Possible differences in alpha-adrenoceptors in rat, guinea-pig and rabbit spleen

Pharmacology of postsynaptic alpha-adrenoreceptors in the mouse anococcygeus muscle

1. The alpha-adrenoreceptor agonists noradrenaline (NA), methoxamine, phenylephrine, naphazoline, oxymetazoline, and xylazine produced contractions of the mouse anococcygeus muscle. All were full agonists. 2. Cocaine (2 microM) increased the pD2 values of NA and phenylephrine. 6-hydroxydopamine pretreatment increased the pD2 values of NA, phenylephrine, and xylazine. After both procedures the order of potency of the agonists was oxymetazoline greater than naphazoline greater than NA greater than phenylephrine greater than methoxamine greater than xylazine. 3. The order of potency of antagonists against all six agonists was prazosin greater than phentolamine greater than yohimbine. However, from differences in the pA2 values and slopes of the Schild plots of the antagonists it was possible to distinguish three distinct groups of agonists: the phenylethylamines; the imidazolines; and the thiazine derivative xylazine. 4. The results suggest that the postsynaptic alpha-adrenoreceptor of the mouse anococcygeus muscle may be broadly classified as alpha 1. However, there may be at least three drug recognition sites on the receptor which interact with agonists of differing chemical structure.

Pharmacological investigation of alpha-adrenoreceptors in guinea-pig splenic capsule

1. A study has been made of alpha-adrenoreceptors in the smooth muscle of the guinea-pig splenic capsule. 2. In this preparation, noradrenaline (NA), adrenaline (Adr) and alpha-methyl-NA were full agonists, while phenylephrine, dopamine and the catechol-imidazolidine (3,4-dihydroxyphenylamino)-2-imidazolidine, (DPI) were partial agonists. 3. The imidazolidines clonidine and oxymetazoline showed no agonist activity, but both inhibited the agonist activity of NA. 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. 4. The alpha-adrenoreceptor antagonists phentolamine, prazosin and yohimbine also inhibited the agonist activity of NA with man 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. 5. The irreversible alpha-adrenoreceptor antagonist phenoxybenzamine at low concentrations (10(-9)M) shifted the log concentration-response curve to NA to the right and greatly reduced the maximum response. 6. It is suggested that the adrenoreceptors in the guinea-pig splenic capsule are probably of the alpha 1-type, and that this tissue possesses relatively few spare receptors.

Are neuronal uptake mechanisms responsible for the difference in sensitivity to noradrenaline between human arteries and veins?

1. The sensitivity of human metacarpal veins and digital arteries obtained post-mortem to noradrenaline and phenylephrine has been tested. 2. pED50 values for noradrenaline were significantly higher in the veins (6.99, s.e.m. = 0.08) than in the arteries (6.56, s.e.m. = 0.09), whereas pED50 values for phenylephrine in the two tissues were not significantly different (arteries: 6.24, s.e.m. = 0.09; veins: 6.26, s.e.m. = 0.05). 3. The addition of propranolol (4 x 10(-6) mol/l) alone, or in combination with hydrocortisone (4 x 10(-5) mol/l), did not affect the responses to either noradrenaline or phenylephrine. The further addition of cocaine (3 x 10(-5) mol/l) slightly shifted the noradrenaline and phenylephrine concentration-effect curves to the left in both arteries and veins, but veins were still found to be more sensitive than arteries to noradrenaline whilst there was still no difference in the sensitivity of veins and arteries to phenylephrine. 4. Cocaine also slightly potentiated responses to barium chloride, potassium chloride and serotonin. 5. It is concluded that the difference in sensitivity to noradrenaline between arteries and veins cannot be explained by differences in neuronal uptake and it is possible that there may be differences in the properties of the postsynaptic alpha-adrenoreceptors of the two tissues. It is also concluded that the potentiation of the contractile effect of noradrenaline produced by cocaine is not solely due to inhibition of neuronal uptake of amines.

Possible subdivision of postsynaptic alpha-adrenoceptors mediating pressor responses in the pithed rat

Additional evidence has been obtained indicating a possible subclassification of postsynaptic alpha-adrenoceptors into alpha 1 - and alpha 2 -subtypes. The pressor responses to the alpha-adrenoceptor agonists L-phenylephrine and guanfacine were quantified after i.v. administration to pithed rats. The alpha-sympatholytic drug yohimbine (1 mg/kg) displaced both dose-response curves to the right, but the effect was greatest for guanfacine. After prazosin (0.1 mg/kg) a 53-fold shift to the right was noticed for the dose-response characteristic of L-phenylephrine. Prazosin antagonized the effect of only the higher doses of guanfacine. The findings indicate that L-phenylephrine and prazosin preferentially interact with alpha 1 -adrenoceptors as agonist and antagonist, respectively. Yohimbine proved less selective than prazosin, but preferentially blocks postjunctional alpha 2 -adrenoceptors in the vascular wall. The results obtained with guanfacine may be interpreted to indicate that this drug acts on alpha 2 -adrenoceptors at lower doses and additionally stimulates alpha 1 -adrenoceptors at higher ones. Preliminary findings with corynanthine and rauwolscine support this interpretation.