Abrupt rate accelerations or premature beats cause life-threatening arrhythmias in mice with long-QT3 syndrome

Deletion of amino-acid residues 1505-1507 (KPQ) in the cardiac SCN5A Na(+) channel causes autosomal dominant prolongation of the electrocardiographic QT interval (long-QT syndrome type 3 or LQT3). Excessive prolongation of the action potential at low heart rates predisposes individuals with LQT3 to fatal arrhythmias, typically at rest or during sleep. Here we report that mice heterozygous for a knock-in KPQ-deletion (SCN5A(Delta/+)) show the essential LQT3 features and spontaneously develop life-threatening polymorphous ventricular arrhythmias. Unexpectedly, sudden accelerations in heart rate or premature beats caused lengthening of the action potential with early afterdepolarization and triggered arrhythmias in Scn5a(Delta/+) mice. Adrenergic agonists normalized the response to rate acceleration in vitro and suppressed arrhythmias upon premature stimulation in vivo. These results show the possible risk of sudden heart-rate accelerations. The Scn5a(Delta/+) mouse with its predisposition for pacing-induced arrhythmia might be useful for the development of new treatments for the LQT3 syndrome.

The effect of external pH on the delayed rectifying K+ current in cardiac ventricular myocytes

The effects of extracellular pH (pHe) on the delayed rectifying K+ current iKr in rabbit ventricular myocytes were studied using the whole-cell-clamp technique. Since a variety of results have been reported on the effect of pH on expressed hERG channels, our aim was to investigate the effects of pH on iKr channels in their native environment. iKr is reduced by extracellular acidification and its deactivation is faster. Extracellular acidification results in a marked shift of the steady-state activation curve to more positive potentials, while alkalinization does not produce a significant shift. E1/2= - 11.3 mV, -20.2 mV, -21.4 mV at pHe 6.5, 7.4, 8.5 respectively; the slope factor is 6.2 mV, and is not affected by pHe. Deactivation of iKr is biexponential, with time constants of the order of 0.5 s and 10 s at -50 mV. Both time constants decrease with external acidification. Also the contribution of the fast component to the total amplitude becomes larger with acidification. Acidification also decreases the fully activated iKr current. Our experiments demonstrate that extracellular acidification reduces iKr by increasing the rate of deactivation, causing a shift of the voltage dependence of activation and producing a voltage-dependent block of the fully activated iKr current.

Downregulation of delayed rectifier K(+) currents in dogs with chronic complete atrioventricular block and acquired torsades de pointes

BACKGROUND

Acquired QT prolongation enhances the susceptibility to torsades de pointes (TdP). Clinical and experimental studies indicate ventricular action potential prolongation, increased regional dispersion of repolarization, and early afterdepolarizations as underlying factors. We examined whether K(+)-current alterations contribute to these proarrhythmic responses in an animal model of TdP: the dog with chronic complete atrioventricular block (AVB) and biventricular hypertrophy.

METHODS AND RESULTS

The whole-cell K(+) currents I(TO1), I(K1), I(Kr), and I(Ks) were recorded in left (LV) and right (RV) ventricular midmyocardial cells from dogs with 9+/-1 weeks of AVB and controls with sinus rhythm. I(TO1) density and kinetics and I(K1) outward current were not different between chronic AVB and control cells. I(Kr) had a similar voltage dependence of activation and time course of deactivation in chronic AVB and control. I(Kr) density was similar in LV myocytes but smaller in RV myocytes (-45%) of chronic AVB versus control. For I(Ks), voltage-dependence of activation and time course of deactivation were similar in chronic AVB and control. However, I(Ks) densities of LV (-50%) and RV (-55%) cells were significantly lower in chronic AVB than control.

CONCLUSIONS

Significant downregulation of delayed rectifier K(+) current occurs in both ventricles of the dog with chronic AVB. Acquired TdP in this animal model with biventricular hypertrophy is thus related to intrinsic repolarization defects.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Repolarizing K+ currents ITO1 and IKs are larger in right than left canine ventricular midmyocardium

BACKGROUND

The ventricular action potential exhibits regional heterogeneity in configuration and duration (APD). Across the left ventricular (LV) free wall, this is explained by differences in repolarizing K+ currents. However, the ionic basis of electrical nonuniformity in the right ventricle (RV) versus the LV is poorly investigated. We examined transient outward (ITO1), delayed (IKs and IKr), and inward rectifier K+ currents (IK1) in relation to action potential characteristics of RV and LV midmyocardial (M) cells of the same adult canine hearts.

METHODS AND RESULTS

Single RV and LV M cells were used for microelectrode recordings and whole-cell voltage clamping. Action potentials showed deeper notches, shorter APDs at 50% and 95% of repolarization, and less prolongation on slowing of the pacing rate in RV than LV. ITO1 density was significantly larger in RV than LV, whereas steady-state inactivation and rate of recovery were similar. IKs tail currents, measured at -25 mV and insensitive to almokalant (2 micromol/L), were considerably larger in RV than LV. IKr, measured as almokalant-sensitive tail currents at -50 mV, and IK1 were not different in the 2 ventricles.

CONCLUSIONS

Differences in K+ currents may well explain the interventricular heterogeneity of action potentials in M layers of the canine heart. These results contribute to a further phenotyping of the ventricular action potential under physiological conditions.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

Modulation of transient outward current by extracellular protons and Cd2+ in rat and human ventricular myocytes

1. The effects of extracellular acidosis and Cd2+ on the transient outward current (Ito) have been investigated in rat and human ventricular myocytes, using the whole-cell patch-clamp technique. 2. In rat myocytes, exposure to acidic extracellular solution (pH 6.0) shifted both steady-state activation and inactivation curves to more positive potentials, by 20.5 +/- 2.7 mV (mean +/- S.E.M.; n = 4) and 19.8 +/- 1.2 mV, respectively. Cd2+ also shifted the activation and inactivation curves in a positive direction in a concentration-dependent manner. 3. In human myocytes, the steady-state activation and inactivation curves were located at more positive potentials. The effect of Cd2+ was similar, but acidosis had less effect than in rat myocytes (e.g. pH 6.0 shifted activation by only 7.2 +/- 2.2 mV and inactivation by 13.7 +/- 0.5 mV; n = 4). 4. In both species, the effect of acidosis decreased with increasing concentrations of Cd2+ and vice versa, suggesting competition between H+ and Cd2+ for a common binding site. 5. The data indicate that acidosis and divalent cations influence Ito via a similar mechanism and act competitively in both rat and human myocytes, but that human cells are less sensitive to the effects of acidosis.

