Antiarrhythmic drugs and the modulation of autonomic control of heart rate in rabbits. 
A model of the components of autonomic control of heart rate was developed and used for the evaluation of quantitative contribution of sympathetic and vagal tone to cardiac function.  In conscious rabbits, sequential inhibition of muscarinic and beta receptors was produced and the relative contributions of vagal and sympathetic tone were characterized.  Based on the model, the magnitude of presynaptic interaction between the vagal and sympathetic nerve endings was evaluated.  From data in the literature, similar analysis of the control of heart rate was performed for the rat, dog, and human subject and compared with that of the rabbit.  The results show that the resting rabbit heart is under less vagal tone than sympathetic tone as compared with other species.  The effects of acute administration of amiodarone on the sympathetic and parasympathetic control of heart rate as well as intrinsic heart rate were investigated.  Amiodarone decreased the heart rate, which resulted from a direct effect on the sinoatrial (SA) node.  In addition, it attenuated the vagal as well as the sympathetic effects on the SA node.  The effect on vagal component was greater.  Further, the effects of other antiarrhythmic drugs on the electrocardiographic PP and PR intervals were studied.  The usefulness of this model for the analysis of the effects of antiarrhythmic drugs is presented.