application of tetraethylammonium, though not by Ba2+ or 4-aminopyridine. Removal of surrounding follicular cells did not reduce the K+ current, indicating that it arises across the oocyte membrane proper. Activation of the K+ conductance was first detected with depolarization to about -12 mV, increased with a limiting voltage sensitivity of 3 mV for an e-fold change in current, and was half-maximally activated at about +10 mV. The current rose following a single exponential timecourse after depolarization, with a time constant that shortened from about 400 ms at -10 mV to about 15 ms at +80 mV. During prolonged depolarization the current inactivated with a time constant of about 4 s, which did not alter greatly with potential. The K+ current was independent of Ca2+, as it was not altered by addition of 10 mM Mn2+ to the bathing medium, or by intracellular injection of EGTA. Noise analysis of K+ current fluctuations indicated that the current is carried by channels with a unitary conductance of about 20 ps and a mean open lifetime of about 300 ms (at room temperature and potential of +10 to +20 mV).     

Glibenclamide depresses the slowly inactivating outward current (ID) in hippocampal neurons.

Sulphonylurea drugs, such as glibenclamide and tolbutamide, are widely used as selective blockers of adenosine triphosphate-sensitive K channels. In experiments on hippocampal slices (from Wistar rats) glibenclamide (and possibly gliquidone and tolbutamide) significantly reduced the highly voltage-dependent, 4-aminopyridine-sensitive D-type outward current of CA3 neurons. Judging by these observations, the sulphonylureas may not be as selective as generally believed.     

Interaction of tetrandrine with slowly inactivating calcium channels. Characterization of calcium channel modulation by an alkaloid of Chinese medicinal herb origin

Tetrandrine, a bis-benzylisoquinoline alkaloid derived from the Chinese medicinal herb Stephania tetrandra, is a putative Ca2+ entry blocker whose mechanism of action is unknown. To investigate this mechanism, the effects of tetrandrine were characterized on binding of three chemical classes of Ca2+ entry blockers in cardiac sarcolemmal membrane vesicles. In the range 25-37 degrees C, tetrandrine completely blocks diltiazem binding, partially inhibits D-600 binding, and markedly stimulates nitrendipine binding, with greatest enhancement occurring at 37 degrees C. The potency of tetrandrine is increased 10-fold as temperature is raised from 25 to 37 degrees C. Scatchard analyses indicate that inhibition of diltiazem binding and stimulation of nitrendipine binding result from changes in ligand affinities while inhibition of D-600 binding is due to both an increase in KD and decrease in Bmax of aralkylamine receptors. Ligand dissociation studies reveal that tetrandrine increases D-600 off-rates, decreases nitrendipine off-rates, but has no effect on diltiazem dissociation kinetics. In addition, tetrandrine reversibly blocks inward Ca2+ currents through L-type Ca2+ channels in GH3 anterior pituitary cells. These results indicate that tetrandrine interacts directly at the benzothiazepine-binding site of the Ca2+ entry blocker receptor complex and allosterically modulates ligand binding at other receptors in this complex. These findings suggest that tetrandrine is a structurally unique natural product Ca2+ entry blocker and provide a rationale explanation for the therapeutic effectiveness of this agent.     

Hippocalcin functions as a calcium sensor in hippocampal LTD

It is not fully understood how NMDAR-dependent LTD causes Ca(2+)-dependent endocytosis of AMPARs. Here we show that the neuronal Ca(2+) sensor hippocalcin binds the beta2-adaptin subunit of the AP2 adaptor complex and that along with GluR2 these coimmunoprecipitate in a Ca(2+)-sensitive manner. Infusion of a truncated mutant of hippocalcin (HIP(2-72)) that lacks the Ca(2+) binding domains prevents synaptically evoked LTD but has no effect on LTP. These data indicate that the AP2-hippocalcin complex acts as a Ca(2+) sensor that couples NMDAR-dependent activation to regulated endocytosis of AMPARs during LTD.     

