Chloride current in mammalian cardiac myocytes. Novel mechanism for autonomic regulation of action potential duration and resting membrane potential

Regulatory volume decrease of cardiac myocytes induced by beta-adrenergic activation of the Cl- channel in guinea pig

A new method was developed to automatically measure the thickness of a single ventricular myocyte of guinea-pig heart. A fine marker was attached on the cell's upper surface and changes in its vertical position were measured by focusing it under the microscope. When the osmolarity of the bath solution was varied, the cell thickness reached a new steady level without any obvious regulatory volume change within the period of observation up to 15 min. The cell thickness was 7.8 +/- 0.2 microns (n = 94) in the control Tyrode solution and was varied to 130.4 +/- 3.1% (n = 10), 119.1 +/- 1.1% (n = 50), 87.2 +/- 1.9% (n = 9), and 75.6 +/- 3.2% (n = 5) of control at 50, 70, 130, and 200% osmolarity, respectively. The application of a Cl- channel blocker, 500 microM anthracene-9-carboxylic acid (9AC) did not modify these osmotic volume changes. We discovered that the application of isoprenaline induced a regulatory volume decrease (RVD) in cells inflated by hypotonic solutions. This isoprenaline-induced RVD was inhibited by antagonizing beta-adrenergic stimulation with acetylcholine. The isoprenaline-induced RVD was mimicked by the external application of 8-bromoadenosine 3':5'-cyclic monophosphate. The RVD was inhibited by blocking the cAMP-dependent Cl- channel (ICl, rAMP) with 9AC but was insensitive to 4,4'-diisothiocyanostilbene-2,2'-dissulphonate (DIDS). Taken together these data suggest an involvement of ICl, cAMP activation in the RVD. Whole cell voltage clamp experiments revealed activation of ICl, cAMP by isoprenaline under the comparable conditions. The cardiac cell volume may be regulated by the autonomic nervous activity.

Role of G proteins in alpha1-adrenergic inhibition of the beta-adrenergically activated chloride current in cardiac myocytes

alpha1-Adrenergic receptor stimulation can inhibit the Cl- current activated by beta-adrenergic receptor agonists in guinea-pig ventricular myocytes. We investigated the role of G proteins in mediating this type of alpha-adrenergic response. The combined alpha- and beta-adrenergic agonist norepinephrine (NE) activated the Cl- current with an EC50 value of 53 nM. Preincubation of myocytes with PTX decreased the EC50 value for NE activation of the Cl- current to 5.9 nM, and addition of the alpha1-adrenergic receptor antagonist prazosin did not cause any further change in sensitivity to NE. These results suggest that the alpha1-adrenergic inhibition of beta-adrenergic responses is mediated through a PTX-sensitive G protein. However, PTX pretreatment also increased the sensitivity of the Cl- current to the selective beta-adrenergic agonist isoproterenol (Iso), which indicates that the PTX treatment increases the sensitivity to beta-adrenergic stimulation alone and that this could account for the PTX-induced change in sensitivity to NE. Consistent with this idea, the selective alpha1-adrenergic receptor agonist methoxamine was still able to inhibit the Cl- current activated by Iso in PTX-treated myocytes. However, the sensitivity to methoxamine was significantly decreased. In control cells, the Cl- current activated by 30 nM Iso was inhibited by methoxamine with an EC50 value of 8.3 microM, but in PTX-treated cells, the EC50 value was 284 microM. The EC50 for methoxamine inhibition was similarly increased when the Cl- current was activated by 300 nM Iso. These data suggest that the effects of PTX on alpha1-adrenergic responses can actually be explained by changes in the sensitivity to beta-adrenergic stimulation. To verify the role for a G protein in mediating the inhibitory alpha1-adrenergic response, we examined the effect of methoxamine on the Cl- current activated in cells dialyzed with the nonhydrolyzable GTP analogue guanosine-5'-O-(3-thio)triphosphate. Pre-exposure to methoxamine resulted in an attenuated response upon subsequent exposure to Iso alone. We conclude that alpha1-adrenergic inhibition of beta-adrenergic responses is mediated by a G protein-dependent mechanism that appears to be PTX-insensitive.

Rebound stimulation of the cAMP-regulated Cl- current by acetylcholine in guinea-pig ventricular myocytes

1. Acetylcholine (ACh)-induced rebound stimulation of the cAMP-regulated Cl- current was studied in isolated guinea-pig ventricular myocytes using dialysing and dialysis-limiting configurations of the whole-cell patch-clamp technique. 2. Exposure to and subsequent washout of ACh produced a transient rebound stimulation of the Cl- current. However, this rebound response was only observed in the presence of submaximally stimulating concentrations of the cAMP-producing agonists isoprenaline (Iso) or histamine. ACh-induced rebound stimulation was not observed in the presence of maximally stimulating concentrations of Iso, nor was it observed in the absence of Iso. 3. To prevent saturation of responses during rebound, the effects of ACh were studied in the presence of a subthreshold concentration of Iso (0.001 microM). Varying the duration of exposure to ACh before washout demonstrated that the stimulatory effect of 1 microM ACh approaches steady state with a time constant of 34 s. Exposing myocytes to varying concentrations of ACh for 90 s demonstrated that the EC50 for the stimulatory effect of ACh was 0.15 microM with a maximum response equal to 67% of that obtained by a maximally stimulating concentration of Iso alone. 4. Rebound stimulation of the Cl- current could also be elicited by washing in 2 microM atropine during exposure to ACh, instead of washing out ACh. Furthermore, ACh-induced rebound was blocked by the M2 muscarinic receptor antagonist methoctramine but not by the M1 receptor antagonist pirenzepine. Rebound was also blocked in pertussis toxin (PTX)-treated myocytes. 5. ACh-induced rebound stimulation was not blocked by: (a) L-NMMA, an inhibitor of nitric oxide synthase activity; (b) Methylene Blue, LY-83583, and ODQ, inhibitors of cGMP production; or (c) milrinone, an inhibitor of cGMP-dependent phosphodiesterase activity. 6. These results indicate that ACh can stimulate cAMP-regulated ion channel activity in cardiac ventricular myocytes by facilitating beta-adrenergic and histaminergic responses. This is opposite to the inhibitory actions more typically associated with muscarinic receptor stimulation in ventricular myocardium. This stimulatory effect of ACh is mediated through M2 muscarinic receptors and a PTX-sensitive G-protein, but it does not appear to involve the production of nitric oxide or cGMP.

Lack of cystic fibrosis transmembrane regulator-type chloride current in pediatric human atrial myocytes

Mature cardiomyocytes have been shown to possess a cyclic AMP-mediated chloride channel (I(Cl)) which is the product of the cystic fibrosis transmembrane regulator (CFTR) gene. Species variability has been demonstrated for other ion channels. This study was designed to evaluate human I(Cl) regulation using the whole-cell patch-clamp bioassay. Atrial tissue obtained from children undergoing congenital heart surgery was enzymatically dispersed into isolated myocytes. The patients ranged in age from 1 day to 11 years (mean 2 years). Isoproterenol was used to activate the cAMP second-messenger system in a potassium-free environment. Membrane calcium and sodium channels were pharmacologically blocked. Of 20 human atrial myocytes obtained from 13 pediatric patients, 80% had a small basal chloride current. The current could be inhibited by the anion transport blocker, 9-anthracene carboxylic acid. In 4 of 20 otherwise viable myocytes, no I(Cl) could be elicited, either at baseline or with beta-adrenergic stimulation. Of the 16 myocytes with a basal I(Cl), the current was unaffected by cAMP stimulation in 15 (94%) cells. There were no significant differences in age, gender or clinical status of patients whose cells conducted Cl- current compared with patients whose myocytes had no measurable I(Cl). Ten mature guinea pig ventricular myocytes were evaluated using the same whole-cell patch-clamp technique. Seven of 10 cells showed a reversible increase in I(Cl) with isoproterenol exposure. Despite presence of the CFTR gene in human cardiomyocytes, functional expression of the cAMP-activated I(Cl) does not appear evident in isolated pediatric atrial myocytes. Whether the pathophysiology of congenital heart disease may influence chloride current modulation via alterations in adrenergic tone, intracellular Ca2+ regulation, and cellular osmotic conditions remains to be established.

