Experimental QT Interval Prolongation" GABRIELLA MALFPLI"TO,' SUSAN F. STEINBERG, TOVE S. ROSEN, PETER DANILO, JR., AND MICHAEL R. ROSEN Departments of Pbarmucob#, Pedialrics and Medicine Columbia University Cbllge of Phyricians and Suvgeons New Ymk, New Ymk 10032
INTRODUCTION The hypotheses formulated to explain the congenital long QT syndrome and/or intrinsic abnorinvoke abnormalities of sympathetic malities in the m y ~ c a r d i u m In . ~ either case, the trigger for malignant arrhythmias would be a sudden increase in sympathetic With respect to the former hypothesis, an imbalance of innervation has been p r o p o ~ e d l - ~ such that the right sympathetic innervation would be incomplete or hnctionally abnormal, leading to a relative dominance of the activity of left-sided cardiac nerves. Previous experiments on animal models have shown that the creation of sympathetic imbalance, by removing the right stellate ganglion or by stimulating the left, not only prolongs the QT interval but induces both arrhythmias and characteristic T wave change^.^.^ Clinically, it has been documented that left thoracic sympathectomy in patients with the congenital long Ql- syndrome is highly protective against the recurrence of lethal arrhythmias.6 Of interest is the fact that in many instances where either left stellectomy or therapy with 0-adrenergic blockers is protective against arrhythmias, the interval shortens but remains prolonged.6 Thus, prolongation may be an abnormality that, while not arrhythmogenic in its own right, may predispose to the possibility of malignant arrhythmias, provided that an appropriate trigger, such as sympathetic hyperactivity, exist^.^.^ In this paper we review some studies previously reported by of a rat model of Ql- prolongation. The model involved the modulation of sympathetic neural development using nerve growth factor or its antiserum. The inhibition of nerve growth in the neonatal rat was associated with the inducUSPHS-NHLBI Grants HL-28958 and HL-38976. Present address and address for correspondence: Centro di Fisiologia Clinica e Ipertensione, Clinica Medica 11, Universiti drgli Studi di Milano; via Francesco Sforza, 35; 20122 Milano, Italy. a These studies were supported by
74
MALFATTO et al.: EXPERIMENTAL QT INTERVAL PROLONGATION
75
tion of a prolonged QT interval on the ECG. Also seen were cellular electrophysiologc evidence of enhanced automaticity and alterations in the receptoreffector coupling pathways of a-,but not 0-adreneqc, receptors.
METHODS Neonatal rats were injected subcutaneously with either nerve growth factor (NGF, 1.0 &day on days 1-5 and 1.5 pg/day on days 6-10), the antiserum to NGF (Ab, 10 pl/day, days 1-5, and 15 $/day, days 6-10) or placebo (saline). The final volume injected was 100 PI. Dosages were based on those reported by Levi Montalcini et al.'O On days 1 and 10, 5 minute ECG rhythm strips were recorded at a paper speed of 100 mm/sec. O n day 11, after C02 narcosis, the rats were decapitated, their hearts were removed via a sternotomy, and the following procedures were performed.
