003 1-39981911290 1-0 104$03.00/0 PEDIATRIC RESEARCH Vol. 29, No. 1, 1991 Copyright O 1990 International Pediatric Research Foundation, Inc. Prinled in (I.S. A. Developmental Cellular Electrophysiologic Effects of d-Sotalol on Canine Cardiac Purkinje Fibers JEFFREY P. MOAK The Lillie Frank Abercrombie Section of Cardiology, Department of Pediatrics, Baylor College of Medicine and Texas Children S Hospital, Houston, Texas 77030 ABSTRACT. d-Sotalol may be a clinically useful class I11 high therapeutic success rate in the treatment of ventricular antiarrhythmic agent for controlling ventricular arrhyth- arrhythmias with antiarrhythmic drugs that significantly prolong mias in children. Because age-related differences in repo- reoolarization. class 111 effect. has broadened clinical interest in larization currents may contribute to developmental differ- thkir use (2, 3). The electrophysiologic effects of class 111 aitiar- ences in response to antiarrhythmic agents that primarily rhythmic agents have been studied in mature animals and adult affect repolarization, the electrophysiologic effects of d- humans (4-7), but little is understood of their effects in younger sotalol were compared in Purkinje fibers from neonatal subjects (8-1 1). Furthermore, important insights into the ionic and adult dogs. Significant age-related changes character- events regulating cardiac repolarization in the immature may be ized the antiarrhythmic profile of d-sotalol. d-Sotalol learned through study of the electrophysiologic effects of class 111 M) significantly prolonged the action potential duration of antiamhythmic agents that inhibit specific potassium currents. adult Purkinje fibers (310 f 8 to 380 f 7 ms, p < 0.01) Selective inhibition of specific potassium currents may under- and neonatal fibers (247 f 5 to 342 f 9 ms, p < 0.01). score their role in the overall repolarization process. However, the lengthening of action potential duration was Previous developmental studies of the cellular electrophysio- significantly greater in the immature age group. d-Sotalol logic effects of antiarrhythmic drugs have observed age-related had no significant effect on maximum diastolic potential, differences in responsiveness. Cardiac tjssues from immature action potential amplitude, or phase zero upstroke velocity animals demonstrate little inhibition of V,,, (an indirect meas- in normally polarized fibers. In contrast, different electro- urement of fast sodium channel activity) after exposure to con- physiologic effects were observed in K+-depolarized Pur- centrations of local anesthetic drugs that depress adult tissues kinje fibers. Superfusion of adult K+-depolarized fibers (12, 13). Similarly, the sensitivity of slow-response action poten- with d-sotalol suppressed excitability in five (38%) of 13 tials to calcium channel blockers is diminished in the neonatal fibers and significantly decreased action potential ampli- age group ( 14, 15). tude (88 f 2 to 83 f 1 mV, p < 0.05) and phase zero Sotalol is a P-adrenergic blocking agent that prolongs action upstroke velocity (180 f 14 to 105 f 3 V/s, p < 0.01) in potential duration and cardiac refractoriness. Its d-isomer has the other eight fibers. The membrane depressant effects little (3-adrenergic blocking activity, except at extremely high observed in the younger age group were significantly less concentrations (16). The purpose of this study was 2-fold: first, [no suppression of excitability and a smaller decrease in to assess age-related differences in the cellular electrophysiologic phase zero upstroke velocity (121 f 22 to 101 f 23 V/s, p effects of d-sotalol, an antiarrhythmic drug that predominantly < 0.05). The magnitude of action potential duration pro- inhibits potassium currents (I 7, 18). Because the electrophysio- longation by d-sotalol in K+-depolarized fibers was less logic effects of many antiarrhythmic drugs are dependent on the than in normally polarized fibers. These results suggest experimental conditions and state of health (level of membrane that at the clinical level the mechanism of d-sotalol's potential) of the tissue studied (19), the second purpose was to antiarrhythmic action will depend not only on the devel- determine whether d-sotalol exerted antiarrhythmic actions in opmental maturity, but also on the resting membrane po- depolarized tissue not recognized in previous electrophysiologic tential of the tissue treated. (Pediatr Res 29: 104-109, studies on healthy myocardium with a normal resting membrane 1991) potential. Abbreviations APA, action potential amplitude MATERIALS AND METHODS APDw, action potential duration to 50% repolarization APD,,, action potential duration to full repolarization Neonatal (0 to 10 d) and adult dogs (I to 5 y, weighing 15 to MDP, maximum diastolic potential 25 kg) were anesthetized with 35 mg/kg sodium pentobarbital, V,.,, phase zero upstroke velocity given i.v. to adults and intraperitoneally to neonates. The heart was quickly removed and placed in cold oxygenated Tyrode's solution. Free-running Purkinje fiber bundles were dissected from the right and left ventricle (disconnected from any muscle Arrhythmia control by lengthening of cardiac repolarization attachments) and placed in a Lucite tissue bath superfused with has emerged as an effective antiarrhythmic mechanism (1). A Tyrode's solution containing (mmol/L): NaCl, 131 ; NaHC03, 18; CaCI,, 2.7; MgC12, 0.5; NaH2P04,1.8; KCI, 4.0, and dextrose, Received April 19, 1990; August 14. 