Effects of stilbene derivatives SITS and DIDS on development of intracellular acidosis during ischemia in isolated guinea pig ventricular papillary muscle in vitro

Using ion-selective microelectrode techniques, we investigated the effects of 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which are known as Cl(-)-HCO3- exchange blockers, on action potentials and intracellular pH (pHi) in guinea pig ventricular papillary muscles subjected to simulated ischemia. Simulated ischemia was produced by stopping the flow of superfusing solution and then covering the preparations with mineral oil. Simulated ischemia induced a progressive decrease in the maximum upstroke rate and resting membrane potentials, shortened action potential duration, and resulted in cessation of action potentials within 10-12 min after the onset of simulated ischemia. The pHi-measurements revealed progressive intracellular acidosis during the period of simulated ischemia. SITS (0.5 mM) or DIDS (0.1 mM) delayed the onset of ischemia-induced deterioration of action potentials and prolonged the time to cessation of action potentials. SITS or DIDS (0.1-0.5 mM) induced an increase in pHi in HCO3(-)-buffered solution and suppressed the development of intracellular acidosis during ischemia. Under the external Cl(-)-free condition, the time to cessation of action potentials caused by ischemia was significantly delayed, and the development of intracellular acidosis during ischemia was attenuated. The present results indicate that activation of the Cl(-)-HCO3- exchange system would be involved, in part, in the development of intracellular acidosis during cardiac ischemia.

Distribution of cAMP-activated chloride current and CFTR mRNA in the guinea pig heart

Guinea pig ventricular myocytes exhibit a Cl(-)-selective current regulated by the cAMP-dependent pathway. We have investigated the distribution of cAMP-activated Cl- channel current density and cystic fibrosis transmembrane-conductance regulator (CFTR) mRNA in three regions of the guinea pig heart: the atrium, and the epicardium and endocardium of the free wall of the left ventricle. The regional differences in the Cl- current density were investigated in enzymatically isolated myocytes using the whole-cell patch-clamp technique. Forskolin (1 mumol/L) activated Cl(-)-selective currents in all ventricular myocytes and 21% of atrial myocytes examined. The conductance density, estimated as the outward chord conductance normalized to cell capacitance, was greatest in epicardial myocytes (79.8 +/- 8.4 pS/pF, n = 21) and significantly lower in endocardial (59.8 +/- 9.5 pS/pF, n = 22) and atrial (10.9 +/- 5.0 pS/pF, n = 38) myocytes. The regional differences in CFTR mRNA expression levels were investigated by competitive reverse-transcribed polymerase chain reaction. The regional distribution of the mRNA levels was similar to that of the Cl- conductance density, ie, highest in the epicardium (23230 +/- 1840 molecules/microgram total RNA, n = 3), significantly lower in endocardium (10610 +/- 780 molecules/microgram total RNA, n = 3), and lowest in atrium (1450 +/- 290 molecules/microgram total RNA, n = 3). The data indicate that regional differences in CFTR mRNA expression in the guinea pig heart are responsible, at least in part, for the regional differences in cAMP-activated Cl- current density.

Cystic fibrosis gene encodes a cAMP-dependent chloride channel in heart

cAMP-dependent chloride channels in heart contribute to autonomic regulation of action potential duration and membrane potential and have been inferred to be due to cardiac expression of the epithelial cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. In this report, a cDNA from rabbit ventricle was isolated and sequenced, which encodes an exon 5 splice variant (exon 5-) of CFTR, with >90% identity to human CFTR cDNA present in epithelial cells. Expression of this cDNA in Xenopus oocytes gave rise to robust cAMP-activated chloride currents that were absent in control water-injected oocytes. Antisense oligodeoxynucleotides directed against CFTR significantly reduced the density of cAMP-dependent chloride currents in acutely cultured myocytes, thereby establishing a direct functional link between cardiac expression of CFTR protein and an endogenous chloride channel in native cardiac myocytes.

Comparison of sarcolemmal calcium channel current in rabbit and rat ventricular myocytes

1. Fundamental properties of Ca2+ channel currents in rat and rabbit ventricular myocytes were measured using whole cell voltage clamp. 2. In rat, as compared with rabbit myocytes, Ca2+ channel current (ICa) was half-activated at about 10 mV more negative potential, decayed slower, was half-inactivated (in steady state) at about 5 mV more positive potential, and recovered faster from inactivation. 3. These features result in a larger steady-state window current in rat, and also suggest that under comparable voltage clamp conditions, including action potential (AP) clamp, more Ca2+ influx would be expected in rat myocytes. 4. Ca2+ channel current carried by Na+ and Cs+ in the absence of divalent ions (Ins) also activated at more negative potential and decayed more slowly in rat. 5. The reversal potential for Ins was 6 mV more positive in rabbit, consistent with a larger permeability ratio (PNa/PCs) in rabbit than in rat. ICa also reversed at slightly more positive potentials in rabbit (such that PCa/PCs might also be higher). 6. Ca2+ influx was calculated by integration of ICa evoked by voltage clamp pulses (either square pulses or pulses based on recorded rabbit or rat APs). For a given clamp waveform, the Ca2+ influx was up to 25% greater in rat, as predicted from the fundamental properties of ICa and Ins. 7. However, the longer duration of the AP in rabbit myocytes compensated for the difference in influx, such that the integrated Ca2+ influx via ICa in response to the species-appropriate waveform was about twice as large as that seen in rat.

Alpha 1-adrenergic inhibition of the beta-adrenergically activated Cl- current in guinea pig ventricular myocytes

alpha-Adrenergic receptor stimulation regulates the activity of a number of different cardiac ion channels, including those underlying one or more distinct Cl- conductances. The whole-cell patch-clamp technique was used in the present study to investigate the effects of alpha-adrenergic stimulation on the beta-adrenergically regulated Cl- current in guinea pig ventricular myocytes. Neither alpha 1-adrenergic receptor stimulation with methoxamine (25 to 500 mumol/L) nor direct activation of endogenous protein kinase C (PKC) with phorbol 12,13-dibutyrate (PDBu, 100 nmol/L) evoked a Cl- current. On the contrary, the Cl- current activated by 30 nmol/L isoproterenol was inhibited by methoxamine, with an EC50 of 6.7 +/- 2.6 mumol/L, and this response was blocked by prazosin, an alpha 1-adrenergic receptor antagonist. Prazosin also decreased the EC50 for current activation by norepinephrine from 53 +/- 7.1 to 18 +/- 3.8 nmol/L, demonstrating that the ability of this endogenous neurotransmitter to activate the Cl- current through beta-adrenergic receptor stimulation is limited by its intrinsic ability to also activate alpha-adrenergic receptors. Methoxamine did not inhibit the Cl- current evoked by either direct activation of adenylate cyclase with forskolin or inhibition of phosphodiesterase activity with 3-isobutyl-1-methylxanthine, indicating that alpha-adrenergic stimulation inhibits beta-adrenergic responses at a point upstream of adenylate cyclase activation. Methoxamine also did not inhibit the Cl- current activated by histamine, suggesting that alpha-adrenergic stimulation specifically inhibits beta-adrenergic receptor-mediated responses. The inhibitory effect of methoxamine was not mimicked by PDBu, and it persisted in the presence of bisindolylmaleimide, a selective PKC inhibitor. However, methoxamine inhibition of the isoproterenol-activated Cl- current was sensitive to pertussis toxin. These results suggest that alpha-adrenergic receptor stimulation inhibits the beta-adrenergically activated Cl- current, demonstrating a novel mechanism by which alpha-adrenergic receptors may regulate ion channel activity in the heart.

Unitary Cl- channels activated by cytoplasmic Ca2+ in canine ventricular myocytes

Recent whole-cell studies have shown that Ca(2+)-activated Cl- currents contribute to the Ca(2+)-dependent 4-aminopyridine-insensitive component of the transient outward current and to the arrhythmogenic transient inward current in rabbit and canine cardiac cells. These Cl(-)-sensitive currents are activated by Ca2+ release from the sarcoplasmic reticulum and are inhibited by anion transport blockers; however, the unitary single channels responsible have yet to be identified. We used inside-out patches from canine ventricular myocytes and conditions under which the only likely permeant ion is Cl- to identify 4-aminopyridine-resistant unitary Ca(2+)-activated Cl- channels, Ca2+ applied to the cytoplasmic surface of membrane patches activated small-conductance (1.0 to 1.3 pS) channels. These channels were Cl- selective, with rectification properties that could be described by the Goldman-Hodgkin-Katz current equation. Channel activity exhibited time independence when cytoplasmic Ca2+ was held constant and was blocked by the anion transport blockers, DIDS and niflumic acid. Ca2+ (ranging from pCa > or = 6 to pCa 3) applied to the cytoplasmic surface of inside-out patches increased, in a dose-dependent manner, NPo, where N is the number of channels opened and Po is open probability. At negative membrane potentials (-60 to -130 mV), an estimate of the dependence of NPo on cytoplasmic Ca2+ yielded an apparent Kd of 150.2 mumol/L. At pCa 3, an average channel density of approximately equal to 3 microns-2 was estimated. Calculations based on these estimates of cytoplasmic Ca2+ sensitivity and channel current amplitude and density suggest that these small-conductance Cl- channels contribute significant whole-cell membrane current in response to changes in intracellular Ca2+ within the physiological range. We suggest that these small-conductance Ca(2+)-activated Cl- channels underlie the transient Ca(2+)-activated 4-aminopyridine-insensitive current, which contributes to phase-1 repolarization, and under conditions of Ca2+ overload, these channels may generate transient inward currents, contributing to the development of triggered cardiac arrhythmias.

