Imidazoline receptors in rat liver cells: a novel receptor or a subtype of alpha 2-adrenoceptors?
An imidazoline/guanidine receptor has been characterized in rat liver cells. Binding of [3H]idazoxan, a selective benzodioxan antagonist, to imidazoline receptor on intact fresh hepatocytes (Bmax = 801 +/- 23 fmol/mg protein, Kd = 11 +/- 0.8 nM) and to liver membranes (Bmax = 400 +/- 38 fmol/mg protein, Kd = 10 +/- 2 nM) was saturable at 4 degrees C within 3.5 h and at 30 degrees C within 30 min, respectively. Rat lung membranes had more imidazoline sites (Bmax = 578 +/- 30 fmol/mg protein, Kd = 14 +/- 1.4 nM) than alpha 2-adrenoceptors (Bmax = 175.0 +/- 20.0 fmol/mg protein, Kd = 4.8 +/- 2.0 nM). We also screened other tissues for imidazoline sites; the ratio of adrenoceptors to total sites labeled with [3H]idazoxan displaced by cirazoline was lower in rat lung compared to rat brain and human platelets. The imidazoline receptor has common pharmacological properties with alpha 2-adrenoceptors, although it is not a subtype of the adrenoceptor, since it bound neither the endogenous agonists norepinephrine and epinephrine, nor the selective alpha 2-antagonists yohimbine and phentolamine. All guanidine type alpha 2-adrenoceptor drugs (e.g. guanbenz, guanoxan) and imidazolines (e.g., UK-14,304, naphazoline) competed with high affinity for the liver imidazoline receptor. The lack of effect by Gpp(NH)p, a non-hydrolysable GTP analogue, on the affinity of guanidine- and imidazoline-type ligands for liver imidazoline receptors suggests that the mode of action of these drugs at imidazoline receptors is different than at conventional alpha 2-adrenoceptors. Ionic changes were considered as a possible mechanism underlying the alpha 2-adrenoceptor effects in various cells. Opening of K+ channels by alpha 2-adrenoceptors agonists is a pathway which might be shared by imidazoline-type agonists at imidazoline sites. Indeed, 4-aminopyridine, a K+ channel blocker, inhibited the specific binding of [3H]idazoxan to liver cells with an IC50 of 0.34 +/- 0.07 mM a concentration which is effective in blocking K+ channels in neuronal cells. Similarly, Cs+ and NH4+ effectively interfered with [3H]idazoxan binding, suggesting a possible coupling of imidazoline sites to K+ gating. The endogenous ligand clonidine-displacing substance (CDS), which was isolated from bovine brain and which binds to alpha 2-adrenoceptors in brain membranes and human platelets competed with idazoxan at rat liver imidazoline receptors.(ABSTRACT TRUNCATED AT 400 WORDS)
Zonnenchein
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R
Department of Biological Chemistry, Hebrew University of Jerusalem, Israel.
Diamant
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Atlas
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D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism; D003000:Clonidine / Q000378:metabolism; D004146:Dioxanes / Q000494:pharmacology; D006146:Guanidines / Q000494:pharmacology; D006801:Humans; D019329:Idazoxan; D054755:Imidazoline Receptors; D066298:In Vitro Techniques; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008168:Lung / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D015221:Potassium Channels / Q000187:drug effects; D051381:Rats; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D011955:Receptors, Drug / Q000378:metabolism*
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D004146:Dioxanes; D006146:Guanidines; D054755:Imidazoline Receptors; D015221:Potassium Channels; D011942:Receptors, Adrenergic, alpha; D011955:Receptors, Drug; D003000:Clonidine; D019329:Idazoxan