Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7
Submitted 3 November 2003 ; accepted in final form 26 June 2004
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
L-type calcium current; magnesium; spermine; ATP
Despite the apparent absence of classic Ito in guinea pig ventricular myocytes, there have been sporadic accounts of "transient outward" K+ currents in these cells. Twenty years ago, Matsuda and Noma (34) recorded a time-dependent outward current at plateau potentials in myocytes dialyzed with a low-K+ (10 mM) pipette solution. The current overlapped with inward L-type Ca2+ current (ICa,L), was suppressed on removal of external K+ (K) or addition of Ba2+, and was attributed to intrinsic voltage-dependent gating of IK1. Later, Inoue and Imanaga (16) described an "A-type" transient outward K+ current that developed upon removal of external divalent cations; however, follow-up investigation by others established that this transient is caused by the outward movement of monovalent cations through L-type Ca2+ channels (9, 30). A time-dependent, Ba2+-sensitive, "patch-duration-dependent," AP-insensitive transient outward K+ current [IK(PDD)] was described by Martin et al. (31). This current had a linear dependence on voltage and was prominent only in myocytes that were dialyzed for prolonged time periods. More recently, Li and colleagues (24, 25) described a 4-AP-insensitive transient outward current that was not dependent on prolonged dialysis, termed Ito,ir. This current was suppressed by removal of K
or addition of 0.2 mM Ba2+, two interventions that are commonly used to suppress IK1. However, Ito,ir differed from IK1 in that it displayed prominent time dependence, relatively weak rectification, and insensitivity to dialysate Mg2+ and spermine. For these reasons, Ito,ir was thought to be carried by a novel voltage-gated K+ channel.
During the course of studying the effects of low-K+ dialysates on membrane currents in guinea pig ventricular myocytes, we observed a 4-AP-insensitive, ICa,L-overlapping, time-dependent outward current that appeared to be similar to the current described by Matsuda and Noma (34). The present study was undertaken to investigate the properties and identity of the overlapping current.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Ventricular Myocytes
Male guinea pigs (250350 g) were killed by cervical dislocation, and single ventricular myocytes were enzymatically isolated as described previously (38). The excised hearts were mounted on a Langendorff column and perfused through the aorta with Ca2+-free Tyrodes solution (37°C) containing collagenase (0.10.15 mg/ml; Yakult Pharmaceutical, Tokyo, Japan) for 1015 min. The cells were dispersed and stored at 22°C in a high-K+, low-Na+ solution supplemented with 50 mM glutamic acid and 20 mM taurine. A few drops of the cell suspension were placed in a 0.3-ml perfusion chamber mounted on an inverted microscope stage. After the cells had settled to the bottom, the chamber was perfused (2 ml/min) with normal Tyrodes solution at 3536°C. Normal Tyrodes solution contained (in mM) 140 NaCl, 5.4 KCl, 1.8 CaCl2, 1 MgCl2, 10 glucose, and 5 N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES; pH 7.4 with NaOH). Na+-free solution contained N-methyl-D-glucamine (NMDG+) instead of Na+, and K+-free solution contained NMDG+ instead of K+. Each of these solutions contained 3 µM E4031 to block rapidly activating IKr and 3 µM glibenclamide to suppress any contribution of ATP-sensitive K+ current to net membrane current.
Whole cell membrane currents were recorded using an EPC-7 amplifier (List Electronic, Darmstadt, Germany). Recording pipettes were fabricated from thick-walled borosilicate glass capillaries (H15/10/137; Jencons Scientific, Leighton Buzzard, UK) and filled with a Na+-free solution to minimize contributions from Na+-K+ pump, Na+/Ca2+ exchanger, and other Na+-dependent currents. The standard filling solution contained (in mM) 10 KCl, 20 CsCl, 110 CsOH, 110 aspartic acid, 5 MgATP, 5 ethylene glycol-bis(-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), and 5 HEPES (pH 7.2 with KOH). NMDG+-based filling solution was made by substituting NMDG+ for Cs+, and sucrose-based solution was made by replacing CsCl and Cs-aspartate with 265 mM sucrose. In some experiments, the calculated free Mg2+ concentration of the standard pipette solution was modified from normal 0.6 mM to 1 µM or 7 mM; in others, the MgATP of standard solution was decreased from 5 to 0 mM with compensatory addition of 0.7 mM MgCl2 and 4 mM Cs aspartate. The pipettes had resistances of 23 M
when filled with pipette solution; series resistance ranged between 3 and 7 M
and was compensated by 6080%. Membrane current signals were filtered at 3 kHz and digitized with an A/D converter (Digidata 1200A; Axon Instruments, La Jolla, CA) and pCLAMP software (Axon Instruments) at a sampling rate of 8 kHz before analysis.
