[Negative inotropic reflex effects in the dog ventricle before and after beta-receptor blockade]

Myocardial contractility of the rabbit ventricle, expressed as contractility index KI = d log (p - p0)/dtmax, was altered by both aortic nerve reflex stimulation and by beta-receptor blockade with propranolol (Obsidan) and talinolol (Cordanum, (+)-Cordanum and (-)-Cordanum). In every case, beta-receptor blockade decreased negative reflex inotropy, whereas reflex hypotension was only reduced by Obsidan. The mean reflex inotropy was 11.8% after Obsidan application and 17.3% after Cordanum injection, the reflex hypotension 28.0% and 34.8%. From the results it is concluded that Cordanum acts in the rabbit ventricle cardioselectively. If neural and pharmacological effects are related to initial effects, we may conclude that the contractile state maintained by the resting sympathetic tone is diminished. The overall reduction of KI by Cordanum blockade and reflex inhibition amounts to 45.5%, that of blood pressure to 42%. Its optical isomers act also cardioselectively, the inotropic effects, however, are smaller than those of the racemate. Obviously, reflex stimulation and beta 1-blocking agents have a similar mode of inotropic action. From this we suggest that reflex inotropy is mediated by sympathetic nerve fibres and beta 1-adrenergic receptors.