Ebctrophywh& Studies The interventricular septa were dissected free and placed in a tissue bath p e h s e d with a modified Tyrode's solution containing (mM): NaCl 131; KC1 2.7; CaCl 2.7; N a H C 0 3 18; NaH2P04 1.8; MgC12 0.5; dextrose 5.5. The Tyrode's was gassed with 95% 02-5% C02 and maintained at 37OC. Standard techniques were used to record electrical activity from the septa, using bipolar silver electrodes. l2 The preparations were allowed to beat spontaneously. Following attainment of a stable automatic rhythm (variability ~ early s t u l e s demonstrated that the creation of an imbalance in cardiac sympathetic innervation, obtained with right stellectomy, significantly reduced the ventricular fibrillation threshold,23 whereas lei+ stellectomy had the opposite effect. These data were obtained in adult animals, but the f i n l n g has been recently confirmed also in 3-4-week-old puppies.24 New experimental models often provides new viewpoints, and while bringing some answers, elicit new questions. This rat model of altered cardiac sympathetic innervation offers the possibility to investigate the links between sympathetic innervation, the modulation of cardiac electrophysiological prop-
82
ANNALS NEW YORK ACADEMY OF SCLENCES
erties and the occurrence of malignant arrhythmias in the long Qrsyndrome. These exciting issues are the subject of several current studies. REFERENCES 1 . SCHWARTZ, P. J . , M. PERITI& A. MALLIANI. 1975. The long QT syndrome. Am. Heart J. 89: 378-384. P. J. 1985. The idiopathic long QT syndrome: Progress and ques2. SCHWARTZ, tions. Am. Heart J. 109: 399-411. 3. SCHWARTZ,P. J., E. LOCATI,S. G. PR~ORI & A. ZAZA. 1990. The idiopathic long QT syndrome. In Cardiac Electrophysiology. From Cell to Bedside. D. P. Zipes & J. Jalife, Eds.: 589-605. W. B. Saunders Co. Philadelphia, PA. 4. SCHWARTZ, P. J. & A. MALLIANI. 1975. Electrical alternation of the T wave. Clinical and experimental evidence of its relationship with the sympathetic nervous system and with the long QT syndrome. Am. Heart J. 89: 45-50. 5. SCHWARTZ, P. J. 1984. Sympathetic imbalance and cardiac arrhythmias. In Nervous Control of Cardiovascular Function. W. C. Randall, Ed.: 225-251. Oxford University Press. New York. 6. SCHWARTZ, P. J., E. LOCATI,A. J. Moss, R S. CRAMPTON,R TRAZZI& U. RUBERTI.1991. Left cardiac sympathetic denervation in the therapy of the congenital long syndrome: A world-wide report. Circulation 84: 503-51 1. 7. MALFAITO,G., T. S. &SEN, S. F. STEINBERG, P. C . URSELL, L. S. SUN, S. DANIEL,P. DANILO,J R . & M. R ROSEN.1990. Sympathetic neural modulation of cardiac impulse initiation and repolarization in the newborn rat. Circ. Res. 66: 427-437. G., L. S. SUN & T. S. ROSEN. 1990. Bradycardia and long QT 8. MALE’ATTO, interval in neonate rats with delayed cardiac sympathetic innervation. J. Autonom. New. Syst. 30: SlOl-Sl02. 9. STEINBERG, S. F., T. S. ROSEN, G. MALFATTO 8c M. R ROSEN. 1991. Beta adrenergic modulation of cardiac rhythm iri a rat model of altered sympathetic neural development. J. Mol. Ccll Cadiol. 23(Suppl. I): 47-52. R & R . BOOKER. 1960. Excessive growth o f t h e sympathetic 10. LEVIMONTALCINI, ganglia evoked by a protein isdated from mouse salivary gland. Proc. Natl. Acad. Sci. USA 46: 373-384. 1 1 . LEVIMONTALCINI, R & R BOOKER.1960. Destruction of sympathetic ganglia in mammals by an antiserum to a nervc growth protein. Proc. Natl. Acad. Sci. USA 46:384-391. E. D., M. R SEN & R B. ROBINSON.1985. Neuronal regulation 12. DRUGGE, of the development of the alpha-adrcnergic chronotropic response in the rat heart. Circ. Res. 57: 415-423. G . 1949. The attraction of proteins for small molecules and ions. 13. SCATCHARD, Ann. N.Y. Acad. Sci. 51: 660-672. 14. ROSEN,M. R , S. F. STEINBERG, Y. K. CHOW,J. P. BILEZIKIAN & P. DANILO, JR. 1988. The role of a pcrtussis toxin sensitive protein in the modulation of canine Purkinje fiber automaticity. Circ. Res. 62: 315-323. 15. ZAR,J. H . 1983. Biostatistical Analysis. Prentice Hall Publishing Co. Englewood Cliffs, NJ. 16. WALLENSTEIN, S., C . L. ZUCKER & J . L. FLEISS.1980. Some statistical methods used in Cc~ulatirmReseavch. Circ. Res. 47: 1-9.