Drugs
E4031 (Eisai, Tokyo, Japan) and glibenclamide (Sigma, St. Louis, MO) were dissolved in the bathing solution. Nisoldipine was kindly provided by Bayer (Etobicoke, ON, Canada), dissolved in dimethyl sulfoxide (0.1 M stock solution; Sigma), and stored in the dark at 20°C. Appropriate amounts of stock solution were freshly added to bathing solutions, and these were protected from the light during experiments. Spermine (Sigma) was dissolved in the pipette solution.
Statistics
Results are expressed as means ± SE, and differences were tested for significance using Students t-test or a Student-Newman-Keuls multiple comparisons test. Differences were considered to be statistically significant at P < 0.05.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Effects of removal of external K+ and addition of Ba2+.
Ventricular myocytes were superfused with 5.4 mM K+ Tyrodes solution and patched with pipettes that contained 10 mM K+ solution. At patch breakthrough, IK1 at the holding potential (Vhp) of 40 mV was a sizable (0.5 nA) outward current; during the next 5 min, the current declined in amplitude and then reversed in direction. These changes indicate that dialysis decreased intracellular K+ concentration ([K+]i) and shifted the K+ equilibrium potential (EK) to a voltage above 40 mV.
Under steady-state conditions, 100-ms depolarizations to 0 mV elicited inward ICa,L, and subsequent repolarizations to 40 mV elicited currents that relaxed (half time 20 ms) in the inward direction (and therefore were not due to deactivation of a voltage dependent K+ current) (Fig. 1A, left). Replacement of 5.4 mM K+ superfusate with K+-free superfusate both suppressed steady-state inward IK1 and increased the amplitude of peak ICa,L from control 0.59 ± 0.05 to 1.35 ± 0.1 nA (n = 9) (P < 0.001) (e.g., Fig. 1A, right). Matsuda and Noma (34) reported a similar response; recordings in their Fig. 7 show that peak ICa,L (+15 mV) increased from 0.6 to 1 nA after the removal of 2.7 mM K
.
|
|
It seemed likely that the enhancement of ICa,L caused by the removal of Kor the addition of Ba2+ was related to inhibition of an outward K+ current that overlapped ICa,L. To unmask the overlapping outward K+ current, ICa,L was blocked by application of 1 mM Cd2+. As illustrated by the superimposed recordings in Fig. 1C, the Cd2+-unmasked current had the appearance of a rapidly activating transient outward current. Although the current was insensitive to 2 mM 4-AP (n = 5, data not shown), both its time-dependent and steady-state components were almost fully suppressed by removal of K
(Fig. 1C) and addition of 0.2 mM Ba2+ (Fig. 1D).
To evaluate the impact of K-sensitive overlapping outward current on peak ICa,L at 0 mV, we measured the effects of K
removal on membrane currents at a fixed depolarization time (3 ms at 0 mV) both before and after the application of Cd2+. In six myocytes, brief removal of K
increased ICa,L by 0.79 ± 0.08 nA and decreased Cd2+-unmasked outward current by 0.68 ± 0.07 nA. A similar correlation was found in four experiments with 0.2 mM Ba2+; addition of the K+-channel blocker increased ICa,L by 0.93 ± 0.09 nA and decreased unmasked outward current by 0.78 ± 0.07 nA.
Effects of increasing Vhp from 40 to 80 mV.
