Calcium-activated transient outward current in calf cardiac Purkinje fibres

From Bernstein's rheotome to Neher-Sakmann's patch electrode. The action potential

The aim of this review was to provide an overview of the most important stages in the development of cellular electrophysiology. The period covered starts with Bernstein's formulation of the membrane hypothesis and the measurement of the nerve and muscle action potential. Technical innovations make discoveries possible. This was the case with the use of the squid giant axon, allowing the insertion of "large" intracellular electrodes and derivation of transmembrane potentials. Application of the newly developed voltage clamp method for measuring ionic currents, resulted in the formulation of the ionic theory. At the same time transmembrane measurements were made possible in smaller cells by the introduction of the microelectrode. An improvement of this electrode was the next major (r)evolution. The patch electrode made it possible to descend to the molecular level and record single ionic channel activity. The patch technique has been proven to be exceptionally versatile. In its whole-cell configuration it was the solution to measure voltage clamp currents in small cells. See also: https://doi.org/10.14814/phy2.13860 & https://doi.org/10.14814/phy2.13862.

The spatial and temporal regulation of the hormonal signal. Role of mitochondria in the formation of a protein complex required for the activation of cholesterol transport and steroids synthesis

The mitochondria are critical for steroidogenesis since the ability of cholesterol to move into mitochondria to be available for cytochrome P450, CYP11A1, determines the efficacy of steroid production. Several proteins kinases, such as PKA, MEK and ERK which are essential to complete steroidogenesis, form a mitochondria-associated complex. The protein-protein interactions between kinases and key factors during the transport of cholesterol takes place in the contact sites between the two mitochondrial membranes; however, no mitochondrial targeting sequence has been described for these kinases. Here we discuss the possibility that mitochondrial reorganization may be mediating a compartmentalized cellular response. This reorganization could allow the physical interaction between the hormone-receptor complex and the enzymatic and lipidic machinery necessary for the complete steroid synthesis and release. The movement of organelles in specialized cells could impact on biological processes that include, but are not limited to, steroid synthesis.

Establishment of functional primary cultures of heart cells from the clam Ruditapes decussatus

Heart cells from the clam Ruditapes decussatus were routinely cultured with a high level of reproducibility in sea water based medium. Three cell types attached to the plastic after 2 days and could be maintained in vitro for at least 1 month: epithelial-like cells, round cells and fibroblastic cells. Fibroblastic cells were identified as functional cardiomyocytes due to their spontaneous beating, their ultrastructural characteristics and their reactivity with antibodies against sarcomeric α-actinin, sarcomeric tropomyosin, myosin and troponin T-C. Patch clamp measurements allowed the identification of ionic currents characteristic of cardiomyocytes: a delayed potassium current (I (K slow)) strongly suppressed (95%) by tetraethylammonium (1 mM), a fast inactivating potassium current (I (K fast)) inhibited (50%) by 4 amino-pyridine at 1 mM and, at a lower level (34%) by TEA, a calcium dependent potassium current (I (KCa)) activated by strong depolarization. Three inward voltage activated currents were also characterized in some cardiomyocytes: L-type calcium current (I (Ca)) inhibited by verapamil at 5 × 10(-4) M, T-type Ca(2+) current, rapidly activated and inactivated, and sodium current (I (Na)) observed in only a few cells after strong hyperpolarization. These two currents did not seem to be physiologically essential in the initiation of the beatings of cardiomyocytes. Potassium currents were partially inhibited by tributyltin (TBT) (1 μM) but not by okadaic acid (two marine pollutants). DNA synthesis was also demonstrated in few cultured cells using BrdU (bromo-2'-deoxyuridine). Observed effects of okadaic acid and TBT demonstrated that cultured heart cells from clam Ruditapes decussatus can be used as an experimental model in marine toxicology.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

The beat-to-beat decay of cardiac contractility from highly potentiated levels is bi-exponential

In order to determine the mode of beat-to-beat decay of contractility from very high levels, we studied the beat-by-beat decay of cardiac contractility following potentiation. Such decay curves are normally analysed using a mono-exponential decay function, which assumes that a fixed fraction of activator calcium ions is recirculated from one beat to the next. We postulated that there might be deviations from such a mono-exponential expression at high levels of contractility. In single sucrose-gap voltage clamp experiments of isolated ferret papillary muscle, we obtained very high contractility by potentiation due to prolonged depolarisations. We found a bi-exponential decay in 9 of 11 muscles studied, in which the initial decay is much faster than the subsequent slower decay, as judged by residual variance of least-squares exponential fitting and by analysis of covariance using a linear equation (force of beat versus force of previous beat), p = 0.0089. In the slower decay period (physiological range), the decay was identical to that following post-extrasystolic potentiation in the same muscles studied with conventional stimulation.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

Involvement of anion channel(s) in the modulation of the transient outward K+ channel in rat ventricular myocytes

The cardiac Ca2+-independent transient outward K+ current ( Ito), a major repolarizing ionic current, is markedly affected by Cl− substitution and anion channel blockers. We reexplored the mechanism of the action of anions on Ito by using whole cell patch-clamp in single isolated rat cardiac ventricular myocytes. The transient outward current was sensitive to blockade by 4-aminopyridine (4-AP) and was abolished by Cs+ substitution for intracellular K+. Replacement of most of the extracellular Cl− with less permeant anions, aspartate (Asp−) and glutamate (Glu−), markedly suppressed the current. Removal of external Na+ or stabilization of F-actin with phalloidin did not significantly affect the inhibitory action of less permeant anions on Ito. In contrast, the permeant Cl− substitute Br− did not markedly affect the current, whereas F− substitution for Cl− induced a slight inhibition. The Ito elicited during Br− substitution for Cl− was also sensitive to blockade by 4-AP. The ability of Cl− substitutes to induce rightward shifts of the steady-state inactivation curve of Ito was in the following sequence: NO3− > Cl− ≈ Br− > gluconate− > Glu− > Asp−. Depolymerization of actin filaments with cytochalasin D (CytD) induced an effect on the steady-state inactivation of Ito similar to that of less permeant anions. Fluorescent phalloidin staining experiments revealed that CytD-pretreatment significantly decreased the intensity of FITC-phalloidin staining of F-actin, whereas Asp− substitution for Cl− was without significant effect on the intensity. These results suggest that the Ito channel is modulated by anion channel(s), in which the actin cytoskeleton may be implicated.

Calcium-activated CL- current is enhanced by acidosis and contributes to the shortening of action potential duration in rabbit ventricular myocytes

Ca2+-activated Cl- current (I(Cl(Ca))) is activated by Ca2+ transient via Ca2+-induced Ca2+ release from sarcoplasmic reticulum in cardiac myocytes and is supposed to play an important role in the repolarization of action potential. It is not well understood, however, how I(Cl(Ca)) is modulated to affect action potential in normal or pathological conditions. In this study we examined the effects of external acidosis on I(Cl(Ca)) and action potential. A whole-cell patch clamp was performed to record action potential and I(Cl(Ca)), using isolated rabbit ventricular myocytes. In the standard solution at pH 7.4, action potential duration (APD) was markedly prolonged by lowering the extracellular Cl- concentration ([Cl-](o)) or by applying an anion channel blocker, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). In the low pH solution at 6.4, APD was markedly shortened and the amplitude of I(Cl(Ca)) was increased at all membrane potentials. At pH 6.4, the apparent steady-state inactivation curves of I(Cl(Ca)) were shifted to more positive potentials compared with those at pH 7.4, but no change in inactivation occurred at a holding potential of -60 mV. The apparent activation curves were not changed between the two sets of conditions. When I(Cl(Ca)) was inhibited at low pH, early afterdepolarizations and triggered activities were induced. The amplitude of I(Cl(Ca)) was suggested to be enhanced by the external acidosis, which may have prevented the induction of early afterdepolarization or triggered activity.

Beat dependent alteration of Ca2+-activated Cl- current during rapid stimulation in rabbit ventricular myocytes

The transient outward currents (Ito) play an important role in action potential repolarization in cardiac myocytes. Two components of Ito have been identified as 4-AP-sensitive but Ca2+-insensitive Ito carried by K, and Ca2+-sensitive but 4-AP insensitive Ito carried by Cl- (I(Cl(Ca))). It is known that the amplitudes of Ito change depending on the stimulation frequency. In this study we investigated the beat dependent alteration of I(Cl(Ca)) during rapid stimulation using the whole cell patch clamp technique in rabbit ventricular myocytes. The cells were internally perfused with a solution containing 0.1 microM free Ca2+ to develop I(Cl(Ca)) and all internal K+ was replaced with Cs+ to block 4-AP-sensitive Ito and other K+ currents. By applying depolarizing pulses at a high frequency of 2.5 Hz, the amplitudes of I(Cl(Ca)) gradually increased as the number of pulses increased following a transient decrease in the 2nd pulse and reached a plateau level at the 20th pulse. The shape of the current-voltage curve of I(Cl(Ca)) was not overly different for different numbers of preceding pulses. The recovery from inactivation of I(Cl(Ca)) could be fitted to a single exponential curve and full recovery was achieved after > 1 sec with a time constant of 368 ms. The ramp clamp experiments showed that the conductance of the background I(Cl(Ca)) increased with the preceding pulse numbers, indicating that the resting level of [Ca2]i increased with the pulses applied. From these results, we conclude that beat dependent alteration of I(Cl(Ca)) is determined by not only its apparent kinetic property, but also the resting level of [Ca2+]i during rapid stimulation.

