Ventricular action potential adaptation to regular exercise: role of β-adrenergic and K channel function.

Regular exercise training is known to affect the action potential duration (APD) and improve heart function, but involvement of β-adrenergic receptor (β-AR) subtypes and/or the ATP-sensitive K (K) channel is unknown. To address this, female and male Sprague-Dawley rats were randomly assigned to voluntary wheel-running or control groups; they were anesthetized after 6-8 wk of training, and myocytes were isolated. Exercise training significantly increased APD of apex and base myocytes at 1 Hz and decreased APD at 10 Hz. Ca transient durations reflected the changes in APD, while Ca transient amplitudes were unaffected by wheel running. The nonselective β-AR agonist isoproterenol shortened the myocyte APD, an effect reduced by wheel running. The isoproterenol-induced shortening of APD was largely reversed by the selective β-AR blocker atenolol, but not the β-AR blocker ICI 118,551, providing evidence that wheel running reduced the sensitivity of the β-AR. At 10 Hz, the K channel inhibitor glibenclamide prolonged the myocyte APD more in exercise-trained than control rats, implicating a role for this channel in the exercise-induced APD shortening at 10 Hz. A novel finding of this work was the dual importance of altered β-AR responsiveness and K channel function in the training-induced regulation of APD. Of physiological importance to the beating heart, the reduced response to adrenergic agonists would enhance cardiac contractility at resting rates, where sympathetic drive is low, by prolonging APD and Ca influx; during exercise, an increase in K channel activity would shorten APD and, thus, protect the heart against Ca overload or inadequate filling. Our data demonstrated that regular exercise prolonged the action potential and Ca transient durations in myocytes isolated from apex and base regions at 1-Hz and shortened both at 10-Hz stimulation. Novel findings were that wheel running shifted the β-adrenergic receptor agonist dose-response curve rightward compared with controls by reducing β-adrenergic receptor responsiveness and that, at the high activation rate, myocytes from trained animals showed higher K channel function.