13th International Congress
THE "NEW FRONTIERS"
OF ARRHYTHMIAS 1998

January 24-31, 1998
Marilleva, Trento, Italy

RT-52

Arrhythmogenic actions of norepinephrine and acetylcholine in sheep cardiac purkinje fibers

Ennio Aceto, Mario Vassalle.
Department of Physiology, State University of New York, New York, USA

This work was supported by a grant from the Am Heart Association, NY Affiliate.

Abstract

Background and methods. The actions and interactions of norepinephrine (NE) and acetylcholine (ACh) on spontaneous activity and on drive patterns were studied in sheep Purkinje fibers by means of a microelectrode technique.

Results. In quiescent fibers, NE decreased the resting potential and increased the size of diastolic depolarization (DD). NE increased the hyperpolarization during fast drives and induced spontaneous activity and overdrive excitation. In quiescent fibers, ACh transiently decreased the resting potential and could induce spontaneous activity which was accelerated by drive. In ACh, fast drives hyperpolarized as much as (or less than) in its absence, were followed by an overshoot of DD and by overdrive excitation. ACh increased DD size and induced the voltage oscillations Th-Vos. In driven Purkinje fibers, ACh decreased the maximum diastolic potential (MDP) and increased force; NE increased slightly MDP and increased force more than ACh. Combined NE and ACh rapidly decreased the resting potential, initiated discharge which was not suppressed by overdrive, decreased MDP (as in ACh alone), and increased markedly force (as with NE alone). In low [Ca²⁺]_o, DD was smaller, NE quickly induced spontaneous activity through a more negative threshold, and ACh induced overdrive excitation through Th-V_os. In high [Ca²⁺]_o, overdrive increased hyperpolarization and force more than in Tyrode and NE exaggerated these effects. In high [Ca²⁺]_o, ACh decreased drive-induced hyperpolarization, increased force and induced spontaneous activity.

Conclusions. In Purkinje fibers, both NE and ACh depolarize the resting membrane, can induce spontaneous discharge and overdrive excitation. NE steepens DD and shifts the threshold to more negative values; ACh increases DD size so that oscillatory potentials may attain the threshold and potentiates NE effects.

Key Words

Norepinephrine, acetylcholine 
Purkinje fibers, arrhythmogenic actions, norepinephrine-acetylcholine interaction, OA