Phenylephrine-induced stimulation of Na+/Ca2+ exchange in rat ventricular myocytes

OBJECTIVE

The effect of an alpha-adrenergic agonist, phenylephrine, on the Na+/Ca2+ exchange current in rat ventricular myocytes was investigated.

METHODS

The Na+/Ca2+ exchange current was measured at room temperature in rat ventricular myocytes as the whole-cell current induced by addition of extracellular Na+ and Ca2+, while blocking Na+ current by setting the holding potential at -30 mV, K+ currents by intracellular Cs+, TEA+ and by extracellular Ba2+, Ca2+ current by nifedipine and Na+ pump current by ouabain or by 0 extracellular K+.

RESULTS

Under these experimental conditions, application of external Na+ and Ca2+ induced a current which was further increased by phenylephrine. Phenylephrine (80 microM) increased the current by up to 31.0 +/- 5.4% of control at all membrane potentials tested both below and above the reversal potential. The reversal potential (+21.0 +/- 3.2 mV), which corresponded with the theoretical reversal potential for the Na+/Ca2+ exchange current under our ionic conditions (+21.3 mV), was not changed by phenylephrine (+23.2 +/- 4.1 mV). Applying phenylephrine in the absence of Na+/Ca2+ exchange (0 Na+e, 0 Ca2+e) did not change the current. The effect was resistant to propranolol, a beta-adrenergic blocker, but prevented by prazosin, an alpha-receptor antagonist, by neomycin, an inhibitor of phospholipase C, and by chelerythrine, a selective inhibitor of protein kinase C. Phorbol 12-myristate 13-acetate failed to stimulate the current. The effect remained similar under conditions of high (HEPESi = 5 mM) and low (HEPESi = 0.5 mM) intracellular pH buffering.

CONCLUSION

Our data indicate that phenylephrine stimulates the Na+/Ca2+ exchange, both in the forward and the reverse modes, probably via a protein kinase C-dependent pathway.

Effects of cetirizine on the delayed K+ currents in cardiac cells: comparison with terfenadine

1. The aim of the present experiments was to analyse the effect of the H1-histamine antagonist, cetirizine, on the delayed K+ currents in cardiac cells and to compare its effects with another H1-histamine antagonist terfenadine, known to possess proarrhythmic effects. 2. Whole cell currents were measured by use of the single electrode patch-clamp technique in rabbit and guinea-pig myocytes. 3. The activation relationship for the IKr current in rabbit ventricular myocytes was depressed and its voltage-dependence shifted in the negative direction with a V1/2 value -13.4+/-2.4 mV under control conditions which changed to -19.1+/-1.9 mV (n=4) in the presence of 0.1 mM cetirizine. 4 In rabbit ventricular myocytes the IC50 for block of IKr was 108+/-8 microM (n=5); in guinea-pig ventricular myocytes this concentration of cetirizine reduced the rapidly activating component IKr to 49+/-4.5% (n=5), while the slowly activating IKs was less affected and only inhibited to 79+/-2.3% (n=5). 5 The block of IKr did not show use-dependence and the time course of the tail current was not changed, suggesting rested-state block or fast activated-state block and no rapid recovery on deactivation. No important difference was found in the activity of the two enantiomers of cetirizine. 6 Terfenadine in comparison was more potent in blocking IKr, the IC50 being 96+/-15 nM (n=6). 7 Based on the present results and information in the literature on binding, it was concluded that cetirizine is a relatively selective H1-histamine receptor antagonist, with minor effects on K+ currents. The IC50 concentration for IKr block in heart cells was 1.000 times higher than the concentrations needed to block H1 histamine receptors. The occurrence of cardiac arrhythmias due to K+ current blockade is therefore unlikely with this drug.

T-type Ca2+ current as a trigger for Ca2+ release from the sarcoplasmic reticulum in guinea-pig ventricular myocytes

1. We have investigated whether Ca2+ entry through T-type Ca2+ channels participates in triggering Ca2+ release from the sarcoplasmic reticulum (SR) in single guinea-pig ventricular myocytes (whole-cell voltage clamp, K5fura-2 as [Ca2+]i indicator; all monovalent cations replaced by impermeant ions to record uncontaminated Ca2+ currents; T = 23 or 36 degrees C). 2. T-type Ca2+ currents were elicited from a holding potential of -90 mV during steps to -50 to -20 mV. For steps to -50 mV, very small [Ca2+]i transients could be recorded with high loading of the SR (peak Delta[Ca2+]i, 67 +/- 41 nM; n = 9). 3. For steps to -40, -30 and -20 mV, we compared the amplitude of Ca2+ release for a holding potential of -50 mV with L-type Ca2+ current only to Ca2+ release for a holding potential of -90 mV with both T- and L-type Ca2+ current. Significantly more Ca2+ release was observed with T-type current present, and both the T-type current and the additional Ca2+ release were suppressed by 50 microM NiCl2. 4. Ca2+ influx through T-type Ca2+ channels triggered less Ca2+ release than a comparable Ca2+ influx through L-type Ca2+ channels. 5. Rapid block of T-type Ca2+ current during the action potential (50 microM NiCl2 during steady-state stimulation at 1 or 2 Hz) did not immediately reduce Ca2+ release, although a small decrease was observed after longer application. 6. We conclude that T-type Ca2+ current can trigger Ca2+ release from the SR albeit less efficiently than L-type Ca2+ current. T-type current is most likely to provide only a small contribution to the trigger for Ca2+ release in normal conditions. These results support the hypothesis that L-type Ca2+ channels have a privileged role in excitation-contraction coupling.

