CGRP-induced activation of KATP channels in follicular Xenopus oocytes

The two-microelectrode voltage-clamp technique was used to monitor K⁺ channel activity in Xenopus oocyte follicular cells, which are electrically coupled to the oocyte itself by gap junctions. Endogenous vasodilators such as calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), prostaglandin E₂ (PGE₂) and adenosine activate glibenclamide-ATP-sensitive K⁺ (K_ATP) channels in Xenopus oocyte follicular cells. The mechanism of action of CGRP was studied in detail. CGRP effects undergo a rapid desensitization. CGRP acts via CGRP_I receptors. Its effects are antagonized by the amino-truncated CGRP analog hCGRP(8–37). The second messenger for CGRP activation of K_ATP channels is cAMP. Phosphodiesterase inhibition by 3-isobutyl-1-methylxanthine enhances the CGRP response while adenyl cyclase inhibition by either 2,5-dideoxyadenosine or progesterone nearly completely depresses the CGRP response. Vasoconstrictors such as ACh and angiotensin II also have receptors in follicular cells. ACh strongly inhibits the CGRP activation of K⁺ channels as it inhibits the activation of K_ATP channels by P1060, but angiotensin II does not. It is concluded that as in vascular smooth muscle cells, CGRP and probably other hyperpolarizing vasodilators open K_ATP channels in follicular cells by protein kinase A activation.Thanks are due to C. Roulinat and F. Aguila for expert technical assistance. This work was supported by the Centre National de la Recherche Scientifique (CNRS).     

Acetylcholine-mediated K+ channel activity in guinea-pig atrial cells is supported by nucleoside diphosphate kinase

We studied the role of nucleoside diphosphate kinase (NDPK) in acetylcholine-mediated muscarinic K⁺ channel activation in inside-out patches of guinea-pig atrial cells. NDPK-catalysed activation of the muscarinic K⁺ channels by adenosine triphosphate-Mg²⁺ (ATP-Mg²⁺) is not prevented by occupation of the muscarinic receptor [by acetylcholine (ACh) or atropine], nor by uncoupling of the receptor from the G protein by pertussis-toxin-catalysed adenosine diphosphate (ADP)-ribosylation of G_K. In the presence of ACh, addition of 0.1 mM guanosine triphosphate (GTP) after activation of the channels by 4 mM ATP alone resulted in a moderate increase of channel activity (in contrast to block in the absence of ACh): NDPK-mediated direct transphosphorylation is uncoupled by the G nucleotide but agonist-induced guanosine diphosphate (GDP)-to-GTP exchange takes over activation of the channels. Moreover, ACh-dependent channel stimulation was possible in inside-out patches while ATP and GDP were present in the bathing solution (in contrast to the complete absence of channel activation in the absence of ACh). This indicates that NDPK synthesises sufficient GTP to support channel activation by exchange. Hence, it is postulated that the main functional role of NDPK under physiological conditions is to provide a local supply of GTP (using GDP and ATP) in the immediate vicinity of the G protein, thereby maintaining a high local GTP/GDP ratio and ensuring adequate receptor-mediated regulation of muscarinic K⁺ channel activity.     

Potassium channel openers act through an activation of ATP-sensitive K+ channels in guinea-pig cardiac myocytes

In a previous article (Escande et al. 1988a), we have shown that cromakalim (BRL 34915), a potassium channel opener (PCO), is a potent activator of ATP-sensitive K⁺ channels in cardiac cells. In the present article, the influence on K⁺ channels of two other potassium channel openers chemically unrelated to cromakalim, RP 49356 and pinacidil, has been investigated in patch-clamped isolated cardiac myocytes. In the whole-cell configuration, K⁺ currents were recorded in the presence of 50 M TTX and 3 M nitrendipine or 3 mM cobalt. Like cromakalim, RP 49356 or pinacidil activated a time-independent outward current at 33–35°C but not at 19–21°C, which showed little voltage-dependency in the potential range –60 to +60 mV. Its amplitude was a function of the agonist concentration, e.g. it was 2.1±0.4 nA at +60 mV with 30 M RP 49356 and 4.3±0.8 nA with 300 M. In control conditions, glibenclamide, a blocker of K⁺-ATP channels in pancreatic and heart cells, affected neither the inward rectifier,i _K1, nor the delayed K⁺ current,i _K. At 3 M, glibenclamide fully prevented the effects of 300 M RP 49356 or pinacidil. At lower concentrations, glibenclamide partially counteracted the activation by PCOs of a K⁺ current. In the cell-attached contiguration, externally applied RP 49356 or pinacidil caused opening of large channels which reversed around 0 mV in a high K⁺ external medium. In inside-out patches, both RP 49356 or pinacidil activated K⁺-ATP channels by increasing the time period for which the channels remained in the open state. It is concluded that, like cromakalim, RP 49356 and pinacidil are potent activators of K⁺-ATP channels in cardiac myocytes.     