MALFA?TO et al. : EXPERIMENTAL QT INTERVAL PROLONGATION
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17. GILMAN, A. G. 1984. Guanine nucleotide-binding proteins and dual control of adenylate cyclase. J. Clin. Invest. 73: 1-4. 18. HAN, H . M., R B. ROBINSON,J. P. BILEZIKIAN & S. F. STEINBERG. 1989. Developmental changes in guanine nuclrotide regulatory proteins in the rat myocardial alpha-1 adrenergic receptor complex. Circ. Res. 65: 1763-1773. 19. ZAZA,A,, R B. KLINE& M . R ROSEN. 1990. Effects ofalpha-adrenergic stimulation o n intracellular sodium activity and automaticity in canine Purkinje fibers. Circ. Rcs. 66: 416-426. S. F., E. D . DRUGGE,J. P. BILEZIKIAN & R B. ROBINSON. 1985. 20. STEINBERG, Acquisition by innervated cardiac myocites of a pertussis-toxin specific regulatory protein linked to the alpha-1 receptor. Science 230: 186-188. 21. STEINBERG, S. F., L. M. KAPLAN,T. INOUYE, J . F. ZHANG& R B. ROBINSON. 1989. Alpha-1 adrenergic stimulation of 1,4,5-inositol triphosphate formation in ventricular myocytes. J. Pharmacol. Exp. Therap. 250: 1141-1148. 22. SUN, L. S., T . S. ROSEN,M . J . LEGATO & M . R. ROSEN. 1991. Sympathetic innervation modulates conduction and the alpha-1 adrenergic effect of repolarization in the rat heart. FASEB J. 5: A1497. P. J., N . G . SNEBOLD & A. M. BROWN.1976. Efects of unilateral 23. SCHWARTZ, cardiac sympathetic denervation o n the ventricular fibrillation threshold. Am. J. Cardiol. 37: 1034-1041. 24. STRAMBA-BADIALE, M., M. LAZZAROTTI & P. J . SCHWARTZ. 1990. Postnatal development of cardiac innervation and susceptibility to malignant arrhythmias in the dog. J. Autonom. New. Syst. 30: S153-Sl54.
INTRODUCTION The hypotheses formulated to explain the congenital long QT syndrome and/or intrinsic abnorinvoke abnormalities of sympathetic malities in the m y ~ c a r d i u m In . ~ either case, the trigger for malignant arrhythmias would be a sudden increase in sympathetic With respect to the former hypothesis, an imbalance of innervation has been p r o p o ~ e d l - ~ such that the right sympathetic innervation would be incomplete or hnctionally abnormal, leading to a relative dominance of the activity of left-sided cardiac nerves. Previous experiments on animal models have shown that the creation of sympathetic imbalance, by removing the right stellate ganglion or by stimulating the left, not only prolongs the QT interval but induces both arrhythmias and characteristic T wave change^.^.^ Clinically, it has been documented that left thoracic sympathectomy in patients with the congenital long Ql- syndrome is highly protective against the recurrence of lethal arrhythmias.6 Of interest is the fact that in many instances where either left stellectomy or therapy with 0-adrenergic blockers is protective against arrhythmias, the interval shortens but remains prolonged.6 Thus, prolongation may be an abnormality that, while not arrhythmogenic in its own right, may predispose to the possibility of malignant arrhythmias, provided that an appropriate trigger, such as sympathetic hyperactivity, exist^.^.^ In this paper we review some studies previously reported by of a rat model of Ql- prolongation. The model involved the modulation of sympathetic neural development using nerve growth factor or its antiserum. The inhibition of nerve growth in the neonatal rat was associated with the inducUSPHS-NHLBI Grants HL-28958 and HL-38976. Present address and address for correspondence: Centro di Fisiologia Clinica e Ipertensione, Clinica Medica 11, Universiti drgli Studi di Milano; via Francesco Sforza, 35; 20122 Milano, Italy. a These studies were supported by
74
MALFATTO et al.: EXPERIMENTAL QT INTERVAL PROLONGATION
75
tion of a prolonged QT interval on the ECG. Also seen were cellular electrophysiologc evidence of enhanced automaticity and alterations in the receptoreffector coupling pathways of a-,but not 0-adreneqc, receptors.