Na(+)-Ca2+ exchange in rat dorsal root ganglion neurons

The role of the Na(+)-Ca2+ exchanger was examined in isolated rat dorsal root ganglion (DRG) neurons. Neurons were dialyzed with the Ca2+ indicator Indo-1. Ca2+ transients were elicited by depolarizing the cells from -80 to 0 mV for 100 ms under voltage clamp conditions. In most cells (45 of 67), the decay of intracellular Ca2+ concentration ([Ca2+]i) could be fitted with a single exponential with a time constant of 2.43 s. In the remaining 22 cells, the decay of [Ca2+]i could be described with a double exponential with time constants of 0.76 and 11.84 s. In cells that displayed a biphasic [Ca2+]i relaxation, Na(+)-free medium caused resting [Ca2+]i to increase from 116 to 186 nM; the slow component of recovery to basal [Ca2+]i was nearly abolished in Na(+)-free medium or by application of 5 mM Ni2+. In 35 of 45 cells displaying a monophasic [Ca2+]i decay, omitting external Na+ increased the time constant of [Ca2+]i decay from 2.02 to 3.63 s. In the remaining 10 cells, Na(+)-free solution did not affect Ca2+ handling. The time constant of [Ca2+]i relaxation was voltage dependent. These findings demonstrate the important role of the Na(+)-Ca2+ exchanger in DRG neurons. Its presence was further confirmed both at the mRNA and the protein level.

Mechanism of L- and T-type Ca2+ channel blockade by flunarizine in ventricular myocytes of the guinea-pig

Flunarizine is a substance known to block voltage-dependent Ca2+ channels in smooth muscle and neuronal cells. Reports on the effect on voltage-dependent cardiac Ca2+ channels are however sparse. Therefore, the mechanism of action of flunarizine on two types of voltage-dependent cardiac Ca2+ channels, the L- and T-type, in single ventricular myocytes of the guinea-pig was investigated using the whole-cell voltage clamp technique. Both channel types can be blocked by flunarizine in a time-, frequency-, voltage-, Ca(2+)-, and proton-dependent way. While the overall mechanism of action on cardiac myocytes is similar to the one reported for other cell types, we found that cardiomyocytes are less susceptible to block (Kd 3.3-11 mM). We also describe a complete analysis of the different components of block, together with evidence for open channel state block and drug-induced changes in channel gating. These findings provide new insights into the mechanism of action of flunarizine on voltage-dependent Ca2+ channels.

Na+ current and Ca2+ release from the sarcoplasmic reticulum during action potentials in guinea-pig ventricular myocytes

1. Ca2+ release from the sarcoplasmic reticulum (SR) was examined in enzymatically isolated single guinea-pig ventricular myocytes by monitoring [Ca2+]i with fura-2 during whole-cell recording of action potentials at room temperature (23-25 degrees C). Modulation of Ca2+ release by the Na+ current (INa) was studied by manipulating Na+ influx through the Na+ channel. 2. For a comparable Ca2+ loading of the SR, brief hyperpolarizing currents applied at the peak of the action potential increased Ca2+ release, while depolarizing pulses had the opposite effect. Similar currents applied before the action potential did not affect Ca2+ release. 3. Application of tetrodotoxin (TTX; 60 microM) moderately reduced Ca2+ release from the SR, but this effect was delayed in comparison with the immediate block of INa. An early effect of TTX was to increase Ca2+ release. 4. Replacement of Na+ with Li did not reduce Ca2+ release, but led to a progressive increase in Ca2+ release, resulting in spontaneous activity. 5. Ca2+ channel blockers (CdCl2, 100 microM; nisoldipine, 20 microM; or nifedipine, 20 microM) drastically reduced Ca2+ release from the SR. 6. Voltage clamp experiments confirmed that TTX blocked INa and its associated [Ca2+]i transient during voltage steps from -90 to -50 mV. INa and its associated [Ca2+]i transient were equally suppressed following replacement of Na+ with N-methyl-D-glucamine (NMDG+), but the [Ca2+]i transient was not suppressed following replacement of Na+ with Li+. 7. The INa-associated transient was sensitive to Ca2+ channel blockers. During steps from -50 to 0 mV, it appeared that the dihydropyridine antagonists often did not provide full block of the calcium current (ICa). 8. During current clamp stimulation at 1 Hz in the presence of TTX (60 microM), the Ca2+ content of the SR was decreased, due to the changes in action potential configuration and to changes in [Na+]i. 9. Our experiments indicate that the Ca2+ entry coupled to Na+ influx via the Na+ channel does not contribute substantially to the trigger for Ca2+ release from the SR during action potentials (23-25 degrees C). However, INa modulates Ca2+ release by affecting the Ca2+ load of the SR.

The alpha-dendrotoxin footprint on a mammalian potassium channel

alpha-Dendrotoxin, a 59-amino acid basic peptide from the venom of Dendroaspis angusticeps (green mamba snake), potently blocks some but not all voltage-dependent potassium channels. Here we have investigated the relative contribution of the individual alpha-subunits constituting functional Kv1.1 potassium channels to alpha-dentroxin binding. Three residues critical for alpha-dentrotoxin binding and located in the loop between domains S5 and S6 were mutated (A352P, E353S, and Y379H), and multimeric cDNAs were constructed encoding homo- and heterotetrameric channels composed of all possible ratios of wild-type and mutant alpha-subunits. Complete mutant channels were about 200-fold less sensitive for the alpha-dendrotoxin block than complete wild-type channels, which is attributable to a smaller association rate. Analysis of the bimolecular reaction between alpha-dendrotoxin and the different homo- and heteromeric channel constructs revealed that the association rate depends on the number of wild-type alpha-subunits in the functional channel. Furthermore, we observed a linear relationship between the number of wild-type alpha-subunits in functional channels and the free energy for alpha-dendrotoxin binding, providing evidence that all four alpha-subunits must interact with alpha-dendrotoxin to produce a high affinity binding site.

