Myocardial flow and function after regional ??-blockade in exercising dogs

This study was designed to examine the contribution of beta 1- and beta 2-adrenergic receptors in modulating coronary blood flow and cardiac function in exercising dogs. Dogs were chronically instrumented to measure left circumflex flow velocity (CFV), heart rate, regional left ventricular function [systolic shortening, (%S) and maximum velocity of shortening (dL/dt(s)max)], and global left ventricular function [left ventricular pressure (LVP and dP/dtmax)]. Exercise significantly increased LVP (31 +/- 4%), dP/dtmax (130 +/- 17%), heart rate (116 +/- 20%), %S (28 +/- 6%), dL/dt(s)max (89 +/- 23%), and CFV (91 +/- 25%). Regional injection of the non-selective beta-blocker propranolol (1.0 mg) into the circumflex artery during exercise was associated with decreases in LVP (-8 +/- 3%), dP/dtmax (-17 +/- 3%), %S (-15 +/- 4), dL/dt(s)max (-13 +/- 4%), and CFV (-22 +/- 4%). Selective beta 1-receptor blockade with atenolol (1.0 mg, i.c.) was associated with similar decreases in LVP (-7 +/- 3%), dP/dtmax (-33 +/- 4%), %S (-12 +/- 3%), dL/dt(s)max (-17 +/- 2%), and CFV (-18 +/- 3%) during exercise. In contrast, selective beta 2-receptor blockade with ICI 118551 (250 micrograms, i.c.) produced significant decreases in only CFV (-11 +/- 2%) during exercise. Thus, the data suggest that the reductions in myocardial contractile function and flow after regional beta-blockade are primarily due to a decrease in myocardial beta 1-receptor stimulation. In addition, there apparently is a small involvement of either coronary vascular or pre-synaptic beta 2-receptors in mediating the coronary vascular flow response during exercise.