Contribution of slowly inactivating potassium current to delayed firing of action potentials in NG108-15 neuronal cells: experimental and theoretical studies

The properties of slowly inactivating delayed-rectifier K⁺ current (I_Kdr) were investigated in NG108-15 neuronal cells differentiated with long-term exposure to dibutyryl cyclic AMP. Slowly inactivating I_Kdr could be elicited by prolonged depolarizations from −50 to +50 mV. These outward K⁺ currents were found to decay at potentials above −20 mV, and the decay became faster with greater depolarization. Cell exposure to aconitine resulted in the reduction of I_Kdr amplitude along with an accelerated decay of current inactivation. Under current-clamp recordings, a delay in the initiation of action potentials (APs) in response to prolonged current stimuli was observed in these cells. Application of aconitine shortened the AP initiation in combination with an increase in both width of spike discharge and firing frequency. The computer model, in which state-dependent inactivation of I_Kdr was incorporated, was also implemented to predict the firing behavior present in NG108-15 cells. As the inactivation rate constant of I_Kdr was elevated, the firing frequency was progressively increased along with a shortening of the latency for AP appearance. Our theoretical work and the experimental results led us to propose a pivotal role of slowly inactivating I_Kdr in delayed firing of APs in NG108-15 cells. The results also suggest that aconitine modulation of I_Kdr gating is an important molecular mechanism through which it can contribute to neuronal firing.     

Daio-Orengedokudo works as a cell-proliferating compound in endothelial cells

Daio-Orengedokuto is a combination drug that has inhibitory effects on HMG-CoA reductase and pancreatic lipase. Here we investigated whether Daio-Orengedokuto has effects on vascular endothelial cells. To determine its effects on blood vessels, we examined roles of Daio-Orengedokuto in cell migration, cell apoptosis and cell cycle progression over bovine aortic endothelial cells (BAECs). Interestingly, Daio-Orengedokuto was shown to work as an anti-apoptotic agent, a cell cycle progressive agent and a cell-migration inducing agent in BAECs, whereas it was known to act as a tumor suppressor in cancer cells (unpublished data). The inducing effect of Daio-Orengedokuto on cell-cycle progression and cell migration in endothelium suggests that Daio-Orengedokuto may be referred to as a drug, inducing angiogenesis, healing wounds, and (or) remodeling vascular tissue. Then we further investigated which signaling molecules were activated by Daio-Orengedokuto and found that extracellular signal-regulated kinase (ERK) phosphorylation and IkappaB degradation were stimulated by the Daio-Orengedokuto treatment in BAECs. More interestingly, pretreatment with PD compound, an ERK inhibitor, blocked the anti-apoptosis induced by Daio-Orengedokuto. In conclusion, Daio-Orengedokuto plays a role in endothelial cell proliferation via activation of MAP kinase.     

Structural basis for calmodulin as a dynamic calcium sensor

Calmodulin is a prototypical and versatile Ca(2+) sensor with EF hands as its high-affinity Ca(2+) binding domains. Calmodulin is present in all eukaryotic cells, mediating Ca(2+)-dependent signaling. Upon binding Ca(2+), calmodulin changes its conformation to form complexes with a diverse array of target proteins. Despite a wealth of knowledge on calmodulin, little is known on how target proteins regulate calmodulin's ability to bind Ca(2+). Here, we take advantage of two splice variants of SK2 channels, which are activated by Ca(2+)-bound calmodulin but show different sensitivity to Ca(2+) for their activation. Protein crystal structures and other experiments show that, depending on which SK2 splice variant it binds to, calmodulin adopts drastically different conformations with different affinities for Ca(2+) at its C-lobe. Such target protein-induced conformational changes make calmodulin a dynamic Ca(2+) sensor capable of responding to different Ca(2+) concentrations in cellular Ca(2+) signaling.     