Effect of acidosis on transient outward potassium current in isolated rat ventricular myocytes

The effect of acidosis on the transient outward K(+) current (I(to)) of rat ventricular myocytes has been investigated using the perforated patch-clamp technique. When the holding potential was -80 mV, depolarizing pulses to potentials positive to -20 mV activated I(to) in subepicardial cells but activated little I(to) in subendocardial cells. Exposure to an acid solution (pH 6.5) had no significant effect on I(to) activated from this holding potential in either subepicardial or subendocardial cells. When the holding potential was -40 mV, acidosis significantly increased I(to) at potentials positive to -20 mV in subepicardial cells but had little effect on I(to) in subendocardial cells. The increase in I(to) in subepicardial cells was inhibited by 10 mM 4-aminopyridine. In subepicardial cells, acidosis caused a +8.57-mV shift in the steady-state inactivation curve. It is concluded that in subepicardial rat ventricular myocytes acidosis increases the amplitude of I(to) as a consequence of a depolarizing shift in the voltage dependence of inactivation.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Atrial electrical remodeling by rapid pacing in the isolated rabbit heart: effects of Ca++ and K+ channel blockade

INTRODUCTION

Electrical remodeling describes atrial electrophysiologic changes that occur following atrial fibrillation. The mechanism(s) responsible for this phenomenon is not well understood. The purpose of this study was to examine the effects of rapid atrial pacing on atrial action potential duration, conduction time and refractoriness in the isolated rabbit heart. The effects of Ca++ and K+ blockade in this model were also studied.

METHODS AND RESULTS

Monophasic action potential recordings were made from 12 epicardial atrial sites in 50 isolated perfused rabbit heart preparations. These recordings were analyzed for activation time (AT), 90% action potential duration (APD) and conduction times (CT) measured at a 250 msec cycle length. Atrial effective refractory periods (ERP) were determined at a 200 msec cycle length. All measurements were made at baseline and repeated after 2 hours of biatrial pacing at 250 msec (control group, n = 10) or 2 hours of rapid biatrial pacing (approximately 80 msec) in 4 groups: rapid pacing alone (rapid pacing group); rapid pacing in the presence of 0.1 mM verapamil (verapamil group) for L-type Ca++ channel blockade; rapid pacing with 1 mM 4-aminopyridine (4-AP group) for K+ channel blockade; and rapid pacing with 50 microM nickel chloride (Ni++ group) for T-type Ca++ channel blockade (n = 10 each group). All baseline and post pacing measurements were taken in the presence of Ca++ or K+ blockers for the respective groups. After rapid atrial pacing alone the average APD shortened by 8.2 +/- 10.4 msec compared to 3.6 +/- 12.5 msec shortening for control group (p = 0.002). The shortening of APD was uniform at all recording sites. For the rapid pacing group, CT was unchanged for right to left atrial conduction but shortened significantly for left to right atrial conduction (26.8 +/- 1.9 msec at baseline to 22.3 +/- 4.1 msec post pacing, p = 0.005). Conduction times were unchanged in the control group. The dispersion of repolarization was unchanged by rapid pacing alone. The decrease in APD from baseline to post rapid pacing was similar to the control group for those hearts treated with verapamil and 4-AP (1.5 +/- 12.3 and 4.7 +/- 10.4 msec, respectively, both p > or = 0.18 vs control group). The decrease in APD was significantly greater for the Ni++ group (11.8 +/- 14.3 msec) than for either the control group or rapid pacing group (both p < or = 0.023). The dispersion of repolarization was increased only in the 4-AP group post rapid pacing (41.7 +/- 6.2 msec at baseline to 53.5 +/- 9.6 msec post pacing, p = 0.01). ERPs were unchanged in any of the 5 groups except for a decrease in left atrial ERP in the Ni++ group after rapid pacing (98 +/- 14 msec at baseline to 88 +/- 8 msec post rapid pacing, p = 0.005).

CONCLUSIONS

In the isolated rabbit heart model: 1) atrial APD is shortened after rapid pacing; 2) the shortening of APD is attenuated by verapamil and 4-AP but exaggerated by Ni++; 3) atrial conduction times are shortened in a direction specific manner after rapid pacing; and 4) shortening of ERP in this model is measured only in the presence of Ni++. These findings suggest that both L-type Ca++ and 4-AP sensitive channels may participate in atrial electrical remodeling.

Unexpected and differential effects of Cl- channel blockers on the Kv4.3 and Kv4.2 K+ channels. Implications for the study of the I(to2) current

The Kv4.3 K+ channel is thought to underlie the Ca(2+)-insensitive transient outward current (I(to1)) in ventricular myocytes of canine and human heart and to contribute to the I(to1) in rat myocytes. It has been suggested that there is a second component of the transient outward current in some species that is contributed by a Ca(2+)-activated Cl- current (known as I(to2)). The evidence for the existence of the I(to2) current is based, in part, on the pharmacological effects of various Cl- channel blockers. To test for possible interactions between these compounds and I(to1), the effect of several different Cl- channel blockers on the Kv4.3 channel was examined. The fenamates (niflumic and flufenamic acid) were found to have large effects on the position of the steady state inactivation curve of the Kv4.3 channel. The disulfonic stilbenes (DIDS and SITS) had markedly different effects and were found to greatly reduce the rate of recovery from inactivation of the Kv4.3 channel without large changes in the position of the activation and steady state inactivation curves. Both classes of drugs produced an apparent blockade of the Kv4.3 channel under some recording conditions. Surprisingly, the closely related Kv4.2 channel was found to be markedly less sensitive to these drugs. Caffeine was found to block both the Kv4.3 and Kv4.2 channels to a similar extent. These nonspecific drug effects have implications for the study of the two components of the transient outward current and suggest that purely pharmacological criteria cannot be used to define the physiological role of I(to2).

Outward potassium currents of supraoptic magnocellular neurosecretory cells isolated from the adult guinea-pig

1. Several types of whole-cell outward K+ current recorded from magnocellular neurosecretory cells (MNCs) dissociated from the supraoptic nucleus of the adult guinea-pig were identified on the basis of their voltage dependence, kinetics, pharmacology and Ca2+ dependence. 2. The predominant K+ current evoked from a holding potential of -40 mV was slowly activating, long-lasting, tetraethylammonium (TEA) sensitive and showed little steady-state inactivation. Also, this current was reduced by extracellular Cd2+. These data suggest that in supraoptic MNCs classical Ca(2+)-insensitive, delayed rectifier channels (KV) and Ca(2+)-sensitive, non-inactivating channels (KCa) both contribute to the sustained current. 3. A transient, low-threshold K+ current, which was 4-aminopyridine (4-AP) sensitive and showed significant steady-state inactivation, was evoked along with the sustained current from a holding potential of -90 mV. Based on these characteristics, this current corresponds to the A-current (IK(A)) described in other neurons. 4. IK(A) was activated when Ca2+ influx was blocked or when Ca2+ was absent from the extracellular medium, suggesting that Ca2+ influx is not necessary for activation of the current. 5. In many recordings, a transient 4-AP-insensitive outward current was evoked from a holding potential of -40 mV. This high-threshold transient K+ current was abolished by extracellular Cd2+ or TEA and was absent when extracellular Ca2+ was replaced by Sr2+, suggesting that it is a transient Ca(2+)-dependent K+ current. 6. We conclude that the presence of multiple types of K+ current may, in part, underlie the complex firing patterns of oxytocinergic and vasopressinergic MNCs.

