Showing papers by "Stanley Nattel published in 1996"

Evidence for Two Components of Delayed Rectifier K+ Current in Human Ventricular Myocytes

Abstract: Previous voltage-clamp studies have suggested that the delayed rectifier current (IK) is small or absent in the human ventricle and, when present, consists only of the rapid component (IKr); however, molecular studies suggest the presence of functionally important IK in the human heart, specific IKr blockers are known to delay ventricular repolarization and cause the long QT syndrome in humans, and we have shown that the expression of IK is strongly influenced by cell isolation techniques. The present experiments were designed to assess the expression of IK in myocytes obtained by arterial perfusion of right ventricular tissue from explanted human hearts. Of 35 cells from three hearts, 33 (94%) showed time-dependent currents typical of IK. The envelope-of-tails test was not satisfied under control conditions but became satisfied in the presence of the benzenesulfonamide E-4031 (5 μmol/L). E-4031 suppressed a portion of IK in 32 of 33 cells, with properties of the drug-sensitive and -resistant components consistent with previous descriptions of IKr and the slow component (IKs), respectively. Action potential duration to 95% repolarization at 1 Hz was prolonged by E-4031 from 336±16 (mean±SEM) to 421±19 ms (n=5, P <.01), indicating a functional role for IK. Indapamide, a diuretic agent previously shown to inhibit IKs selectively, suppressed E-4031–resistant current. The presence of a third type of delayed rectifier, the ultrarapid delayed rectifier current (IKur), was evaluated with the use of depolarizing prepulses and low concentrations (50 μmol/L) of 4-aminopyridine. Although these techniques revealed clear IKur in five of five human atrial cells, no corresponding component was observed in any of five human ventricular myocytes. We conclude that a functionally significant IK, with components corresponding to IKr and IKs, is present in human ventricular cells, whereas IKur appears to be absent. These findings are important for understanding the molecular, physiological, and pharmacological determinants of human ventricular repolarization and arrhythmias.

Protein Kinase C Activates ATP-Sensitive K+ Current in Human and Rabbit Ventricular Myocytes

Abstract: Mediators involved in ischemic preconditioning, such as adenosine and norepinephrine, can activate protein kinase C (PKC), and a variety of observations suggest that both PKC and ATP-sensitive K+ current (IKATP) play essential roles in ischemic preconditioning. PKC is therefore a candidate to link receptor binding to IKATP activation, but it has not been shown whether and how PKC can activate IKATP in the heart. The present study was designed to determine whether PKC can activate IKATP in rabbit and human ventricular myocytes. Under conditions designed to minimize Na+ and Ca2+ currents, dialysis of rabbit ventricular myocytes with pipette solutions containing reduced [ATP] elicited IKATP, with a 50% effective concentration (EC50) of 260 μmol/L. In cells that failed to show IKATP under control conditions, superfusion with 1 μmol/L phorbol 12,13-didecanoate (PDD) elicited IKATP in a fashion that depended on pipette [ATP], with an [ATP] EC50 of 601 μmol/L. PDD-induced IKATP activation was concentration dependent, with an EC50 of 7.1 nmol/L. The highly selective PKC inhibitor bisindolylmaleimide totally prevented IKATP activation by PDD, and in blinded experiments, 1 μmol/L PDD elicited IKATP in eight of nine cells, whereas its non–PKC-stimulating analogue 4α-PDD failed to elicit IKATP in any of the five cells tested ( P =.003). Similar experiments were conducted in human ventricular myocytes and showed that 0.1 μmol/L PDD elicited IKATP at pipette \[ATP] of 100 and 400 μmol/L (five of five cells at each concentration) but not at 1 mmol/L [ATP\] (none of five cells). We conclude that PKC activates IKATP in rabbit and human ventricular myocytes by reducing channel sensitivity to intracellular ATP. This finding has potentially important implications for understanding the mechanisms of ischemic preconditioning.

Transient outward and delayed rectifier currents in canine atrium: properties and role of isolation methods

Abstract: Although the dog is the principal species used for in vivo studies of atrial arrhythmias, little is known about currents governing canine atrial repolarization. Cells were isolated from dog atria b...