[Ca2+]i-dependent membrane currents in guinea-pig ventricular cells in the absence of Na/Ca exchange

Transient inward currents (Iti) during oscillations of intracellular [Ca2+] ([Ca2+]i) in ventricular myocytes have been ascribed to Na/Ca exchange. We have investigated whether other Ca2+-dependent membrane currents contribute to Iti in single guinea-pig ventricular myocytes, by examining membrane currents during [Ca2+]i oscillations and during caffeine-induced Ca2+ release from the sarcoplasmic reticulum in the absence of Na+. Membrane currents were recorded during whole-cell voltage clamp and [Ca2+]i measured simultaneously with fura-2. In the absence of Na/Ca exchange, i.e., with Li+, Cs+ or N-methyl-D-glucamine (NMDG+) substituted for Na+, the cell could be loaded with Ca2+ by repetitive depolarizations to +10 mV, resulting in spontaneous [Ca2+]i oscillations. During these oscillations, no inward currents were seen, but instead spontaneous Ca2+ release was accompanied by a shift of the membrane current in the outward direction at potentials between -40 mV and +60 mV. This [Ca2+]i-dependent outward current shift was not abolished when NMDG+ was substituted for internal monovalent cations, nor was it sensitive to substitution of external Cl-. It was however, sensitive to the blockade of ICa by verapamil. These results suggest that the transient outward current shift observed during spontaneous Ca2+ release represents [Ca2+]i-dependent transient inhibition of ICa. Similarly, during the [Ca2+]i transients induced by brief caffeine (10 mM) applications, we could not detect membrane currents attributable to a Ca2+-activated nonselective cation channel, or to a Ca2+-activated Cl- channel; however, transient Ca2+-dependent inhibition of ICa was again observed. We conclude that neither the Ca2+-activated nonselective cation channel nor the Ca2+-activated Cl- channel contribute significantly to the membrane currents during spontaneous [Ca2+]i oscillations in guinea-pig ventricular myocytes. However, in the voltage range between -40 mV and +60 mV Ca2+-dependent transient inhibition of ICa will contribute to the oscillations of the membrane current.

Intracellular citrate induces regenerative calcium release from sarcoplasmic reticulum in guinea-pig atrial myocytes

Ca2+ release from the sarcoplasmic reticulum was studied in voltage-clamped guinea-pig atrial myocytes. Cells were dialysed with a pipette solution containing the Ca2+ indicator 1- [2-amino-5-(6-carboxyindol-2-yl) phenoxy]-2-(2'-amino-5'-methylphenoxy) ethane-N,N,N',N'-tetraacetic acid] (Indo-1, 100 microM) and as main anion either chloride or the low-affinity Ca2+ buffer citrate. Intracellular Ca2+ transients (Cai transients) were elicited by depolarizations from a holding potential of -50 mV. In chloride-dialysed cells, Cai transients showed a bell-shaped dependence on the amplitude of the depolarizing pulse. In citrate-dialysed cells, membrane depolarizations were associated with a small rise in [Ca2+]i. These small changes in [Ca2+]i were either followed by a large Cai transient or failed to induce large changes in [Ca2+]i. The peak amplitude of the large Cai transient did not vary with the amplitude of the depolarizing pulse. These results demonstrate that in the presence of intracellular chloride, Ca2+ release in atrial cells is a graded process triggered by Ca2+ influx. Using citrate as the main intracellular anoin, Ca2+ release triggered by Ca2+ entry was no longer graded but occurred in a regenerative manner. The results are discussed in terms of two models in which citrate, affects the spatial distribution of [Ca2+]i or the loading state of the sarcoplasmic reticulum.

Two components of [Ca2+]i-activated Cl- current during large [Ca2+]i transients in single rabbit heart Purkinje cells

1. Single Purkinje cells, enzymatically isolated from rabbit ventricle, were studied under whole-cell voltage clamp conditions and internally perfused with the fluorescent Ca2+ indicator fura-2(100 microM). 2. Ca2+ release from the sarcoplasmic reticulum was either induced by external application of caffeine or occurred spontaneously in Ca2+i-overloaded cells. Membrane currents accompanying these Ca(2+)-release signals were studied at steady membrane potentials. 3. [Ca2+]i transients were accompanied by transient membrane currents. In the absence of Na(+)-Ca2+ exchange, two current components could be observed. The first component peaked well before the [Ca2+]i transient (Ifast) and relaxed before peak [Ca2+]i. The second component, on the other hand, peaked at the time when [Ca2+]i was maximal (Islow). 4. In symmetrical Cl- solutions both current components had a reversal potential close to O mV. A reduction of external or internal [Cl-] shifted this reversal potential in accordance with the change of the Cl- equilibrium potential. 5. Each [Ca2+]i transient was accompanied by Ifast. Properties of Ifast suggest that this current component is the [Ca2+]i-dependent Cl- current, ICl(Ca), previously observed during depolarizing pulses. 6. Islow was only detected in cells that displayed a large [Ca2+]i transient with or without elevated resting [Ca2+]i. 7. It is concluded that during large [Ca2+]i transients a slow component of ICl(Ca) can be activated. This second component may arise from the same channel population as the previously described fast component and be related to the presence of spatial and temporal inhomogeneities of [Ca2+]i. Alternatively, this current component may arise from a different Cl- channel population with a different Ca2+ sensitivity.

