Implications of imidazolines and imidazoline receptors role at the vascular level

There are both post- and pre-synaptic vascular imidazoline (IM) binding sites. The importance of direct IM actions and that of peripheral imidazoline receptors (IRs) are shadowed by the central effects of IMs and by their interaction with alpha 2 adrenoceptors. Since the discovery of clonidine the many studies on IMs have been focused on their hypotensive effect, with rilmenidine and moxonidine as representative drugs. Formerly called IM preferring alpha 2 or IM/guanidium sites, the IRs (idazoxan-sensitive) are the plasmalemmal I1 (clonidine-sensitive) and the various I2 (one structure identified as MAO). I1 signaling includes activation of phosphatidylcholine-selective phospholipase C and inhibition of some ligand-gated channels. Inhibitory IRs on postganglionic sympathetic terminals, are not alpha 2, H3, I1 or I2. Some IMs directly affect CaL, while others inhibit K+ efflux. Clonidine-displacing substances including agmatine are endogenous ligands at IRs and alpha 2 and may participate in arterial pressure control. Beside few speculations, the roles of vascular IRs are largely unknown.