Adrenergic Modulation of Ultrarapid Delayed Rectifier K+ Current in Human Atrial Myocytes

Abstract: The ultrarapid delayed rectifier K+ current (IKur) in human atrial cells appears to correspond to Kv1.5 cloned channels and to play an important role in human atrial repolarization. Kv1.5 channels have consensus sites for phosphorylation by protein kinase A and C, suggesting possible modulation by adrenergic stimulation. The present study was designed to assess the adrenergic regulation of IKur in human atrial myocytes. Isoproterenol increased IKur in a concentration-dependent manner, with significant effects at concentrations as low as 10 nmol/L. The effects of isoproterenol were reversible by washout or by the addition of propranolol (1 μmol/L). Isoproterenol’s effects were mimicked by the direct adenylate cyclase stimulator, forskolin, and by the membrane-permeable form of cAMP, 8-bromo cAMP. Isoproterenol had no effect on IKur when the protein kinase A inhibitor peptide, PKI(6-22)amide, was included in the pipette solution; in a separate set of experiments in which isoproterenol alone increas...

Characterization of an ultrarapid delayed rectifier potassium channel involved in canine atrial repolarization.

Abstract: 1. Depolarizing pulses positive to 0 mV elicit a transient outward current (Ito) and a sustained 'pedestal' current in canine atrial myocytes. The pedestal current was highly sensitive to 4-aminopyridine (4-AP) and TEA, with 50% inhibitory concentrations (EC50) of 5.3 +/- 0.7 and 307 +/- 25 microM, respectively. When the pedestal current was separated from Ito with prepulses or by studying current sensitive to 10 mM TEA, it showed very rapid activation and deactivation. We therefore designated the current IKur,d, for 'ultrarapid delayed rectifier, dog'. IKur,d inactivation was bi-exponential, with mean time constants of 609 +/- 91 and 5563 +/- 676 ms during a 20 s pulse to +40 mV. 2. The reversal potential of IKur,d tail currents are dependent on extracellular potassium concentration ([K+]o; slope, 54.7 mV decade-1). The envelope of tails test was satisfied and the current inwardly rectified at > or = +40 mV. The current was insensitive to E-4031, dendrotoxin and chloride substitution, but was inhibited by barium, with an EC50 of 1.65 mM. Lanthanum ions caused a positive shift in voltage dependence without producing direct inhibition. 3. Single-channel activity was observed in cell-attached, inside-out and outside-out patches. Upon depolarization from -50 to +30 mV, single channels had similar time constants and [K+]o dependence to whole-cell current. Channel open probability (Po) increased with depolarization in a saturable fashion and the Po-voltage relation had a half-activation voltage and slope factor similar to whole-cell IKur,d. 4. Unitary channel current was linearly related to depolarization potential to +40 mV; at more positive potentials, inward rectification occurred. The unitary conductance was 20.3 and 35.5 pS for an [K+]o of 5.4 and 130 mM, respectively. Single-channel activity was strongly inhibited by 50 microM 4-AP or 10 mM TEA. Both 4-AP and TEA decreased open time, suggesting open-channel block. 5. Selective inhibition of IKur,d with 50 microM 4-AP or 0.3-5 mM TEA prolonged canine atrial action potentials, indicating that IKur,d contributes to canine atrial repolarization. The single-channel and macroscopic properties of IKur,d have many similarities to those of currents carried by Kv3.1 cloned channels and our findings thus suggest a possible role for Kv3.1 channels in cardiac repolarization.