Inhibition and rapid recovery of Ca2+ current during Ca2+ release from sarcoplasmic reticulum in guinea pig ventricular myocytes

We have investigated the modulation of the L-type Ca2+ channel by Ca2+ released from the sarcoplasmic reticulum (SR) in single guinea pig ventricular myocytes under whole-cell voltage clamp. [Ca2+]i was monitored by fura 2. By use of impermeant monovalent cations in intracellular and extracellular solutions, the current through Na+ channels, K+ channels, nonspecific cation channels, and the Na+-Ca2+ exchanger was effectively blocked. By altering the amount of Ca2+ loading of the SR, the time course of the Ca2+ current (ICa) could be studied during various amplitudes of Ca2+ release. In the presence of a large Ca2+ release, fast inhibition of ICa occurred, whereas on relaxation of [Ca2+]i, fast recovery was observed. The time course of this transient inhibition of ICa reflected the time course of [Ca2+]i. However, the inhibition seen in the first 50 ms, ie, the time of net Ca2+ release from the SR, exceeded the inhibition observed later during the pulse, suggesting the existence of a higher [Ca2+] near the channel during this time. Transient inhibition of ICa during Ca2+ release was observed to a similar degree at all potentials. It could still be observed in the presence of intracellular ATP-gamma-S and of cAMP. Therefore, we conclude that the modulation of ICa by Ca2+ release from the SR is not related to dephosphorylation. It could be related to a reduction in the driving force and to a direct inhibition of the channel by [Ca2+]i. The observation that the degree of inhibition does not depend on membrane potential suggests that the Ca2+ binding site for this modulation is located outside the pore.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversal of rectification and alteration of selectivity and pharmacology in a mammalian Kv1.1 potassium channel by deletion of domains S1 to S4

1. A possible relation between the family of inwardly rectifying K+ channels and the Shaker superfamily of K+ channels was investigated using a deletion mutant (DelS1-S4) of a delayed rectifier Kv1.1 (RCK1) K+ channel. 2. The mutant DelS1-S4 was made by eliminating the sequence coding for transmembrane domains S1 to S4 of the Kv1.1 K+ channel, and re-ligating the sequence coding for the cytoplasmic amino terminus to transmembrane domain S5. Microelectrode voltage-clamp and patch-clamp experiments were performed on Xenopus laevis oocytes after injection of in vitro transcribed mRNA coding for mutant and wild-type channels. 3. The lack of transmembrane domains S1 to S4 converts a depolarization-activated wild-type Kv1.1 K+ channel with outward rectification into a hyperpolarization-activated channel with inward rectification. Although the pore region of the deletion mutant is identical to the wild-type channel, the mutant channel is a non-selective cation channel and is characterized by an altered pharmacology profile.

Resting and action potentials of nonischemic and chronically ischemic human ventricular muscle

INTRODUCTION

The effect of chronic ischemia on the electrical properties of human cardiac tissue is not well understood.

METHODS AND RESULTS

Membrane potentials were studied using microelectrode techniques in isolated human ventricular tissues obtained from nonischemic (n = 17) or chronically ischemic (n = 7) myocardium. In normal Tyrode's solution, resting potential (Vr) was lower in ischemic (-70.1 +/- 2.12 mV) than in nonischemic muscles (-77.6 +/- 0.93 mV; mean +/- SEM; P < 0.05). In high [K]o (> 10 mM) media, Vr was of similar magnitude in both types of tissue (in 21.6 mM [K]o, Vr was -53.1 +/- 2.24 mV in nonischemic and -49.6 +/- 2.03 mV in ischemic preparations; n = 7 each; P > 0.05). Lowering [K]o caused persistent hyperpolarization in nonischemic muscles, but caused depolarization in chronically ischemic preparations (in 2.7 mM [K]o, Vr was -84.9 +/- 2.74 mV and -61.7 +/- 7.72 mV, respectively; n = 7; P < 0.05). Pinacidil (100 microM) normalized the response of chronically ischemic preparations to [K]o. Action potentials (APs) from nonischemic tissues varied in shape and could show aberrations. Epinephrine (1.5 microM) and 4-aminopyridine (3 mM) increased the AP duration, while butanedione monoxime (20 mM) and tetrodotoxin (1 microM) shortened it. In chronically ischemic muscles, the AP was characterized by the absence of a plateau and the presence of a slow phase of final repolarization.

CONCLUSION

The differential effect of low [K]o on the resting membrane potential of nonischemic and chronically ischemic tissues suggests a change in the properties or the regulation of background K+ channels during chronic ischemia.

Effects of intracellular sodium and hydrogen ion on the sodium activated potassium channel in isolated patches from guinea pig ventricular myocytes

OBJECTIVE

The Na+ activated K+ channel (IK(Na)) may be activated during ischaemia when the intracellular Na+ concentration is raised. As ischaemia is also associated with intracellular acidification, the influence of intracellular pH on this K+ channel was investigated.

METHODS

The effects of intracellular Na+ and H+ on the IK(Na) channel were investigated in isolated patches from guinea pig ventricular myocytes by the patch clamp technique.

RESULTS

Increase of intracellular Na+ increased the open probability of the channel. Intracellular acidification had no effect on the single channel conductance, but significantly decreased the open probability of the channel in the pH range 7.5-6.5. The effect of intracellular pH on open probability was about the same in a wide range of intracellular Na+ concentrations. The lower open probability induced by acidification seemed to be caused by prolonged closed times.

CONCLUSIONS

Because the effects of increased intracellular Na+ and decreased pH on open probability are opposite, it is suggested that the IK(Na) channel might be important under conditions of raised intracellular Na+ with relatively unchanged intracellular pH. Consequently, it is hypothesised that this channel may be involved in adaptation of action potential duration at high heart rate and in action potential shortening in the border zone of regional ischaemic areas.

