Cardiac β-adrenergic responsiveness with exercise

Left ventricular performance is enhanced with chronic exercise training. Alterations in cardiomyocyte β-adrenergic responsiveness (BAR) may, in part, mediate this response. In this study, cardiac BAR and the expression of some key cardiac hypertrophic signaling molecules following 3 months of treadmill training were examined. Four-month old, female, Wistar Kyoto (WKY) rats were randomly assigned into either a sedentary (WKY-SED, n = 15) or an exercise-trained (WKY-TRD, n = 11) group. All rats were maintained on a 12-h light/dark cycle, and fed ad libitum. Exercise training consisted of motorized treadmill training at 25 m/min, 0% grade, 60 continuous minutes, 5 days/week for a period of 12 weeks. RT-PCR was used to establish basal cardiac calcineurin A, ANP, and AKT mRNA expression. In vitro cardiac BAR responsiveness was determined in Langendorff, isolated hearts. Following baseline, isoproterenol (ISO) was incrementally infused at concentrations ranging from 1 × 10⁻¹⁰ to 1 × 10⁻⁷ mol/L. There were no group differences for heart weight, heart to body weight ratio, calcineurin A, ANP, or AKT mRNA levels between WKY-SED and WKY-TRD. WKY-TRD showed enhanced cardiac BAR relative to WKY-SED (at ISO 1 × 10⁻⁷ mol/L; P < 0.05). Moderate intensity treadmill exercise improved cardiac BAR responsiveness to a high concentration of isoproterenol. This adaptation was independent of training-induced alterations in cardiac hypertrophy or hypertrophic marker expression.