Abstract
1. 1. The inotropic responses to phenylephrine and methoxamine were compared in isolated left and right atria, ventricles and papillary muscles from rat, guinea-pig and rabbit. The responses were not consistent to both alpha-adrenoceptor agonists and depended on the type of preparation. 2. 2. The positive inotropic effect of phenylephrine was much greater than that of methoxamine in most of the preparations. However, in the rat atria, the inotropic effect of both agonists was equivalent. 3. 3. Phenolamine treatment (10−6M) shifted the dose response curve for phenylephrine to the right in all the preparations except for guinea-pig ventricles and papillary muscles. However, after combined treatment with phentolamine and propranolol (both 10−6M), the maximum tension increase produced by phenylephrine was inhibited. 4. 4. On the other hand, phentolamine nearly abolished the methoxamine-induced positive inotropic effect in rat ventricles and papillary muscles and reversed the methoxamine response to a negative inotropic effect in rabbit left atria. Even in the absence of phentolamine, methoxamine was able to elicit a negative inotropic effect in guinea pig ventricles and papillary muscles. 5. 5. Propranolol (10−6M) did not have apparent effect on any of the methoxamine responses. 6. 6. In the atria from the different animals, methoxamine could elicit the positive inotropic effect at different frequencies of stimulation (0.5–2.0 Hz). 7. 7. These results suggested that the existence of the regional and species differences should be considered in a study of the nature of myocardial adrenoceptors and that the alpha receptor is capable of eliciting an inotropic effect.
Published Version
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