Regional and functional factors determining induction and maintenance of atrial fibrillation in dogs

Abstract: The present study was designed to determine the factors governing the ability of premature beats at various atrial sites to initiate atrial fibrillation (AF) and the determinants of the duration of AF in dogs at 1-10 Hz. The site of atrial extrasystoles determined their ability to induce AF. Regional differences in AF inducibility were due to differences in local refractoriness. Premature beats initiated AF by blocking in regions of greater refractoriness, causing macroreentrant activation with subsequent disorganization producing fibrillation. The atrial refractory period and wavelength during rapid 1:1 atrial pacing were weak predictors of AF duration (r = 0.24 and 0.23, respectively), which depended strongly on variability in regional refractoriness measured by the standard deviation in local refractory periods (r = 0.80, P < 0.001) and on the heterogeneity of activation during AF (r = 0.74, P < 0.001). Thus, premature beats cause AF by initiating a single macroreentrant cycle that degenerates into multiple wavefronts, regional refractoriness is the primary determinant of AF induction by premature beats, and variability in refractoriness may be an important determinant of the ability of AF to sustain itself.

Transmembrane chloride currents in human atrial myocytes.

Abstract: 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...

Relative Role of Alkalosis and Sodium Ions in Reversal of Class I Antiarrhythmic Drug–Induced Sodium Channel Blockade by Sodium Bicarbonate

Abstract: Background Hypertonic sodium salts are used to treat sodium channel–blocking drug cardiotoxicity. The relative roles of alkalinization and increased sodium concentration ([Na+]o) for various drugs are incompletely known. Methods and Results The effects of four class I drugs on action potential characteristics of canine Purkinje fibers at equi-effective concentrations (disopyramide 30 μmol/L, mexiletine 80 μmol/L, flecainide 7 μmol/L, imipramine 5 μmol/L) were studied in the presence of normal Tyrode solution and one altered solution (increased [Na+]o, increased bicarbonate concentration, or both) in each experiment. Combined increases in sodium and bicarbonate concentration significantly reduced the depressant effects of flecainide, imipramine, and mexiletine on phase 0 upstroke (Vmax) but did not alter the effects of disopyramide. The effects of sodium bicarbonate were entirely due to alkalinization in the case of imipramine, but both alkalinization and increased [Na+]o contributed to the interaction wit...

Properties of sodium and potassium currents of cultured adult human atrial myocytes.

Abstract: Cultured cell systems are valuable for the study of regulation of phenotypic expression, but little is known about the electrophysiological properties of human cardiac tissues in culture. The present studies were designed to determine the feasibility of maintaining human atrial myocytes in primary culture and to assess changes in Na+ (INa) and K+ (Ito, transient outward, and Ikur, ultra-rapid delayed rectifier) currents. Within 24 h of culture, cells assumed an avoid shape, which they maintained for up to 7 days. The voltage dependence, kinetics, and density of INa were unchanged in culture. The activation properties of Ito (kinetics and voltage dependence) were not altered, but Ito density (current normalized to cell capacitance) was reduced and inactivation properties were altered (negative shift in voltage dependence and slowed kinetics) in cultured compared with fresh cells. The absolute current amplitude, kinetics, voltage dependence, and 4-aminopyridine sensitivity of IKur were unchanged, but current density was increased. All changes in ionic currents occurred within 24 h of culture and remained stable for the next 4 days. We conclude that human atrial myocytes can be maintained in primary culture, that the qualitative properties of INa, Ito, and IKur remain constant but that some quantitative changes occur, and that cultured human atrial myocytes may be valuable for studies of the molecular mechanisms and regulation of cardiac channel function in humans.

Demonstration of an Inward Na+‐Ca2+ Exchange Current in Adult Human Atrial Myocytes

Abstract: In the heart, Nat-Ca2+ exchange has been considered to play a major role in extruding Ca2+ and maintaining a low intracellular Ca2+ level.’ Since the exchange ratio is 3Nat : 1Ca2+, Na+-Ca2’ exchange is e le~t rogenic .~ .~ Ca2+ released from the sarcoplasmic reticulum (SR) store is believed to contribute importantly to the electrogenic Na+-Ca2+ exchange current (INa.ca).4 It has been demonstrated that can contribute to action potential duration (APD) in cardiac muscles from a variety of animal however, whether INa-Ca contributes to APD in human cardiac muscles has not been carefully evaluated.’ The goal of the present study is to determine whether an electrogenic INa.Ca is present in adult human atrial myocytes, and if so, whether it contributes to APD.