The relationship between the adrenoceptor and nonadrenoceptor-mediated effects of imidazoline- and imidazole-containing compounds

Imidazoline I(2)-receptors and spinal reflexes in the decerebrated rabbit

Idazoxan potentiates spinal reflexes in the decerebrated rabbit, an effect that has been attributed to antagonism of tonic noradrenergic inhibition. It is now known that this drug has a higher affinity for I(2)-imidazoline receptors than alpha(2)-receptors. The roles of I(2)-receptors in modulating transmission in spinal reflex pathways have been investigated using the selective ligands RX 821029 and RS-45041-190, and, as I(2)-receptors are closely associated with monoamine oxidase (MAO), the MAO inhibitors pargyline and clorgyline. In decerebrated rabbits with an intact spinal cord, intrathecal doses of 5-365 microg (cumulative) of the I(2)-ligands augmented, to 150-180% of pre-drug levels, the reflex responses of medial gastrocnemius motoneurones to electrical stimulation of the sural nerve, and significantly increased mean arterial blood pressure by approximately 10 mmHg over pre-drug values. Neither MAO inhibitor had significant effects on reflexes, but the highest dose of the MAO-A selective agent clorgyline (365 microg cumulative) caused a significant rise in blood pressure of 6 mmHg. Neither the I(2)-ligands nor the MAO inhibitors prevented the further enhancement of reflexes or blood pressure by subsequent administration of the selective alpha(2)-adrenoceptor antagonist RX 821002. In decerebrated, spinalized rabbits, intrathecal RS-45041-190 (60 microg, single dose) increased spinal reflex responses to 109% of pre-drug values, a significantly smaller effect than that seen in non-spinal preparations, and had no effect on blood pressure. These data show that imidazoline I(2)-receptors can influence somatic and autonomic motor outflows. These effects should be taken into account when interpreting the spinal effects of imidazoline-based adrenoceptor-active drugs.

Antagonism of alpha1-adrenoceptor agonist-induced responses by rilmenidine in vascular smooth muscle

The effect of the centrally acting antihypertensive agent, rilmenidine, was examined on the contractile properties of isolated rat portal vein strips and on the free cytosolic [Ca2+] ([Ca2+]i) in isolated myocytes. Rilmenidine (1-30 microM) relaxed strips precontracted with noradrenaline. This effect was not inhibited by the alpha2-adrenoceptor antagonist, yohimbine, and was not mimicked by the alpha2-adrenoceptor agonist, 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14,304). Rilmenidine dose dependently shifted to the right the concentration-response curves to noradrenaline and to phenylephrine but not that to carbachol. Rilmenidine alone (0.1-30 microM) caused a contraction which maximally corresponded to 18% of the maximal noradrenaline-induced contraction. This effect was not produced by UK 14,304, was not affected by yohimbine, but was inhibited by the alpha1-adrenoceptor antagonist, prazosin. In isolated myocytes, rilmenidine reduced the noradrenaline-induced [Ca2+]i increase but alone, it produced a rise in [Ca2+]i, the peak amplitude of which averaged 15% of the noradrenaline-induced transient [Ca2+]i rise. It is concluded that rilmenidine acts as a partial agonist of alpha1-adrenoceptors of vascular smooth muscle, causing relaxation of vessels precontracted by full agonists of alpha1-adrenoceptors.