METHODS Neonatal rats were injected subcutaneously with either nerve growth factor (NGF, 1.0 &day on days 1-5 and 1.5 pg/day on days 6-10), the antiserum to NGF (Ab, 10 pl/day, days 1-5, and 15 $/day, days 6-10) or placebo (saline). The final volume injected was 100 PI. Dosages were based on those reported by Levi Montalcini et al.'O On days 1 and 10, 5 minute ECG rhythm strips were recorded at a paper speed of 100 mm/sec. O n day 11, after C02 narcosis, the rats were decapitated, their hearts were removed via a sternotomy, and the following procedures were performed.
Ebctrophywh& Studies The interventricular septa were dissected free and placed in a tissue bath p e h s e d with a modified Tyrode's solution containing (mM): NaCl 131; KC1 2.7; CaCl 2.7; N a H C 0 3 18; NaH2P04 1.8; MgC12 0.5; dextrose 5.5. The Tyrode's was gassed with 95% 02-5% C02 and maintained at 37OC. Standard techniques were used to record electrical activity from the septa, using bipolar silver electrodes. l2 The preparations were allowed to beat spontaneously. Following attainment of a stable automatic rhythm (variability ~ early s t u l e s demonstrated that the creation of an imbalance in cardiac sympathetic innervation, obtained with right stellectomy, significantly reduced the ventricular fibrillation threshold,23 whereas lei+ stellectomy had the opposite effect. These data were obtained in adult animals, but the f i n l n g has been recently confirmed also in 3-4-week-old puppies.24 New experimental models often provides new viewpoints, and while bringing some answers, elicit new questions. This rat model of altered cardiac sympathetic innervation offers the possibility to investigate the links between sympathetic innervation, the modulation of cardiac electrophysiological prop-
82
ANNALS NEW YORK ACADEMY OF SCLENCES
erties and the occurrence of malignant arrhythmias in the long Qrsyndrome. These exciting issues are the subject of several current studies. REFERENCES 1 . SCHWARTZ, P. J . , M. PERITI& A. MALLIANI. 1975. The long QT syndrome. Am. Heart J. 89: 378-384. P. J. 1985. The idiopathic long QT syndrome: Progress and ques2. SCHWARTZ, tions. Am. Heart J. 109: 399-411. 3. SCHWARTZ,P. J., E. LOCATI,S. G. PR~ORI & A. ZAZA. 1990. The idiopathic long QT syndrome. In Cardiac Electrophysiology. From Cell to Bedside. D. P. Zipes & J. Jalife, Eds.: 589-605. W. B. Saunders Co. Philadelphia, PA. 4. SCHWARTZ, P. J. & A. MALLIANI. 1975. Electrical alternation of the T wave. Clinical and experimental evidence of its relationship with the sympathetic nervous system and with the long QT syndrome. Am. Heart J. 89: 45-50. 5. SCHWARTZ, P. J. 1984. Sympathetic imbalance and cardiac arrhythmias. In Nervous Control of Cardiovascular Function. W. C. Randall, Ed.: 225-251. Oxford University Press. New York. 6. SCHWARTZ, P. J., E. LOCATI,A. J. Moss, R S. CRAMPTON,R TRAZZI& U. RUBERTI.1991. Left cardiac sympathetic denervation in the therapy of the congenital long syndrome: A world-wide report. Circulation 84: 503-51 1. 7. MALFAITO,G., T. S. &SEN, S. F. STEINBERG, P. C . URSELL, L. S. SUN, S. DANIEL,P. DANILO,J R . & M. R ROSEN.1990. Sympathetic neural modulation of cardiac impulse initiation and repolarization in the newborn rat. Circ. Res. 66: 427-437. G., L. S. SUN & T. S. ROSEN. 