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2-adrenoceptor responsiveness and prevents ventricular fibrillation in animals susceptible to sudden death
1Department of Physiology and Cell Biology and 2Davis Heart and Lung Research Institute, The Ohio State University, Columbus, Ohio
Submitted 18 November 2005 ; accepted in final form 13 December 2005
Enhanced cardiac 
2-adrenoceptor (2-AR) responsiveness can increase susceptibility to ventricular fibrillation (VF). Exercise training can decrease cardiac sympathetic activity and could, thereby, reduce 2-AR responsiveness and decrease the risk for VF. Therefore, dogs with healed myocardial infarctions were subjected to 2 min of coronary occlusion during the last minute of a submaximal exercise test; VF was observed in 20 susceptible, but not in 13 resistant, dogs. The dogs were then subjected to a 10-wk exercise-training program (n = 9 susceptible and 8 resistant) or an equivalent sedentary period (n = 11 susceptible and 5 resistant). Before training, the 2-AR antagonist ICI-118551 (0.2 mg/kg) significantly reduced the peak contractile (by echocardiography) response to isoproterenol more in the susceptible than in the resistant dogs: –45.5 ± 6.5 vs. –19.2 ± 6.3%. After training, the susceptible and resistant dogs exhibited similar responses to the 2-AR antagonist: –12.1 ± 5.7 and –16.2 ± 6.4%, respectively. In contrast, ICI-118551 provoked even greater reductions in the isoproterenol response in the sedentary susceptible dogs: –62.3 ± 4.6%. The 2-AR agonist zinterol (1 µM) elicited significantly smaller increases in isotonic shortening in ventricular myocytes from susceptible dogs after training (n = 8, +7.2 ± 4.8%) than in those from sedentary dogs (n = 7, +42.8 ± 5.8%), a response similar to that of the resistant dogs: +3.0 ± 1.4% (n = 6) and +3.2 ± 1.8% (n = 5) for trained and sedentary, respectively. After training, VF could no longer be induced in the susceptible dogs, whereas four sedentary susceptible dogs died during the 10-wk control period and VF could still be induced in the remaining seven animals. Thus exercise training can restore cardiac -AR balance (by reducing 2-AR responsiveness) and could, thereby, prevent VF.
-adrenergic receptor; myocardial infarction; myocardial ischemia
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