Sites of action of alpha 2 agonists and antagonists

Effect of N-dicyclopropylmethyl-amino-2-oxazoline (S-3341) on antioxidant status and nitric oxide in hypertensive patients

Defective endothelium-dependent vascular relaxation has been found in animal models of hypertension and in hypertensive patients. An imbalance due to reduced production of nitric oxide (NO) or increased production of free radicals, mainly the superoxide anion, may facilitate the development of an arterial functional spasm. Although it has been shown that many antihypertensive drugs can normalise both the antioxidant activity and NO, the antioxidant effect of N-dicyclopropylmethyl-amino-2-oxazoline (S-3341), an alpha-adrenoreceptor agonist, has not been investigated. In this study we investigated the antioxidant and NO status in hypertensive patients and whether there was any effect of S-3341 on these parameters. Eleven patients with mild hypertension (mean systolic blood pressure 159.5 +/- 2.5 mmHg) were administered S-3341 (1 mg/day) for 4 weeks. Plasma vitamin E, nitrite-nitrate and MDA levels, and catalase activity, were measured both before and after treatment with S-3341. There was significant reduction in both mean systolic and diastolic blood pressure during the treatment. We found an increase in catalase activity (p < 0.05), a decrease in malondialdehyde (MDA) levels (p < 0.01) and an insignificant increase in vitamin E levels in hypertensive patients following the S-3341 treatment. We propose that S-3341 may prevent oxidant stress in hypertensive patients by inhibiting free-radical formation.

Responses to alpha 2-adrenoceptor blockade by idazoxan in healthy male and female volunteers

Seven male and five female volunteers underwent double-blind infusions of the alpha 2-adrenoceptor antagonist idazoxan (100 and 200 micrograms/mg) and placebo in random order. Blood pressure, plasma norepinephrine, growth hormone and subjective responses were measured. The higher dose of idazoxan produced increases in blood pressure, norepinephrine and growth hormone and slight increases in anxiety. Both subject age and sex appeared to influence the magnitude of responses.

Bronchial effects of alpha 2-adrenoceptor agonists and of other antihypertensive agents in asthma

The respective prevalence of hypertension and asthma is sufficient for their combined existence to be far from rare. The effects of certain antihypertensive drugs, e.g., alpha 2-adrenoceptor agonists, on the bronchi may be either harmful or beneficial. When inhaled, alpha 2-agonists reduce the immediate bronchial response to allergens, whereas when ingested they aggravate the bronchial response to histamine and all the more so when their effect on the central nervous system is greater. Therefore, there has been much interest in agents such as the new oxazoline derivative, rilmenidine, which has less central effects than clonidine, an imidazoline compound of reference. Calcium antagonists inhibit smooth muscle contraction and release of mast cell inflammatory mediators. In asthmatic subjects, their short-term administration leads to a modest improvement in spontaneous bronchial obstruction, has only a partial protective action against various nonspecific or allergenic stimuli, and slightly reinforces the beneficial effect of beta 2-agonists. Beta-adrenoceptor antagonists aggravate bronchial obstruction and nonspecific bronchial hyperreactivity in asthmatic subjects. These harmful effects are dose-dependent, have even been reported after the administration of eyedrops, and are common to all beta-blockers. Angiotensin-converting enzyme inhibitors increase bronchial hyperreactivity in patients who develop cough during treatment and may, in certain cases, worsen or even induce asthma, probably by opposing inactivation by hydrolysis of tachykinins and of bradykinins.

Alpha-adrenoceptor subtypes in blood vessels: physiology and pharmacology

The existence of alpha 1- and alpha 2-adrenoceptors subtypes is now well established. Presynaptically, alpha 2-adrenergic autoreceptors modulate norepinephrine release. In vascular smooth muscle both alpha 1- and alpha 2-adrenoceptors exist postjunctionally, the alpha 1 subtype being innervated preferentially in most vascular beds. alpha 2-Adrenoceptors may be present on the smooth muscle cell as well as the endothelial cell where they may participate in the release of endothelium derived relaxing and constricting factors. Developing insights into the role of alpha-adrenoceptor subtypes in the cardiovascular system, and into the classification of subtypes of alpha 1- and alpha 2-adrenoceptors may be expected to have, if confirmed, significant therapeutic implications for drug discovery in the cardiovascular system.

Comparative effects of rilmenidine and clonidine on bronchial responses to histamine in asthmatic subjects

1. The effects of pretreatment with clonidine and rilmenidine, a new alpha 2-adrenoceptor agonist, on the bronchial responses to inhaled histamine were studied on 3 different days in a controlled, double-blind, randomized study in 12 asymptomatic asthmatic subjects. Clonidine and rilmenidine were orally administered as single and equipotent doses of 150 micrograms and 1 mg, respectively. All the subjects were non-smokers with normal lung function tests (forced expiratory volume in one second (FEV1) = 97 +/- 10% predicted FEV1). 2. Histamine (first dose = 543 nmol) was delivered by a breath activated dosimeter (DeVilbiss no. 646 nebulizer) every 5 min; FEV1 was measured in triplicate after each dose and the largest value was analyse. The three dose-response curves were compared by analysis of variance. 3. Both clonidine and rilmenidine decreased arterial blood pressure in all subjects. There was no difference in baseline values and pre-challenge values of FEV1 after placebo, clonidine and rilmenidine on the 3 study days. Compared with placebo, both rilmenidine and clonidine significantly increased the bronchial responses to histamine (P less than 0.05 and P less than 0.01 respectively) an effect which was significantly more marked with clonidine than rilmenidine (P less than 0.05). 4. We suggest that the enhancement of bronchial responsiveness to histamine by clonidine and rilmenidine may result from their effects on both central and peripheral alpha 2-adrenoceptors, and that the lesser aggravation of histamine-induced bronchial obstruction in asthmatic subjects on rilmenidine might be explained by its lesser central and/or greater peripheral effects than clonidine.