Mode of regulation by G protein of the ATP-sensitive K+ channel in guinea-pig ventricular cell membrane

1. The effect of G protein activation on the ATP-sensitive K+ (K+ATP) channel was examined in inside-out patches from guinea-pig ventricular myocytes. At low (0.3 mM) intracellular ATP concentration ([ATP]i) in the bathing solution, in the absence of agonists in the pipette, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) or AlF4- applied to the intracellular side of the patch membrane gradually activated the K+ATP channel. The activation by GTP gamma S was irreversible, although high [ATP]i could completely close the channel. 2. In ATP-free media GTP gamma S did not increase further the activity of the fully active channel, and was unable to reactivate the channel in the non-operative state after rundown. [ATP]i-channel activity curves constructed before and after GTP gamma S application demonstrated that GTP gamma S shifts the half-inhibitory [ATP]i from 19.5 to 110 microM without changing the Hill coefficient. 3. When acetylcholine or adenosine was included in the pipette, intracellular GTP reversibly activated the K+ATP channel which was partially inhibited by [ATP]i. 4. These results indicate that G protein may stimulate myocardial K+ATP channels in the operative state by reducing the potency of ATP inhibition. The possible coupling of the G protein with muscarinic as well as A1 adenosine receptors is suggested.

Mechanisms and control of repolarization

This paper describes and discusses interventions that may lead to prolongation of the action potential. Special emphasis is given to regulation by drugs. The results of studies focusing on the modulation of inward and outward currents are presented in detail. Specific attention is given to Na+, Ca2+, K+ and Cl- channel currents and their components.

Use-dependent block and use-dependent unblock of the delayed rectifier K+ current by almokalant in rabbit ventricular myocytes

A voltage-clamp analysis of the effect of almokalant on the delayed rectifier K+ current (IK) was made in rabbit ventricular myocytes. The two-suction pipette method was used, and appropriate voltage-clamp protocols were used to study more specifically use dependence, block development, and recovery from block. Almokalant interacted with the IK in two ways: it shifted the activation curve in the hyperpolarizing direction (stimulatory effect) and blocked the open IK channel in a use-dependent way (inhibitory effect). For 2-second voltage clamps to +20 mV, half-maximum block was obtained at 5 x 10(-8) mol/L, with a Hill coefficient of 1.76. Use-dependent block was related to an open-channel block that occurred at 0 mV with a time constant of 1.07 second and a rather slow recovery from block: at -50 mV, recovery time constant was approximately 10 seconds; at -75 mV, recovery was practically absent. The absence of an important recovery at negative membrane potentials is consistent with the hypothesis of the drug being trapped in the channel. A limited frequency-dependent block could be demonstrated. Use-dependent unblock was demonstrated by a rapid recovery from block during stimulation following complete washout of the drug. It is concluded that almokalant shifts the activation curve of IK in the hyperpolarizing direction, blocks the open channel, and is trapped by the closure of the activation gate.

Use-dependent block of the delayed K+ current in rabbit ventricular myocytes

Block of the delayed K+ current, iK, and the concomitant increase in refractoriness is considered an alternative to a decrease of conduction in the treatment of reentry arrhythmias. Ideally an agent should selectively prolong the action potential at high frequencies. A minimum requirement is use-dependent block. A number of drugs were tested for the existence of use dependence by applying a train of depolarizing clamps to single cardiac myocytes of the rabbit ventricle. Development of block during a long depolarizing clamp and recovery from block were also measured. Five of the nine drugs tested, i.e., disopyramide, encainide, quinidine, sotalol, and tedisamil, did not show use dependence. When a train of depolarizing clamps was applied, block was already present for the first depolarization and did not increase with repetition of the pulse. This result suggests block of the channel in the rested state or a very fast block of the open channel. Almokalant and amiodarone, and to a lesser extent dofetilide and E4031, showed use-dependent block, i.e., block increased during the train of depolarizing clamps. The time constant for the open channel block was 1.07 seconds for almokalant and 0.67 seconds for amiodarone. Recovery from block for almokalant and amiodarone was very slow: time constants measured at -50 mV were 13.9 and 12.7 seconds, respectively. For dofetilide it was in the order of minutes. The existence of this slow recovery explains why frequency-dependent changes in block were negligible or absent for frequencies above 0.5 Hz. Future research should be aimed to select drugs with a slower onset of active state block and faster recovery from block.

[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)

K+ channels and control of ventricular repolarization in the heart

K+ channels form a large family, in which voltage-operated and ligand-operated channels can be distinguished. Under physiological conditions, four K+ currents contribute to the repolarization process and their role is discussed: i) the transient outward current (ito) is responsible for the rapid initial repolarization process from the crest of the action potential to the plateau level; ii) the delayed K+ current (iK) is involved in the overall repolarization process during the plateau; iii) the inward rectifier (iK1) is responsible for the final rapid repolarization and the maintenance of the resting potential; iv) a ligand-operated channel activated by acetylcholine and adenosine participates in the repolarization process and the maintenance of the resting potential in nodal, atrial and Purkinje cells. In the context of antiarrhythmic interventions, block of outward K+ current and prolongation of refractoriness is currently considered as an alternative to block of the Na+ current and reduction of conduction velocity. Although some of these drugs show use-dependent block, the frequency-dependent changes in current and action potential duration are not ideal.

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

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

Intracellular protons inhibit inward rectifier K+ channel of guinea-pig ventricular cell membrane

The effect of intracellular protons (Hi+) on the inward rectifier K+ channel of the guinea-pig ventricular cell membrane was examined, using the patch-clamp technique. The inward single-channel current was recorded in "inside-out" and "outside-out" patch configurations, while the pH of the solution perfusing the intra- and extracellular side, respectively, was varied. Low intracellular pH (pHi), but not low extracellular pH, inhibited the channel. Low pHi reduced the unit amplitude, which was about 20% smaller at pHi 6.0 than that at pHi 7.4 at every voltage tested. The slope conductance decreased from 41.7 pS at pHi 7.4 to 35.1 pS at pHi 6.0. Low pHi also reduced the channel activity without apparent voltage dependence. The concentration/response curve indicated the half-maximum inhibition at pHi 6.11 and a Hill coefficient of 2.52. Lowering the pHi from 7.4 to 6.0 did not affect the distributions of the open times and the closed times below 50 ms, while the time constant of the histogram constructed from closings longer than 50 ms was approximately doubled. These results indicate that the inward rectifier K+ channel in ventricular myocytes is inhibited by H+ from the intracellular side. This might contribute to the depolarization of the resting membrane potential induced by intracellular acidosis during myocardial ischaemia.