Phorbol ester activation of chloride current in guinea-pig ventricular myocytes

1. Although earlier studies with phorbol esters indicate that protein kinase C (PKC) may be an important regulator of Cl- current (Icl) in cardiac cells, there is a need for additional quantitative data and investigation of conflicting findings. Our objectives were to measure the magnitude, time course, and concentration-dependence of Icl activated in guinea-pig ventricular myocytes by phorbol 12-myristate 13-acetate (PMA), evaluate its PKC dependence, and examine its modification by external and internal ions. 2. The whole-cell patch clamp technique was used to apply short depolarizing and hyperpolarizing pulses to myocytes superfused with Na(+)-, K(+)-, Ca(2+)-free solution (36 degrees C) and dialysed with Cs+ solution. Stimulation of membrane currents by PMA (threshold < or = 1nM, EC50 approximately equal to 14 nM, maximal 40% increase with > or = 100 nM) plateaued within 6-10 min. 3. PMA-activated current was time-independent, and suppressed by l mM 9-anthracenecarboxylic acid (9-AC). Its reversal potential (Erev) was sensitive to changes in the Cl- gradient, and outward rectification of the current-voltage (I-V) relationship was more pronounced with 30 mM than 140 mM Cl- dialysate. 4. The relative permeability of PMA-activated channels estimated from Erev measurements was I- > Cl- > > aspartate. Channel activation was independent of external Na+. 5. PMA failed to activate Icl in myocytes pretreated with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) or dialysed with pCa 10.5 solution. Lack of response to 4 alpha-phorbol 12, 13-didecanoate (alpha PDD) was a further indication of mediation by PKC. 6. Icl induced by 2 microM forskolin was far larger than that induced by PMA, suggesting that endogenous protein kinase A is a much stronger Cl- channel activator than endogenous PKC in these myocytes. 7. The macroscopic properties of PMA-induced Icl appear to be indistinguishable from those of PKA-activated Icl. We discount stimulation of PKA by PMA as an explanation, and conclude that endogenous PKC may activate PKA-regulated Cl- channels in these myocytes.

Transmembrane chloride currents in human atrial myocytes

The present study was designed to evaluate the presence of basal, swelling-induced, and cAMP-dependent Cl- currents in human atrial myocytes studied with the whole cell patch-clamp technique. Under basal conditions, a small outwardly rectifying background conductance was noted that reversed close to 0 mV and was not altered by Cl- replacement. Isoproterenol (1 microM), forskolin (3 microM), and 8-bromoadenosine 3',5'-cyclic monophosphate (50 microM) did not increase membrane conductance, even when responsiveness to isoproterenol was confirmed by an increase in Ca2+ current and when perforated-patch techniques (nystatin) were used. Exposure to hyposmotic solutions increased cell volume and induced a whole cell conductance that showed outward rectification, was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (100 microM), and responded to changes in Cl- gradient in a fashion consistent with a Cl(-)-selective conductance, with estimated relative permeabilities of 1, 0.25, and 0.07 for Cl-, methanesulfonate, and aspartate, respectively. The results suggest that human atrial cells lack basal and adenosine 3',5'-cyclic monophosphate-dependent Cl- current but manifest a substantial Cl- conductance in the presence of cell swelling.

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.

beta-Adrenergic modulation of the inwardly rectifying potassium channel in isolated human ventricular myocytes. Alteration in channel response to beta-adrenergic stimulation in failing human hearts

The beta-adrenergic modulation of the inwardly-rectifying K+ channel (IK1) was examined in isolated human ventricular myocytes using patch-clamp techniques. Isoproterenol (ISO) reversibly depolarized the resting membrane potential and prolonged the action potential duration. Under the whole-cell C1- -free condition, ISO applied via the bath solution reversibly inhibited macroscopic IdK1. The reversal potential of the ISO-sensitive current was shifted by approximately 60 mV per 10-fold change in the external K+ concentration and was sensitive to Ba2+. The ISO-induced inhibition of IK1 was mimicked by forskolin and dibutyrl cAMP, and was prevented by including a cAMP-dependent protein kinase (PKA) inhibitor (PKI) in the pipette solution. In single-channel recordings from cell-attached patches, bath applied ISO could suppress IK1 channels by decreasing open state probability. Bath application of the purified catalytic sub-unit of PKA to inside-out patches also inhibited IK1 and the inhibition could be antagonized by alkaline phosphatase. When beta-adrenergic modulation of IK1 was compared between ventricular myocytes isolated from the failing and the nonfailing heart, channel response to ISO and PKA was significantly reduced in myocytes from the failing heart. Although ISO inhibited IK1 in a concentration-dependent fashion in both groups, a half-maximal concentration was greater in failing (0.12 microM) than in nonfailing hearts (0.023 microM). These results suggest that IK1 in human ventricular myocytes can be inhibited by a PKA-mediated phosphorylation and the modulation is significantly reduced in ventricular myocytes from the failing heart compared to the nonfailing heart.

Chronotropic action of Na+, K+, Cl- cotransport inhibition in the isolated rat heart

The chronotropic actions of Na+, K+, Cl- cotransport were investigated by studying the effects of the loop diuretics bumetanide and furosemide, specific inhibitors of the cotransporter, on an isolated rat sino-atrial node preparation. Application of bumetanide decreased the cycle length from 0.334 s (+/- 0.087 S.D.) to 0.279 s (+/- 0.083, n = 16, P = 6.5 x 10(-6)) in Hepes-buffered physiological salt solution (PSS). Similar decreases were recorded in bicarbonate-buffered PSS. Chloride channel blockers indicate that the tachycardia evoked by loop diuretics is not due their blocking of chloride channels. Thus, 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) had a negative chronotropic action and 2-[(2-cyclopentyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxo-1H-inden -5-yl) oxy] acetic acid (IAA-94) produced no change in cycle length. Pharmacological manoeuvres indicate that the positive chronotropic action of loop diuretics is associated with catecholamine release. The positive chronotropic action of bumetanide was inhibited by the beta-adrenoceptor antagonists, propranolol and atenolol, but was unaffected by atropine.

Prolonged repolarization during hypoxemia in epicardial electrogram: difference from ischemia and a competitive action of cyclic AMP

The effects of regional hypoxemia and ischemia on epicardial electrogram were studied in anesthetized, open-chest dogs. The left circumflex artery (LCx) was cannulated and perfused with either arterial blood or hypoxic solution. A contact electrode for recording monophasic action potential (MAP) was applied to the epicardial site of the LCx area. Epicardial electrograms and MAP in the LCx perfusion territory were recorded 1) just before and at the end of a 2-min coronary occlusion (ischemia) and 2) just before and at the end of a 2-min perfusion of hypoxic solution (hypoxemia). The activation-recovery interval (ARI), defined as an interval from the minimum derivative of the QRS complex to the maximum derivative of the T-wave in the unipolar electrogram, changed linearly with MAP duration during above interventions. The ARI decreased by 29% from 189 +/- 14 to 134 +/- 30 ms during ischemia (p < .001), and it increased by 39% from 183 +/- 11 to 254 +/- 31 ms during hypoxemia (p < .001). Hypoxemia produced a giant negative T-wave whose pattern was not modified by pretreatments with autonomic nerve blockers (propranolol and atropine), a Ca2+ channel blocker (verapamil), an ATP-sensitive K+ channel (KATP blocker (5-hydroxydecanoate or transient outward K+ current (I(to) blocker (4-aminopyridine). Isoproterenol, forskolin or aminophylline inhibited both the appearances of giant negative T and the ARI prolongation. Accordingly, unlike ischemia, hypoxemia prolongs repolarization process and this prolongation is inhibited by the augmentation of intracellular cyclic AMP.

[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.