1990. 5.5. The Tyrode's solution was bubbled with 95% 0,-5% CO, Correspondence: Jeffrey P. Moak, M.D., Texas Children's Hospital, Section of k Pediatric Cardiology, 662 1 Fannin, Houston, TX 77030. and warmed to 37"C, (pH 7.33 0.15). Supported in part by the American Heart Association. Texas Affiliate and the Stimuli were delivered with bipolar silver wires that were John A. Hanford Foundation. insulated to the tips with Teflon. The stimulus pulse width was ELECTROPHYSIOLOGY OF d-SOTALOL 105 2 ms and the amplitude was 1.5 to 2.0 times diastolic threshold. repolarization (Fig. 1). d-Sotalol significantly prolonged APDSo The Purkinje fibers were impaled with 3 M KC1-filled glass in both adult and neonatal Purkinje fibers. The effect was similar capillary microelectrodes having tip resistances of 10-30 M for each age group. In the 12 adult fibers studied, d-sotalol 1 x ohms. The Purkinje fibers were paced at a cycle length of 500 M significantly prolonged APDSofrom a control value of rns and allowed to stabilize for 1 h before control measurements 202 + 8 to 229 & 7 ms (p < 0.01). In the 12 neonatal fibers were made. After equilibration, the following transmembrane studied, d-sotalol prolonged APD,, from a control value of 15 1 action potential characteristics were measured from photo- 2 2 to 177 + 11 ms (p<O.OI). graphic recordings: MDP; APA; V,.,, which was differentiated A significant age-related difference in response of full repolar- electronically; plateau height (measured from 0 potential to the ization to d-sotalol was observed (Fig. 1). After exposure to d- peak of the plateau); APD,,; and APDI,. The slopes of phases 2 sotalol l x M, the APDloo of neonatal Purkinje fibers and 3 of repolarization were measured digitally from photo- increased from a control value of 247 + 5 to 342 2 9 ms (p < graphic enlargements of recorded transmembrane action poten- 0.01). Although d-sotalol 1 x lo-' M prolonged the APDlooof tials. Plateau duration was measured from the end of phase I to adult fibers significantly [3 10 f 8 to 380 + 7 ms (p < 0.01)], the the beginning of phase 3. The methods used for measuring degree of action potential prolongation was significantly different transmembrane action potential characteristics, calibrating the than that observed for the neonatal age group (p < 0.01). In the equipment, and recording the action potential data have been neonatal age group, d-sotalol I x M prolonged APDlooby described previously (20). 27%, in contrast to only 18% in the older group of fibers. The cellular electrophysiologic effects of d-sotalol were studied Superfusion of Purkinje fibers with concentrations of d-sotalol in three groups of Purkinje fibers: adult free-running, adult that prolonged repolarization had no significant depressant effect subendocardial, and neonatal free-running fibers. To assess age- on MDP, APA, or V,,, (Table 1). related differences in the electrophysiologic effects of d-sotalol in The response of plateau height, plateau duration, and the slope similar regions of the conduction system, the response of free- of phases 2 and 3 of repolarization to d-sotalol superfusion was running Purkinje fibers from adult and neonatal animals was analyzed to further characterize the age-related differences in compared. Previous microelectrode studies have shown adult repolarization observed. d-Sotalol superfusion had no significant Purkinje fibers to have significantly longer action potential du- effect on plateau height in either age group: adult fibers, 0 + 1 rations, compared to neonatal fibers (14). In an attempt to mV (control) versus -2 + 1 mV (d-sotalol 1 x M); neonatal minimize differences in action potential duration as a variable fibers, 1 1 + 3 mV (control) versus 1 1 + 3 mV (d-sotalol 1 x explaining developmental differences in drug response, compar- M). Figure 2A demonstrates a concentration-dependent prolon- ison of the electrophysiologic effects of d-sotalol superfusion was gation of plateau duration after d-sotalol superfusion in both age made between adult subendocardial Purkinje fibers (shorter ac- groups studied; however, the magnitude of increase in plateau tion potential than adult free-running fibers) and neonatal free- duration was significantly greater for the younger age group (p running Purkinje fibers (2 1). < 0.01). In addition, the threshold concentration for prolonging The Purkinje fibers were superfused with graded concentra- plateau duration in the neonatal fibers was lower (d-sotalol 1 x tions of d-sotalol hydrochloride (Bristol-Myers Squibb, Walling- M). d-Sotalol had no significant effect on the slope of phase ford, CT): 1 x lo-', 1 x 1 x and 1 x M.