A flagellum-specific calcium sensor

The flagellar calcium-binding protein (FCaBP) of the flagellated protozoan Trypanosoma cruzi associates with the flagellar membrane via its N-terminal myristate and palmitate moieties in a calcium-modulated, conformation-dependent manner. This mechanism of localization is similar to that described for neuronal calcium sensors, which undergo calcium-dependent changes in conformation, which modulate the availability of the acyl groups for membrane interaction and partner association. To test whether FCaBP undergoes a calcium-dependent conformational change and to explore the role of such a change in flagellar targeting, we first introduced point mutations into each of the two EF-hand calcium-binding sites of FCaBP to define their affinities. Analysis of recombinant EF-3 mutant (E151Q), EF-4 mutant (E188Q), and double mutant proteins showed EF-3 to be the high affinity site (Kd approximately 9 microM) and EF-4 the low affinity site (Kd approximately 120 microM). These assignments also correlated with partial (E188Q), nearly complete (E151Q), and complete (E151Q,E188Q) disruption of calcium-induced conformational changes determined by NMR spectrometry. We next expressed the FCaBP E151Q mutant and the double mutant in T. cruzi epimastigotes. These transproteins localized to the flagellum, suggesting the existence of a calcium-dependent interaction of FCaBP that is independent of its intrinsic calcium binding capacity. Several proteins were identified by FCaBP affinity chromatography that interact with FCaBP in a calcium-dependent manner, but with differential dependence on calcium-binding by FCaBP. These findings may have broader implications for the calcium acyl switch mechanism of protein regulation.     

Characterization of slowly inactivating KV{alpha} current in rabbit pulmonary neuroepithelial bodies: effects of hypoxia and nicotine

Pulmonary neuroepithelial bodies (NEB) form innervated cell clusters that express voltage-activated currents and function as airway O(2) sensors. We investigated A-type K(+) currents in NEB cells using neonatal rabbit lung slice preparation. The whole cell K(+) current was slowly inactivating with activation threshold of approximately -30 mV. This current was blocked approximately 27% by blood-depressing substance I (BDS-I; 3 microM), a selective blocker of Kv3.4 subunit, and reduced approximately 20% by tetraethylammonium (TEA; 100 microM). The BDS-I-sensitive component had an average peak value of 189 +/- 14 pA and showed fast inactivation kinetics that could be fitted by one-component exponential function with a time constant of (tau1) 77 +/- 10 ms. This Kv slowly inactivating current was also blocked by heteropodatoxin-2 (HpTx-2; 0.2 microM), a blocker of Kv4 subunit. The HpTx-2-sensitive current had an average peak value of 234 +/- 23 pA with a time constant (tau) 82 +/- 11 ms. Hypoxia (Po(2) = 15-20 mmHg) inhibited the slowly inactivating K(+) current by approximately 47%, during voltage steps from -30 to +30 mV, and no further inhibition occurred when TEA was combined with hypoxia. Nicotine at concentrations of 50 and 100 microM suppressed the slowly inactivating K(+) current by approximately 24 and approximately 40%, respectively. This suppression was not reversed by mecamylamine suggesting a direct effect of nicotine on these K(+) channels. In situ hybridization experiments detected expression of mRNAs for Kv3.4 and Kv4.3 subunits, while double-label immunofluorescence confirmed membrane localization of respective channel proteins in NEB cells. These studies suggest that the hypoxia-sensitive current in NEB cells is carried by slowly inactivating A-type K(+) channels, which underlie their oxygen-sensitive potassium currents, and that exposure to nicotine may directly affect their function, contributing to smoking-related lung disease.     

Fast inactivating potassium current in cultured hippocampal neurons

Using a voltage-clamp whole-cell technique, we studied transmembrane currents in hippocampal neurons after their long-lasting cultivation. Based on the activational characteristics and data on pharmacological sensitivity, we isolated and described an A-type voltage-activated fast inactivating potassium current (FIPC). This transient FIPC was activated at −50… −40 mV and was rather sensitive to 4-aminopyridine (4-AP). Extracellular application of 5 mM 4-AP decreased the FIPC amplitude by 75%, while application of 10 mM tetraethylammonium evoked no significant changes in it. Kinetics of FIPC activation could be described by one exponent in the fourth degree. With variations of the membrane potential from −40 to 30 mV, the activation time constant varied from 2.8 to 1.5 msec. Inactivation kinetics was described by one exponent with the time constant varying from 37 msec at −45 mV to 18 msec at 40 mV. Stationary activation and inactivation curves could be described by Boltzmann's equation; a half value of the level of stationary inactivation was reached at −80 mV, while stationary activation was attained at −25 mV. Kinetics of deinactivation (from stationary inactivation) was monoexponential with the time constant of 41 msec. It is supposed that FIPC through the membrane of hippocampal neurons is provided by the type Kv4.2 potassium channels.     