A surprising finding in experiments with myocytes with Vhp fixed at 80 mV was that ICa,L elicited by depolarization from prepulse potential 40 mV was substantially larger than ICa,L recorded in myocytes with Vhp fixed at 40 mV. To investigate this phenomenon, myocytes held at 40 mV for 10 min after patch breakthrough were subsequently held at 80 mV for several minutes. In response to the change in Vhp, peak ICa,L upon depolarizations from 40 to 0 mV increased with time and reached a new steady-state amplitude after 23 min (Fig. 2, A and B); at 3 min, the increase in ICa,L was 133 ± 9% (n = 20 myocytes). The stimulation was not due to a Vhp-induced increase in the availability of Ca2+ channels, because there was no detectable effect of Vhp when myocytes were pretreated with 0.2 mM Ba2+ (Fig. 2C); i.e., peak ICa,L were 1.37 ± 0.14 nA when Vhp was 40 mV and 1.49 ± 0.14 nA when Vhp was changed to 80 mV (n = 9 myocytes).
|
An important observation in the Vhp trials was that the direction of IK1 at 40 mV was dependent on Vhp; at steady state, the current was inward when Vhp was 40 mV and outward when Vhp was 80 mV (Fig. 2E). Because inwardly rectifying channels are strongly selective for K+ and do not conduct Cs+ ions (33), these changes in current direction can be attributed to changes in EK. EK estimated from the reversal potential of Ba2+-sensitive current was 30 ± 3 mV when Vhp was 40 mV and 55 ± 5 mV when Vhp was 80 mV (n = 5). The estimated EK for Vhp 40 mV corresponds to a mean [K+]i of 17 mM, a value higher than the [K+] (10 mM) of the dialysate. A similar discrepancy was noted by Matsuda and Noma (34), and the likely reason is that K+ "efflux" from cytoplasm to pipette failed to keep pace with K+ influx generated by steady-state inward IK1. The imbalance increased as a consequence of larger inward IK1 when Vhp was changed to 80 mV (e.g., Fig. 2, A and B), resulting in an increase in calculated mean [K+]i to 43 mM.
Time- and Voltage-Dependent Properties of the Outward and Inward K+ Currents
The voltage dependencies of peak and steady-state outward currents were determined by measuring Ba2+-K-sensitive currents in Cd2+-treated myocytes that were pulsed from Vhp 40 mV to higher potentials. The representative recordings and average current-voltage (I-V) relationships shown in Fig. 3 indicate that the peak current increased to a saturating level at 0 mV, whereas the steady-state current increased up to 10 mV and declined at higher potentials. We note that these results should be interpreted with caution because they may have been affected by a voltage-dependent block of outward current by intracellular Cs+ (see Ref. 33). On the other hand, similar results were obtained when myocytes were dialyzed with either NMDG+- or sucrose-based low-K+ solution (n = 2 each; data not shown) instead of the standard Cs+-based solution.
|
|
As noted earlier (see also DISCUSSION), the inward tail currents cannot have been due to deactivation of a voltage-dependent conductance activated on depolarization. Rather, the results in Fig. 4A suggest that the tail current was due to a time-dependent recovery from rectification that occurred during the preceding depolarization to 0 mV. Further information on the recovery process was obtained from myocytes that were repolarized to voltages between 30 and 70 mV after 200-ms depolarizations to 0 mV. The recordings in Fig. 4B indicate that both the amplitude and the rate of rise of the tail current increased with negative voltage in a manner consistent with earlier findings regarding the "activation" of guinea pig IK1 at voltages negative to EK (19, 22, 32).
Modulation by Mg2+, Spermine, and ATP
For additional information regarding the K+ current under investigation, we evaluated the effects of established modulators of inwardly rectifying K+ currents, including intracellular Mg2+ (18, 19, 44, 45), spermine (6, 8, 18), and ATP (21, 42, 46).