Comparative roles of ATP-sensitive K channels and Ca-activated BK channels in posthypoxic hyperexcitability and rapid hypoxic preconditioning in hippocampal CA1 pyramidal neurons in vitro

The aim of this study was to investigate the comparative effects of glibenclamide (GC), a selective blocker of K⁺_ATP channels, and iberiotoxin (IbTX), a selective blocker of BK⁺_Ca channels, on the repeated brief hypoxia-induced posthypoxic hyperexcitability and rapid hypoxic preconditioning in hippocampal CA1 pyramidal neurons in vitro. The method of field potentials measurement in CA1 region of the rat hippocampal slices was used. In contrast to GC (10 μM), IbTX (10 nM) significantly abolished both posthypoxic hyperexcitability and rapid hypoxic preconditioning induced by brief hypoxic episodes. These effects of IbTX did not depend on its ability to reduce the hypoxia-induced decrease of population spike (PS) amplitude during hypoxic episodes since GC (10 μM), comparatively with IbTX (10 nM), significantly reduced the depressive effect of hypoxia on the PS amplitude during hypoxic episodes but did not abolish both posthypoxic hyperexcitability and rapid hypoxic preconditioning in CA1 pyramidal neurons. Our results indicated that BK⁺_Ca channels, in comparison with K⁺_ATP channels, play a more important role in such repeated brief hypoxia-induced forms of neuroplasticity in hippocampal CA1 pyramidal neurons as posthypoxic hyperexcitability and rapid hypoxic preconditioning.     

Metabolic inhibition and low internal ATP activate K-ATP channels in rat dopaminergic substantia nigra neurones

The patch-clamp technique was used to study whole-cell currents of acutely dissociated rat substantia nigra (SN) neurones. In perforated-patch current-clamp recordings, inhibition of mitochondrial metabolism by rotenone (5 M) produced a hyperpolarisation and inhibited electrical activity. These effects were reversed by the sulphonylureas tolbutamide (0.5 mM) or glibenclamide (0.5 M). Under voltageclamp conditions, rotenone induced a timeand voltage-independent K⁺ current which was selectively blocked by sulphonylureas. The glibenclamide-sensitive current reversed at –81.7±2.7 mV (n=5) and showed marked inward rectification. Intracellular dialysis with 0.3 mM adenosine 5-triphosphate (ATP), but not 2 mM or 5 mM ATP, in standard whole-cell recordings also resulted in activation of a sulphonylurea-sensitive K⁺ current with similar properties (reversal potential, –81.9±2.5 mV, n=5). The close similarity in the properties of the ATP-sensitive K⁺ current observed in whole-cell recordings and the K⁺ current activated by metabolic inhibition in perforated-patch recordings suggest that they both result from activation of the same type of ATP-sensitive K⁺ channel. Sulphonylureas had no effect on electrical activity or membrane currents in the absence of rotenone in perforated-patch recordings, or in cells dialysed with 5 mM ATP, indicating that in SN neurones these drugs are selective for the ATP-sensitive K⁺ current.     

K+-dependent stability and ion conduction of Shab K+ channels: a comparison with Shaker channels

K⁺ depletion exerts dramatically variable effects on different potassium channels. Here we report that Shab channels are rather stable in the absence of either internal or external K⁺ alone; however, its stability is greater with K⁺ outside the cell. In contrast, with 0 K⁺ (non-added) solutions on both sides of the membrane, the conductance (G_K) is rapidly and irreversibly lost. G_K is lost with the channels closed and regardless of the composition of the 0 K⁺ solutions. In comparison, it is known that the Shaker B G_K collapses only if the channels are gated in 0 K⁺, Na⁺-containing solutions. In order to compare the behavior of Shab to that of Shaker, we show that after extensively gating the channels in 0 K⁺ N-methyl-D-glucamine solutions, most Shaker channels remain stable, and in a conformation where G_K collapses as soon as there is Na⁺ in the solutions. Regarding ion conduction, in contrast to Kv2.1 and Shaker A463C that have a sizable G_Na in 0 K⁺, Shab, which shares a 463-cysteine and an identical signature sequence with these channels, does not appreciably conduct Na⁺, although it presents a significant Cs⁺ conductance. The observations suggest that there are at least two sites where K⁺ binds and thus maintains Shab G_K stable, one internal and the other(s) most likely located outside the selectivity filter.     

