Structure-activity relationships in the adrenergic-blocking agents

Alpha-adrenoceptor blocking properties of vesamicol

The side-effects of vesamicol, an inhibitor of acetylcholine storage, on alpha 1- and alpha 2-adrenoceptors have been studied in the isolated rat vas deferens. Antagonism of alpha 1-adrenoceptors was determined from the ability of vesamicol to reduce contractions elicited by exogenous noradrenaline. Antagonism of alpha 2-adrenoceptors was determined from the ability of vesamicol to block the inhibitory effects of the alpha 2-adrenoceptor agonist clonidine on electrically evoked twitches. In the absence of noradrenaline uptake block, (-)-vesamicol, the isomer active at cholinergic synapses, produced a leftward shift of the noradrenaline concentration-effect curve. This effect was abolished by desipramine suggesting that it is due to an ability of (-)-vesamicol to block uptake1. In the presence of noradrenaline uptake blockers, (-)-vesamicol produced a competitive, non-selective block of both alpha 1- and alpha 2-adrenoceptors with a Kd of around 40 microM (pA2 4.4). (+)-Vesamicol, the isomer that has no activity at cholinergic synapses, was equipotent with the (-)-isomer for blocking alpha 2-adrenoceptors. In addition to its alpha-adrenoceptor antagonist activity, (-)-vesamicol augmented the maximum response of the tissue to exogenous and endogenous noradrenaline. This study was unable to determine the exact nature of this effect. We suggest that the alpha-adrenoceptor blocking activity of vesamicol is a function of the phenylpiperidino moiety of the molecule.

Pharmacological studies of lycorenine, an alkaloid of Lycoris radiata Herb.: Vasodepressor mechanism in rats

Vasodepressor mechanism of lycorenine (an alkaloid of Lycoris radiata Herb.) was investigated in anesthetized rats. Lycorenine (1--10 mg/kg i.v.) produced dose-related decreases in blood pressure and heart rate and tachyphylaxis developed with repeated injections. In the blood-perfused rat hindquarters, lycorenine (62.5--500 micrograms i.a.) produced dose-related decreases both in mean blood pressure and in perfusion pressure, and the lycorenine-induced decrease in perfusion pressure was abolished by phenoxybenzamine or hexamethonium. Lycorenine (more than 1 mg/kg i.v.) blocked the pressor response to sympathetic nerve stimulation, but failed to block the tachycardia induced by sympathetic nerve stimulation. Lycorenine (7.5 or 15 mg/kg i.v.) reduced the spontaneous splanchnic nerve activity. Lycorenine when given intracerebroventricularly produced decreases in blood pressure and heart rate only in large doses (over 500 micrograms). The maximal bradycardia induced by lycorenine was abolished by bilateral vagotomy. It is suggested that lycorenine may produce a decrease in blood pressure as the result of alpha-adrenergic blockade in conjunction with the reduction of the spontaneous sympathetic nerve activity, and produce bradycardia by modifying vagal activity.

Phenoxybenzamine in neurogenic bladder dysfunction after spinal cord injury. I. Voiding dysfunction

Phenoxybenzamine is a welcomed adjunct in the pharmacologic management of patients with neurogenic bladder dysfunction resulting from spinal cord trauma. Side effects are infrequent. Bladder balance was predictably and effectively achieved and maintained in patients with 1) lower motor neuron lesions, 2) upper motor neuron lesions and coexisting autonomic dysreflexia and 3) intact sympathetic innervation and upper motor neuron bladders. The response was unpredictable in patients with upper motor neuron lesions without coexisting dysreflexia.

Beta-adrenoceptor stimulating properties of para-dimethylaminobenzaldehyde

1. Studies on the various isolated tissues indicate that para-dimethylaminobenzaldehyde (DMAB) is a beta-adrenoceptor stimulant. DMAB is antagonized by the beta-adrenoceptor blocking agent, MJ 1999, but not by the alpha-adrenoceptor blocking agent, phentolamine.2. A study of dose-response relationships suggests a competitive interaction between MJ 1999 and DMAB.3. DMAB was about 122 times less potent than isoprenaline on the isolated guinea-pig tracheal preparation. The effects of DMAB on isolated rabbit atria were not only very weak, but were also very brief. On this tissue, DMAB was respectively 72,000 and 55,400 times less active than isoprenaline in producing positive chronotropic and inotropic effects. DMAB caused the relaxation of the guinea-pig taenia coli and the rabbit ileum. These actions were very weak in comparison with those of isoprenaline.4. These results suggest that a compound (DMAB) structurally different from isoprenaline may mimic isoprenaline responses by stimulating beta-adrenoceptors through different mechanisms.