Modulation by Mg2+. Cd2+-treated myocytes were dialyzed with a solution that had a free Mg2+ concentration of 1 µM, 0.6 mM, or 7 mM and pulsed from Vhp 40 to 0 mV for measurement of peak (5 ms) and steady-state (200 ms) outward current amplitudes (Fig. 5). After 10 min of dialysis, the amplitudes of the peak currents were 0.74 ± 0.06 nA in the 1 µM trials (n = 18), 0.56 ± 0.07 nA in the 0.6 mM trials (n = 16), and 0.37 ± 0.05 nA in the 7 mM trials (n = 6) (P < 0.05 or 0.01 for each pair of mean values). Similarly, the amplitudes of the steady-state currents declined with higher Mg2+ concentration (P < 0.01 between 1 µM and 0.6 or 7 mM) (Fig. 5C). Additional observations were that the high-Mg2+ dialysate had little effect on steady-state inward IK1 but slowed the time course of the inward tail current (Fig. 5, A and B).
|
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
ICa,L and Overlapping Time-Dependent Outward IK1
The amplitude of peak ICa,L on depolarizations from 40 to 0 mV was markedly increased by the following three interventions: removal of K, addition of 0.2 mM Ba2+, and changing of Vhp from 40 to 80 mV. The sensitivity of peak ICa,L to the first two of these interventions suggested a mechanism that involved an ICa,L-overlapping K+ current, and this was confirmed by the unmasking of a time-dependent outward current when ICa,L was blocked by the application of Cd2+. The time course of the unmasked current resembled that of a rapidly activating transient outward current, and this raises the question of whether the overlapping current was carried by one or more of the voltage-dependent K+ channels (Kv1.4, Kv4.2, Kv4.3) responsible for transient outward currents in cardiac cells (4, 35, 50). Although involvement of Kv channels in the overlapping current is not excluded by the finding that the overlapping current was sensitive to K
and Ba2+ (39), it can be ruled out on other grounds. First, a recent study of Kv-subunit mRNA and protein in rabbit and guinea pig ventricular tissue concluded that Kv1.4, Kv4.2, and Kv4.3 are not expressed in guinea pig tissue (54). Second, the effects of changing Vhp from 40 to 80 mV on the overlapping current (Fig. 2) argue against involvement of rapidly activating/inactivating Kv current of any origin. As indicated by the marked increase in ICa,L at 0 mV, the more negative Vhp resulted in a marked decrease of the overlapping outward current, which in turn was linked to the decay of a large time-dependent outward current during the prepulse from Vhp 80 to 40 mV. The latter decay was not due to voltage-dependent inactivation because orders-longer conditioning at 40 mV (e.g., when Vhp was set to 40 mV) resulted in larger (not smaller) overlapping outward current at 0 mV, even in the face of the marked decline in outward K+ driving force that occurred when Vhp was 40 mV.
The anomalous behavior of overlapping outward current when Vhp was changed is most easily explained by identifying the overlapping current as time-dependent outward IK1. Then, the decay of time-dependent outward current during the prepulse to 40 mV was due to a strong time-dependent rectification that partially occluded time-dependent rectification on the subsequent test pulse to 0 mV; a similar "conditioning" at 40 mV did not occur when Vhp was 40 mV, because EK, a major determinant of the rectification of IK1 (28, 29), was now more positive than 40 mV (Fig. 2B).
There were a number of other findings that support the view that the overlapping outward current was time-dependent IK1. First, the current was completely suppressed by two interventions known to inhibit IK1, removal of Ko+ and addition of Ba2+. Second, repolarizations from 0 mV elicited tail currents whose configuration and dependence on duration of the preceding depolarization were consistent with those expected for IK1 recovering from rectification. Third, the overlapping outward current responded to dialysates with modified Mg2+, spermine, and ATP in the manner expected for IK1. For example, low-Mg2+ dialysates enhanced the current, whereas high-Mg2+ and spermine dialysates inhibited it (see below).
