PHARMACOLOGICAL INVESTIGATION OF α-ADRENORECEPTORS IN GUINEA-PIG SPLENIC CAPSULE

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Abstract
A study was made of .alpha.-adrenoreceptors in the smooth muscle of the guinea-pig splenic capsule. In this preparation, norepinephrine (NE), epinephrine (E) and .alpha.-methyl-NE were full agonists, while phenylephrine, dopamine and the catechol-imidazolidine (3,4-dihydroxyphenylamino)-2-imidazolidine, (DPI) were partial agonists. The imidazolidines clonidine and oxymetazoline showed no agonist activity, but both inhibited the agonist activity of NE pA2 values calculated from Arunlakshana and Schild plots were 6.88 and 6.95 for clonidine and oxymetazoline, respectively. Slopes of the Schild plots were close to unity, indicating competitive antagonism. The .alpha.-adrenoreceptor antagonists phentolamine, prazosin and yohimbine also inhibited the agonist activity of NE with mean pA2 values of 8.32, 9.22 and 6.90, respectively. Slopes of Schild plots were significantly less than 1, suggesting that the inhibition was not truly competitive in nature. The irreversible .alpha.-adrenoreceptor antagonist phenoxybenzamine at low concentrations (10-9 M) shifted the log concentration-response curve to NE to the right and greatly reduced the maximum response. The adrenoreceptors in the guinea-pig splenic capsule are probably of the .alpha.1-type, and that this tissue possesses relatively few spare receptors.