ENHANCED BETA-ADRENERGIC-MEDIATED CARDIOVASCULAR-RESPONSES IN ENDURANCE ATHLETES

To determine whether the adaptive increase in left ventricular systoli c function in the trained state is mediated by enhanced responses to b eta-adrenergic stimulation, we studied eight male endurance athletes [ age 27 +/- 1.8 yr; maximal O-2 uptake (VO2max) 60 +/- 0.9 (SE) ml . kg (-1). min(-1)] and eight sedentary menage 27 +/- 1.4 yr; VO2max 43.1 /- 1.7 ml . kg(-1). min(-1)). Left ventricular function was evaluated with two-dimensionial echocardiography and pulsed Doppler transmitral flow velocity profile in the basal state, after parasympathetic blocka de by atropine, and during infusion of dobutamine. Cardiac output and stroke volume, determined with the acetylene rebreathing technique, du ring maximal exercise were significantly higher in the endurance athle tes than in the sedentary men (28.9 +/- 1.7 vs. 23 +/- 1.23 l/min, P = 0.019, and 162 +/- 12 vs. 125 +/- 7 ml/min, P = 0.029). Endurance ath letes showed physiological volume overload-left ventricular hypertroph y and greater enhancements of left. ventricular systolic function and filling dynamics in response to dobutamine than did the sedentary men as reflected in 1) a steeper slope of the fractional shortening-end-sy stolic wall stress relationship (-0.986 +/- 0.16 vs. -0.508 +/- 0.054, P = 0.014, athletes vs. controls) and 2) a higher early-to-late trans mitral diastolic Doppler velocity ratio (2.14 +/- 0.14 vs. 1.74 +/- 0. 12, P = 0.016) at a comparable heart rate. Although endurance athletes had a significantly greater inotropic response to dobutamine, they de monstrated a markedly attenuated chronotropic response to beta(1)-adre nergic stimulation compared with sedentary subjects. Our findings sugg est that, even with a blunted chronotropic response, endurance-trained young men show an augmented inotropic response to a beta(1)-adrenergi c agonist that, along with physiological volume overload hypertrophy a nd increased diastolic filling, can contribute to a larger stroke volu me during maximal exercise in the trained state.