Effects of chloride ion substitutes and chloride channel blockers on the transient outward current in rat ventricular myocytes

The Cai(2+)-insensitive transient outward current, ilo was studied at 20-24 degrees C in rat ventricular myocytes with the whole cell recording patch-clamp technique. The current was recorded before and after replacement of chloride by methanesulfonate or aspartate or in the absence and the presence of chloride channel blockers, SITS or 9-anthracene carboxylic acid. In control conditions (in the presence of external divalent cations, Ca2+ and Cd2+, Cd2+ being used to suppress Ca2+ current), ilo inactivation was composed of a fast and a slow component. When methanesulfonate was substituted for external Cl-, the peak current decreased to a variable extent, but the inactivation of the remaining current was still composed of a fast and a slow component. In contrast, the inactivation of the difference current was well fitted by a single exponential. The time to peak of the difference current was shorter than that of the current recorded either in the absence or the presence of methanesulfonate. Both activation- and steady-state inactivation-voltage curves were either unchanged (n = 4) or shifted by a few mV (5.5 mV, n = 14) towards positive potentials when methanesulfonate was substituted for Cl-. The current remaining in methanesulfonate reversed at potentials closed to EK. The difference current was composed of a peak and a steady-state component. The peak was suppressed by 4-aminopyridine whereas the steady-state component was not. The peak was also suppressed when pipette solution contained Cs+ instead of K+ but was still present when the Hepes concentration in both external and pipette media was increased 5-fold (50 mM vs. 10 mM). When aspartate was substituted for Cl- or when 2 mM SITS was added to the external solution (in the absence of Ca2+ and Cd2+ because aspartate is known to chelate Ca2+ ions and possibly other divalent cations), ilo was reduced to a similar extent in the two cases and the difference current was composed of a peak (inactivation fitted by a single exponential) and a steady-state component. The SITS-sensitive transient current reversed at a potential close to ECl. When 5 mM 9-anthracene carboxylic acid was added to external solution (in the presence of Ca2+ and Cd2+), the peak of the difference current was similar to that observed when Cl- was substituted by methanesulfonate. The difference current resulting from the substitution of methanesulfonate for chloride was not changed when the pipette solution contained either 50 mM EGTA (instead of 5 mM) or 10 mM EGTA and 10 mM BAPTA. The nature of Cs(+)- and 4-aminopyridine-sensitive transient outward current suppressed by chloride ion substitutes or chloride channel blockers is discussed.

Signaling from synapse to nucleus: postsynaptic CREB phosphorylation during multiple forms of hippocampal synaptic plasticity

Phosphorylation of the transcription factor CREB is thought to be important in processes underlying long-term memory. It is unclear whether CREB phosphorylation can carry information about the sign of changes in synaptic strength, whether CREB pathways are equally activated in neurons receiving or providing synaptic input, or how synapse-to-nucleus communication is mediated. We found that Ca(2+)-dependent nuclear CREB phosphorylation was rapidly evoked by synaptic stimuli including, but not limited to, those that induced potentiation and depression of synaptic strength. In striking contrast, high frequency action potential firing alone failed to trigger CREB phosphorylation. Activation of a submembranous Ca2+ sensor, just beneath sites of Ca2+ entry, appears critical for triggering nuclear CREB phosphorylation via calmodulin and a Ca2+/calmodulin-dependent protein kinase.

Activation mechanism of Ca(2+)-sensitive transient outward current in rabbit ventricular myocytes

1. The mechanism of activation of the Ca(2+)-sensitive and 4-aminopyridine (4-AP)-insensitive transient outward current, I(to)(Ca), was examined in single rabbit ventricular myocytes using the whole-cell patch-clamp technique. 2. When the steady-state intracellular Ca2+ (Ca2+i) concentration ([Ca2+]i) was < 1 nM, I(to)(Ca) could not be activated by applying pulses at 0.1 Hz. When [Ca2+]i was increased to > or = 10 nM, I(to)(Ca) was activated by 0.1 Hz depolarizing pulses in all control experiments. 3. I(to)(Ca) was completely blocked by an anion transport blocker, DIDS, or by replacement of NaCl with sodium aspartate. Upon changing extracellular [Cl-], the reversal potential was shifted as predicted for a chloride-selective conductance. When intracellular K+ was replaced with Cs+, I(to)(Ca) was also observed. From these results it was concluded that I(to)(Ca) was carried by Cl-. 4. Anion selectivity of I(to)(Ca) was investigated by the replacement of C.- with various anions. The sequence of permeability was SCN- > I- > Br- > Cl-. 5. The amplitude of I(to)(Ca) was enhanced in a [Ca2+]i-dependent manner between 10 nM and 1 microM Ca2+i, while steady-state inactivation curves and the voltage-dependent activation curves were unchanged. The half-inactivation and half-activation potentials were -35 mV and +37 mV, respectively, at all [Ca2+]i. 6. I(to)(Ca) was inhibited by blocking Ca2+ influx or Ca2+ release from sarcoplasmic reticulum, suggesting that a 'Ca(2+)-induced Ca(2+)-release' mechanism is essential for the activation of I(to)(Ca). 7. A steady-state Ca(2+)-activated Cl- current with a linear I-V relationship was observed at 1 microM Ca2+, while the current activated by depolarization was strictly dependent on Ca2+ entry or Ca2+ release from the sarcoplasmic reticulum. These results suggest that the I(to)(Ca) channel is purely ligand (Ca2+) gated and its time course reflects the concentration of Ca2+i.

Activation of protein kinase A partially reverses the effects of 2,3-butanedione monoxime on the transient outward K+ current of rat ventricular myocytes

The transient outward K+ current (Ito) was assessed in single rat ventricular myocytes with the whole-cell patch-clamp technique. Extracellular application of the chemical phosphatase 2,3-butanedione monoxime (BDM) inhibited Ito in a concentration-dependent manner. The IC50 was 14 mM. The on-set of this effect occurred within 20 s after BDM application. Ito recovered almost completely at 2 min after washout of BDM. Application of 20 mM BDM shifted the steady-state inactivation curve of Ito by 9 +/- 2 mV (n = 8) in the negative direction. Addition of 5 microM isoproterenol enhanced Ito amplitude by 16.2 +/- 1.8%. This concentration of isoproterenol partially reversed the BDM-induced inhibition of Ito. Furthermore, application of 10 mM 8-bromoadenosine 3':5'-cyclic monophosphate enhanced the amplitude of Ito and also significantly reversed the BDM-induced suppression of Ito. In contrast, intracellular dialysis with guanosine 3':5'-cyclic monophosphate (cGMP, 1-10 mM) did not affect the BDM-induced inhibition of Ito. The inward rectifier K+ current (Ik1) was relatively insensitive to BDM; i.e., 20 mM BDM inhibited Ito and Ik1 to 35.5 +/- 4.3% (n = 8) and 92.9 +/- 4.0% (n = 4) of the control, respectively. These results indicate that BDM suppressed Ito but not Ik1 of rat ventricular myocytes. We attribute the BDM suppression of Ito to dephosphorylation of the channel protein.

Hyperpolarization induced by sodium removal in rabbit sinoatrial node cells. Possible role of electrogenic sodium-calcium exchange

Spontaneously active rabbit sinoatrial node (SAN) cells were bathed in K-free solution or in K-free ouabain (20 microM)-containing solution to depress the electrogenic Na(+)-K+ pump activity. In SAN cells exposed to K-free solution, the automatic action potentials ceased with gradual depolarization, followed by an eventual steady-state membrane potential of -32 +/- 1 mV. Under conditions where the Na(+)-K+ pump was blocked, removal of external Na+ produced a large and rapid hyperpolarization in the membrane potential and the membrane was hyperpolarized by 23 +/- 0.5 mV. When the external Na+ was lowered, Na+ was replaced by Li+. The Na-free hyperpolarization was not affected by applications of verapamil (4 microM), lidocaine (1 mM), and quinidine (50 microM), but was inhibited by either quinacrine (50 microM) or Cd2+ (10 mM), which are blockers of Na(+)-Ca2+ exchange. In the absence of external K+, replacement of external NaCl by sucrose produced a hyperpolarization similar to that seen in the replacement of external Na+ by Li+. In the K-free ouabain (20 microM)-containing solution, removal of external Na+ also produced a hyperpolarization, and the membrane potential dropped from -29 +/- 1 to -48 +/- 1 mV. The intracellular acidification due to NH4Cl removal after exposure to NH4Cl (20 mM) produced a decrease in Na-free hyperpolarization, which in the presence of ouabain was inhibited by the application of Cd2+ (10 mM). Removal of external Ca2+ nearly completely blocked Na-free hyperpolarization. It can be concluded that Na-free hyperpolarizations are related to the functioning of an electrogenic Na(+)-Ca2+ exchange mechanism.

Tetrandrine: a new ligand to block voltage-dependent Ca2+ and Ca(+)-activated K+ channels

Extensive pharmacological investigations on tetrandrine, one of the traditional medicinal alkaloids, are reviewed. Tetrandrine has been used clinically in China for centuries in the treatment of many diseases. A recent series of studies has revealed major mechanisms underlying its multiple pharmacological and therapeutic actions. One of the most interesting discoveries is that tetrandrine is a new kind blocker of the voltage-activated, L-type Ca2+ channel in a variety of excitable cells, such as cardiac, GH3 anterior pituitary and neuroblastoma cells, as well as in rat neurohypophysial nerve terminals. Although tetrandrine does not belong to any of the three classical Ca2+ channel blocker groups, electrophysiological and radioligand binding studies show that tetrandrine is an L-type Ca2+ channel blocker with its binding site located at the benzothiazepine receptor on the alpha 1-subunit of the channel. In addition, tetrandrine is a blocker of the voltage-dependent T-type Ca2+ channel. It is clear that tetrandrine's actions in the treatment of cardiovascular diseases, including hypertension and supraventricular arrhythmia, are due primarily to its blocking of voltage-activated L-type and T-type Ca2+ channels. Furthermore, this alkaloid is a potent blocker of the Ca(2+)-activated K+ (K(Ca)) channels of neurohypophysial nerve terminals. The blocking kinetics of tetrandrine on the K(Ca) channel is quite different from that of typical K(Ca) channel blockers such as tetraethylammonium and Ba2+. Although the clinical role of tetrandrine as a blocker of the K(Ca) channels is unclear, it is a promising ligand for the study of K(Ca) channel function.

