Identification and characterization of platelet α2-adrenoceptors and imidazoline receptors in rats, rabbits, cats, dogs, cattle, and horses

This study aimed to pharmacologically identify and characterize alpha(2)-adrenoceptors and imidazoline (1) receptors (I-1- and I-2-subtype) on canine, feline, bovine, equine, murine, and leporine platelet membranes. Saturation binding studies with both H-3-yohimbine and H-3-clonidine showed that alpha(2)-adrenoceptors were expressed on canine, leporine, feline, and murine platelets but not on bovine and equine platelets. In competition studies, the rank order of affinity of 6 compounds for canine platelet alpha(2)-adrenoceptors was similar to that of potency at alpha(2A)-subtype reported in human platelets. Saturation binding studies in the presence of norepinephrine showed that canine, feline, bovine, and equine platelets had I-1-receptors defined by H-3-clonidine binding, but neither murine nor leporine platelets had I-1-receptors; whereas, platelets of all species had I-2-receptors defined by H-3-idazoxan binding. In competition studies, more potent compounds displayed biphasic competition curves with H-3-clonidine. The rank orders of affinity of I-1 compounds for high affinity components of I-1-receptors of canine, feline, bovine, and equine platelets and I-2-receptors of all species platelets were similar to those of compounds for high affinity components reported in human I-1- and I-2-receptors, respectively. Guanine nucleotides inhibited the high affinity component of naphazoline binding to canine I-1-receptors, but not to I-2-receptors. Furthermore, guanine nucleotides dose dependently inhibited H-3-clonidine binding to I-1-receptors; whereas, they did not interfere with H-3-idazoxan binding to I-2-receptors, supporting the notion that I-1 receptors may belong to a G protein coupled receptor superfamily in canine platelets. Interspecific variations of platelet alpha(2)-adrenoceptor and imidazoline receptor expressions may explain different platelet responses to catecholamines and imidazoline a-adrenergic agents. (C) 2013 Elsevier B.V. All rights reserved.