Alpha-adrenergic control of volume-regulated Cl- currents in rabbit atrial myocytes. Characterization of a novel ionic regulatory mechanism

alpha-Adrenergic stimulation is known to play a role in cardiac arrhythmogenesis and to modulate a variety of cardiac K+ currents. The effects of alpha-adrenergic stimulation on Cl- currents are largely unknown. Many cardiac cell types show a volume-sensitive Cl- current induced by cell swelling (ICl.swell). The present experiments were designed to assess the potential alpha-adrenergic modulation of ICl.swell in rabbit atrial myocytes. ICl.swell was induced with the use of a hypotonic superfusate, under conditions designed to prevent currents carried by K+, Na+, and Ca2+ ions. A basal Cl- current (ICl.b) was observed under isotonic conditions in 128 of 150 cells (85%), had the same dependency on [Cl-]o as ICl.swell, and was reduced by cell shrinkage induced by hypertonic superfusion, suggesting that ICl.b is carried by the same volume-sensitive Cl- conductance as ICl.swell. Phenylephrine produced a concentration-dependent and near-complete inhibition of ICl.b and ICl.swell, with EC50 values of 86 +/- 5 and 72 +/- 7 (mean +/- SEM) mumol/L, respectively, at +20 mV. Norepinephrine (administered in the presence of 1 mumol/L propranolol) also inhibited ICl.b and ICl.swell, with EC50 values of 2.6 +/- 0.1 and 2.8 +/- 0.4 mumol/L, respectively. The concentration-response curve for phenylephrine was shifted significantly (P < .001) to the right by the alpha 1-adrenoceptor antagonist prazosin and by the alpha 1A-receptor antagonists (+)-niguldipine and 5-methylurapidil but was unaltered by the alpha 1B-receptor antagonist chloroethylclonidine (100 mumol/L). Inhibition of protein kinase C (PKC) with staurosporine, H-7, or 18-hour preincubation with the phorbol ester 4 beta-phorbol 12-myristate 13-acetate (PMA, 500 nmol/L) blocked the effects of phenylephrine on ICl.swell, and the highly selective PKC inhibitor bisindolylmaleimide blocked the effects of norepinephrine on ICl.swell and ICl.b. Both PMA and 1-oleoyl-2-acetylglycerol inhibited ICl.swell in a concentration-dependent fashion. In blinded studies, the phorbol ester phorbol 12,13-didecanoate (PDD) reduced ICl.swell by 91 +/- 3%; its inactive analogue 4 alpha-PDD had no effect (mean change, 3 +/- 1%). Preincubation with pertussis toxin (PTX) prevented the actions of phenylephrine on ICl.swell, indicating a role for a PTX-sensitive guanine nucleotide-binding (G) protein. We conclude that alpha-adrenergic agonists inhibit volume-sensitive Cl- currents in rabbit atrial cells by interacting with an alpha 1A-adrenoceptor mechanism that is coupled to PKC via a PTX-sensitive G protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Glibenclamide, an ATP-sensitive K+ channel blocker, inhibits cardiac cAMP-activated Cl- conductance

Stimulation of the beta-adrenoceptor activates a time-independent Cl- conductance that is known to be regulated via phosphorylation by cAMP-dependent protein kinase in guinea pig ventricular myocytes. Since epithelial cystic fibrosis transmembrane conductance regulator Cl- channels are known to be sensitive to an antidiabetic sulfonylurea, glibenclamide, we tested whether the drug modulates cardiac cAMP-activated Cl- conductance. Bath application of isoproterenol (1 mumol/L, n = 11) or forskolin (1 mumol/L, n = 17) or the intracellular application of cAMP (1 mmol/L, n = 9) activated whole-cell Cl- currents recorded from single myocytes at 36 degrees C. External glibenclamide (> or = 10 mumol/L, n = 26) inhibited the Cl- current induced by either of the stimulants in a concentration-dependent manner. The half-maximal inhibition concentration (IC50) of glibenclamide and the Hill coefficient were 24.5 to 37.9 mumol/L and 1.6 to 2.2, respectively. During current-clamp experiments, forskolin was found to shorten the action potential significantly (250 +/- 45 to 201 +/- 52 milliseconds, P < .05) in 7 of 11 cells tested. Glibenclamide antagonized the forskolin-induced shortening (to 243 +/- 54 milliseconds, n = 7, P < .05). Intracellular administration of sodium orthovanadate (0.5 to approximately 1 mmol/L, n = 6) brought about persistent activation of Cl- current after brief bath application of forskolin. This Cl- current was not affected by H-89 (100 mumol/L, n = 3), a specific inhibitor of cAMP-dependent protein kinase, and was suppressed by glibenclamide similarly, with an IC50 of 29.7 mumol/L.(ABSTRACT TRUNCATED AT 250 WORDS)

Forskolin stimulates swelling-induced chloride current, not cardiac cystic fibrosis transmembrane-conductance regulator current, in human cardiac myocytes

Whole-cell patch clamp was used to look for cystic fibrosis transmembrane-conductance regulator (CFTR)-like chloride currents in calcium-tolerant human cardiac myocytes. Potassium-containing solutions were used initially. Steady state currents were measured with hyperpolarizing ramps (-16.25 mV/s). Peak net inward currents during voltage steps from -50 to +5 mV were used as an index of L-type calcium current. Isoproterenol (1 mumol/L) or forskolin (10 mumol/L) were used in attempts to evoke CFTR-like chloride current. No forskolin- or isoproterenol-induced steady state current was found in any of 17 atrial cells from seven patients in the absence of cell swelling. Every cell exhibited a large increase in net inward current in response to forskolin, suggesting that cAMP-dependent stimulation of L-type calcium current was present. Swelling with osmotic stress induced an outwardly rectifying steady state current with a reversal potential close to the chloride equilibrium potential. Once this current was activated, exposure to forskolin caused a further increase that subsided on washout (four of four cells, two patients). The atrial swelling-induced current was studied in more detail by using cesium-containing solutions. The current was determined to be a chloride current because the reversal potential was sensitive to changes in intracellular chloride and outward currents were blocked by 150 mumol/L DIDS. Ventricular cells were isolated from five failing human hearts. No CFTR-like current was found in any of 17 cells. In eight of eight ventricular cells, a swelling-induced current was found. The amplitude of the swelling-induced current was enhanced by forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning of a swelling-induced chloride current related protein from rabbit heart

Recently, pIcln has been reported to be a regulator of a swelling-induced chloride conductance. We have cloned a cDNA RCL-H1 from rabbit heart, of which primary structure is highly homologous to that of pIcln. Outwardly rectifying currents were recorded in oocytes expressing RCL-H1, which is consistent with the result of pIcln. RNA blot analysis revealed the widespread expression of RCL-H1 mRNA in rabbit tissues. RCL-H1 may play an important role in regulating cell volume and give a clue to revealing molecular structure of swelling-induced chloride channel(s).

Angiotensin II activation of a chloride current in rabbit cardiac myocytes

1. The effects of angiotensin II (Ang II) on membrane currents were investigated in single ventricular myocytes from the rabbit heart by the whole-cell voltage-clamp method. 2. In the presence of an inhibitor of Ca2+ currents (nifedipine at 3 microM or CdCl2 at 0.3 mM) and a beta-adrenoceptor blocker (bupranolol at 1 microM), 1 microM Ang II significantly increased the membrane conductance. 3. After elimination of K+ from external and internal solutions and its replacement by Cs+, Ang II at 0.1 microM increased an outwardly rectifying current that reached a maximum after about 40 min. The effect was concentration dependent (10(-9)-10(-6) M) and was inhibited by saralasin, an antagonist of Ang II receptors. 4. The reversal potential of the Ang II-induced current in the absence of K+ was compatible with the Cl- equilibrium potential at various external concentrations of Cl-. 5. A Cl- channel blocker, 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS, at 5 mM), reversibly decreased the Ang II-induced current. 6. The Ang II-induced current developed when the internal solution contained Ca2+ (pCa 7.2 or 7.0) but not when it contained 10 mM EGTA without Ca2+. 7. Besides developing a Cl- current, Ang II at 1 microM increased the inwardly rectifying K+ current (IK1) and this effect reached maximum within 3 min. 8. The effect of Ang II on the action potential was biphasic: the duration of the action potential was initially reduced and then it was increased. 9. These results suggest that Ang II induces a Cl- current that appears likely to modulate the action potential in rabbit ventricular myocytes.

Beta-adrenergic blocking property of dl-sotalol maintains class III efficacy in guinea pig ventricular muscle after isoproterenol

BACKGROUND

Catecholamines antagonize the efficacy of several class III antiarrhythmic agents. To determine the role of the intrinsic beta-adrenergic blocking property of dl-sotalol in maintaining class III efficacy during a high-catecholamine state, we compared the electrophysiological properties of dl-sotalol with those of d-sotalol, which is devoid of significant beta-adrenergic blocking effect, before and after isoproterenol infusion.

