| Abstract: | The blocking effects of 50 mg and 100 mg of propanolol, a beta-receptor blocker, upon the beta-adrenergic receptors of cattle heart were verified by means of 250 microgram of isoprenaline, a maximum dose. The 50-mg-propanolol dose inhibited between 77 and 80 per cent of the isoprenaline-caused QT reduction, whic action was considered as being sufficiently safe to rule out any beta-adrenergic cardiac effect. Blockade was further increased to the order of between 89 and 92 per cent by using 100 mg of propanolol, but certain side-effects occurred (expiratory dyspnoe, quinidinlike action in terms of PQ extension, and vagovasal collapse). Therefore, 50 mg of propanolol are recommended as the proper dosage with which to examine cardiac and extracardiac beta-adrenergic receptors in cattle. Application of propanolol to non-anaesthetised, nonatropinised cattle resulted in higher bloackade of isoprenaline-caused QT reduction, as compared to the parameters PQ, TP, TQ, time of cardiac action, PP, and momentary heart rate. This phenomenon is being analysed. With reference being made to the peculiarities of sympathico-parasympathetic interactions on the heart, QT time is considered as a more safe parameter by which to assess substrate competition between propanolol and isoprenaline at cardiac beta-receptors, while the ventriculodiastolic ECG recordings TQ, TP, and PQ as well as time of cardiac action, PP and momentary heart rate might come under a somewhat stronger influence of the parasympathetic system. |
