Class III action of -blocking agents

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Abstract
We have studied the effect of the intravenous administration of the following drugs on ventricular refractoriness in 35 experiments in open chested beagle dogs: atenolol 0.2 mg·kg−1; nadolol 0.05 mg·kg−1; oxprenolol 0.2 mg·kg−1; pindolol 0.04 mg·kg−1; propranolol 0.2 mg·kg−1; sotalol 0.6 mg·kg−1 and timolol 0.1 mg·kg−1. The animals were anaesthetised with chloralose and urethane. Measurements were made of left ventricular epicardial monophasic action potentials (MAP) (n=35) and the left ventricular paced evoked response (PER) (n=25) at a fixed paced cycle length of between 220 and 310 ms. The animals were initially β-blocked with iv pindolol or propranolol and the drug under study then administered iv 10 min later. The results were as follows: (control and post drug administration) MAP: nadolol 151±12 SD to 172±13 SD (pPER: nadolol 172±14 to 190±20 (p<0.001); oxprenolol 164± 16 to 188± 15 (p<0.001); propranolol 166±22 to 173± 18 (NS). There was insufficient data on the PER for sotalol, atenolol, pindolol and timolol for statistical analysis but the trend of observation followed the respective MAP. These results show that nadolol and oxprenolol, like sotalol, have class III action, although in view of the fast intrinsic heart rates the possibility that the effects of different β-blockers on MAP duration is due, at least in part, to competitive (β-blockade cannot be disregarded.