Membrane ionic currents and properties of freshly dissociated rat brainstem neurons

It is well known that neuronal firing properties are determined by synaptic inputs and inherent membrane functions such as specific ionic currents. To characterize the ionic currents of brainstem cardio-respiratory neurons, cells from the hypoglossal (XII) nucleus and the dorsal motor nucleus of the vagus (DMX) were freshly dissociated and membrane ionic currents were studied under whole-cell voltage and current clamp. Both of these neurons showed a TTX-sensitive Na+ current with a much larger current density in XII than DMX neurons. This Na+ current had two (fast and slow) distinct inactivation decay components. The ratio of the magnitudes of the fast to slow component was roughly two-fold greater in DMX than in XII cells. Both DMX and XII neurons also showed a high voltage-activated Ca2+ current, but this current density was significantly greater (three-fold) in DMX than XII neurons. A relatively small amount of low-voltage activated Ca2+ current was also observed in DMX neurons, but not in the majority of XII cells. A transient and a sustained outward current components were observed in DMX cells, but only sustained currents were present in XII neurons. These outward currents had a reversal potential of about -70 mV with 3 mM external K+ and -30 mV with 25 mM K+, and substitution of K+ with cesium and tetraethylammonium suppressed more than 90% the outward currents, indicating that most outward currents were carried by K+. The transient outward current consisted of two components with one sensitive to 4-aminopyridine and the other to intracellular Ca2+. In XII neurons, BRL 38227 (lemakalim), an ATP-sensitive K+ (KATP) channel activator, increased the sustained K+ currents by 10% of control, and glibenclamide, a KATP channel blocker, decreased the sustained K+ currents by 20%. Evidence for the presence of an inward rectifier K+ current was also obtained from both XII and DMX neurons. These results on XII and DMX neurons indicate that (1) the methods used to dissociate neurons provide a useful means to overcome voltage clamp technical difficulties; (2) ion channel characteristics such as density and biophysical properties of DMX neurons are very different from those of XII neurons; and (3) several newly discovered membrane ionic currents are present in these cells.

Persistent T-wave changes after alteration of the ventricular activation sequence. New insights into cellular mechanisms of 'cardiac memory'

BACKGROUND

"Cardiac memory" refers to persistent T-wave changes on ECG that follow resumption of sinus rhythm after a period of altered activation sequence. Previous studies demonstrated that cardiac memory in intact dogs was abolished by 4-aminopyridine (4-AP), which blocks both the transient outward potassium current, Ito, and IK.

METHODS AND RESULTS

We used standard microelectrode techniques to study the mechanism for cardiac memory in canine ventricular subepicardial and subendocardial slabs measuring 15 x 30 x 1 to 2 mm. Bipolar electrodes were used to stimulate slabs parallel to fiber axis, simulating normal activation, and perpendicular to fiber axis, simulating ventricular pacing. Four 30-minute periods of normal activation at a basic cycle length of 650 milliseconds were interrupted by three 20-minute periods of ventricular pacing at a basic cycle length of 450 milliseconds. We first recorded action potentials differentially from epicardial and endocardial slabs. The stimulation protocol induced changes in the "T" wave of the difference signals that mimicked cardiac memory and that could be explained on the basis of the transmural gradient in repolarization between epicardium and endocardium. This result was not obtainable with slow and rapid pacing from one site only. In subsequent experiments, action potential characteristics of epicardial and endocardial slabs were studied by the same pacing protocol with alternation between simulated normal activation and ventricular pacing. During ventricular pacing, the epicardial phase 1 notch and plateau amplitude decreased compared with normal activation. 4-AP (3 mmol/L) decreased notch size and plateau amplitude during normal activation in epicardium but not endocardium. In contrast, the local anesthetic lidocaine did not change notch size or plateau amplitude in epicardium or endocardium.

CONCLUSIONS

These results suggest that the contribution to repolarization of specific potassium channels influences the memory phenomenon and that by blocking Ito and reducing the transmural voltage gradient for repolarization, 4-AP abolishes cardiac memory.

Mechanical and electrophysiological studies on the positive inotropic effect of 2-phenyl-4-oxo-hydroquinoline in rat cardiac tissues

1. The pharmacological and electrophysiological effect of 2-phenyl-4-oxo-hydroquinoline (YT-1), a new synthetic agent, were determined in rat isolated cardiac tissues and ventricular myocytes. 2. YT-1 was found to have a positive inotropic effect in both atria and ventricular muscles but did not cause significant increases in the spontaneously beating rate of right atria. 3. The positive inotropic effect of YT-1 was antagonized neither by beta-nor by alpha-adrenoceptor antagonists but was partially antagonized by a Ca2+ channel blocker (verapamil) and a K+ channel blocker (4-AP). 4. The action potential duration and amplitude of ventricular cells were progressively increased as the concentration of YT-1 was increased from 3 to 30 microM. 5. A voltage clamp study revealed that the prolongation of action potential duration by YT-1 was associated with a prominent inhibition of 4-AP-sensitive transient outward current (I(to)). At potentials negative to the reversal potential of K1-channels, the inward current through these channels was partially reduced by YT-1. At potentials positive to the reversal potential, the outward current through these channels was affected very little. 6. Although YT-1 blocked the amplitude of I(to), the voltage-dependence of the steady-state inactivation of I(to), was unaffected. 7. Apart from the inhibition of K+ currents, YT-1 also inhibited the sodium inward current. 8. The evidence suggests that YT-1 increases the slow inward Ca2+ current (ICa) significantly. 9. It is concluded that the positive inotropic effect of YT-1 is due predominantly to the increase of ICa and inhibition of I(to).

[Ca2+]i transients and [Ca2+]i-dependent chloride current in single Purkinje cells from rabbit heart

1. Single Purkinje cells, enzymatically isolated from rabbit ventricle, were studied under whole-cell voltage clamp and internally perfused with the fluorescent Ca2+ indicator, indo-1 (100 microM). 2. Fast [Ca2+]i transients were elicited by brief depolarizations from a holding voltage of -45 mV and by repolarization from very positive potentials. The peak [Ca2+]i-voltage relation was bell-shaped with a peak around +10 mV. 3. [Ca2+]i transients were completely blocked by the Ca2+ channel antagonist, nisoldipine (10 microM) and were very small when Ca2+ release from the sarcoplasmic reticulum (SR) was prevented by superfusion of cells by caffeine (1 mM) or ryanodine (10 microM). A fast application of caffeine induced a transient increase in [Ca2+]i. These results suggest [Ca2+]i transients are due to Ca(2+)-induced Ca2+ release from the SR. 4. Rate of decline of the [Ca2+]i transient was voltage dependent, suggesting contribution of the Na(+)-Ca2+ exchanger to Ca2+ efflux. At very positive potentials (> +60 mV), Ca2+ influx through the Na(+)-Ca2+ exchanger could be observed. 5. A transient outward current was observed at potentials positive to +10 mV, but only if depolarizing pulses were accompanied by a [Ca2+]i transient. 6. When the amplitude of the [Ca2+]i transient was changed by (1) changes in [Ca2+]o, (2) changes in frequency of depolarization or (3) conditioning prepulses, the amplitude of the outward current changed in the same direction. This suggests activation of the current is dependent on and graded by [Ca2+]i. 7. The outward current was observed in K(+)-free solutions, in the presence of Cs+ and TEA+, and was not blocked by 4-aminopyridine (10 mM). In contrast, DIDS (100 microM) decreased the outward current by 70 +/- 20% (mean +/- S.D., n = 9), without affecting [Ca2+]i. 8. When external Cl- was lowered, the amplitude of the outward current decreased; when internal Cl- was replaced by aspartate, it became apparent at more negative potentials. These interventions strongly suggest the current was carried by Cl-; it can therefore be referred to as a [Ca2+]i-activated Cl- current or ICl(Ca). 9. When ICl(Ca) was maximally activated during a conditioning step, steps to negative potentials revealed inward currents through ICl(Ca) (in symmetrical Cl- solutions). The fully activated I-V relation was linear. 10. ICl(Ca) could be activated at membrane potentials between -80 and +80 mV by a fast application of caffeine (10 mM), inducing Ca2+ release from the SR, demonstrating that ICl(Ca) does not require membrane depolarization or Ca2+ influx through the Ca2+ channel for its activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium current in single human cardiac myocytes

INTRODUCTION

Significant species-, tissue-, and age-dependent differences have been described for the L-type calcium current (ICa). Therefore, extrapolation of data obtained from the many animal models to human cardiac physiology is difficult. In this study, we have characterized the voltage-dependent properties of ICa from pediatric and adult, atrial and ventricular human heart tissue.