ATP-Dependent inhibition of Ca2+-activated K+ channels in vascular smooth muscle cells by neuropeptide Y

Neuropeptide Y(NPY) inhibits Ca²⁺-activated K⁺ channels reversibly in vascular smooth muscle cells from the rat tail artery. NPY (200 M) had no effect in the absence of intracellular adenosine 5triphosphate (ATP) and when the metabolic poison cyanide-M-chlorophenyl hydrozone (10 M) was included in the intracellular pipette solution. NPY was also not effective when ATP was substituted by the non-hydrolysable ATP analogue adenosine 5-[, -methylene]-triphosphate (AMP-PCP). NPY inhibited Ca²⁺-activated K⁺ channel activity when ATP was replaced by adenosine 5-O-(3-thiotriphosphate) (ATP [-S]) and the inhibition was not readily reversed upon washing. Protein kinase inhibitor (1 M), a specific inhibitor of adenosine 3, 5-cyclic monophosphatedependent protein kinase, had no significant effect on the inhibitory action of NPY. The effect of NPY on single-channel activity was inhibited by the tyrosine kinase inhibitor genistein (10 M) but not by daidzein, an inactive analogue of genistein. These observations suggest that the inhibition by NPY of Ca²⁺-activated K⁺ channels is mediated by ATP-dependent phosphorylation. The inhibitory effect of NPY was antagonized by the tyrosine kinase inhibitor genistein.     

ATP maintains ATP-inhibited K+ channels in an operational state

In patch-clamp records of K⁺ _ATP channels in an insulin-secreting cell line (RINm5F) inhibition evoked by exposing the internal surface of the membrane to ATP is followed not just by the recovery of K⁺ _ATP channel activity when the ATP is removed but by a marked activation of K⁺ _ATP channels. This phenomenon is not a direct consequence of channel closure as inhibition induced by quinidine and quinine is followed upon the removal of the drug only by the recovery of K⁺ _ATP channel activity and not by post-inhibitory activation. If ATP is applied to the exposed internal surface of a membrane patch when all of its K⁺ _ATP channel have run down subsequent removal of the ATP causes their activation. The magnitude and duration of the reactivation of K⁺ _ATP channels is shown to depend upon both the concentration of ATP and the length of time for which the membrane is exposed to ATP. We therefore have a paradoxical situation in that K⁺ channels which are inhibited by intracellular ATP require intracellular ATP to retain the ability to open.     

Functional linkage of the cardiac ATP-sensitive K+ channel to the actin cytoskeleton

The role of the cytoskeleton in the rundown and reactivation of adenosine triphosphate (ATP) sensitive K⁺ channels (KATP channels) was examined by perturbing selectively the intracellular surface of inside-out membrane patches excised from guinea-pig ventricular myocytes. Actin filament-depolymerizing agents (cytochalasins and desoxyribonuclease I) accelerated channel rundown, while actin filament stabilizer (phalloidin) or phosphatidylinositol biphosphate (PIP₂; inhibitor of F-actin-severing proteins) inhibited spontaneous and/or Ca²⁺-induced rundown. When rundown was induced by cytochalasin D or by long exposure to high Ca²⁺, channel activity could not be restored by exposure to MgATP, but application of F-actin with MgATP could reinstitute channel activity. The processes of rundown and reactivation of cardiac K_ATP channels may thus be influenced by the assembly and disassembly of the actin cytoskeletal network, which provides a novel regulatory mechanism of this channel.     

