Accentuated antagonism in canine subendocardium is not altered by chronic exercise

Acetylcholine often affects cardiac action potential repolarization only during augmented adrenergic tone, i.e., the phenomenon of accentuated antagonism. Since chronic exercise involves repeated changes in autonomic outflow, we determined whether it also influenced adrenergic/cholinergic interactions in isolated canine cardiac tissue. Using standard micro-electrode techniques in thin ventricular subendocardial slices isolated from exercised (EX: 8-10 wk daily exercise) and sedentary (SED): 8-10 wk cage rest) dogs, we examined transmembrane potential responses to isoproterenol (ISO: 10(-8), 10(-7), 10(-6) M) and to ISO in the presence of ACH (10(-5) M). Control transmembrane characteristics at BCL = 500 ms were similar for EX (N = 8 dogs) and SED (N = 9 dogs). ISO (10(-6) M) decreased action potential duration at 50% repolarization (APD50): EX = -29 +/- 15 ms; SED = 11 ms and at 90% repolarization (APD90): EX = -37 +/- 17 ms; and SED = -24 +/- 14 ms (P > 0.05, EX vs SED). ACH alone did not alter APD. With ACH (10(-5) M), delta APD50 with ISO (10(-6) M) was -5 +/- ms and 0 +/- 5 ms for EX and SED, respectively; delta APD90 was -8 +/- 4 ms and -8 +/- 7 ms for EX and SED, respectively (P > 0.05, EX vs SED). Thus, ACH antagonized ISO-mediated acceleration of repolarization equally in both groups. Chronic daily exercise does not influence adrenergic/cholinergic interactions at the cellular level.