benzofurans and 4-iodoclonidine

The rostral ventrolateral medulla (RVLM) is the primary region maintaining vasomotor tone, and a site of action for central antihypertensive agents. In vitro [125I]p-iodoclonidine binding studies showed that moxonidine was selective for I1-imidazoline over alpha 2-adrenergic receptors in the RVLM. We identified efaroxan and SK&F 86466 as selective I1- and alpha 2-antagonists, respectively. We tested moxonidine's action within the RVLM of spontaneously hypertensive rats (SHRs) on I1-imidazoline or alpha 2-adrenergic receptors, and determined whether the RVLM mediates the action of systemic moxonidine. SHRs were anesthetized, paralyzed, and ventilated and the RVLM was localized by testing for a pressor response to 2 nmol glutamate. To test whether I1 or alpha 2 mediates hypotensive effects of moxonidine, the I1/alpha 2 antagonist efaroxan (4 nmol) or the alpha 2-blocker SK&F 86466 (10 nmol) was administered 15 min before 4 nmol moxonidine. Efaroxan elevated blood pressure and abolished the action of moxonidine, whereas alpha 2-blockade with SK&F 86466 slightly lowered blood pressure and only partially attenuated moxonidine's effect. The depressor effect of intravenous moxonidine (40 micrograms/kg) was reversed within 10 min by microinjection of 10 nmol efaroxan into the RVLM. Prior bilateral microinjections of efaroxan (10 nmol in 80 nl/site) into the RVLM prevented the hypotensive action of moxonidine given i.v. (40 micrograms/kg). Pharmacokinetic studies showed that at the peak vasodepressor response (8 min post-injection), [3H]moxonidine spread less than 1 mm from the injection site. Moxonidine is a centrally acting antihypertensive with a selective action on I1-imidazoline receptors in RVLM.

Topics: Adrenergic alpha-Antagonists; Affinity Labels; Animals; Antihypertensive Agents; Benzazepines; Benzofurans; Binding, Competitive; Blood Gas Analysis; Blood Pressure; Blood Pressure Determination; Cattle; Clonidine; Disease Models, Animal; Heart Rate; Hypertension; Imidazoles; Imidazoline Receptors; In Vitro Techniques; Medulla Oblongata; Microinjections; Radioligand Assay; Rats; Rats, Inbred SHR; Receptors, Drug

The effect on insulin release of efaroxan, an alpha 2-adrenoceptor antagonist and a highly potent drug at imidazoline I1 receptors, and the effects of seven other imidazoline compounds selective for the imidazoline I1 or I2 receptors, were studied in the rat insulinoma cell line RIN-5AH. The cells released insulin in response to glucose (0.3-10 mM), and efaroxan (100 microM) potentiated glucose-induced insulin release. (-)-Adrenaline completely displaced the binding of [125I]p-iodoclonidine to membranes of RIN-5AH cells, indicating that these cells do not express imidazoline I1 receptors. Cirazoline and idazoxan (100 microM), both highly potent drugs at imidazoline I2 receptors, and the guanidines guanoxan and amiloride (200 microM), also promoted insulin release from RIN-5AH cells. Irreversible blockade of imidazoline I2 receptors with 10 microM clorgyline did not prevent the stimulatory effects of cirazoline or idazoxan; however, these compounds completely reversed the inhibition by diazoxide (250 microM), an opener of ATP-dependent K+ channels (K+ATP channels), of glucose-induced insulin release. These data indicate that the imidazoline/guanidine compounds promote insulin release from RIN-5AH cells, by interacting with a novel binding site related to K+ATP channels that does not represent any of the known imidazoline I1 or I2 receptors.

Topics: Adrenergic alpha-Agonists; Adrenergic alpha-Antagonists; Adrenergic beta-Agonists; Affinity Labels; Animals; Benzofurans; Binding, Competitive; Clonidine; Clorgyline; Diazoxide; Dioxanes; Guanidines; Idazoxan; Imidazoles; Imidazoline Receptors; Insulin; Insulinoma; Ligands; Pancreatic Neoplasms; Potassium Channels; Rats; Receptors, Drug; Regression Analysis; Software; Tumor Cells, Cultured