METHODS AND RESULTS

Action potential duration at 90% repolarization (APD90) was prolonged in isolated guinea pig papillary muscles perfused with d-sotalol and dl-sotalol 10(-4) mol/L over stimulation cycle lengths from 200 to 2000 ms. The increases in APD90 for d-sotalol and dl-sotalol over control were 10.9 +/- 2.5 to 23.7 +/- 4.8 ms and 27.9 +/- 4.0 to 39.0 +/- 5.6 ms, respectively. APD90 shortened to less than control in papillary muscles treated with d-sotalol but not dl-sotalol on addition of isoproterenol 10(-6) mol/L: -31.2 +/- 3.5 to -18.3 +/- 4.8 ms and 10.5 +/- 3.6 to 33.3 +/- 7.8 ms, respectively, P < .003. Single guinea pig ventricular myocytes were studied by the whole-cell patch clamp method. Time-dependent (Iout) and total (Itot) outward current in response to a 300-ms pulse to 20 mV and tail current (Itail) to -35 mV were measured after Ca2+ channel block and Na+ channel inactivation. Iout, Itail, and Itot were reduced in myocytes perfused with d-sotalol and dl-sotalol 10(-4) mol/L: Iout, -36.1 +/- 4.1%, -40.5 +/- 3.3%; Itail, -59.3 +/- 4.6%, -62.2 +/- 11.1%; Itot, -27.3 +/- 4.3%, -50.0 +/- 11.8%. Iout and Itot increased to a greater degree in myocytes treated with d-sotalol than dl-sotalol on addition of isoproterenol 10(-6) mol/L: Iout, 100.3 +/- 20.6%, 11.3 +/- 7.6%, P = .002; Itot, 86.8 +/- 39.2%, -41.1 +/- 20.9%, P = .01. Itail tended to increase more in myocytes treated with d-sotalol than dl-sotalol on addition of isoproterenol, but the difference was not significant (-9.1 +/- 13.5%, -28.0 +/- 9.0%).

CONCLUSIONS

The beta-adrenergic blocking property of dl-sotalol maintains APD prolongation and repolarizing outward current block during isoproterenol infusion in guinea pig ventricular muscle. Extrapolation of these data to a clinical setting may explain the efficacy of dl-sotalol in diminishing ventricular arrhythmia recurrence.

Pharmacologic properties of the swelling-induced chloride current of dog atrial myocytes

INTRODUCTION

Swelling-induced chloride currents may contribute to cardiac electrical activity and cell volume regulation. Identification of selective blockers would aid in understanding the functional contribution(s) of this current.

METHODS AND RESULTS

Dog atrial cells were used to investigate the pharmacologic properties of the swelling-induced chloride current. Whole cell patch clamp was used. Swelling-induced chloride current was activated by osmotic stress. Initially, the chloride selectivity and calcium independence of the swelling-induced current in dog atrial cells was demonstrated. Subsequently, a number of putative chloride channel blockers were examined. Anthracene-9-carboxylic acid (1 mM) and dideoxyforskolin (100 microM) and extracellular cAMP (5 mM) were found to partially inhibit the swelling-induced chloride current (approximately 50%, 80%, and 10% inhibition, respectively). Niflumic acid (100 microM), nitrophenylpropylamino benzoate (NPPB; 10 to 40 microM), and (+) 2-[(2-cyclopentyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxy-1H-inden -5-yl)oxy d acetic acid (indanyloxyacetic acid; IAA-94; 100 microM) could fully inhibit the swelling-induced chloride current without decreasing cell size. DIDS (100 microM) and dinitrostilbene disulfonic acid (DNDS; 5 mM) fully inhibited outward currents but only partially inhibited inward current.

CONCLUSIONS

Niflumic acid, IAA-94, and NPPB were identified as full blockers of cardiac swelling-induced chloride current. Nonspecific effects were identified for each of the full blockers. Experiments that use these agents as functional antagonists should be carefully designed and interpreted with caution.

Cardiomyocytes differentiated in vitro from embryonic stem cells developmentally express cardiac-specific genes and ionic currents

Cardiomyocytes differentiated in vitro from pluripotent embryonic stem (ES) cells of line D3 via embryo-like aggregates (embryoid bodies) were characterized by the whole-cell patch-clamp technique during the entire differentiation period. Spontaneously contracting cardiomyocytes were enzymatically isolated by collagenase from embryoid body outgrowths of early, intermediate, and terminal differentiation stages. The early differentiated cardiomyocytes exhibited an outwardly rectifying, transient K+ current sensitive to 4-aminopyridine and an inward Ca2+ current but no Na+ current. The Ca2+ current showed all features of L-type Ca2+ current, being highly sensitive to 1,4-dihydropyridines but not to omega-conotoxin. Cardiomyocytes of intermediate stage were characterized by the additional expression of cardiac-specific Na+ current, the delayed K+ current, and If current. Terminally differentiated cardiomyocytes expressed a Ca2+ channel density about three times higher than that of early stage. In addition, two types of inwardly rectifying K+ currents (IK1 and IK,Ach) and the ATP-modulated K+ current were found. During cardiomyocyte differentiation, several distinct cell populations could be distinguished by their sets of ionic channels and typical action potentials presumably representing cardiac tissues with properties of sinus node, atrium, and ventricle. Reverse transcription polymerase chain reaction revealed the transcription of alpha- and beta-cardiac myosin heavy chain (MHC) genes synchronously with the first spontaneous contractions. Transcription of embryonic skeletal MHC gene at intermediate and terminal differentiation stages correlated with the expression of Na+ channels. The selective expression of alpha-cardiac MHC gene in ES cell-derived cardiomyocytes was demonstrated after ES cell transfection of the LacZ construct driven by the alpha-cardiac MHC promoter region followed by ES cell differentiation and beta-galactosidase staining. In conclusion, our data demonstrate that ES cell-derived cardiomyocytes represent a unique model to investigate the early cardiac development and permit pharmacological/toxicological studies in vitro.

Inhibition of the cardiac protein kinase A-dependent chloride conductance by endothelin-1

Endothelin-1 is a peptide hormone constitutively secreted by vascular and endocardial endothelial cells. Secretion of endothelin-1 is increased under certain pathophysiological conditions, including coronary vasospasm, cardiac ischaemia and myocardial infarction. We have examined the effect of endothelin-1 on the protein kinase A (PKA)-dependent chloride current in voltage-clamped guinea pig ventricular myocytes. This conductance, induced by catecholamines through beta-adrenergic receptors, counteracts the simultaneously increased L-type calcium current by shortening the action potential duration. We report here that endothelin-1, acting through ETA (endothelin-1-selective) receptors, inhibited the current through a pertussis toxin-sensitive mechanism, analogous to muscarinic receptors, by reducing the intracellular cyclic AMP concentration. This effect of endothelin-1 should help protect the ventricle against potentially arrhythmogenic shortening of the action potential during ischaemia when the circulating levels of catecholamines are increased.

Effects of extracellular Cl- on the inotropic response to alpha-adrenoceptor stimulation

This study was designed to determine if the sustained positive inotropic action of alpha-adrenergic stimulation is affected by the absence of extracellular chloride ion (Clo-). Atrial and papillary muscle were isolated from adult male rats, bathed in Krebs-Henseleit solution (30 degrees C) with and without Cl- (methane-sulfonate substitution), and stimulated at 0.5 Hz. Isometric developed tension was monitored during cumulative addition of phenylephrine, isoproterenol and Ca2+. The dose-dependent positive inotropic effects of isoproterenol and Ca2+ were not altered by the absence of Clo-. However, the magnitude of the response to phenylephrine was diminished in both tissues. In atrial muscle, the maximum positive inotropic effect of phenylephrine was reduced from 2.05 +/- 0.17 g in the presence of Clo- to 0.39 +/- 0.06 g in the absence of Clo-; control developed tension was 0.60 +/- 0.08 and 0.47 +/- 0.10 g in these two groups before exposure to the alpha-adrenoceptor agonist. In papillary muscle, control developed tension was 1.40 +/- 0.11 and 1.17 +/- 0.18 g in the presence and absence of Clo-, respectively; and the maximum inotropic responses to phenylephrine were 0.71 +/- 0.12 and 0.27 +/- 0.13 g. EC50 values for phenylephrine were not significantly affected by substitution for Cl-. Similar results were observed in a Hepes-buffered bathing solution without bicarbonate (HCO3-). These results indicate that the positive inotropic action of alpha-adrenergic stimulation is mediated in part by a mechanism requiring Cl-. Furthermore, data suggest that the antagonistic effect of Clo- removal is not mediated via Cl-/HCO3- exchange.