Membrane-bound nucleoside diphosphate kinase activity in atrial cells of frog, guinea pig, and human

Muscarinic K+ channels in inside-out patches of atrial cells from guinea pig or rabbit can be activated by Mg(2+)-ATP in the absence of acetylcholine and GTP or GDP. The ATP-dependent activation involves a phosphorylation and is postulated to be due to the association of a membrane-bound nucleoside diphosphate kinase (NDPK) with the G protein GK: direct phosphorylation of the GK-bound GDP into GTP, catalyzed by NDPK, would result in activation of the G protein and, hence, activation of the channels. The aim of this study was to identify the presence of NDPK activity in atrial membranes by investigating the phosphate transfer between tritium-labeled nucleotides. We show that frog, guinea pig, and human atrial membranes contain a substantial NDPK activity since they catalyze the conversion from [3H]GDP+nucleoside triphosphate (NTP or NTP gamma S) to [3H]GTP (or [3H]GTP gamma S), from [3H]ADP+NTP to [3H]ATP, and from [3H]GTP+nucleoside diphosphate (NDP) to [3H]GDP. The phosphate transfer rates for the [3H]GDP+ATP to [3H]GTP conversion are 1.8, 0.5, and 2.4 mumol inorganic phosphate formation/mg per 10 minutes at 37 degrees C in frog, guinea pig, and human, respectively. The order of substrate efficiency for different NTPs was ATP greater than ITP approximately equal to GTP greater than UTP greater than CTP, which parallels the efficiency of these nucleotides in their activation of the muscarinic K+ channels. Addition of other nucleotides blocked the transphosphorylation reaction, indicating that the NTP-NDP conversion mechanism is aspecific, as is expected for an NDPK-catalyzed reaction. In conclusion, the data support the concept of NDPK involvement in the atrial muscarinic signal transduction cascade.

Voltage- and time-dependent block of the delayed K+ current in cardiac myocytes by dofetilide

The delayed K+ current (ik) and its change by dofetilide was studied in single myocytes from the guinea pig and rabbit heart using the two-electrode voltage clamp technique. In rabbit myocytes, ik consisted of only one component (Kr), which developed for moderate depolarizations and with a fast time course. In guinea pig myocytes, activation consisted of a rapid and a slow component, and the latter (Ks) only became manifest for depolarizations positive to 0 mV. Ks was resistant to block by dofetilide. Kr, however, was very sensitive: Kd 3.9 x 10(-9) M, Hill coefficient 2.0 (n = 5). The effect was voltage-dependent block increasing at depolarized levels. Block development was time dependent and occurred in two phases: a first fast and voltage-dependent phase was followed by a second much slower phase (time constant of 4.4 +/- 0.48 sec (n = 11). Recovery from block was slower as the membrane potential became more negative. This resulted in the absence of a steady-state frequency-dependent effect at negative membrane potentials. It is concluded that dofetilide is an efficient blocker of the fast component of ik. The block, as well as recovery, are voltage and time dependent. Block is greater at more depolarized levels, recovery is slower at more hyperpolarized levels.

Calcium transients caused by calcium entry are influenced by the sarcoplasmic reticulum in guinea-pig atrial myocytes

1. Single atrial myocytes obtained by enzyme perfusion from hearts of adult guinea-pigs were investigated using whole-cell voltage clamp and Indo-1 micro-fluorometry. 2. In myocytes loaded with a solution containing citrate as a low-affinity, non-saturable Ca2+ chelator, two types of [Ca2+]i transients could be recorded during repetitive activation of L-type Ca2+ current. Both large and small [Ca2+]i transients occurred; large transients reached peak values of about 1 microM, and small transients were about 100 nM or less in amplitude. 3. In the case of the large transients, peak [Ca2+]i was usually reached with a variable delay after repolarization from a voltage step that activated calcium current (ICa). For the small transients the rise in [Ca2+]i paralleled ICa. Upon repolarization [Ca2+]i started to decay. 4. The small transients reflect entry of Ca2+ through Ca2+ channels (entry transients), whereas the large transients are due to entry and release from the sarcoplasmic reticulum (release transients). 5. The entry transients displayed a positive staircase pattern during trains of depolarizing voltage steps despite constant or even decreasing amplitude of ICa. The steepness of the staircase was increased by elevation of [Ca2+]o. Entry transients were always smallest immediately after a release transient. 6. After functional removal of the sarcoplasmic reticulum by caffeine (1-5 mM) the staircase pattern of the transients reflecting Ca2+ entry was abolished. 7. It is concluded that the staircase pattern is due to rapid uptake by the sarcoplasmic reticulum of Ca2+ entering the cell, resulting in an attenuation of the signal. The attenuation is strongest shortly after a release signal, when the rate of sequestration of Ca2+ by the SR should be highest. 8. Evidence is provided that a compartment of the SR is involved in attenuation of the entry transients. This compartment has been identified recently as a peripheral release compartment.

