Cardiac adrenergic responses and electrophysiology during ischemia: effect of exercise

Whether exercise protects the myocardium from arrhythmias during ischemia (ISC) or alters electrophysiology is controversial. We used microelectrode techniques in isolated cardiac fibers from exercise-trained (ET: 8-10 wk daily exercise) and sedentary (SED: 8-10 wk cage-rest) dogs to examine the effect of exercise on cellular electrophysiology during simulated ISC. We superfused fibers first with normal Tyrode's, then "ischemic Tyrode's" ([K+]o = 10 mM, pH = 6.7, pO2 < 25 mm Hg), and then again with normal Tyrode's. In automatic fibers, maximum diastolic potential in normal Tyrode's was -98 +/- 1 mV (ET, N = 22) and -97 +/- 1 mV (SED, N = 23); rates were 20 +/- 2 and 18 +/- 3 bpm for ET and SED, respectively. All fibers depolarized to -61 +/- 2 mV with ISC. Abnormal rhythms (abnormal automaticity with or without delayed afterdepolarizations) during ISC alone were seen in 0% of ET and 33% of SED; during ISC with alpha-adrenergic stimulation with 5 x 10(-8) M phenylephrine the incidence was 25% of ET and 0% of SED; during ISC with isoproterenol it was 75% for ET (P < 0.05 vs control) and 38% for SED. Transmembrane potentials in paced subendocardial fibers were similar for ET and SED during control, ISC, and reperfusion. Exercise did not alter cellular electrophysiology but did influence ectopic rhythms seen with beta-stimulation during ISC.