Synaptotagmin I functions as a calcium sensor to synchronize neurotransmitter release

To characterize Ca(2+)-mediated synaptic vesicle fusion, we analyzed Drosophila synaptotagmin I mutants deficient in specific interactions mediated by its two Ca(2+) binding C2 domains. In the absence of synaptotagmin I, synchronous release is abolished and a kinetically distinct delayed asynchronous release pathway is uncovered. Synapses containing only the C2A domain of synaptotagmin partially recover synchronous fusion, but have an abolished Ca(2+) cooperativity. Mutants that disrupt Ca(2+) sensing by the C2B domain have synchronous release with normal Ca(2+) cooperativity, but with reduced release probability. Our data suggest the Ca(2+) cooperativity of neurotransmitter release is likely mediated through synaptotagmin-SNARE interactions, while phospholipid binding and oligomerization trigger rapid fusion with increased release probability. These results indicate that synaptotagmin is the major Ca(2+) sensor for evoked release and functions to trigger synchronous fusion in response to Ca(2+), while suppressing asynchronous release.     

Expression of T-type calcium current precedes neurite extension in neuroblastoma cells.

1. N1E-115 mouse neuroblastoma cells morphologically differentiate by extending neurites in a period of seven days after addition of 2% DMSO to the culture medium. We used the whole-cell patch clamp technique to measure calcium currents in these cells under conditions where voltage clamp of the whole membrane was assured. 2. Current densities of both T and L type calcium currents were identical in cells included to differentiate with dibutyryl cyclic AMP and cells induced to differentiate with dimethylsulphoxide (DMSO). Cells differentiated with DMSO were used for all subsequent experiments. 3. All morphologically differentiated cells showed a T type calcium current. In contrast, a minority of morphologically undifferentiated cells did not show a T current. 4. Once expressed, both T and L currents did not change either in current density or in behaviour over a period of five days. 5. These data demonstrate that expression of a T current always precedes neurite extension, and suggest a role for calcium currents in triggering morphological differentiation.     

Inactivation properties of T-type calcium current in canine cardiac Purkinje cells.

The kinetic behavior of T-type Ca2+ current (ICa-T) was studied in canine cardiac Purkinje cells using a single suction-pipette whole-cell voltage clamp method. ICa-T was studied without contamination of conventional L-type Ca2+ current (ICa-L). Ca2+, Sr2+, or Ba2+ were used as the charge carrier. During maintained depolarization ICa-T decayed rapidly, and under most conditions the decay showed a voltage-dependent single exponential time course that did not depend on the species of charge carrier. The development of inactivation did not depend on Ca2+, but the time course required more than a single exponential process. Just negative to the threshold voltage for activating ICa-T, inactivation slowly developed and there was a delay in its onset. The time course of recovery from inactivation was dependent on the protocol used to measure it. As the duration of an inactivating voltage step was increased, recovery slowed markedly and there was a delay in its onset. The time course of recovery could be fit as a biexponential. The fast and slow time constants of recovery were relatively constant, however, the relative amplitudes were dependent on the duration of the inactivating voltage step. Recovery was not dependent on Ca2+, and it was slower at a less negative voltage. These results suggest that the T-type Ca2+ channel in cardiac Purkinje cells follows a complex kinetic scheme dependent only on voltage. This behavior can be accounted for by incorporating into a Markovian model several inactivated and closed states.     

Aminopyridines block an inactivating potassium current having slow recovery kinetics.