Blockade of dopamine storage,but not of dopamine synthesis,prevents activation of a tolbutamide-sensitive K+ channel in the guinea-pig substantia nigra

The substantia nigra has one of the highest levels of ATP-sensitive K⁺ channel in the brain. Since this channel is controlled by cell metabolism, the aim of this study was to see how closely it is associated with nigral dopamine systems, which are decreased in Parkinson's disease. In a sub-population of neurons within the rostral substantia nigra pars compacta of the guinea-pig, a brief period of hypoxia resulted in a tolbutamide (100–500 M) sensitive hyperpolarisation [input resistance (IR) decrease from 144.88±14.04 M pre-hypoxia to 105.91±13.25 M during hypoxia]. Maximal blockade of this decrease was seen in presence of 500 m tolbutamide [IR decrease only from 161.35±32.82 M to 155.02±34.29 M]. Reserpine (which depletes dopamine stores) but not -methyl-para-tyrosine (which decreases de novo synthesis of dopamine) caused a marked attenuation of this hyperpolarisation [IR decrease only from 163.32±44.42 M pre-hypoxia to 154.42±50.97 M during hypoxia]. This observation suggests that blockade of dopamine storage, but not of de novo synthesis, leads to a loss of responsiveness of certain mid-brain neurons to hypoxia, rendering them potentially more susceptible to subsequent degeneration. The possible link between nigral dopamine systems and ATP-sensitive K⁺ channels is discussed.     

GTP and GDP activation of K+ channels that can be inhibited by ATP

K⁺ channels that can be inhibited by intracellular ATP have been found in many different cell types. In the insulin-secreting pancreatic islet cells these channels are of crucial importance for stimulus-secretion coupling as glucose stimulation closes the ATP-sensitive channels which leads to depolarization and firing of Ca²⁺ action potentials. We now demonstrate that nucleotides other than ATP also influence the gating of these K⁺ channels. In contrast to the action of ATP, GTP (10 M – 1 mM) and GDP (100 M to 1 mM) evoke dose-dependent channel activation and this effect is immediately reversible. Phosphorylation is not directly involved as non-hydrolysable GTP-and GDP-analogues also evoke channel opening. ATP reversibly inhibits opening of the GTP- or GDP- activated K⁺ channels.     

Small and maxi K+ channels in the basolateral membrane of isolated crypts from rat distal colon: single-channel and slow whole-cell recordings

The patch-clamp technique was used to characterize K⁺ channel activity in the basolateral membrane of isolated crypts from rat distal colon. In cell-attached patches with KCl in the pipette, channels with conductances ranging from 6 pS to 80 pS appeared. With NaCl in the pipette and KCl in the bath in excised inside-out membrane patches a small-conductance channel with a mean conductance of 12±6 pS (n=18) was observed. The channel has been identified as K⁺ channel by its selectivity for K⁺ over Na⁺ and by its sensitivity to conventional K⁺ channel blockers, Ba²⁺ and tetraethylammonium (TEA⁺). Changes of cytosolic pH did not attenuate channel activity. Activity of the 12-pS channel was increased by membrane depolarization and elevated cytosolic Ca²⁺ concentration. In addition, a maxi K⁺ channel with a mean conductance of 187±15 pS (n=4) in symmetrical KCl solutions was only occasionally recorded. The maxi K⁺ channel could be blocked by Ba²⁺ (5 mmol/l) on the cytosolic side. Using the slow-whole cell recording technique under control conditions, a cell membrane potential of –70±10mV (n=18) was measured. By application of various K⁺ channel blockers such as glibenclamide, charybdotoxin, apamin, risotilide, Ba²⁺ and TEA⁺ in the bath, only Ba²⁺ and TEA⁺ depolarized the cell membrane. The present data suggest that the small K⁺ channel (12 pS) is involved in the maintenance of the cell membrane resting potential.     

The flickery block of ATP-dependent potassium channels of skeletal muscle by internal 4-aminopyridine

We have examined the effects of 4-aminopyridine (4-AP) on single ATP-dependent potassium channels in patches excised from frog skeletal muscle. 4-AP applied to the internal face of the membrane caused a flickery block. We could not detect any flickery block when 10 mM 4-AP was applied to the external surface of the membrane. The reduction in mean unitary current by internal 4-AP was consistent with 11 binding with a K _d of 3.3 mM at 0 mV. The block was voltage-dependent, increasing with depolarization with an effective valency of 0.57. Rate constants for blocking and unblocking by 4-AP were obtained by fitting functions to the distribution of current amplitudes. Both rate constants were voltage-dependent. At 0 mV they were 17 mM^–1 ms^–1 and 61 ms^–1. Simulation of the block using these rate constants produced a flickery block very similar to that observed experimentally.     

