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

MASS, H. and P. A. GWIRTZ. Myocardial flow and function after regional β-blockade in exercising dogs. Med. Sci. Sports Exerc., Vol. 19, No. 5, pp. 443-450, 1987. This study was designed to examine the contribution of β₁- and β₂-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 β-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 β₁-receptor blockade with atenolol (1.0 mg, i.e.) was associated with similar decreases in LVP (-7 t 3%), dP/dtmax (-33 ± 4%), %S (-12 ± 3%), dL/dt(s)max (-17 ± 2%), and CFV (-18 ± 3%) during exercise. In contrast, selective β₂-receptor blockade with ICI 118551 (250 μg, i.e.) 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 β-blockade are primarily due to a decrease in myocardial β₁-rcceptor stimulation. In addition, there apparently is a small involvement of cither coronary vascular or pre-synaptic β₂-receptors in mediating the coronary vascular flow response during exercise.