The blocking action of aminopyridines on an inactivating K current (lKi) in GH3 pituitary cells was studied before and after altering the macroscopic decay of the current with N-bromoacetamide (NBA). The first depolarizing pulse delivered either seconds or minutes after beginning 4-aminopyridine (4AP) application, elicited a current with both a more rapid decay and a reduced peak amplitude. The rapid decay (or time-dependent block) was especially prominent in NBA-treated cells. With continued drug application, subsequent test pulses revealed a stable block of peak current, greater in NBA-treated than control cells. Recovery from block was enhanced by hyperpolarizing holding potentials and by the first depolarizing pulse delivered after prolonged recovery intervals. Unlike aminopyridine block of other K currents, there was no convincing evidence for voltage shifts in activation or inactivation, or for voltage and frequency-dependent unblock. Increasing the open probability of the channels did, however, facilitate the block. Although the behavior of currents in 4AP was suggestive of "open channel block," the block was not produced by 4-aminopyridine methiodide, a positively charged aminopyridine. Moreover, because partial block and recovery occurred without opening the channels we suggest that aminopyridines bind to, or near, this K channel, that this binding is enhanced by opening the channel, and that a conformational change is induced which mimics inactivation. Because recovery from block is enhanced by negative potentials, we suggest that aminopyridine molecules may become "trapped" by inactivation awaiting the slow process of reactivation to escape their binding sites.     

Tuning of a neuronal calcium sensor

Recoverin is a Ca(2+)-regulated signal transduction modulator expressed in the vertebrate retina that has been implicated in visual adaptation. An intriguing feature of recoverin is a cluster of charged residues at its C terminus, the functional significance of which is largely unclear. To elucidate the impact of this segment on recoverin structure and function, we have investigated a mutant lacking the C-terminal 12 amino acids. Whereas in myristoylated recoverin the truncation causes an overall decrease in Ca(2+) sensitivity, results for the non-myristoylated mutant indicate that the truncation primarily affects the high affinity EF-hand 3. The three-dimensional structure of the mutant has been determined by x-ray crystallography. In addition to significant changes in average coordinates compared with wild-type recoverin, the structure provides strong indication of increased conformational flexibility, particularly in the C-terminal domain. Based on these observations, we propose a novel role of the C-terminal segment of recoverin as an internal modulator of Ca(2+) sensitivity.     

A transient outward current dependent on external calcium in rat cerebellar granule cells

Summary The outward potassium current of rat cerebellar granule cells in culture was studied with the whole-cell patch-clamp method. Two voltage-dependent components were identified: a slow current, resembling the classical delayed rectifier current, and a fast component, similar to anI_A-type current. The slow current was insensitive to 4-aminopyridine and independent of external Ca²⁺, but significantly inhibited by 3mM tetraethylammonium. The fast current was depressed by external 4-aminopyridine, with an ED₅₀=0.7mM, and it was abolished by removal of divalent cations from the external medium. The sensitivity of the transient outward current to different divalent cations was investigated by equimolar substitution of Ca²⁺, Mn²⁺ and Mg²⁺. In 2.8mM Mn²⁺, the transient potassium conductance was comparable to that in 2.8mM Ca²⁺, while in 2.8mM Mg²⁺ the transient component was drastically reduced, as in the absence of any divalent cations. However, when Ca²⁺ was present, Mg²⁺ up to 5mM had no effect. The transient current increased with increasing concentrations of external Ca²⁺, [Ca²⁺]_o, and the maximum conductancevs. [Ca²⁺]_o curve could be approximated by a one-site model. In addition, the current recorded with 5.5mM BAPTA in the intracellular solution was not different from that recorded in the absence of any Ca²⁺ buffer. These results suggest that divalent cations modulate the potassium channel interacting with a site on the external side of the cell membrane.     

Osmotic induction of calcium accumulation in human embryonic kidney cells detected with a high sensitivity FRET calcium sensor

Calcium serves as a second messenger in glucose-triggered insulin secretion of pancreatic cells. Less is known about sugar signaling in non-excitable cells. Here, the high sensitivity FRET calcium sensor TN-XXL was used to characterize glucose-induced calcium responses in non-excitable human embryonic kidney HEK293T cells. HEK293T cells responded to perfusion with glucose with a sustained and concentration-dependent increase in cytosolic calcium levels. Sucrose and mannitol triggered comparable calcium responses, suggesting that the increase of the calcium concentration was caused by osmotic effects. HEK293T cells are characterized by low endogenous glucose uptake capacity as shown with a high sensitivity glucose sensor. Consistently, when glucose influx was artificially increased by co-expression of GLUT glucose transporters, the glucose-induced calcium increase was significantly reduced. Neither calcium depletion, nor gadolinium or thapsigargin were able to inhibit the calcium accumulation. Taken together, membrane impermeable osmolytes such as sucrose and mannitol lead to an increase in calcium levels, while the effect of glucose depends on the cell's glucose uptake capacity and will thus vary between cell types in the body that differ in their glucose uptake capacity.     