METHODS AND RESULTS

ICa was measured in single human heart muscle cells using the "whole cell," voltage clamp method. Single myocytes were isolated from myocardial specimens obtained intraoperatively from both pediatric and adult patients (ages 3 months to 75 years) undergoing cardiac surgery. Cells obtained for these experiments appeared to be healthy; the resting potential was between -80 and -85 mV. The action potential shape and duration and current-voltage relationship for ICa were similar to that reported by others for human heart cells. The steady-state activation variable, d infinity, was found to be similar in both pediatric atrial and ventricular cells but shifted approximately 5 mV negative in the adult atrial and ventricular cells. ICa of all cells displayed biexponential inactivation and steady-state inactivation was incomplete at positive potentials (steady-state inactivation curves turned up at positive potentials) consistent with inactivation arising from voltage-dependent and calcium-dependent processes as reported in heart cells from many species. The potential of maximal inactivation was more negative for adult cells (around -10 mV) than pediatric cells (around 0 mV). Estimates of the calcium "window" current, using a modified Hodgkin-Huxley model, could explain measured differences in action potential shape and duration.

CONCLUSION

Human cardiac ICa can be investigated using whole cell, voltage clamp methods and a modified Hodgkin-Huxley model. Quantitative characterization of many of the properties of ICa in human heart tissue suggests that important species differences do exist and that further investigations are required to characterize the dependence of inactivation on [Ca2+]i in human heart cells. Since the array of characteristics of ICa in different species varies, the study of human myocardial cells per se continues to be important when examining human cardiac physiology.

Spontaneous myocardial calcium oscillations: are they linked to ventricular fibrillation?

The physiological oscillation of cytosolic [Ca2+] that underlies each heart beat is generated by the sarcoplasmic reticulum (SR) in response to an action potential (AP) and occurs relatively synchronously within and among cells. When the myocardial cell and SR Ca2+ loading become sufficiently high, the SR can also generate spontaneous, i.e., not triggered by sarcolemmal depolarization, Ca2+ oscillations (S-CaOs). The purpose of this review is to describe properties of S-CaOs in individual cells, myocardial tissue, and the intact heart, and to examine the evidence that may link S-CaOs to the initiation or maintenance of ventricular fibrillation (VF). The SR Ca2+ release that generates S-CaOs occurs locally within cells and spreads within the cell via Ca(2+)-induced Ca2+ release. The localized increase in cytosolic [Ca2+] due to S-CaOs may equal that induced by an AP and causes oscillatory sarcolemmal depolarizations of cells in which it occurs. These oscillatory depolarizations are due to Ca2+ activation of the Na/Ca exchanger and of nonspecific cation channels. Asynchronous occurrence of diastolic S-CaOs among cells within the myocardium causes inhomogeneity of diastolic SR Ca2+ loading; this leads to inhomogeneity of the systolic cytosolic [Ca2+] transient levels in response to a subsequent AP, which leads to heterogeneity of AP repolarization, due to heterogeneous Ca2+ modulation of the Na/Ca exchanger, nonspecific cation channels, and of the L-type Ca2+ channel. In a tissue in which asynchronous S-CaOs are occurring in diastole, the subsequent AP temporarily synchronizes SR Ca2+ loading and release within and among cells. Varying extents of synchronized S-CaOs then begin to occur during the subsequent diastole. The partial synchronization of this diastolic S-CaOs among cells within myocardial tissue produces aftercontractions and diastolic depolarizations. When S-CaOs are sufficiently synchronized, the resultant depolarizations summate and can be sufficient to trigger a spontaneous AP.S-CaOs occurrence within some cells during a long AP plateau also modulates the removal of voltage inactivation of L-type Ca2+ channels and increases the likelihood for "early afterdepolarizations" to occur in myocardial tissue. S-CaOs have an apparent modulatory role in the initiation of VF in the Ca2+ overload model and in the reflow period following ischemia. Likewise, in non-a priori Ca2+ overloaded hearts, S-CaOs modulate the threshold for VF induction (induced typically by alternating current) but may not be essential for VF induction.(ABSTRACT TRUNCATED AT 400 WORDS)

Background potassium current active during the plateau of the action potential in guinea pig ventricular myocytes

Background outward K+ currents in guinea pig ventricular myocytes were characterized over a broad range of membrane potentials, including those corresponding to the plateau of the action potential. The background current that is blocked by 1 mM Ba2+ (IK,p) activates within 5 msec at positive potentials, does not inactivate, and deactivates very rapidly on repolarization. IK,p is insensitive to Cl- channel blockers, internal or external [Cl-], dihydropyridines, and sulfonylureas. In contrast, the delayed rectifier K+ current (IK) was not completely blocked even by 30 mM Ba2+. Ba(2+)-sensitive current density increased progressively from 0.16 +/- 0.04 pA/pF at 0 mV to 0.52 +/- 0.21 pA/pF at +80 mV (n = 13, mean +/- SEM). The background current remains present when [K+]o is reduced to 0 mM, which suppresses the inward rectifier K+ current (IK1). These and other features suggest that IK,p is generated by K+ channels that are distinct from IK1 or IK. The kinetics and voltage dependence of IK,p render it capable of modulating both the height and duration of the cardiac action potential.

Ionic bases for electrophysiological distinctions among epicardial, midmyocardial, and endocardial myocytes from the free wall of the canine left ventricle

Recent studies from our laboratory involving syncytial preparations have delineated electrophysiological distinctions between epicardium, endocardium, and a unique population of cells in the deep subepicardial to midmyocardial layers (M region) of the canine ventricle. In the present study, we used standard microelectrode, single microelectrode switch voltage-clamp, and whole-cell patch-clamp techniques to examine transmembrane action potentials, steady-state current-voltage relations, and the 4-aminopyridine-sensitive transient outward current (Ito1) in myocytes enzymatically dissociated from discrete layers of the free wall of the canine left ventricle. Action potential characteristics of myocytes isolated from the epicardium, M region, and endocardium were very similar to those previously observed in syncytial preparations isolated from the respective regions of the ventricular wall. A prominent spike and dome was apparent in myocytes from epicardium and the M region but not in myocytes from endocardium. Action potential duration-rate relations were considerably more pronounced in cells isolated from the M region. Current-voltage relations recorded from cells of epicardial, M region, and endocardial origin all displayed an N-shaped configuration with a prominent negative slope-conductance region. The magnitude of the inward rectifier K+ current (IK1) was 392 +/- 86, 289 +/- 65, and 348 +/- 115 pA in epicardial, M region, and endocardial myocytes, respectively, when defined as steady-state current blocked by 10 mM Cs+. Similar levels were obtained when IK1 was defined as the steady-state difference current measured in the presence (6 mM) and absence of extracellular K+. Ito1 was significantly greater in epicardial and M region myocytes than in endocardial myocytes. At a test potential of +70 mV (holding potential, -80 mV), Ito1 amplitude was 4,203 +/- 2,370, 3,638 +/- 1,135, and 714 +/- 286 pA in epicardial, M region, and endocardial cells, respectively. No significant differences were observed in the voltage dependence of inactivation of Ito1 in the three cell types. The time course of reactivation of Ito1 was slower in cells from the M region compared with either epicardial or endocardial cells. Our data suggest that prominent heterogeneity exists in the electrophysiology of cells spanning the canine ventricular wall and that differences in the intensity of the transient outward current contribute importantly, but not exclusively, to this heterogeneity. These findings should advance our understanding of basic heart function and the ionic bases for the electrocardiographic J wave, T wave, U wave, and long QT intervals as well as improve our understanding of some of the complex factors contributing to the development of cardiac arrhythmias.

