Pharmacologic and molecular discrimination of I2-imidazoline receptor subtypes

I-2-imidazoline receptors (I2IR) are characterized by their high affinity for imidazolines and guanidines and medium affinity for imidazolidines. The differential recognition of I-2-IR by amiloride led to subtype these sites as amiloride-sensitive (I-2A-IR) and amiloride-insensitive (I-2B-IR). I-2-IR labeled with [H-3]idazoxan or [H-3]2-BFI in the rabbit cerebral cortex (I-2A-IR) displayed higher affinities for amiloride and amiloride analogs than in the;rat cerebral cortex (I-2B-IR) Other drugs tested displayed biphasic curves in competition experiments, indicating the existence of high and low affinity Sites for both I-2-TR subtypes. The drugs (+)- and (-)-medetomidine, bromoxidine, moxonidine, and clorgyline, were more potent on the high and/or low affinity sites of I-2B-IR than on I-2A-IR Preincubation (30 min at 25 degrees C) with 10(-6) M isothiocyanotobenzyl imidazoline (IBI) or with 10(-6) M clorgyline reduced by 40% and 26%, respectively, the binding of BFI to I-2B-I BFI to I-2B-IR, but it did not alter the binding of the radioligand to I-2A-IR. These results indicated that the FIR subtypes differ in their pharmacologic profiles and in the nature of the imidazoline binding site involved in clorgyline and IBI alkylation, In rat cortical membranes, western blot detection of immunoreactive imidazoline receptor proteins revealed a double band of similar to 29/30 kD and three less intense bands of similar to 45; similar to 66, and similar to 85 kD. In rabbit cortical membranes the antibody detected proteins of similar to 30, similar to 57, similar to 66, and similar to 85 kD. It is suggested that I-2-IR may be related to more than one receptor protein and that I-2-IR subtypes differ in the nature of the proteins implicated.