K+ channels stimulated by glucose: a new energy-sensing pathway

Insights into how sugar can turn off cell activity are emerging from studies of hypothalamic neurons. Brain states are coordinated by hypothalamic orexin/hypocretin neurons, whose loss leads to narcoleptic instability of consciousness and inability to rouse when hungry. Recent studies indicate that glucose blocks the electrical activity of orexin cells by opening K⁺ channels in their membrane. This new energy-sensing mechanism is so sensitive that even small changes in glucose levels, of the type occurring between meals, can turn orexin cells on and off. Glucose-stimulated K⁺ channels share biophysical properties with “leak” (two-pore domain) K⁺ channels, the newest and least understood K⁺ channel family. A hypothesis is outlined whereby the stimulation of brain K⁺ channels by sugar could relieve stress and enhance reward, although probably at a cost of increased sleepiness.     

K+ channels in PC12 cells are affected by propofol

The effect on K⁺ currents (I _K) of the general anaesthetic propofol (PR) (2,6-diisopropylphenol) was tested in undifferentiated clonal pheochromocytoma (PC12) cells using the patch-clamp technique in whole-cell and single-channel configurations. PR decreased macroscopic I _K amplitudes in a concentration-dependent way from 50 M to 1 mM. The blocking effect was unchanged by repetitive depolarizing pulses and it was independent of the holding potential. Whereas activation of I _K in control conditions was fitted by sigmoidal plus exponential time courses, only the sigmoidal time course gave an adequate fit with PR in the bath. The above effects were reversible. PR concentrations below 140 M decreased single-channel activity for K⁺ channels with unitary conductance of 22 pS, in the voltage range between –40 and 60 mV from a holding potential of –50 mV. In contrast, the anaesthetic had nearly no effect on the opening probability of a channel with conductance of 10 pS. The unitary current amplitudes were unaffected in both channel types. These results suggest that PR action on I _K may depend on the different blocking mechanisms of the K⁺ channels.     

L-alanine evokes opening of single Ca2+-activated K+ channels in rat liver cells

Cellular uptake of neutral amino acids via Na⁺ cotransporters is known to be associated with an increased membrane K⁺ conductance mediated by an unknown mechanism that is essential for avoiding excessive cell swelling. We now demonstrate by patch-clamp single-channel current recording that exposure of rat liver cells to L-alanine, but not the poorly transported D-stereoisomer, evokes opening of single K⁺ channels and that this effect is reversible upon removal of the amino acid. The nature of the conductance pathways opened in the intact cell by L-alanine has been investigated in cell-free excised membrane patches where it can be shown that the K⁺-selective channels are opened by Ca²⁺ acting from the inside of the membrane at a concentration as low as 0.1 M.     

Roles of K+ channels in regulating tumour cell proliferation and apoptosis

K⁺ channels are a most diverse class of ion channels in the cytoplasmic membrane and are distributed widely in a variety of cells including cancer cells. Cell proliferation and apoptosis (programmed cell death or cell suicide) are two counterparts that share the responsibility for maintaining normal tissue homeostasis. Evidence has been accumulating from fundamental studies indicating that tumour cells possess various types of K⁺ channels, and that these K⁺ channels play important roles in regulating tumour cell proliferation and apoptosis, i.e. facilitating unlimited growth and promoting apoptotic death of tumour cells. The potential implications of K⁺ channels as a pharmacological target for cancer therapy and a biomarker for diagnosis of carcinogenesis are attracting increasing interest. This review aims to provide a comprehensive overview of current status of research on K⁺ channels/currents in tumour cells. Focus is placed on the roles of K⁺ channels/currents in regulating tumour cell proliferation and apoptosis. The possible mechanisms by which K⁺ channels affect tumour cell growth and death are discussed. Speculations are also made on the potential implications of regulation of tumour cell proliferation and apoptosis by K⁺ channels.     

Activation of K+ currents in cultured Schwann cells is controlled by extracellular pH

We analyzed the pH dependence of K⁺ currents recorded with the patch-clamp technique from cultured Schwann cells obtained from mouse dorsal root ganglia. Currents were activated at potentials more positive than –50 mV which was close to the resting membrane potential. Current amplitudes were affected by a change in extracellular pH (pH_o), being increased at alkaline, and decreased at acidic pH_o. The strongest effect of a pH_o change was observed on currents activated close to the resting membrane potential suggesting a functional role for the pH sensitivity of K⁺ currents. Analysis of the time course of current activation at different pH_o values led to the conclusion that the pH-sensitivity of K⁺ currents in Schwann cells is due to changes in surface charges shifting the potential sensed by the gating process of the channel. The reversal potential of the currents was not affected by a change in pH_o. This observation and the finding that even a strong acidification to a pH_o value of 5.0 did not lead to a blockade of the fully activated channel, indicate that the pH-sensitive charges are not located in the channel pore. Under the assumption that pH_o changes in a peripheral nerve are associated with nerve activity as in the optic nerve, the pH-sensitive K⁺ channel in Schwann cells could serve to facilitate the spatial buffering of extracellular K⁺.     