Properties of a transient K+ current in chemoreceptor cells of rabbit carotid body

1. Adult rabbit carotid body chemoreceptor cells, enzymatically dispersed and short-term cultured, exhibit an inactivating outward K+ current that is reversibly inhibited by low PO2. In the present work we have characterized the biophysical and pharmacological properties of this current using the whole-cell voltage clamp recording technique. 2. Inactivating current was recorded after blockage of Ca2+ currents with extracellular Co2+, Cd2+, or after complete washing out of Ca2+ channels. 3. The threshold of activation of this inactivating current was about -40 mV. Current activated very quickly (mean rise time 4.8 +/- 0.42 ms at +60 mV) but inactivated more slowly. Inactivation was well fitted by two exponentials with time constants of 79.7 +/- 6.6 and 824 +/- 42.8 ms (at +40 mV). The inactivation process showed a little voltage dependence. 4. The steady-state inactivation was well fitted by a Boltzman function. Inactivation was fully removed at potentials negative to -80 mV and was complete at voltages near -10 mV; 50% inactivation occurred at -41 mV. 5. Recovery from inactivation had several components and was voltage dependent. Initial recovery was fast, but full recovery, even at -100 mV, required more than 30 s. 6. Inactivating current was selectively blocked by 4-aminopyridine (4-AP), in a dose-dependent manner (IC50, 0.2 mM). The duration of chemoreceptor cells action potentials was augmented by 1 mM 4-AP from 2.3 +/- 0.36 to 7.0 +/- 0.25 ms at 0 mV. Tetraethylamonium (TEA), at concentrations above 5 mM, blocked inactivating and non-inactivating components of the whole K+ current. 7. Inactivating current was modulated by cyclic AMP (cAMP). Bath application of 2 mM dibutyryl cAMP reduced peak amplitude by 18.7 +/- 2.9% (at +30 mV) and slowed down the rise time of the current. The effect was not voltage dependent. Forskolin (10-20 microM) also affected inactivating current, by accelerating the inactivation process. In the same preparations neither dibutyryl cAMP nor forskolin affected Ca2+ currents. 8. It is concluded that modulation of K+ channels by cAMP might play a physiological role potentiating the low PO2 inhibition of K+ channels.

Caffeine inhibits depolarization-activated outward currents in rat ventricular myocytes

The effects of caffeine (10 mM) on depolarization-activated, calcium-independent outward K+ currents were investigated in isolated rat ventricular myocytes, using whole-cell clamping. The external solution contained CoCl2 2 mM and the internal solution contained ethylene glycol-bis(-aminoethyl ether) N,N,N',N'-tetraacetic acid 10 mM. Caffeine decreased the peak amplitude of the total current and the sustained plateau current. Caffeine did not modify the steady state inactivation curve, which was fitted by two Boltzmann functions. Caffeine blocked the tetraethylammonium-sensitive slowly activating and inactivating outward current by 32% and the 4-aminopyridine-sensitive rapidly activating and inactivating transient outward current by 19%. Caffeine did not modify the inactivation rate or the time course of the recovery from inactivation of the transient current. Ryanodine 10 microM did not modify any of the current components and the effect of caffeine was not modified by ryanodine pretreatment. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine 100 microM, did not modify the depolarization-activated calcium-independent outward currents.

Transient outward currents in cochlear ganglion neurons of the chick embryo

Cochlear ganglion neurons were isolated from chick embryos and membrane currents recorded using the patch-clamp technique. Depolarizing voltage steps elicited transient outward currents whose inactivation was best fitted by a double-exponential function with time constants < 30 ms and > 100 ms. The fast inactivating transient outward current (Ito,f) had a threshold for activation of -61 +/- 5.5 mV; steady-state inactivation was voltage-dependent between -90 and -60 mV, with half-inactivation near -75 mV. The slowly inactivating outward current (Ito,s) showed an activation threshold of 34 +/- 4 mV. Half-inactivation was at -67 +/- 3 mV. Ito,f was blocked by 4-aminopyridine which did not affect Ito,s. The effect was concentration- and voltage-dependent. Tetraethylammonium had no effect on either fast or slow transient currents but reduced the amplitude of the non-inactivating outward current in a dose-dependent manner. Ito,f was strongly inhibited by removing Ca2+ from the extracellular bathing solution. Cobalt ions inhibited Ito,f in a dose-dependent manner between 2 and 20 mM. The inhibitory effect of Co2+ was voltage-dependent, displaying a bell-shaped inhibition curve as a function of membrane voltage, maximal inhibition occurring between -20 and 0 mV. Ca2+ removal did not affect Ito,s and partially reduced the amplitude of the steady-state current. These results provide kinetic and pharmacological evidence for the presence of two distinct transient outward currents in cochlear neurons. These currents may play a role in the first synaptic relay of sound transmission.

Hyperthyroidism selectively modified a transient potassium current in rabbit ventricular and atrial myocytes

1. Transient outward potassium currents (I(t)) were compared in single cardiac myocytes obtained from normal and hyperthyroid rabbits. Currents were recorded using the suction electrode whole-cell voltage clamp technique. 2. In ventricular myocytes from hyperthyroid animals (at 22 degrees C and a stimulation rate of 0.2 Hz), I(t) was 4- to 5-fold larger than in normal myocytes, in a potential range of -20 to +60 mV. As in normal myocytes, I(t) in hyperthyroid myocytes was calcium insensitive, and was more than 90% suppressed by 2 mM 4-aminopyridine. 3. The increase in I(t) was observed over a wide range of stimulation rates, even at rates sufficiently slow to enable complete reactivation of the I(t) channels. However, there was a major change in the rate dependence of I(t) in hyperthyroid myocytes, with significant I(t) current still present at rates (e.g. 1-2 Hz) at which it is normally completely suppressed. 4. The augmentation of I(t) in the hyperthyroid myocytes could not be accounted for by changes in the voltage dependence or the kinetics of channel activation or inactivation. There was no change in the reversal potential of I(t), implying no change in the selectivity of the channel. 5. Single-channel activity was recorded using the cell-attached mode of recording. In myocytes from hyperthyroid rabbit we observed the following: (a) active patches (often containing two channels) were obtained more frequently in comparison to control; (b) the unitary conductance of the channel was the same; (c) single-channel openings persisted at high stimulation rates. 6. In contrast to hyperthyroid ventricular cells, I(t) in atrial cells from the same hearts was not substantially changed. 7. The rate dependence of I(t) in atrial cells was also unaffected by hyperthyroidism, in contrast to the large changes observed in ventricular cells. Thus, in atrial cells from hyperthyroid hearts the current was totally suppressed at rates of 1-2 Hz, as in euthyroid conditions. 8. Single-channel recordings in the cell-attached mode showed a unitary conductance similar to that found in normal atrial cells. Channel activity was suppressed at 2 Hz, in contrast to hyperthyroid ventricular cells. 9. In conclusion, I(t) is drastically changed in hyperthyroid rabbit ventricle cells. The changes are in the magnitude of the macroscopic current and its rate dependence. Since the unitary conductance is unchanged (and the peak open probabilities are normally high at positive membrane potential(s) the number of active channels in the membrane must be increased. In atrial cells from the same hyperthyroid hearts no changes are apparent.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic mechanisms of transient inward current in the absence of Na(+)-Ca2+ exchange in rabbit cardiac Purkinje fibres

1. Membrane currents were measured with a two-microelectrode technique in voltage clamped rabbit cardiac Purkinje fibres under conditions known to cause intracellular calcium overload and to eliminate or minimize Na(+)-Ca2+ exchange. 2. Increasing [Ca2+]o from 2.5 to 5 mM or above and substituting external sodium with either sucrose, choline or Li+ induced an oscillatory transient inward current (TI) which peaked 200-300 ms after repolarization from a previous depolarizing pulse. The TI quickly disappeared upon return to normal Tyrode solution. Both the rate and configuration of action potentials of Purkinje fibres also returned to control upon return to Tyrode solution after 30 min of high Ca2+ exposure, if the Ca2+ concentration was 30 mM or less. 3. The TI in Na(+)-free solution was Ca2+ dependent. Either zero or low (2.5 mM) [Ca2+]o, or replacement of [Ca2+]o by BaCl prevented induction of the TI current upon repolarization from a previous depolarizing pulse. 4. In the presence of 30 mM-CaCl2 and with choline chloride as the substitute for NaCl, TI had a distinct reversal potential (Erev) of -25 mV. The time-to-peak TI, either inward or outward, did not shift significantly with change in voltage. Both inward and outward TI were simultaneously abolished by exposure to 1 microM-ryanodine, suggesting they were both activated by transient release of Ca2+ from the sarcoplasmic reticulum. The occurrence of TI in the absence of [Na+]o is not compatible with an electrogenic Na(+)-Ca2+ exchange mechanism. The existence of a clear-cut reversal potential suggests that an ionic channel may be responsible for the TI under these conditions. 5. Both the magnitude of peak TI and the Erev were affected by changes of CaCl2 concentration. (i) Under steady-state conditions, peak inward TI was significantly increased when the [Ca2+]o was elevated from 5 to 15 mM. The peak TI in the outward direction was significantly increased when [Ca2+]o was elevated from 15 to 30 mM; however, the difference in peak inward TI at 15 and 30 mM [Ca2+]o was small. (ii) Clear-cut reversals of TI were found at Ca2+ concentrations of 10 mM (Erev = -19.5 mV) or greater, and elevation of [Ca2+]o to 20, 30, 50 and 105 mM shifted the Erev to more negative potentials. (iii) In the presence of 5 mM [Ca2+]o the inward TI declined to zero at about -30 mV, and test voltages between -55 and +5 mV failed to reveal a distinct outward TI.(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular mechanisms of differential action potential duration restitution in canine ventricular muscle cells during single versus double premature stimuli

BACKGROUND

We tested the hypothesis that action potential duration (APD) restitution of normal ventricular muscle cells is different during double premature stimuli (S3) compared with a single premature stimulus (S2). We propose a possible ionic mechanism for such a difference.