Transient Outward Currents in Guinea Pig Ventricular Myocytes
After the Matsuda and Noma (34) report of a time-dependent outward current that overlapped ICa,L, Saigusa and Matsuda (40) studied myocytes bathed with high-K+ solution and observed rapidly decaying outward transients (
5 ms at EK + 15 mV) whose amplitude was linearly dependent on voltage. Subsequently, Ishihara et al. (19) investigated K+ currents in myocytes using the oil gap voltage-clamp method and observed rapidly decaying outward transients (
3 ms at EK + 15 mV; <1 ms at EK + 40 mV) that had a linear I-V relationship; experimental problems noted by these authors included the presence of restricted oil membrane extracellular spaces, K
-insensitive outward transients, and rapid washout of ICa,L (19). Later, Martin et al. (31) described IK(PDD), which developed after prolonged dialysis via large-tipped (
1 M
) pipettes. In contrast to the time-dependent outward current described in the present study, IK(PDD) had a linear I-V relationship, was unaffected by variation (1 nM1 mM) of free Mg2+ in the dialysate, and was not observed when 10 mM Cs+ was included in the dialysate.
More recently, Li et al. (25) investigated a transient outward current, Ito,ir, that was independent of patch duration. Ito,ir was insensitive to 4-AP, inhibited by the removal of Kor the addition of 0.2 mM Ba2+, and had a peak amplitude that saturated near EK +60 mV. In other words, Ito,ir had properties similar to those attributed to time-dependent outward IK1 in the present study. However, Li et al. (25) had several grounds for concluding that Ito,ir was most likely a novel, long-overlooked, voltage-dependent current and not IK1. These grounds were as follows: 1) Ito,ir was insensitive to intracellular Mg2+, 2) Ito,ir was insensitive to intracellular spermine, and 3) Ito,ir was a time-dependent weakly rectifying current, whereas outward IK1 has long been described as a virtually time-independent, strongly rectifying current (12, 37). These grounds and their relationship to the present findings are considered below.
Responses to Mg2+. Li et al. (25) evaluated the sensitivity of Ito,ir to intracellular Mg2+ by comparing the amplitude of Ito,ir just after patch breakthrough with that measured after 10-min dialysis with Mg2+-free solution. They found that there was no significant difference between these values in four test myocytes and concluded that Ito,ir was insensitive to Mg2+. That was not the case for the time-dependent outward current that we examined in the present study; its amplitude was significantly increased by low (1 µM) Mg2+ solution, and significantly decreased by high (7 mM) Mg2+ solution. High Mg2+ also induced a marked slowing of the inward tail at 40 mV, an effect that may be due to a negative shift in the "activation" time constant (17, 4345).
Responses to spermine. Li et al. (25) used a 10 µM spermine dialysate to evaluate whether the polyamine had an inhibitory effect on Ito,ir and concluded that it did not. However, the following considerations suggest that the 10 µM concentration they used for this test may have been too low: 1) Stadnicka et al. (43) found that a 10-min dialysis of guinea pig ventricular myocytes with 25 µM spermine solution had no effect on classic outward IK1, 2) the normal cytoplasmic concentration of free spermine estimated for various cell types ranges from 10 to 75 µM (2, 6, 51), 3) free spermine in the dialysate may be lower than total spermine because of binding by ATP (51), and 4) intracellular spermine is buffered by nucleic acids and proteins (2, 6, 51). It seems likely that these are among the reasons why investigators of spermine action on whole cell Kir2.1 current (17, 18) and other inwardly rectifying currents (1) have used concentrations between 100 µM and 1 mM. In the present study, 100 µM spermine solution had little effect on steady-state inward IK1 at 40 mV; however, it partially inhibited time-dependent outward current at 0 mV (suggesting enhanced block) and increased the amplitude of the inward tail current at 40 mV (suggesting recovery from larger block). These results, as well as the pronounced inhibition of all current components by 1 mM spermine dialysate, are consistent with earlier findings regarding Kir2.1 current (11, 17, 28).
Time and voltage dependence of IK1. Li et al. (25) pointed out that Ito,ir is a time-dependent current, whereas outward IK1 is a time-independent current. Clearly, this distinction between the two currents depends on whether outward IK1 really is a time-independent current. It is important to remember that the (now entrenched) description of outward IK1 is based on a theoretical formulation of an instantaneously rectifying current that, at the time, provided a useful framework for interpretation of membrane currents in cardiac Purkinje fibers (37). Thereafter, the true time- and voltage-dependent nature of the current is likely to have gone unnoticed in studies that used "unfavorable" experimental protocols/conditions such as voltage-ramp commands, depolarizing steps from holding potentials above EK, nonblocked INa, and nonblocked ICa,L. We have elucidated some of the time- and voltage-dependent properties of outward IK1, and the similarity of these properties to those ascribed to Ito,ir (25) is noteworthy.