Protein kinase-dependent Cl- currents in feline ventricular myocytes

A Cl- current (ICl) induced by isoproterenol (ISO) has been identified in isolated guinea pig ventricular myocytes. This ISO-induced ICl can be inhibited by propranolol and mimicked by forskolin (FSK), suggesting that beta-receptors, cAMP, and protein kinase A (PKA) are involved in regulating the involved Cl- channel. Because activation of protein kinase C (PKC) mediated via alpha-adrenergic receptor stimulation is also known to regulate several ion channels, the idea that activation of PKC also can induce ICl was investigated by using isolated feline ventricular myocytes and the whole-cell patch-clamp technique. We found that extracellularly applied phorbol 12-myristate 13-acetate (PMA) could activate ICl in feline ventricular cells. Control experiments indicated that in the absence of PMA or other interventions, the steady-state current-voltage relation of patches maintained for more than 40 minutes was unchanged over a voltage range from -100 to +80 mV. This suggests that the present findings are not complicated by the development over time after patching of a steady-state ICl, similar to the findings reported for canine atrial myocytes. When induced by PMA, ICl was noninactivating and outwardly rectifying; it reversed polarity at approximately the equilibrium potential for Cl- and was sensitive to the Cl- channel blocker 9-anthracene carboxylic acid. In contrast, PMA failed to induce ICl when either staurosporine or calphostin C was added to the patch pipette solution used to internally dialyze the myocytes. The kinetic properties of PMA- and FSK-induced ICl were similar. When supramaximal concentrations of both ISO (1 mumol/L) and PMA (6 mumol/L) were applied simultaneously, the size of the induced ICl was the same as that induced by the same concentrations of either agonist applied alone. In addition, maximal induction of ICl with PMA (6 mumol/L) prevented the effects of FSK (1 mumol/L, the concentration causing approximately 40% of the maximal response [approximately EC40]), yet the effects of simultaneously applied submaximal concentrations (eg, approximately EC25 to approximately EC40) of both 0.5 mumol/L PMA and 1 mumol/L FSK were roughly additive. The results suggest that (1) both PMA and ISO or FSK can induce ICl with approximately equal efficacy, (2) the PMA- and ISO- or FSK-induced ICls are similar, and (3) they all flow through the same set of Cl- channels, implying that channel phosphorylation via either PKA or PKC can activate this feline cardiac ICl.

A dynamic model of the cardiac ventricular action potential. I. Simulations of ionic currents and concentration changes

A mathematical model of the cardiac ventricular action potential is presented. In our previous work, the membrane Na+ current and K+ currents were formulated. The present article focuses on processes that regulate intracellular Ca2+ and depend on its concentration. The model presented here for the mammalian ventricular action potential is based mostly on the guinea pig ventricular cell. However, it provides the framework for modeling other types of ventricular cells with appropriate modifications made to account for species differences. The following processes are formulated: Ca2+ current through the L-type channel (ICa), the Na(+)-Ca2+ exchanger, Ca2+ release and uptake by the sarcoplasmic reticulum (SR), buffering of Ca2+ in the SR and in the myoplasm, a Ca2+ pump in the sarcolemma, the Na(+)-K+ pump, and a nonspecific Ca(2+)-activated membrane current. Activation of ICa is an order of magnitude faster than in previous models. Inactivation of ICa depends on both the membrane voltage and [Ca2+]i. SR is divided into two subcompartments, a network SR (NSR) and a junctional SR (JSR). Functionally, Ca2+ enters the NSR and translocates to the JSR following a monoexponential function. Release of Ca2+ occurs at JSR and can be triggered by two different mechanisms, Ca(2+)-induced Ca2+ release and spontaneous release. The model provides the basis for the study of arrhythmogenic activity of the single myocyte including afterdepolarizations and triggered activity. It can simulate cellular responses under different degrees of Ca2+ overload. Such simulations are presented in our accompanying article in this issue of Circulation Research.

Properties of a protein kinase C-activated chloride current in guinea pig ventricular myocytes

Cardiac ventricular myocytes from several species, including the guinea pig, possess a cAMP-dependent protein kinase A (PKA)-activated Cl- channel. In the present study, the properties of a protein kinase C (PKC)-activated Cl- current were studied in isolated guinea pig ventricular myocytes using the whole-cell arrangement of the patch-clamp technique. Intracellular dialysis of ventricular cells with PKC resulted in the activation of a large background current that displayed time-independent kinetics. In the presence of 146 mmol/L external Cl- and 71 mmol/L internal Cl-, the reversal potential (Erev) of the background current (-17 +/- 1 mV) was close to that of the Cl- equilibrium potential (-18 mV), and the current versus voltage relation for the current was outward rectifying in shape. When [Cl-]i or [Cl-]o was reduced by substitution of Cl- with aspartic acid, Erev for the background current shifted in a manner expected for a Cl(-)-selective channel. Based on Erev measurements, the permeability sequence for this PKC-activated Cl- channel was determined to be SCN- > I- > Br- congruent to Cl-. The PKC-activated Cl- current was not inhibited by the Cl- channel blocker 4,4'-dinitrostilbene-2,2'-disulfonic acid (100 mumol/L) but could be blocked by anthracene-9-carboxylic acid (1 mmol/L). Activation of the current was abolished in the presence of the PKC inhibitor staurosporine (2.5 mumol/L). Under conditions designed to cause a maximal activation of the Cl- channels by PKC, the addition of forskolin (1 mumol/L) to stimulate PKA caused only a slight further increase in the amplitude of the Cl- current. Thus, PKC activates a Cl- channel in guinea pig ventricular cells with properties similar but not identical to the PKA-activated channel.

A chloride current associated with swelling of cultured chick heart cells

1. Cultured chick heart cells challenged by hyposmotic stress underwent regulatory volume decrease (RVD) that was attenuated by prior depletion of intracellular chloride. 2. During hyposmotic swelling, cell aggregates experienced an initial increase in spontaneous contractile activity followed by eventual quiescence. Conventional microelectrode studies revealed an underlying increase in spontaneous electrical activity, followed by a sustained depolarization beyond threshold. 3. Whole-cell patch clamp studies, with K+ currents blocked, indicated that exposure of cells to hyposmotic solution (NaCl reduction) resulted in a rapid osmotic swelling followed by a substantial increase in whole-cell conductance which persisted for the duration of hyposmotic exposure and was almost completely reversed on return to isosmotic bath solution. 4. For a variety of Cl- concentrations, the reversal potentials (Erev) of the measured swelling-activated current closely followed the calculated Cl- equilibrium potential (ECl) with a linear regression slope of 0.82. When estimated by the Nernst equation, the relationship between Erev and the [Cl-]i/[Cl-]o ratio fitted well with a slope of 51 mV per decade change in the concentration ratio, consistent with a Cl(-)-selective conductance. 5. The permeability ratios of this swelling-activated conductance to chloride, methanesulphonate (MSA) and aspartate (Asp) were calculated as PCl:PMSA:PASP = 1:0.36:0.02, with the ion selectivity sequence of Cl- > MSA- >> Asp-, which suggests the swelling-activated conductance is slightly permeable to other anions. 6. Application of a Cl- channel blocker, diphenylamine-2-carboxylate (DPC, 200 microM), substantially suppressed the swelling-activated current without shifting the Erev of this current. The effect of DPC was independent of membrane potential. 7. This evidence demonstrates that hyposmotic swelling of cultured chick heart cells activates a channel-mediated Cl- conductance which may be associated with the integrated response of volume-regulatory mechanisms.

Tetramethylammonium activation of muscarinic receptors in cardiac ventricular myocytes

Replacement of extracellular Na+ with tetramethylammonium (TMA) reduces the magnitude of the Cl- current activated by beta-adrenergic receptor stimulation in guinea pig ventricular myocytes. However, the effects of replacing Na+ appear to be associated with the presence of TMA, rather than the absence of Na+. Direct addition of TMA to extracellular solutions, without changing the Na+ concentration, was able to inhibit the Cl- current activated by isoproterenol (Iso) in a concentration-dependent manner. The concentration of TMA that caused half-maximal inhibition was 327 microM when the Cl- current was activated by 1 microM Iso and 29 microM when the Cl- current was activated by 0.03 microM Iso. The effect of TMA was also blocked by atropine, suggesting that TMA exerts its effect through stimulation of the muscarinic receptors. Furthermore, TMA inhibited the Iso-activated Ca2+ current, as would be expected for an effect involving muscarinic receptor stimulation. The response to complete Na+ replacement with TMA could not be overcome by increasing the concentration of Iso 1,000-fold, and direct addition of TMA was able to antagonize the Cl- current activated independently of the beta-adrenergic receptor, using forskolin and histamine. These results are consistent with the hypothesis that TMA does not exert its effects through a competitive mechanism at the beta-adrenergic receptor. It is concluded that TMA is able to antagonize adenosine 3',5'-cyclic monophosphate-dependent activation of ion channels in the heart through activation of muscarinic receptors.