1990. Bradycardia and long QT 8. MALE’ATTO, interval in neonate rats with delayed cardiac sympathetic innervation. J. Autonom. New. Syst. 30: SlOl-Sl02. 9. STEINBERG, S. F., T. S. ROSEN, G. MALFATTO 8c M. R ROSEN. 1991. Beta adrenergic modulation of cardiac rhythm iri a rat model of altered sympathetic neural development. J. Mol. Ccll Cadiol. 23(Suppl. I): 47-52. R & R . BOOKER. 1960. Excessive growth o f t h e sympathetic 10. LEVIMONTALCINI, ganglia evoked by a protein isdated from mouse salivary gland. Proc. Natl. Acad. Sci. USA 46: 373-384. 1 1 . LEVIMONTALCINI, R & R BOOKER.1960. Destruction of sympathetic ganglia in mammals by an antiserum to a nervc growth protein. Proc. Natl. Acad. Sci. USA 46:384-391. E. D., M. R SEN & R B. ROBINSON.1985. Neuronal regulation 12. DRUGGE, of the development of the alpha-adrcnergic chronotropic response in the rat heart. Circ. Res. 57: 415-423. G . 1949. The attraction of proteins for small molecules and ions. 13. SCATCHARD, Ann. N.Y. Acad. Sci. 51: 660-672. 14. ROSEN,M. R , S. F. STEINBERG, Y. K. CHOW,J. P. BILEZIKIAN & P. DANILO, JR. 1988. The role of a pcrtussis toxin sensitive protein in the modulation of canine Purkinje fiber automaticity. Circ. Res. 62: 315-323. 15. ZAR,J. H . 1983. Biostatistical Analysis. Prentice Hall Publishing Co. Englewood Cliffs, NJ. 16. WALLENSTEIN, S., C . L. ZUCKER & J . L. FLEISS.1980. Some statistical methods used in Cc~ulatirmReseavch. Circ. Res. 47: 1-9.
MALFA?TO et al. : EXPERIMENTAL QT INTERVAL PROLONGATION
83
17. GILMAN, A. G. 1984. Guanine nucleotide-binding proteins and dual control of adenylate cyclase. J. Clin. Invest. 73: 1-4. 18. HAN, H . M., R B. ROBINSON,J. P. BILEZIKIAN & S. F. STEINBERG. 1989. Developmental changes in guanine nuclrotide regulatory proteins in the rat myocardial alpha-1 adrenergic receptor complex. Circ. Res. 65: 1763-1773. 19. ZAZA,A,, R B. KLINE& M . R ROSEN. 1990. Effects ofalpha-adrenergic stimulation o n intracellular sodium activity and automaticity in canine Purkinje fibers. Circ. Rcs. 66: 416-426. S. F., E. D . DRUGGE,J. P. BILEZIKIAN & R B. ROBINSON. 1985. 20. STEINBERG, Acquisition by innervated cardiac myocites of a pertussis-toxin specific regulatory protein linked to the alpha-1 receptor. Science 230: 186-188. 21. STEINBERG, S. F., L. M. KAPLAN,T. INOUYE, J . F. ZHANG& R B. ROBINSON. 1989. Alpha-1 adrenergic stimulation of 1,4,5-inositol triphosphate formation in ventricular myocytes. J. Pharmacol. Exp. Therap. 250: 1141-1148. 22. SUN, L. S., T . S. ROSEN,M . J . LEGATO & M . R. ROSEN. 1991. Sympathetic innervation modulates conduction and the alpha-1 adrenergic effect of repolarization in the rat heart. FASEB J. 5: A1497. P. J., N . G . SNEBOLD & A. M. BROWN.1976. Efects of unilateral 23. SCHWARTZ, cardiac sympathetic denervation o n the ventricular fibrillation threshold. Am. J. Cardiol. 37: 1034-1041. 24. STRAMBA-BADIALE, M., M. LAZZAROTTI & P. J . SCHWARTZ. 1990. Postnatal development of cardiac innervation and susceptibility to malignant arrhythmias in the dog. J. Autonom. New. Syst. 30: S153-Sl54.
