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1 The present study investigated the eects of the preferential b 3 -AR agonist BRL 37344 (BRL) on force of contraction (FOC), Ca 2+ -transient and eNOS-activity in human right atrial myocardium. 2 BRL concentration-dependently caused an increase in FOC that was paralleled by an increase in Ca 2+ -transient and a shortening of time to half peak relaxation (T0.5T). These eects were abolished in the presence of propranolol (0.3 mM).3 BRL acted as a competitive antagonist towards isoprenaline and in binding experiments it was shown to have a distinct anity towards b 1/2 -AR. 4 In immunohistochemical experiments BRL (10 mM) increased detection of activated eNOS. This eect remained constant in the presence of propranolol (0.3 mM). 5 BRL increased directly detected NO in DAF-staining experiments. This increase was signi®cantly smaller in the presence of the NO-inhibitor L-NAME. 6 The inotropic eects of BRL were not changed in the presence of L-NMA. 7 These results suggest that the inotropic eects of BRL in human atrium are mediated via b 1/2 -AR, whereas the increase of atrial eNOS-activity is due to b 3 -adrenergic stimulation. This increase in eNOS-activity did not in¯uence atrial myocardial contractility. In conclusion, this study shows that b 3 -adrenergic stimulation is present in human atrium, but may not be functionally as signi®cant as in the left ventricle.

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