Ractopamine hydrochloride, a beta-adrenoceptor agonist, is a phenethanolamine salt approved for use as a feed additive. Recently published studies indicate that the RR-isomer (butopamine) is the stereoisomer with the most activity at the beta-adrenoceptor. Butopamine was shown to be a non-selective ligand at the beta1 and beta2-adrenoceptors, but signal transduction is more efficiently coupled through the b2-adrenoceptor than the beta1 adrenoceptor. Therefore, the RR-isomer of ractopamine is considered to be a full agonist at the beta2-adrenoceptor and a partial agonist at the beta1¬adrenoceptor. These results are consistent with the pharmacological characterization of racemic ractopamine in isolated cardiac (atria) and smooth muscle (costo-uterine, vas deferens, trachea), which shows a maximal response at beta2- and a submaximal response at beta1¬adrenoceptors when compared with the full beta1 and beta2-adrenoceptor agonist isoproterenol. Butopamine is chemically similar to dobutamine but, unlike dobutamine, it is not a catecholamine. Butopamine induces a positive inotropic response in patients with congestive heart failure but for equal increments in cardiac output, butopamine increases heart rate more than dobutamine. Butopamine inproved cardiac performance in patients with ventricular dysfunction and congestive heart failure. Butopamine was prepared by Tuttle et al (unpublished data) and has a structure similar to dobutamine. This compound is refractory to the action of catechol-O-methyl transferase and thus it is orally active and has a longlasting action. Clinical findings in acute heart failure cases have been reported by Thompson et al. Intravenous administration produced an increase in the cardiac index and heart rate and shortening of systolic time intervals. A few patients experienced ventricular ectopy, especially with the higher doses used. No data pertaining to oral administration are available.

Clorprenaline is a β2-adrenergic receptor agonist. As a bronchodilator it has been used for the treatment of bronchial asthma, bronchitis and other respiratory diseases. It is a potential new lean meat-boosting feed additive because it can promote animal muscular mass growth and decrease fat accumulation.

Nordefrin is a catecholamine sympathomimetic used as a topical nasal decongestant and vasoconstrictor. Nordefrin is a racemic mixture of levo- and dextro- isomers and levonordefrin to be 100 to 200 times more active than dextro-nordefrin in raising blood pressure. Nordefrin is a structural isomer of epinephrine and levonordefrin produced a greater increase in heart rate than epinephrine.

Etilefrine is a cardiac stimulant used as an antihypotensive. Intravenous infusion of this compound increases cardiac output, stroke volume, venous return and blood pressure in man and experimental animals, suggesting stimulation of both α and β adrenergic receptors. However, in vitro studies indicate that etilefrine has a much higher affinity for β1 (cardiac) than for β2 adrenoreceptors. Intravenous etilefrine increases the pulse rate, cardiac output, stroke volume, central venous pressure and mean arterial pressure of healthy individuals. Marked falls in pulse rate, cardiac output, stroke volume and peripheral bloodflow, accompanied by rises in mean arterial pressure, occur when etilefrine is infused after administration of intravenous propranolol 2,5 mg. These findings indicate that etilefrine has both β1 and α1 adrenergic effects in man. The French Health Products Agency concluded that etilefrine and heptaminol have an unfavourable harm-benefit balance, and also placed restrictions on the use of midodrine.