Reflex vagal withdrawal and the hemodynamic response to intravenous isoproterenol in the presence of beta-antagonists.

To investigate the contribution of reflex vagal tone to the hemodynamic response after intravenous isoproterenol, 12 healthy subjects received isoproterenol by both bolus injection and continuous infusion before and after atropine, and during intravenous infusion of the beta 1-selective antagonist atenolol and the nonselective beta-antagonist, propranolol. With bolus injections, atropine displaced the heart rate dose-response curve for atenolol to the right, implying reflex withdrawal of cardiac vagal tone, but did not alter the heart rate dose-response curve for propranolol. With continuous infusions of isoproterenol, atropine displaced the heart rate dose-response curves for both atenolol and propranolol to the left, implying the presence of a reflex increase rather than withdrawal in cardiac vagal tone. These reflex changes in cardiac vagal tone can be partly understood by changes in mean arterial pressure and pulse pressure. As the two methods of isoproterenol administration are associated with contrasting contributions from reflex vagal tone, dose ratios obtained for the displacement of the heart rate dose-response curve by beta-antagonists may differ.