ABSTRACT
The influence of beta-adrenoceptor antagonists on the spontaneous rate and on force of contraction of the myocardium was studied independently on spontaneously beating right and electrically driven isolated left atria of rabbit. On spontaneous rate, practolol had sympathomimetic effect only, N-isopropylmethoxamine (IMA), had both sympathomimetic as well as depressant effects, whereas alprenolol, procinolol, bunolol and H 35/25 had depressant effects only in higher concentrations. The order of potency was procinolol greater than bunolol greater than alprenolol greater than H 35/25 greater than and IMA. On the contractions of isolated left atria, all beta-adrenoceptor antagonists produced concentration-dependent depressant effect. In relation to procinolol, these agents were 5-125 times less potent for depressing the contractions of left atria by 15% and the order of beta-potency was procinolol greater than alprenolol greater than bunolol = H 35/25 greater than IMA greater than and practolol. The present results indicate that the depressant effects of beta-adrenoceptor antagonists on spontaneous rate of right atria and on contraction of isolated left atria are not related to each other.
Subject(s)
Adrenergic beta-Antagonists/pharmacology , Alprenolol/pharmacology , Animals , Depression, Chemical , Ephedrine/analogs & derivatives , Heart Atria/drug effects , Levobunolol/pharmacology , Methoxamine/analogs & derivatives , Myocardial Contraction/drug effects , Phenoxypropanolamines , Practolol/pharmacology , Propanolamines/pharmacology , RabbitsABSTRACT
The study was made with the help of a chemitrode placed in various areas of the hypothalamus by the stereotaxic technique, for electrical and chemical stimulation (noradrenaline or isoprenaline) before and after microinjection of respective blockers (phenoxybenzamine or practalol). The results indicated the presence of both alpha and beta-adrenoreceptors in the anterior and dorsomedial hypothalamus producing a depressor response, and, the presence of alpha adrenoreceptors in the posterior and lateral hypothalamus producing a pressor response.
Subject(s)
Adrenergic beta-Antagonists/pharmacology , Animals , Blood Pressure/drug effects , Cats , Female , Hypothalamus/drug effects , Isoproterenol/pharmacology , Male , Norepinephrine/pharmacology , Phenoxybenzamine/pharmacology , Practolol/pharmacology , Receptors, Adrenergic/physiology , Receptors, Adrenergic, alpha/drug effects , Receptors, Adrenergic, beta/drug effectsSubject(s)
Abdominal Muscles/drug effects , Acetylcholine/pharmacology , Adrenergic beta-Antagonists/pharmacology , Animals , Carbachol/pharmacology , Cholinesterase Inhibitors/pharmacology , Metoprolol/pharmacology , Muscle Contraction/drug effects , Physostigmine/pharmacology , Practolol/pharmacology , Propranolol/pharmacology , Ranidae , Sotalol/pharmacologySubject(s)
Adrenergic beta-Antagonists/pharmacology , Anesthetics, Local , Animals , Anti-Arrhythmia Agents , Anura , Cyclopropanes/pharmacology , Ephedrine/analogs & derivatives , Guinea Pigs , Heart Conduction System/drug effects , Levobunolol/pharmacology , Neural Conduction/drug effects , Phenoxypropanolamines , Practolol/pharmacology , Propanolamines/pharmacology , RabbitsABSTRACT
Phenylephrine exerted a positive chronotropic and inotropic effect on isolated, spontaneously beating, atria of reserpinised rabbits. Addition of phenoxybenzamine and phentolamine resulted in a depression of control contractile amplitude. Practolol, however, was devoid of this effect. The positive inotropic response to phenylephrine was significantly antagonised by all the three blockers used, while positive chronotropic response was annulled by phentolamine and practolol, but not with phenoxybenzamine. It is, therefore, suggested that phenylephrine exerts its cardiostimulant effects through mediation of both alpha and beta-1 adrenoceptors. A probable mechanism of action could be, that phenylephrine acts on some specific chemical group, shared by alpha and beta1 receptors. This specific group is probably blocked by both alpha and betaceptor antagonists separately, so phenylephrine becomes ineffective in presence of these antagonists.