Rilmenidine differs from clonidine in that it lacks histamine-like activity

A pharmacological and biochemical investigation of the venom from the platypus (Ornithorhynchus anatinus)

In this study several activities of the venom of Ornithorhynchus anatinus have been investigated. Whole venom induced local oedema after subplantar injection and produced relaxation of the rat uterus in vitro. The relaxant activity was partially purified by gel permeation HPLC and subsequent analyses by SDS-PAGE revealed that this activity was associated with a 4200 mol. wt peptide. The N-terminal partial sequence of this peptide exhibited substantial identity with human and porcine C-type natriuretic peptide (CNP). Three other major proteins isolated from the venom had mol. wts of 140,000, 55,000 and 16,000. None was found to have any sequence homology with proteins listed in the SwissProt database. The 140,000 mol. wt protein exhibited hyaluronidase activity but the nature of the 55,000 and 16,000 mol. wt proteins remains to be determined. Platypus venom also exhibits protease activity, although the concentration of proteolytic enzymes was too low to be visualised by SDS-PAGE using Coomassie staining.

Mediation of the hypotensive action of systemic clonidine in the rat by alpha 2-adrenoceptors

1. During the past few years it has been shown that the sympatholytic effect resulting from localized microinjection of clonidine and other imidazolines into the rostral ventrolateral medulla (RVL) results from activation of 'imidazoline' receptors (I1 receptors) rather than from an alpha 2-adrenoceptor-mediated effect. 2. The relative contributions of these two receptor systems to the hypotensive action of systemically administered clonidine have not been studied. Clonidine has affinity for both I1 and alpha 2-adrenoceptors; guanabenz represents a useful pharmacological tool, since it activates only the alpha 2-adrenoceptor. 3. Antagonists acting at both I1 and alpha 2-adrenoceptors (idazoxan) and at only alpha 2-adrenoceptors (SK&F 86466; 6-chloro-3-methyl-2,3,4,5-tetrahydro-3-benzazepine) are available. Idazoxan (1 mg kg-1, i.v.) and SK&F 86466 (3 mg kg-1, i.v.) produced an equivalent degree of blockade of the pressor response to guanabenz or clonidine in the pithed rat, a response mediated by the alpha 2-adrenoceptor. 4. Guanabenz (30 micrograms kg-1, i.v.) and clonidine (10 micrograms kg-1, i.v.) lowered blood pressure in the chloralose-anaesthetized spontaneously hypertensive rat by 48 +/- 4.6 mmHg and 44 +/- 5.4 mmHg, respectively; this response, for either agonist, was blocked by both idazoxan and SK&F 86466. 5. These data show that the hypotensive effect of intravenously administered clonidine results from activation of central alpha 2-adrenoceptors, with no significant contribution from an I1-mediated effect. Thus clonidine can lower blood pressure by different receptor mechanisms, dependent on the route of administration.

Study on the histamine-like activity of guanfacine

The effects of guanfacine have been studied on guinea-pig isolated atria and diethylstilboestrol-treated rat isolated uterus to determine whether it possesses histamine-like activity. Guanfacine produced a concentration-dependent negative chronotropic effect which was not modified by ranitidine (0.1 microM). In rat isolated uterus contracted by KCl, clonidine (5-5000 microM) produced concentration-dependent relaxation which was blocked by ranitidine (0.1 microM), but guanfacine only produced relaxation at high concentrations (100-1000 microM), and this was not affected by ranitidine (0.1 microM). It is concluded that guanfacine, unlike clonidine, does not produce effects due to activation of H2-receptors in either guinea-pig atria or rat uterus.