![]() |
GRANTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
ACKNOWLEDGMENTS |
---|
Current address of T. Asai: Department of Information Science, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan.
![]() |
FOOTNOTES |
---|
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
2. Bowie D and Mayer ML. Inward rectification of both AMPA and kainate subtype glutamate receptors generated by polyamine-mediated ion channel block. Neuron 15: 453462, 1995.[ISI][Medline]
3. Coraboeuf E, Coulombe A, Deroubaix E, Hatem S, and Mercadier JJ. Transient outward potassium current and repolarization of cardiac cells. Bull Acad Natl Med 182: 325335, 1998 (Article in French).[ISI][Medline]
4. Dixon JE, Shi W, Wang HS, McDonald C, Yu H, Wymore RS, Cohen IS, and McKinnon D. Role of the Kv4.3 K+ channel in ventricular muscle: a molecular correlate for the transient outward current. Circ Res 79: 659668, 1996.
5. Dukes ID and Morad M. The transient K+ current in rat ventricular myocytes: evaluation of its Ca2+ and Na+ dependence. J Physiol 435: 395420, 1991.[Abstract]
6. Fakler B, Brandle U, Glowatzki E, Weidemann S, Zenner HP, and Ruppersberg JP. Strong voltage-dependent inward rectification of inward rectifier K+ channels is caused by intracellular spermine. Cell 80: 149154, 1995.[ISI][Medline]
7. Fedida D and Giles WR. Regional variations in action potentials and transient outward current in myocytes isolated from rabbit left ventricle. J Physiol 442: 191209, 1991.[Abstract]
8. Ficker E, Taglialatela M, Wible BA, Henley CM, and Brown AM. Spermine and spermidine as gating molecules for inward rectifier K+ channels. Science 266: 10681072, 1994.[ISI][Medline]
9. Findlay I. Is there an A-type K+ current in guinea pig ventricular myocytes? Am J Physiol Heart Circ Physiol 284: H598H604, 2003.
10. Gintant GA. Characterization and functional consequences of delayed rectifier current transient in ventricular repolarization. Am J Physiol Heart Circ Physiol 278: H806H817, 2000.
11. Guo D and Lu Z. Mechanism of IRK1 channel block by intracellular polyamines. J Gen Physiol 115: 799814, 2000.
12. Harvey RD and Ten Eick RE. Characterization of the inward-rectifying potassium current in cat ventricular myocytes. J Gen Physiol 91: 593615, 1988.[Abstract]
13. Hoppe UC, Johns DC, Marbán E, and ORourke B. Manipulation of cellular excitability by cell fusion: effects of rapid introduction of transient outward K+ current on the guinea pig action potential. Circ Res 84: 964972, 1999.
14. Hume JR and Uehara A. Ionic basis of the different action potential configurations of single guinea pig atrial and ventricular myocytes. J Physiol 368: 525544, 1985.[Abstract]
15. Ibarra J, Morley GE, and Delmar M. Dynamics of the inward rectifier K+ current during the action potential of guinea pig ventricular myocytes. Biophys J 60: 15341539, 1991.[Abstract]
16. Inoue M and Imanaga I. Masking of A-type K+ channel in guinea pig cardiac cells by extracellular Ca2+. Am J Physiol Cell Physiol 264: C1434C1438, 1993.
17. Ishihara K. Time-dependent outward currents through the inward rectifier potassium channel IRK1: the role of weak blocking molecules. J Gen Physiol 109: 229243, 1997.