Anion and cation modulation of the guinea-pig ventricular action potential during beta-adrenoceptor stimulation

Modulation of the ventricular action potential by beta-adrenergic activation of Ca2+, K+ and cyclic adenosine monophosphate (cAMP)-dependent Cl- channels was assessed in enzymatically isolated guinea-pig ventricular myocytes. The effectiveness and relative selectivity of 9-anthracene carboxylic acid (9-AC), as an antagonist of cAMP-dependent Cl- channels was also tested. Membrane currents and action potentials were recorded using the conventional whole-cell variant of the patch-clamp technique or with the amphotericin B perforated-patch technique. The beta-adrenergic agonist isoproterenol either increased or decreased action potential duration depending on whether the dominant effect was on inward Ca2+ currents or on outward K+ or Cl- currents. When Ca2+ and K+ channel modulation was prevented by nisoldipine and low temperature respectively, beta-adrenergic activation of Cl- channels caused a significant reduction in action potential duration and a slight depolarization of the membrane potential. The beta-adrenergic-mediated effects were reversed by the Cl- channel blocker, 9-AC. In the absence of beta-adrenergic stimulation, 9-AC had no detectable effects on action potentials or Ca2+ currents. These results suggest that beta-adrenergic activation of Cl- channels is a potent mechanism for regulation of action potential duration and that 9-AC may be a useful, relatively specific, pharmacological tool for evaluating the physiological role of cAMP-activated Cl- channels in heart. 9-AC also reversed the ability of isoproterenol to antagonize prolongation of action potential duration by the class III antiarrhythmic agent E-4031.

Masking of A-type K+ channel in guinea pig cardiac cells by extracellular Ca2+

Removal of extracellular Ca2+ induced transient outward currents (Io) at membrane potentials more positive than 0 mV in the guinea pig cardiac cell. This current reached a peak within a few milliseconds of stimulation, then decreased exponentially. External Cd2+ (0.1 mM) mimicked the inhibitory effect of Ca2+ on Io. Addition of D 600 (1 microM) or quinidine (0.1 mM) in the perfusate produced a reversible suppression, and replacement of internal K+ with tetraethylammonium induced a complete inhibition of Io. The steady-state inactivation of the transient component of Io was expressed by a Boltzmann relation with a half-inactivation voltage of -33.5 mV and a slope factor of 7.5 mV. This transient component was completely or almost completely inhibited by substitution of 4-aminopyridine for external cations. We conclude that in guinea pig cardiac cells, extracellular Ca2+ at physiological concentrations is masking the activity of an A-type K+ channel. This finding implies that even should a channel gene or transcript be identified using molecular biological techniques, the channel may not necessarily function under physiological conditions.

Functionally distinct phospho-forms underlie incremental activation of protein kinase-regulated Cl- conductance in mammalian heart

The regulation of cardiac Cl- conductance by cAMP-dependent protein kinase (PKA) and cellular phosphatases was studied in isolated guinea pig ventricular myocytes by using wide-tipped, perfused pipettes to record whole-cell currents. Exposure to forskolin (Fsk) or isoproterenol (Iso) elicits a Cl- conductance that results exclusively from PKA-dependent phosphorylation because it can be completely abolished, or its activation fully prevented, by switching to pipette solution containing PKI, a synthetic peptide inhibitor of PKA. The Cl- conductance activated by micromolar concentrations of either agonist reached its steady-state amplitude in 1-2 min and was deactivated promptly and entirely, usually within 2 min, upon washing out the agonist, implying a continuous high level of activity of endogenous protein phosphatases. Accordingly, intracellular application of okadaic acid or microcystin, both potent inhibitors of protein phosphatases 1 and 2A, during exposure to Fsk enhanced the steady-state Cl- conductance and slowed its deactivation after washing out the Fsk. Maximal potentiation of the conductance, by approximately 60%, was obtained with pipette concentrations of approximately 10 microM okadaic acid (or approximately 5 microM microcystin) and did not result from an increase in the apparent affinity for Fsk. In the presence of maximally effective concentrations of okadaic acid and/or microcystin, deactivation of the enhanced Cl- conductance upon washout of agonist was incomplete, with about half of the conductance persisting indefinitely. That residual conductance did not reflect continued action of PKA because it was insensitive to PKI, but was identified as a fraction of the activated Cl- conductance by its biophysical characteristics. The results suggest that complete deactivation of the PKA-regulated cardiac Cl- conductance requires dephosphorylation by a type 1 and/or 2A phosphatase, but that partial deactivation can be accomplished by activity of some other phosphatase(s). These findings are consistent with sequential phosphorylation of a protein, probably the Cl- channel itself, at two different kinds of sites. The resulting phosphoproteins can be distinguished on the basis of their different contributions to whole-cell Cl- conductance.

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.

Effects of stilbenedisulfonic acid derivatives on the cAMP-regulated chloride current in cardiac myocytes

Stilbenedisulfonic acid derivatives have been shown to block Cl- channels directly in many different preparations. Therefore, the utility of these compounds as tools for studying the cAMP-dependent Cl- current (ICl) in guinea-pig ventricular myocytes was examined using the patch-clamp technique to record whole-cell Cl- currents at room temperature. It was found that 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) increased, rather than decreased, the isoproterenol (ISO)-activated Cl- current. However, SITS alone stimulated little or no sustained current, suggesting that SITS activates the Cl- current through a synergistic effect with ISO. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) also enhanced the ISO-activated Cl- current. However, 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS) did not have any effect. SITS also exhibited a synergistic effect on the ISO-enhanced Ca2+ current in the same cells, suggesting that it affects the pathway involved in beta-adrenergic regulation of both Cl- and Ca2+ channels. SITS had no effect on the Cl- current stimulated by direct activation of adenylate cyclase with forskolin or exposure to the membrane-permeable cAMP derivative 8-bromoadenosine 3',5'-cyclic monophosphate. This suggests that SITS and DIDS may enhance the ISO-activated Cl- current via an effect on the beta-adrenergic receptor. It is concluded that these stilbenedisulfonic acid derivatives are not effective antagonists of cAMP-activated Cl- channels in cardiac ventricular myocytes.

Cyclic AMP- and calcium-activated chloride currents in Leydig cells isolated from mature rat testis

Using the whole-cell recording technique, we have investigated some properties of the membrane currents induced by depolarization of Leydig cells isolated from mature rat testis. Internal perfusions with cyclic AMP or calcium induce currents which reveal the properties of the cyclic AMP-activated and calcium-activated chloride conductances. Both currents are sensitive to EC1 displacement and are similarly affected by SITS and DIDS, two stilbene-derived chloride channel blockers. However, at the resting membrane potential, the cyclic AMP-dependent, not the calcium dependent conductance, is activated. We discuss the involvement of these results in the physiology of the Leydig cells.

The protein kinase A-regulated cardiac Cl- channel resembles the cystic fibrosis transmembrane conductance regulator

Stimulation of beta-adrenoceptors in cardiac ventricular myocytes activates a strong chloride ion conductance as a result of phosphorylation by cyclic AMP-dependent protein kinase (PKA). This Cl- conductance, which is time- and voltage-independent, counters the tendency of the simultaneously enhanced Ca2+ channel current to prolong the ventricular action potential. Using inside-out giant patches excised from guinea-pig myocytes, we show here that phosphorylation by the PKA catalytic subunit plus Mg-ATP elicits discrete Cl- channel currents. In almost symmetrical Cl- solutions (approximately 150 mM), unitary current amplitude scales with membrane potential, and reverses sign near 0 mV, to yield a single channel conductance of approximately 12 pS. Opening of the phosphorylated channels requires hydrolysable nucleoside triphosphate, indicating that phosphorylation by PKA is necessary, but not sufficient, for channel activation. The properties of these PKA-regulated cardiac Cl- channels are very similar, if not identical, to those of the cystic fibrosis transmembrane conductance regulator (CFTR), the epithelial cell Cl- channel whose regulation is defective in patients with cystic fibrosis. The full cardiological impact of these Cl- channels and of their possible malfunction in patients with cystic fibrosis remains to be determined.

