© 1993 by European Society of Cardiology
Copyright © 1993, European Society of Cardiology
Parasympathetic control of cycle length dependence of endocardial ventricular repolarisation in the intact feline heart during steady state conditions
Universitá degli Studi di Milano, Via Francesco Sforza 35, 20122 Milan, Italy - Istituto di Clinica Medica II: G Malfatto; Dipartimento di Fisiologia e Biochimica Generali: A Zaza; Dipartimento di Medicina, Universitá degli Studi di Pavia, Italy: P J Schwartz.
Correspondence to Dr Malfatto.
Objective: Parasympathetic modulation of the rate dependence of ventricular repolarisation in vivo during steady state conditions was investigated by analysing the effects of removing vagal activity in three groups of anaesthetised cats. Methods: Bilateral vagotomy was performed while different levels of background sympathetic tone were present in the control periods of each group: in group 1 (n=6), cardiac sympathetic nerves were intact; in group 2 (n=7), bilateral stellate ganglionectomy was performed; and in group 3 (n=7), β adrenergic blockade (propranolol, 0.5 mg·kg–1) was performed after stellate ganglionectomy. The duration of a left ventricular endocardial monophasic action potential (APD) was measured during atrial pacing at 7-10 cycle lengths (CL). The APD/CL relation was fitted to a hyperbolic function: APD=CL/[(axCL)+b]. Two parameters were considered: APDmax (1/a, ie, APD extrapolated at infinite cycle length, a rate-independent measure of APD) and CL50 (bxAPDmax, ie, the cycle length at which 50% of APDmax is reached). Results: In control conditions, APDmax and CL50 were longer in groups 2 and 3, when cardiac sympathetic effects were reduced or absent. Vagotomy reduced APDmax and CL50 similarly in groups 1 and 2 (APDmax, –24% and –18%; CL50, –36% and –27%) (p < 0.05 v control). In group 3, vagotomy did not affect APDmax and CL50. Conclusions: No direct parasympathetic influence on the rate dependence of endocardial ventricular repolarisation was observed. The vagal modulation of sympathetic effects may take place either through vago-sympathetic reflexes or via the antagonism of circulating catecholamines distal to the β receptor.
Cardiovascular Research 1993;27:823-827
KEYWORDS ventricular repolarisation; monophasic action potentials; autonomic nervous system; sympathetic-parasympathetic interaction; muscarinic stimulation
We are grateful to Ms Pinuccia De Toniasi for her care in the preparation of the manuscript.
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