Intrinsic sympathomimetic activity and coronary blood flow

1 The effects of four beta-adrenoceptor blocking agents, propranolol, pindolol, practolol and atenolol, used in doses inducing the same degree of beta 1-adrenoceptor blockade, have been investigated on regional myocardial blood flow (microspheres) and coronary vascular resistance in the normal and ischaemic myocardium of the anaesthetized dog. 2 In the ischaemic and non-ischaemic myocardial areas, transmural blood flow was significantly reduced by propranolol and atenolol which are devoid of intrinsic sympathomimetic activity (ISA) while it remained unaffected by beta-adrenoceptor blocking drugs endowed with ISA, whether cardioselective (practolol) or not (pindolol). In non-ischaemic areas, only propranolol and atenolol increased coronary vascular resistance. 3 Regarding coronary blood flow distribution between the epicardium and endocardium, only propranolol and pindolol induced a favourable redistribution towards the endocardium increasing the endo/epi flow ratio, while practolol and atenolol were ineffective. 4 These results suggest that ISA is of major importance in maintaining coronary flow especially in the ischaemic myocardium but that it is not involved in the mechanisms underlying coronary blood flow redistribution between the different myocardial layers.