18. Ishihara K, Hiraoka M, and Ochi R. The tetravalent organic cation spermine causes the gating of the IRK1 channel expressed in murine fibroblast cells. J Physiol 491: 367381, 1996.[Abstract]
19. Ishihara K, Mitsuiye T, Noma A, and Takano M. The Mg2+ block and intrinsic gating underlying inward rectification of the K+ current in guinea pig cardiac myocytes. J Physiol 419: 297320, 1989.[Abstract]
20. Josephson IR, Sánchez-Chapula J, and Brown AM. Early outward current in rat single ventricular cells. Circ Res 54: 157162, 1984.[Abstract]
21. Kakei M, Noma A, and Shibasaki T. Properties of adenosine-triphosphate-regulated potassium channels in guinea pig ventricular cells. J Physiol 363: 441462, 1985.[Abstract]
22. Kurachi Y. Voltage-dependent activation of the inward-rectifier potassium channel in the ventricular cell membrane of guinea pig heart. J Physiol 366: 365385, 1985.[Abstract]
23. Li GR, Feng J, Yue L, Carrier M, and Nattel S. Evidence for two components of delayed rectifier K+ current in human ventricular myocytes. Circ Res 78: 689696, 1996.
24. Li GR, Sun H, and Nattel S. Characterization of a transient outward K+ current with inward rectification in canine ventricular myocytes. Am J Physiol Cell Physiol 274: C577C585, 1998.
25. Li GR, Yang B, Sun H, and Baumgarten CM. Existence of a transient outward K+ current in guinea pig cardiac myocytes. Am J Physiol Heart Circ Physiol 279: H130H138, 2000.
26. Lindblad DS, Murphey CR, Clark JW, and Giles WR. A model of the action potential and underlying membrane currents in a rabbit atrial cell. Am J Physiol Heart Circ Physiol 271: H1666H1696, 1996.
27. Liu DW and Antzelevitch C. Characteristics of the delayed rectifier current (IKr and IKs) in canine ventricular epicardial, midmyocardial, and endocardial myocytes: a weaker IKs contributes to the longer action potential of the M cell. Circ Res 76: 351365, 1995.
28. Lopatin AN and Nichols CG. [K+] dependence of polyamine-induced rectification in inward rectifier potassium channels (IRK1, Kir2.1). J Gen Physiol 108: 105113, 1996.[Abstract]
29. Lopatin AN and Nichols CG. Inward rectifiers in the heart: an update on IK1. J Mol Cell Cardiol 33: 625638, 2001.[CrossRef][ISI][Medline]
30. Macianskiene R, Moccia F, Sipido KR, Flameng W, and Mubagwa K. Channels involved in transient currents unmasked by removal of extracellular calcium in cardiac cells. Am J Physiol Heart Circ Physiol 282: H1879H1888, 2002.
31. Martin RL, Barrington PL, and Ten Eick RE. A 3,4-diaminopyridine-insensitive, Ca2+-independent transient outward K+ current in cardiac ventricular myocytes. Am J Physiol Heart Circ Physiol 266: H1286H1299, 1994.
32. Martin RL, Koumi S, and Ten Eick RE. Comparison of the effects of internal [Mg2+] on IK1 in cat and guinea pig cardiac ventricular myocytes. J Mol Cell Cardiol 27: 673691, 1995.[ISI][Medline]
33. Matsuda H. Rb+, Cs+ ions and the inwardly rectifying K+ channels in guinea pig ventricular cells. Pflügers Arch 432: 2633, 1996.[CrossRef][ISI][Medline]
34. Matsuda H and Noma A. Isolation of calcium current and its sensitivity to monovalent cations in dialysed ventricular cells of guinea pig. J Physiol 357: 553573, 1984.[Abstract]
35. Nerbonne JM. Molecular basis of functional voltage-gated K+ channel diversity in the mammalian myocardium. J Physiol 525: 285298, 2000.
36. Nichols CG, Makhina EN, Pearson WL, Sha Q, and Lopatin AN. Inward rectification and implications for cardiac excitability. Circ Res 78: 17, 1996.
