title: Effects of the beta-adrenergic blocking agents propranolol and timolol on canine cardiac refractory periods.
abstract: The effects of d,1-propranolol and 1-timolol on cardiac refractory periods (RP) were compared in 14 phentobarbital-anesthetized dogs using endocavitary His bundle electrograms and programmable electrical stimulation. Beta-blocking agents were injected at cumulative doses in each dog at 3 day intervals. A control group (6 dogs) received 4 successive saline injections at the same time intervals. RP measurements at a constant drive rate were done before and 10 minutes after each dose of either drug or saline. Propranolol and timolol produced a dose-dependent increase of atrial and atrioventricular nodal refractory periods; dose-response curves were parallel. Depending on the parameter chosen timolol exerted an 8--36 times more potent effect than propranolol. The effects of propanolol and timolol on ventricular effective (VERP) and functional (VFRP) RP were measured in 6 dogs. Both drugs increased VFRP significantly, but saline had the same effect. Only the higher doses of timolol increased VERP significantly. These data confirm that blockade of myocardial beta-adrenergic receptors exerts predominant effects on supraventricular refractoriness and that in anesthetized dogs timolol has more potent beta-blocking properties than does propranolol.
mesh_terms: D000818:Animals; D001283:Atrioventricular Node / Q000187:drug effects; D004285:Dogs; D005260:Female; D006329:Heart Conduction System / Q000187:drug effects; D006339:Heart Rate / Q000187:drug effects*; D066298:In Vitro Techniques; D008297:Male; D009431:Neural Conduction / Q000187:drug effects*; D011412:Propanolamines / Q000494:pharmacology*; D011433:Propranolol / Q000494:pharmacology*; D012032:Refractory Period, Electrophysiological / Q000187:drug effects*; D013999:Timolol / Q000494:pharmacology*
chemical_list: D011412:Propanolamines; D013999:Timolol; D011433:Propranolol