METHODS AND RESULTS

Action potentials and isometric tension were recorded simultaneously from isolated canine right ventricular trabeculae (2 x 2 x 10 mm) (n = 35). APD and tension restitution curves (APD) and peak tension versus diastolic interval [DI] of S2 and S3 were constructed by the extrastimulus method during pacing at 1,500 msec. The following results were obtained. 1) The APD restitution curve of S2 was different from that of S3. During the restitution of S2, an early biphasic upward hump was present at short DIs. In contrast, a smooth exponential rise was consistently seen during S3 restitution. 2) Peak tension remained significantly (p less than 0.001) lower during the restitution of S2 than during S3 restitution at all DIs tested. 3) The variation of APD during the initial 100 msec of DI was significantly longer during S3 than S2 (22 +/- 5 msec versus 41 +/- 5 msec, p less than 0.001). 4) Caffeine (2 mM, n = 5) and ryanodine (10 microM, n = 5) blocked cyclic variations of tension, presumably by blocking cyclic variations of intracellular calcium ion concentrations ([Ca2+]i), and eliminated the differences in APD restitution between S2 and S3. 5) Nisoldipine at high (5 microM) but not at lower (2 microM, n = 5) concentration eliminated the differences in restitution of both APD and tension between S2 and S3. 6) BAY K 8644 (100 nM, n = 5) had no effect on this difference.

CONCLUSIONS

Greater variations of APD occur during the restitution of S3 than during S2 at short DIs. These differences appear to be caused by cyclic variations in tension and thus in [Ca2+]i. Calcium-sensitive outward currents could explain these differences in APD restitution.

Properties of the calcium-activated chloride current in heart

We used the whole cell patch clamp technique to study transient outward currents of single rabbit atrial cells. A large transient current, IA, was blocked by 4-aminopyridine (4AP) and/or by depolarized holding potentials. After block of IA, a smaller transient current remained. It was completely blocked by nisoldipine, cadmium, ryanodine, or caffeine, which indicates that all of the 4AP-resistant current is activated by the calcium transient that causes contraction. Neither calcium-activated potassium current nor calcium-activated nonspecific cation current appeared to contribute to the 4AP-resistant transient current. The transient current disappeared when ECl was made equal to the pulse potential; it was present in potassium-free internal and external solutions. It was blocked by the anion transport blockers SITS and DIDS, and the reversal potential of instantaneous current-voltage relations varied with extracellular chloride as predicted for a chloride-selective conductance. We concluded that the 4AP-resistant transient outward current of atrial cells is produced by a calcium-activated chloride current like the current ICl(Ca) of ventricular cells (1991. Circulation Research. 68:424-437). ICl(Ca) in atrial cells demonstrated outward rectification, even when intracellular chloride concentration was higher than extracellular. When ICa was inactivated or allowed to recover from inactivation, amplitudes of ICl(Ca) and ICa were closely correlated. The results were consistent with the view that ICl(Ca) does not undergo independent inactivation. Tentatively, we propose that ICl(Ca) is transient because it is activated by an intracellular calcium transient. Lowering extracellular sodium increased the peak outward transient current. The current was insensitive to the choice of sodium substitute. Because a recently identified time-independent, adrenergically activated chloride current in heart is reduced in low sodium, these data suggest that the two chloride currents are produced by different populations of channels.

Possible involvement of a chloride conductance in the transient outward current of whole-cell voltage-clamped ferret ventricular myocytes

The transient outward current was studied, using the whole-cell patch-clamp technique, in isolated ventricular cells from the ferret heart. In the presence of 4-aminopyridine and cadmium chloride which respectively blocked the Ca-insensitive and the Ca-dependent outward currents, a residual transient outward current was observed in about 30% of the cells tested. This current was suppressed in external hypochloride solution, completely inhibited by SITS (3 mM) and reversed at the equilibrium potential for chloride ions. This suggests the presence of a chloride permeability which could contribute to the repolarization phase of the cardiac action potential.

The control of the contraction of myocytes from guinea-pig heart by the resting membrane potential

1. The influence of different holding potentials (-120 to -70 mV) on the contraction of enzymatically dispersed myocytes from guinea-pig hearts was evaluated. Contractions were elicited by repetitive depolarizations to 0 mV at 0.5 Hz. 2. While ineffective at 140 and 5 mmol l-1 [Na+]o and pipette Na+, respectively, depolarization of the resting membrane with the holding potential increased myocyte shortening at reduced Na+ gradients ([Na+]o 70 or [Na+]i 10-15 mmol l-1). Elevated intracellular Na+ after Na(+)-pump inhibition with ouabain 1-10 mumol l-1 was similarly effective with regard to the inotropic response to different holding potentials. 3. At -70 mV holding potential, reduction of [Na+]o from 140 to 70 mmol l-1 increased myocyte shortening and induced an inwardly directed component of the holding current which peaked at -44 +/- 10 pA and declined thereafter in parallel with the inotropic effect. The relation of this inward current to [Ca2+]i was confirmed by experiments at high Ca2+ buffer capacity where [Na+]o reduction induced a Ni(2+)-insensitive, outwardly directed component (36 +/- 15 pA) of the holding current. The observed inward current is suggested to reflect the extrusion of [Ca2+]i in exchange for [Na+]o as a counter-regulatory mechanism which limits the increase of [Ca2+]i. 4. The interventions which increased the strength of the contraction also enhanced the transient tail current after repolarization, suggesting its close relation to [Ca2+]i. This finding confirmed the pattern found with cell shortening. 5. It is concluded that under certain conditions, voltage-dependent and Na(+)-dependent Na(+)-Ca2+ exchange during the interval between the contractions is relevant to the diastolic concentration of [Ca2+]i which in turn determines the accumulation of Ca2+ in the sarcoplasmic reticulum and the magnitude of the subsequent contraction.

Regional variations in action potentials and transient outward current in myocytes isolated from rabbit left ventricle

1. Regional variations in the shape of early repolarization of the action potential have been correlated to differences in transient outward K+ current, I(t), in myocytes isolated from the epicardial surface, the endocardial trabeculae and the papillary muscles of rabbit left ventricles. Temperature was 35 degrees C during whole-cell, and 22-23 degrees C during cell-attached experiments. 2. Membrane resting potentials were very similar regionally. At 0.1 Hz stimulation the action potential plateau amplitude in papillary muscle cells was significantly higher (104.7 mV) than in epicardial cells (96.47 mV). Exposure to 4-aminopyridine or increases in the rate of stimulation from 0.1 Hz to 3.3 Hz increased plateau height and diminished the initial notch on repolarization. These effects were correlated to the magnitude of I(t) in these cells. At low rates of stimulation I(t) caused a 'spike and dome' morphology of the action potential. 3. Voltage clamp experiments confirmed a higher current density of I(t) in epicardial cells (7.66 pA/pF at +20 mV) than in endocardial (6.45 pA/pF) or papillary muscle cells (3.69 pA/pF). I(t) at 35 degrees C was faster and larger than previously reported and individual currents inactivated almost completely during 100 ms pulses to plateau potentials. No differences in the kinetics or voltage dependence of whole-cell currents were found. Thus, the half-inactivation potential was -37.8 mV in cells from all three regions. 4. Cell-attached recordings from endocardial and epicardial cells showed very similar single-channel amplitudes, burst open probabilities and ensemble averages. The peak channel open probability soon after the start of depolarizing voltage clamp pulses did not change between cell types (P approximately 0.8). The slope conductance of I(t) channels was 13.0 pS with an intercept near the resting potential of the cell. 5. We conclude that regional variations in the shape of initial repolarization in cells from rabbit left ventricle are caused by variations in the magnitude of the transient outward K+ current, I(t). Epicardial cells have the largest, and papillary muscle cells the smallest I(t). The differences are not explained by alterations in the whole-cell kinetics or single-channel kinetics and conductance. The most likely explanation for variations in whole-cell current density is therefore a decrease in channel density in endocardium and papillary muscle compared with epicardial tissue. We estimate the density of I(t) channels per cell to be 1495 (one per 3-4 micron2) in epicardium, 1175 (one per 4-5 micron2) in endocardium, and 875 (one per 6 micron2) in papillary muscle cells.

