Heterogeneity of the interaction between alpha 1- and alpha 2-adrenoceptor agonists with their respective receptors in the vascular system of the pithed rat

Cardiovascular differences between phenylpropanolamine and its related norephedrine isomers in the rat

Various studies have confused the norephedrine and norpseudoephedrine isomers with phenylpropanolamine (PPA, d,l-norephedrine). This confusion has led us to investigate the pharmacological activity of the norephedrine (NOR) and norpseudoephedrine (NORP) isomers in the cardiovascular system of the urethane:chloralose anesthetized rat. Following intravenous administration, in a cumulative-dose fashion, l-NOR and PPA were the most potent compounds at increasing arterial blood pressure, while d-NOR, d-NORP, and l-NORP were relatively inactive at the doses tested (0.31-10 mg/kg). Prior reserpinization did not significantly shift the cumulative dose-response curves for l-NOR and PPA. Repetitive injections of PPA and l-NOR (1 mg/kg, 4 doses at 5-min intervals) failed to produce tachyphylaxis to the pressor response. On the other hand, when d-NORP was administered in a similar fashion, tachyphylaxis to the second and subsequent doses was observed. These studies demonstrate that significant mechanistic differences exist between these norephedrine isomers.

Mechanism of action of B-HT 933 (azepexole) in rat vas deferens and guinea-pig ileum

The mechanism of action of B-HT 933 (azepexole) was studied on the rat vas deferens and myenteric plexus-longitudinal muscle (MP-LM) of the guinea-pig ileum. The drug caused a concentration-dependent inhibition of the twitch response in both preparations. The maximal inhibition in both preparations was 80-90%. B-HT 933 did not affect the cumulative dose-response curves of the vas deferens and of MP-LM to noradrenaline (NA) and acetylcholine (Ach) respectively. Yohimbine antagonized in a competitive way the twitch inhibitory effect of B-HT 933 on vas deferens and on MP-LM; the pA2 values were 8.62 and 8.5, respectively. The twitch inhibitory effects of B-HT 933 were not antagonized by propranolol. The results suggest that the action of B-HT 933 is mediated by stimulation of presynaptic alpha 2-receptors.

Influence of respiratory acidosis or alkalosis on pressor responses mediated by alpha 1- and alpha 2-adrenoceptors in pithed normotensive rats

The effect of respiratory acidosis and alkalosis on the vasoconstriction to alpha 1- and alpha 2-adrenoceptor stimulation was studied in pithed normotensive rats. The selective alpha 1-adrenoceptor agonists (-)amidephrine, cirazoline, (+/-)erythro methoxamine, (-)phenylephrine, Sgd 101/75 and St 587 were used, as well as the selective alpha 2-adrenoceptor agonists B-HT 920, B-HT 933, DP-6,7-ADTN, M-7 and UK 14,304. The non-selective alpha-adrenoceptor agonists xylazine, noradrenaline and adrenaline were included as well. The latter two were also studied under selective doses of the antagonists rauwolscine and prazosin, thus yielding the respective alpha 1- and alpha 2-adrenoceptor components of the vasoconstriction to these agonists. The effect of acid-base balance disturbances on presynaptically released noradrenaline elicited by electrical stimulation of preganglionic nerves was studied as well. Dose response curves for the agonists were generated under various conditions of ventilation, yielding either alkalotic, normal or acidotic values of arterial blood pH. Pressor responses to all agonists were maximally affected by changes in acid-base status at the low doses of the agonists. Acidosis was found to inhibit increases in diastolic pressure mediated by the alpha 1-as well as the alpha 2-adrenoceptor agonists studied, although not to the same extent. Alkalosis exerted either an obvious potentiation or did not significantly influence alpha 1-adrenoceptor mediated pressor responses. On the basis of acid-base sensitivity the following groups of agonists were distinguished: Cirazoline, phenylephrine, methoxyamine, electrically released noradrenaline from presynaptic sites, of which pressor responses are obviously potentiated and attenuated by alkalosis and acidosis, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)