Nonadrenergic imidazoline binding sites on human platelets.
Human platelets are shown to possess at least two high-affinity, imidazol(in)e-preferring binding sites that are pharmacologically distinct from alpha-2 adrenoceptors. These nonadrenergic sites were radiolabeled even in the presence of a 10 microM norepinephrine mask of alpha-2 adrenoceptors. Heterogeneity at the nonadrenergic sites was demonstrated by comparing [3H]idazoxan (IDX) binding vs. [125I]p-iodoclonidine (PIC) binding. Nonadrenergic [125I]PIC-labeled sites were enriched in platelet plasma membranes, whereas the nonadrenergic sites labeled by [3H] IDX were codistributed between plasma and internal membranes (nonadrenergic [125I]PIC-labeled sites had Bmax = 62 fmol/mg in plasma membranes and 20 fmol/mg in internal membranes vs. the [3H]IDX-labeled sites had Bmax = 141 fmol/mg in plasma membranes and 192 fmol/mg in internal membranes). Furthermore, competition binding studies in the presence of a 10 microM norepinephrine mask revealed major (approximately 75%) and minor (approximately 25%) binding components on plasma membranes for [125I]PIC. Affinities for the major nonadrenergic [125I]PIC binding site were highly comparable to human subtype-I1 imidazol(in)e receptor sites in the brain stem (rank order: moxonidine > clonidine > cirazoline > IDX > amiloride). However, the minor component of [125I]PIC binding was similar to a site reported in kidney, having low affinities for all compounds tested, except guanabenz. Finally, a third nonadrenergic internal membrane site, labeled by [3H]IDX, was consistent with a subtype-I2 imidazol(in)e receptor site (rank order: cirazoline > IDX >> amiloride > moxonidine > clonidine). Thus, based on differential subcellular distributions and affinity constants, human platelets appear to possess imidazoline receptors (subtype-I1 imidazol(in)e receptor and subtype-I2 imidazol(in)e receptor), plus a novel guanabenz-sensitive site, as well as an alpha-2A adrenoceptor. These nonadrenoceptor binding sites may explain certain novel platelet aggregatory properties previously ascribed to clonidine and endogenous clonidine-displacing substance(s), and may serve as markers of imidazoline receptors in humans.
T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T18 T19 T20 T21 T22 T23 T24 T25 T26 T27 T28 T29 T30 T31 T32 T33 T34 T35 T36 T37 T38 T39 T40 T41 T42 T43 T44
CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL CHEMICAL GENE-Y GENE-N GENE-N GENE-N GENE-Y GENE-N GENE-Y GENE-N GENE-N
[125I]PIC [125I]PIC imidazol(in)e moxonidine clonidine cirazoline IDX amiloride [125I]PIC guanabenz [3H]IDX imidazol(in)e cirazoline amiloride moxonidine clonidine imidazoline imidazol(in)e imidazol(in)e guanabenz clonidine clonidine imidazoline norepinephrine [3H]idazoxan IDX [125I]p-iodoclonidine PIC [125I]PIC [3H] IDX [125I]PIC imidazol(in)e [3H]IDX norepinephrine imidazoline human subtype-I1 imidazol(in)e receptor alpha-2 adrenoceptors subtype-I2 imidazol(in)e receptor imidazoline receptors subtype-I1 imidazol(in)e receptor subtype-I2 imidazol(in)e receptor alpha-2A adrenoceptor imidazoline receptors alpha-2 adrenoceptors