RUNNING HEAD: Gender differences in 4-receptor function after exercise Corresponding Author:

Gender differences in neuroendocrine and metabolic responses to prolonged strenuous exercise (PSE) have been well documented. The aim of this investigation was to examine gender differences in left ventricular function and cardiac 4-receptor responsiveness following a single bout of PSE. Nine male and eight female triathletes were examined during three separate sessions: before, immediately after, and 24 h following a half-ironman triathlon using dobutamine stress echocardiography. Steady-state graded infusions of dobutamine were used to assess 4-receptor responsiveness. Slopes calculated from linear regressions between dobutamine doses and changes in heart rate and contractility for each participant were used as an index of 4-receptor responsiveness. Despite no change in preload, fractional area change decreased from baseline after the race in both males and females, with a greater decrease in males (males: 54.1 ± 2.1 to 50.7 ± 3.4 % vs. females: 55.4 ± 2.7 to 53.3 ± 2.5 %; p<0.05). The amount of dobutamine necessary to increase HR 25 beats.min-1 (males: 29.6 ± 6.6 to 42.7 ± 12.9 µg.kg-1.min-1 vs. females: 23.5 ± 4.0 to 30.0 ± 7.8 µg.kg-1.min-1; p<0.05) and contractility 10 mmHg.cm-2 (males: 20.9 ± 5.1 to 37.0 ± 11.5 µg.kg-1.min-1 vs. females: 22.6 ± 6.4 to 30.7 ± 7.2 µg.kg-1.min-1; p<0.05) was greater in both males and females post-race