ACCENTUATED ANTAGONISM IN CANINE SUBENDOCARDIUM IS NOT ALTERED BY CHRONIC EXERCISE

Acetylcholine often affects cardiac action potential repolarization on ly during augmented adrenergic tone, i.e., the phenomenon of accentuat ed antagonism. Since chronic exercise involves repeated changes in aut onomic outflow, we determined whether it also influenced adrenergic/ch olinergic interactions in isolated canine cardiac tissue. Using standa rd microelectrode techniques in thin ventricular subendocardial slices isolated from exercised (EX: 8-10 wk daily exercise) and sedentary (S ED: 8-10 wk cage rest) dogs, we examined transmembrane potential respo nses 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 B CL = 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 (APD(50)): EX = -29 +/- 15 ms; SED = -17 +/- 11 ms and at 90% repolari zation (APD(90)): EX = -37 +/- 17 ms; and SED = -24 +/- 14 ms (P > 0.0 5, EX vs SED). ACH alone did not alter APD. With ACH (10(-5) M), Delta APD(50) with ISO (10(-6) M) was -5 +/- 4 ms and 0 +/- 5 ms for EX and SED, respectively; Delta APD(90), 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. C hronic daily exercise does not influence adrenergic/cholinergic intera ctions at the cellular level.