Synaptotagmin VII is targeted to secretory organelles in PC12 cells, where it functions as a high-affinity calcium sensor

Synaptotagmin (syt) I is thought to act as a Ca2+ sensor that regulates neuronal exocytosis. Fifteen additional isoforms of syt have been identified, but their functions are less well understood. Here, we used PC12 cells to test the idea that different isoforms of syt impart cells with distinct metal (i.e., Ca2+, Ba2+, and Sr2+) requirements for secretion. These cells express syt's I and IX (syt IX sometimes referred to as syt V), which have low apparent metal affinities, at much higher levels than syt VII, which we show has a relatively high apparent affinity for metals. We found that syt I and VII partially colocalize on large dense core vesicles and that upregulation of syt VII produces a concomitant increase in the divalent cation sensitivity of catecholamine release from PC12 cells. Furthermore, RNA interference-mediated knockdown of endogenous syt VII reduced the metal sensitivity of release. These data support the hypothesis that the complement of syt's expressed by a cell, in conjunction with their metal affinity, determines the divalent cation sensitivity of exocytosis.     

A rapidly activating and slowly inactivating potassium channel cloned from human heart. Functional analysis after stable mammalian cell culture expression

The electrophysiological properties of HK2 (Kv1.5), a K+ channel cloned from human ventricle, were investigated after stable expression in a mouse Ltk- cell line. Cell lines that expressed HK2 mRNA displayed a current with delayed rectifier properties at 23 degrees C, while sham transfected cell lines showed neither specific HK2 mRNA hybridization nor voltage-activated currents under whole cell conditions. The expression of the HK2 current has been stable for over two years. The dependence of the reversal potential of this current on the external K+ concentration (55 mV/decade) confirmed K+ selectivity, and the tail envelope test was satisfied, indicating expression of a single population of K+ channels. The activation time course was fast and sigmoidal (time constants declined from 10 ms to < 2 ms between 0 and +60 mV). The midpoint and slope factor of the activation curve were Eh = -14 +/- 5 mV and k = 5.9 +/- 0.9 (n = 31), respectively. Slow partial inactivation was observed especially at large depolarizations (20 +/- 2% after 250 ms at +60 mV, n = 32), and was incomplete in 5 s (69 +/- 3%, n = 14). This slow inactivation appeared to be a genuine gating process and not due to K+ accumulation, because it was present regardless of the size of the current and was observed even with 140 mM external K+ concentration. Slow inactivation had a biexponential time course with largely voltage-independent time constants of approximately 240 and 2,700 ms between -10 and +60 mV. The voltage dependence of slow inactivation overlapped with that of activation: Eh = -25 +/- 4 mV and k = 3.7 +/- 0.7 (n = 14). The fully activated current-voltage relationship displayed outward rectification in 4 mM external K+ concentration, but was more linear at higher external K+ concentrations, changes that could be explained in part on the basis of constant field (Goldman-Hodgkin-Katz) rectification. Activation and inactivation kinetics displayed a marked temperature dependence, resulting in faster activation and enhanced inactivation at higher temperature. The current was sensitive to low concentrations of 4- aminopyridine, but relatively insensitive to external TEA and to high concentrations of dendrotoxin. The expressed current did not resemble either the rapid or the slow components of delayed rectification described in guinea pig myocytes. However, this channel has many similarities to the rapidly activating delayed rectifying currents described in adult rat atrial and neonatal canine epicardial myocytes.(ABSTRACT TRUNCATED AT 400 WORDS)