Basolateral K+ efflux is largely independent of maxi-K+ channels in rat submandibular glands during secretion

The involvement of large-conductance, voltage- and Ca²⁺-activated K⁺ channels (maxi-K⁺ channels) in basolateral Ca²⁺-dependent K⁺-efflux pathways and fluid secretion by the rat submandibular gland was investigated. Basolateral K⁺ efflux was monitored by measuring the change in K⁺ concentration in the perfusate collected from the vein of the isolated, perfused rat submandibular gland every 30 s. Under conditions in which the Na⁺/K⁺-ATPase and Na⁺-K⁺-2Cl^– cotransporter were inhibited by ouabain (1 mmol/l) and bumeta-nide (50 mol/l) respectively, continuous stimulation with acetylcholine (ACh) (1 mol/l) caused a transient large net K⁺ efflux, followed by a smaller K⁺ efflux, which gradually returned to the basal level within 10 min. These two components of the K⁺ efflux appear to be dependent on an increase in cytosolic Ca²⁺ concentration. The initial transient K⁺ efflux was not affected by charybdotoxin (100 nmol/l) or tetraethylammonium (TEA) (5 mmol/l) but the smaller second component was strongly and reversibly inhibited by charybdotoxin (100 nmol/l) and TEA (0.1 and 5 mmol/l). The initial K⁺ efflux transient induced by ACh was inhibited by quinine (0.1–3 mmol/l), quinidine (1–3 mmol/l) and Ba²⁺ (5 mmol/l), but not by verapamil (0.1 mmol/l), lidocaine (1 mmol/l), 4-aminopyridine (1 mmol/l) or apamin (1 mol/l). Ca²⁺-dependent transient large K⁺ effluxes induced by substance P (0.01 mol/l) and A23187 (3 mol/l) were not inhibited by TEA (5 mmol/l or 10 mmol/l). A23187 (3 mol/l) evoked a biphasic fluid-secretory response, which was not inhibited by TEA (5 mmol/l). Patch-clamp studies confirmed that the whole-cell outward K⁺ current attributable to maxi-K⁺ channels obtained from rat submandibular endpiece cells was strongly inhibited by the addition of TEA (1–10 mmol/l) to the bath. It is concluded that maxi-K⁺ channels are not responsible for the major part of the Ca²⁺-dependent basolateral K⁺ efflux and fluid secretion by the rat submandibular gland.     

Contribution of potassium accumulation in narrow extracellular spaces to the genesis of nicorandil-induced large inward tail current in guinea-pig ventricular cells

The mechanism of nicorandil-induced large inward tail current (I _tail) in single guinea-pig ventricular cells was investigated using the whole-cell patch-clamp technique. In the presence of 0.5–1.0 mM nicorandil, an activator of adenosine 5-triphosphate (ATP)-sensitive K⁺ current (I _KATP), a depolarization pulse causing a large outward current was followed by a large inward I _tail on the repolarization step to the holding potential at-85 mV. The larger the outward current, the greater the I _tail. The amplitude of I _tail increased as a single exponential function (=74.9 ms) as the duration of preceding depolarization was prolonged. Both the outward current and I _tail were inhibited nearly completely after application of glibenclamide (1 M), a specific blocker of I _KATP. Substitution of K⁺ with Cs⁺ in both the external and internal solutions resulted in a virtual elimination of I _tail. I _tail was well preserved under the condition where Ca²⁺ entry during the preceding depolarization was largely inhibited or where external Na⁺ was replaced by Li⁺. A transient positive shift of reversal potential for the net current was observed at the peak of I _tail. At 30 mM external K⁺ concentration, I _tail was almost eliminated. From these findings, it is concluded that the I _tail is a K⁺ current associated with an alteration of the K⁺ equilibrium potential (E _K) following a substantial K⁺ efflux. This E _K change is most likely explained by an accumulation of K⁺in transverse tubules (T-tubules) since I _tail was not induced in atrial cells in which T-tubules are poorly developed.