Chloride conductance pathways in heart

Nonelectrogenic movement of Cl- is believed to be responsible for the active accumulation of intracellular Cl- in cardiac muscle. The electro-neutral pathways underlying this nonpassive distribution of Cl- are believed to include Cl(-)-HCO3- exchange, Na(+)-dependent cotransport (operating as Na(+)-Cl- and Na(+)-K(+)-2Cl- cotransport), and K(+)-Cl- cotransport. The electrogenic movement of Cl- in cardiac muscle is particularly interesting from a historical perspective. Until recently, there was some doubt as to whether Cl- carried any current in the heart. Early microelectrode experiments indicated that a Cl- conductance probably played an important role in regulating action potential duration and resting membrane potential. Subsequent voltage-clamp experiments identified a repolarizing, transient outward current that was believed to be conducted by Cl-, yet further investigation suggested that this transient outward current was more likely a K+ current, not a Cl- current. This left some doubt as to whether Cl- played any role in regulating membrane potential in cardiac muscle. More recent studies, however, have identified a highly selective Cl- conductance that is regulated by intracellular adenosine 3',5'-cyclic monophosphate, and it appears that this Cl- current may play an important role in the regulation of action potential duration and resting membrane potential.

Divalent cations modulate the transient outward current in rat ventricular myocytes

The modulation of the transient outward K+ current (Ito) by divalent cations was studied in enzymatically isolated rat ventricular myocytes with the whole cell patch-clamp technique. At holding potentials negative to -70 mV, 1 mM Cd2+ suppressed Ito, whereas, at potentials positive to -50 mV, the current was augmented. These effects were caused by shifts in the voltage dependence of both activation and inactivation of Ito toward more positive potentials. Cd2+ also slowed the activation kinetics of Ito by shifting the voltage dependence of its rate of activation, but the rate of inactivation was unaffected. Other divalent cations produced similar shifts but at markedly different concentrations. Thus, in the millimolar range, a rightward shift of approximately 20 mV was produced by 3 Co2+, 5 Ni2+, and 10 Ca2+, whereas 10 microM concentrations of Cu2+ and Zn2+ produced equivalent shifts. Similar effects were seen in hippocampal neurons with micromolar concentrations of Zn2+. Thus divalent cations have marked and specific effects on the kinetics and voltage dependence of Ito and may serve as a regulatory mechanism in its activation, particularly in cells with resting potentials positive to -60 mV.

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

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 an IA-type current. The slow current was insensitive to 4-aminopyridine and independent of external Ca2+, but significantly inhibited by 3 mM tetraethylammonium. The fast current was depressed by external 4-aminopyridine, with an ED50 = 0.7 mM, 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 2.8 mM Mn2+, the transient potassium conductance was comparable to that in 2.8 mM Ca2+, while in 2.8 mM Mg2+ the transient component was drastically reduced, as in the absence of any divalent cations. However, when Ca2+ was present, Mg2+ up to 5 mM had no effect. The transient current increased with increasing concentrations of external Ca2+, [Ca2+]o, and the maximum conductance vs. [Ca2+]o curve could be approximated by a one-site model. In addition, the current recorded with 5.5 mM 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.

A subpopulation of cells with unique electrophysiological properties in the deep subepicardium of the canine ventricle. The M cell

Recent studies have shown that canine ventricular epicardium and endocardium differ with respect to electrophysiological characteristics and pharmacological responsiveness and that these differences are in large part due to the presence of a prominent transient outward current Ito and a spike-and-dome morphology of the action potential in epicardium but not endocardium. In attempting to quantitate these differences and assess their gradation across the ventricular wall, we encountered a subpopulation of cells in the deep subepicardial layers with electrophysiological characteristics different from those of either epicardium or endocardium. These cells, which we have termed M cells, display a spike-and-dome morphology typical of epicardium but a maximal rate of rise of the action potential upstroke that is considerably greater than that of either epicardium or endocardium. Using the restitution of the amplitude of phase 1 of the action potential as a marker for the reactivation of Ito, we showed M cells to possess a prominent 4-aminopyridine-sensitive Ito with a reactivation time course characterized by two components with fast and slow time constants. The rate dependence of action potential duration of M cells was considerably more accentuated than that of epicardium or endocardium and more akin to that of Purkinje fibers (not observed histologically in this region). Phase 4 depolarization was never observed in M cells, not even after exposure to catecholamines and/or low [K+]o. In summary, our study presents evidence for the existence of a unique subpopulation of cells in the deep subepicardium of the canine left and right ventricles with electrophysiological features intermediate between those of conducting and myocardial cells. Although their function is unknown, M cells may facilitate conduction in epicardium and are likely to influence or mediate the manifestation of electrocardiographic J waves, T waves, U waves, and long QT intervals and contribute importantly to arrhythmogenesis.

The cytosolic calcium transient modulates the action potential of rat ventricular myocytes

1. The modulation of the action potential by the cytosolic Ca2+ (Cai2+) transient was studied in single isolated rat ventricular myocytes loaded with the acetoxymethyl ester form of the Ca(2+)-sensitive fluorescent dye Indo-1. Stimulation following rest and exposure to ryanodine were used to change the amount of Ca2+ released from the sarcoplasmic reticulum and thus the size of the Cai2+ transient. The Cai2+ transient was measured as the change, upon stimulation, in the ratio of Indo-1 fluorescence at 410 nm to that at 490 nm (410/490) and action potentials or membrane currents were recorded using patch-type microelectrodes. 2. When stimulation was initiated following rest, the magnitude of the Cai2+ transient decreased in a beat-dependent manner until a steady state was reached. The negative staircase in the Cai2+ transient was accompanied by a similar beat-dependent decrease in the duration of the action potential, manifested primarily as a gradual loss of the action potential plateau (approximately -45 mV). A slow terminal phase of repolarization of a few millivolts in amplitude was found to parallel the terminal decay of the Cai2+ transient. 3. The terminal portion of phase-plane loops of membrane potential (Vm) vs. Indo-1 ratio from all of the beats of a stimulus train followed a common linear trajectory even though the individual beats differed markedly in the duration and amplitude of the action potential and Cai2+ transient. 4. When the stimulation dependence of the Cai2+ transient was titrated away with submaximal exposure to ryanodine, the stimulation-dependent changes in the action potential plateau and terminal phase of repolarization were also eliminated. The same effect was noted in cells which, fortuitously, did not show a staircase in the Cai2+ transient following a period of rest. 5. When action potentials were triggered immediately following spontaneous release of Ca2+ from the sarcoplasmic reticulum, which results in a small depolarization at the resting potential, phase-plane loops (Vm vs. Indo-1 ratio) of the spontaneous events followed the same linear trajectory as the terminal phase of repolarization in the loops of the stimulated beats. 6. Following repolarization from brief voltage clamp pulses (to minimize time and voltage-dependent currents associated with depolarization), an inward current was observed that rose and fell in phase with the Cai2+ transient. This current was present at -70 mV, near the resting potential, and at -40 mV, a potential relevant to the plateau of the action potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of two distinct depolarization-activated K+ currents in isolated adult rat ventricular myocytes

Depolarization-activated outward K+ currents in isolated adult rat ventricular myocytes were characterized using the whole-cell variation of the patch-clamp recording technique. During brief depolarizations to potentials positive to -40 mV, Ca(2+)-independent outward K+ currents in these cells rise to a transient peak, followed by a slower decay to an apparent plateau. The analyses completed here reveal that the observed outward current waveforms result from the activation of two kinetically distinct voltage-dependent K+ currents: one that activates and inactivates rapidly, and one that activates and inactivates slowly, on membrane depolarization. These currents are referred to here as Ito (transient outward) and IK (delayed rectifier), respectively, because their properties are similar (although not identical) to these K+ current types in other cells. Although the voltage dependences of Ito and IK activation are similar, Ito activates approximately 10-fold and inactivates approximately 30-fold more rapidly than IK at all test potentials. In the composite current waveforms measured during brief depolarizations, therefore, the peak current predominantly reflects Ito, whereas IK is the primary determinant of the plateau. There are also marked differences in the voltage dependences of steady-state inactivation of these two K+ currents: IK undergoes steady-state inactivation at all potentials positive to -120 mV, and is 50% inactivated at -69 mV; Ito, in contrast, is insensitive to steady-state inactivation at membrane potentials negative to -50 mV. In addition, Ito recovers from steady-state inactivation faster than IK: at -90 mV, for example, approximately 70% recovery from the inactivation produced at -20 mV is observed within 20 ms for Ito; IK recovers approximately 25-fold more slowly. The pharmacological properties of Ito and IK are also distinct: 4-aminopyridine preferentially attenuates Ito, and tetraethylammonium suppresses predominantly IK. The voltage- and time-dependent properties of these currents are interpreted here in terms of a model in which Ito underlies the initial, rapid repolarization phase of the action potential (AP), and IK is responsible for the slower phase of AP repolarization back to the resting membrane potential, in adult rat ventricular myocytes.