Modulation of beta-adrenergic responses of chloride and calcium currents by external cations in guinea-pig ventricular cells

1. The catecholamine-induced Cl- current and the Ca2+ current were recorded in the single ventricular cells of guinea-pig hearts, using the whole-cell patch clamp technique combined with internal perfusion. Dependence of the beta-adrenergic responses on external monovalent cations was investigated. The Cl- current was recognized by measuring the reversal potential of the agonist-induced current. 2. The amplitude of the Cl- current, activated by 1 microM adrenaline or 0.01-0.1 microM isoprenaline, was decreased when the external Na+ concentration ([Na+]o) was reduced by replacement with Tris+. The conductance of the catecholamine-induced Cl- current was proportional to the logarithm of the [Na+]o over a range of 15-140 mM. When the conductance was plotted against the concentration of Tris+, a dose-dependent inhibition of the Cl- response by Tris+ was suggested with a half-maximum concentration of 95 mM. 3. The inhibitory effect of the Na+ substitute TEA+ on the Cl- current was not affected by either increasing the buffer for the internal Ca2+ (10 mM BAPTA) or for the pH (50 mM HEPES). 4. In the relationship between agonist concentration and the Cl- conductance, the half-maximum concentration (K1/2) of isoprenaline was 0.013 microM in the control Na+ solution, and was shifted to 0.07, 0.08, 0.1 and 0.3 microM in the Li+, Cs+, TEA+ and Tris+ external solutions, respectively. The maximum slope conductance was not significantly affected, except for a slight depression on the Tris+ solution. When the current was induced by adrenaline, qualitatively the same finding was obtained; K1/2 was 0.15 and 3.2 microM in the Na+ and Tris+ solutions, respectively. 5. As a substitute for the external Na+, sucrose seemed to be inert. The activation of the inward Cl- current was conserved in the 300 mM sucrose solution ([Cl-]o = 8 mM) with a K1/2 value of 0.015 microM isoprenaline. 6. The Cl- current, when activated by either an external application of forskolin (0.2-10 microM) or an internal perfusion of cyclic AMP (100-500 microM), was not affected by replacing external Na+ with other cations. Activation of the Cl- current by 0.2-5 microM histamine was also insensitive to a substitution of Na+. These findings indicate that the inhibition by the Na+ substitute is at a point before the activation of GTP-binding protein. 7. The effects of Na+ substitution were not affected by varying the Na+ concentration (0-115 mM) in the internal solution, excluding an involvement of a change in the [Na+]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Classifying antiarrhythmic actions: by facts or speculation

Classification of antiarrhythmic actions is reviewed in the context of the results of the Cardiac Arrhythmia Suppression Trials, CAST 1 and 2. Six criticisms of the classification recently published (The Sicilian Gambit) are discussed in detail. The alternative classification, when stripped of speculative elements, is shown to be similar to the original classification. Claims that the classification failed to predict the efficacy of antiarrhythmic drugs for the selection of appropriate therapy have been tested by an example. The antiarrhythmic actions of cibenzoline were classified in 1980. A detailed review of confirmatory experiments and clinical trials during the past decade shows that predictions made at the time agree with subsequent results. Classification of the effects drugs actually have on functioning cardiac tissues provides a rational basis for finding the preferred treatment for a particular arrhythmia in accordance with the diagnosis.

Expression of cystic fibrosis transmembrane regulator Cl- channels in heart

Cyclic AMP (cAMP)-dependent chloride channels modulate changes in resting membrane potential and action potential duration in response to autonomic stimulation in heart. A growing body of evidence suggests that there are marked similarities in the properties of the cAMP-dependent chloride channels in heart and cystic fibrosis transmembrane regulator (CFTR) chloride channels found in airway epithelia or in cells expressing the CFTR gene product. We isolated poly A+ mRNA from rabbit ventricle and converted it to cDNA for amplification using the polymerase chain reaction (PCR). A fragment corresponding to the nucleotide-binding domain 1 (NBD1) of the CFTR transcript was cloned. Comparison of the amino acid sequence of NBD1 of human CFTR with the deduced sequence of the rabbit heart PCR product indicated 98% identity. Northern blot analysis, using the heart amplification product as a cDNA probe, demonstrated expression of homologous transcripts in human atrium, guinea pig and rabbit ventricle, and dog pancreas. Xenopus oocytes injected with poly A+ mRNA extracted from rabbit and guinea pig ventricle or dog pancreas expressed robust time-independent chloride currents in response to an elevation of cAMP. We conclude that CFTR chloride channels are expressed in heart and are responsible for the observed cAMP-dependent chloride conductance.

Stretch-activated anion currents of rabbit cardiac myocytes

1. Stretch-activated anion currents were studied in sino-atrial and atrial cells using the whole-cell patch clamp technique. With continuous application of positive pressure (5-15 cmH2O) through the patch clamp electrode, the cell was inflated and the membrane conductance was increased. 2. Voltage clamp steps revealed that the stretch-activated currents had time-independent characteristics. The increase in membrane conductance was reversible on subsequent application of negative pressure to the electrode. 3. The reversal potential of the stretch-activated currents was shifted by 60 mV for a 10-fold change in intracellular Cl- concentration, while it was unaffected by replacement of Na+ in the extracellular solution by N-methyl-D-glucamine. Cell superfusion with Cl(-)-deficient solution (10 mM Cl-) reduced the amplitude of outward current. These findings indicate that the stretch-activated conductance is Cl- selective. 4. The sequence of anion permeability through the stretch-activated conductance was determined to be I-(1.7) > NO3-(1.5) > Br-(1.2) > Cl-(1.0) > and F-(0.6). SCN- appeared to be more permeant than I-. 5. The stretch-activated conductance was reduced by the Cl- channel blockers, 4,4'-dinitrostilbene-2,2'-disulphonic acid disodium salt, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid or anthracene-9-carboxylate (9-AC). Administration of furosemide or bumetanide had no effect. 6. The stretch-activated Cl- current was recorded even though intracellular Ca2+ ions were chelated by including 10 mM EGTA in the pipette solution. Neither the specific peptide inhibitor of cyclic AMP-dependent protein kinase (50 microM), nor the non-selective blocker of protein kinases, H-7 (20 microM), was effective in reducing the stretch-activated Cl- current, suggesting that the stretch-activated Cl- current is a novel type of cardiac Cl- current, which shows a different modulatory mechanism from that of other cardiac Cl- currents.

Arrhythmogenic effects of isoproterenol-activated Cl- current in guinea-pig ventricular myocytes

Possible arrhythmogenic effects of the isoproterenol-activated Cl- current were examined in isolated guinea-pig ventricular myocytes under various intra- and extracellular Cl- concentrations. Experiments were carried out with external K+ concentration ([K+]o) decreased to 2 or 3 mM. Under symmetrical concentrations of Cl- in intra- and extra-cellular solutions (ECl = 0 mV), 1 microM isoproterenol (ISP) depolarized resting membrane potential (RMP) by 6.2 +/- 1.1 mV and slowed repolarization with induction of early afterdepolarizations (EADs) in 9 out of 9 cells. EADs appeared at voltages positive to -40 mV, where L-type Ca2+ current is assumed to be activated. When Cl- concentrations were settled near physiological conditions (ECl = -40 - -50 mV), ISP depolarized RMP by 2.8 +/- 0.4 mV and elicited abnormal repolarization with occasional EADs in 6 out of 19 cells. When ECl was set to -80 mV, however, ISP depolarized RMP by only 0.5 +/- 0.5 mV without induction of abnormal activities. Thus, depolarizing effects of ISP and incidence of repolarization abnormalities including EADs were increased as ECl shifted to more positive potential levels. At [K+]o = 4 mM, no abnormal activities were observed when ECl was around -50 mV (0/8), and 6 out of 6 cells showed abnormal activities when ECl was set to 0 mV. ISP-elicited abnormal activities were abolished by 1 mM DNDS (4,4'-dinitrostilbene-2,2'-disulphonic acid), a blocker for Cl- channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of K+ channel blockers on the action potential of hypoxic rabbit myocardium

1. In order to assess the role of different ionic currents in hypoxia-induced action potential shortening, we investigated the effects of blockers of voltage-dependent and ATP-sensitive K(+)-channel on the membrane potential of hypoxic rabbit hearts and papillary muscles. The response to blocking of the inward rectifier was studied at three external K+ concentration: 2.5, 5, and 7.5 mM. 2. Hypoxia produced a progressive decline in action potential duration (APD) that levelled off after 15 to 20 min. Steady state APD values at 25% and 95% repolarization (APD25 and APD95) were 26.0 +/- 1.9% and 42.2 +/- 2.4% of controls respectively. 3. Tetraethylammonium (TEA, 10 mM) delayed but did not reduce APD shortening at the steady state. 4. Blocking of IK1 with a mixture of 0.2 mM Ba2+ and 4 mM Cs+ lengthened APD in normoxia and prevented APD95 shortening in hypoxia. The APD25 shortening was significantly attenuated at all [K]o. 5. Glibenclamide (Glib, 30 microM) did not prevent APD shortening, but produced a progressive action potential (AP) lengthening after 15 min of hypoxia. Steady levels of 48 +/- 3.5% and 62 +/- 5.0% of controls for APD25 and APD95 respectively were reached after 45 min. 6. The relation between APD25 and pacing rate was determined in normoxic and hypoxic papillary muscles and the effects of 2 mM 4-aminopyridine (4-AP) were examined. Hypoxia attenuated the APD25 shortening currently observed when the stimulation rate was lowered from 1 to 0.1 Hz without altering the plateau reduction occurring at frequencies above 2 Hz. These effects were potentiated by 4-AP.7. Our data suggest that the accelerated AP repolarization in hypoxic rabbit myocardium represents a delicate balance of several outward currents: IKI, IK-ATP. and at least one yet unidentified current component rather insensitive to changes in [K]o and to K+ channel blockers.