Effects of the enantiomers of disopyramide and its major metabolite on the electrophysiological characteristics of the guinea-pig papillary muscle

Disopyramide, a Class Ia antiarrhythmic drug, is clinically used as a racemic mixture; R(-)disopyramide and S(+)disopyramide. The major metabolite in man is desisopropyldisopyramide: R(-)desisopropyldisopyramide and S(+)desisopropyldisopyramide. The effects of the four compounds were compared on the electrophysiological characteristics of the guinea-pig papillary muscle using the standard microelectrode technique. At an external K+ concentration of 5.4 mmol/l and a stimulation frequency of 1 Hz, S(+)disopyramide (20 mumols/l) increased action potential duration (APD) by more than 18%, while it was diminished by 6% in the presence of R(-)disopyramide. Resting membrane potential amounted to -87.1 +/- 0.5 mV (n = 14) and -85.6 +/- 1.2 mV (n = 10), respectively. Also a small but significant difference in effect on the maximal rate of depolarization was observed, R(-)disopyramide being more potent, related with a slower recovery of the maximal rate of depolarization. The enantiomers of the metabolite appeared to be three times less potent than those of the parent drug in their effect on the maximal rate of depolarization. The characteristics of the enantiomers of the metabolite correlated with those of the parent drug: also the R(-)enantiomer was more potent in decreasing the maximal rate of depolarization and caused more shortening of the action potential than the S(+)enantiomer. Time constants for onset and recovery of/from rate dependent block of the maximal rate of depolarization were dependent upon the external K+ concentration, both for the enantiomers of the parent drug and those of the metabolite. Onset slowed down while recovery accelerated when external K+ was increased. Time constants were lower for the metabolite. When stimulation interval was shortened, the effect on the maximal rate of depolarisation increased. Only for the metabolite statistical significant stereoselective differences were observed at all stimulation intervals. The effects on the action potential duration were dependent upon stimulation interval; for all enantiomers the action potential duration tended to be relatively (% of control) higher at short stimulation intervals than at large stimulation intervals. The effect on the maximal rate of depolarization was also voltage dependent, but no significant differences were observed between the enantiomers, for the parent drug as well as for the metabolite.

Ion channel agonists: expectations for therapy

Some animal or plant toxins and man-made drugs exert agonist activity on Na+, Ca2+ and K+ channels. The increase in current through these channels is essentially due to an increase of 'open probability' and not of single channel conductance. The enhanced open probability is caused by a prolongation of the open time. In the case of voltage-operated channels this change in open time can be accompanied by increased reopenings and thus slowing of inactivation, or a shift in the activation process to more negative potentials. In the case of the ligand-operated K+ channel, a decrease in the affinity for the normal physiological ligand, ATP, is the mechanism underlying the enhancement of open probability. Agonists show potential clinical applications for Na+ and Ca2+ channels more specifically as positive inotropic agents in cardiac tissue. For K+ channels, the potential therapeutic field is even broader and spans from relaxation of smooth muscle (hypertension, asthma, bladder, uterus), reduction in excitability (arrhythmias, certain skeletal muscle myopathies) to inhibition of neurotransmitter release (depression, epilepsy).

Atrial membranes contain nucleoside diphosphate kinase (NDPK) activity: its role in regulation of muscarinic K+ channels

Guinea pig or rabbit atrial muscarinic K+ channels in cell-free inside-out patches can be activated in the absence of extracellular agonist and cytoplasmic G nucleotides by intracellular ATP-Mg2+. This ATP-dependent activation is compatible with the existence of a membrane-bound nucleoside diphosphate kinase (NDPK), which directly phosphorylates GK-bound GDP. We show that this ATP-dependent activation is also possible in frog atrial cells, and that atrial membranes of frog and guinea pig contain NDPK activity. The relative order of different nucleoside triphosphates (NTPs) as phosphate donors parallels the observed efficiency of these nucleotides in activation of the channels. Thus, atrial membranes contain NDPK activity, which can be responsible for the ATP-dependent activation of muscarinic K+ channels, seen in patches of atrial cells. Under physiological conditions, NDPK can act as a GTP supply in the immediate vicinity of the G protein to ensure reliable signal transduction.

Differential electrophysiologic and inotropic effects of phenylephrine in atrial and ventricular heart muscle preparations from rats

Stimulation of alpha 1-adrenoceptors evokes a different pattern of inotropic responses in atrial and ventricular heart muscle preparations from rats. The inotropic effects are accompanied by different changes in membrane potential. In an attempt to clarify the question whether or to which extent these events are causally related, the effects of phenylephrine on force of contraction, transmembrane potential, Ca2+ current (ICa) and K+ currents were comparatively studied in either tissue. In atrial preparations, phenylephrine 10 mumol/l caused an increase in force of contraction, a marked prolongation of the action potential duration and a depolarization of the membrane at rest. In the ventricle, however, the addition of phenylephrine 10 mumol/l produced first a decline in force of contraction associated with a hyperpolarization of the membrane and a reduction in the action potential duration. These changes were followed by an increase in force of contraction and a slight prolongation of the action potential, whereas the resting membrane potential remained increased. The hyperpolarization was eliminated in the presence of ouabain 100 mumol/l. In enzymatically isolated atrial and ventricular myocytes, the whole-cell voltage clamp technique was used to study membrane currents on exposure to phenylephrine. Phenylephrine 30 mumol/l did not affect the magnitude of ICa in either cell type. Transient and steady state K+ outward currents, however, were significantly diminished to a similar extent in atrial and in ventricular myocytes. It is concluded that the positive inotropic effect of alpha 1-adrenoceptor stimulation in the rat atrium is related to an increase in action potential duration and a decrease in resting membrane potential due to a decrease in K+ currents.(ABSTRACT TRUNCATED AT 250 WORDS)

Flunarizine inhibits a high-threshold inactivating calcium channel (N-type) in isolated hippocampal neurons

The action of flunarizine on the high-threshold inactivating calcium channel (N-type) in hippocampal neurons of the rat was investigated using the whole-cell voltage clamp technique. Flunarizine reduced the currents at all test potentials, without shifting the peak of the current-voltage relationship along the voltage-axis. The drug did not affect the activation curve, but drastically decreased the slope conductance in the linear region of the current-voltage relationship. Block of the current by flunarizine occurred in a use-dependent way. Flunarizine was without effect when applied intracellulary, and the onset of action, when applied extracellularly, was slow (range of minutes). The Kd for the block by flunarizine obtained after 6 repetitive depolarizations at 0.2 Hz (pulse duration 150 ms) from -90 mV to 0 mV was 0.8 microM. In conclusion, we present electrophysiological evidence that flunarizine blocks the high-threshold inactivating Ca channel of hippocampal neurons of the rat. We discuss the possibility that flunarizine might inhibit neuronal transmitter release by means of this effect.