37. Noble D. The Initiation of the Heartbeat. Oxford, UK: Oxford University Press, 1975.
38. Ogura T, Shuba LM, and McDonald TF. Action potentials, ionic currents and cell water in guinea pig ventricular preparations exposed to dimethyl sulfoxide. J Pharmacol Exp Ther 273: 12731286, 1995.[Abstract]
39. Pardo LA, Heinemann SH, Terlau H, Ludewig U, Lorra C, Pongs O, and Stühmer W. Extracellular K+ specifically modulates a rat brain K+ channel. Proc Natl Acad Sci USA 89: 24662470, 1992.[Abstract]
40. Saigusa A and Matsuda H. Outward currents through the inwardly rectifying potassium channel of guinea pig ventricular cells. Jpn J Physiol 38: 7791, 1988.[ISI][Medline]
41. Sanguinetti MC and Jurkiewicz NK. Two components of cardiac delayed rectifier K+ current: differential sensitivity to block by class III antiarrhythmic agents. J Gen Physiol 96: 195215, 1990.[Abstract]
42. Schackow TE and Ten Eick RE. Enhancement of ATP-sensitive potassium current in cat ventricular myocytes by -adrenoreceptor stimulation. J Physiol 474: 131145, 1994.[Abstract]
43. Stadnicka A, Bosnjak ZJ, Kampine JP, and Kwok WM. Effects of sevoflurane on inward rectifier K+ current in guinea pig ventricular cardiomyocytes. Am J Physiol Heart Circ Physiol 273: H324H332, 1997.
44. Stanfield PR, Davies NW, Shelton PA, Khan IA, Brammar WJ, Standen NB, and Conley EC. The intrinsic gating of inward rectifier K+ channels expressed from the murine IRK1 gene depends on voltage, K+ and Mg2+. J Physiol 475: 17, 1994.[Abstract]
45. Stanfield PR, Davies NW, Shelton PA, Sutcliffe MJ, Khan IA, Brammar WJ, and Conley EC. A single aspartate residue is involved in both intrinsic gating and blockage by Mg2+ of the inward rectifier, IRK1. J Physiol 478: 16, 1994.[Abstract]
46. Takano M, Qin DY, and Noma A. ATP-dependent decay and recovery of K+ channels in guinea pig cardiac myocytes. Am J Physiol Heart Circ Physiol 258: H45H50, 1990.
47. Trautwein W and McDonald TF. Current-voltage relations in ventricular muscle preparations from different species. Pflügers Arch 374: 7989, 1978.[ISI][Medline]
48. Varro A, Lathrop DA, Hester SB, Nanasi PP, and Papp JG. Ionic currents and action potentials in rabbit, rat, and guinea pig ventricular myocytes. Basic Res Cardiol 88: 93102, 1993.[ISI][Medline]
49. Visentin S, Zaza A, Ferroni A, Tromba C, and DiFrancesco C. Sodium current block caused by group IIb cations in calf Purkinje fibres and in guinea pig ventricular myocytes. Pflügers Arch 417: 213222, 1990.[ISI][Medline]
50. Wang Z, Feng J, Shi H, Pond A, Nerbonne JM, and Nattel S. Potential molecular basis of different physiological properties of the transient outward K+ current in rabbit and human atrial myocytes. Circ Res 84: 551561, 1999.
51. Watanabe S, Kusama-Eguchi K, Kobayashi H, and Igarashi K. Estimation of polyamine binding to macromolecules and ATP in bovine lymphocytes and rat liver. J Biol Chem 266: 2080320809, 1991.
52. Watanabe T, Rautaharju PM, and McDonald TF. Ventricular action potentials, ventricular extracellular potentials, and the ECG of guinea pig. Circ Res 57: 362373, 1985.[Abstract]
53. Yue L, Feng J, Li GR, and Nattel S. Transient outward and delayed rectifier currents in canine atrium: properties and role of isolation methods. Am J Physiol Heart Circ Physiol 270: H2157H2168, 1996.
54. Zicha S, Moss I, Allen B, Varro A, Papp J, Dumaine R, Antzelevich C, and Nattel S. Molecular basis of species-specific expression of repolarizing K+ currents in the heart. Am J Physiol Heart Circ Physiol 285: H1641H1649, 2003.
HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Visit Other APS Journals Online |