TY - JOUR
T1 - Imidazoline receptors in rat liver cells
T2 - A novel receptor or a subtype of α2-adrenoceptors?
AU - Zonnenschein, Ruth
AU - Diamant, Sophia
AU - Atlas, Daphne
PY - 1990/11/6
Y1 - 1990/11/6
N2 - 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°C within 3.5 h and at 30°C within 30 min, respectively. Rat lung membranes had more imidazoline sites (Bmax = 578 ± 30 fmol/mg protein, Kd = 14 ± 1.4 nM) than α2-adrenoceptors (Bmax = 175.0 ± 20.0 fmol/mg protein,Kd = 4.8 ± 2.0 nM). We also screened other tissues for imidazoline 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 α2-adrenoceptors, although it is not a subtype of the adrenoceptor, since it bound neither the endogenous agonists norepinephrine and epinephrine, nor the selective α2-antagonists yohimbine and phentolamine. All guanidine type α2-adrenoceptor drugs (e.g. guanabenz, 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 α2-adrenoceptors. Ionic changes were considered as a possible mechanism underlying the α2-adrenoceptor effects in various cells. Opening of K+ channels by α2-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 α2-adrenoceptors in brain membranes and human platelets competed with idazoxan at rat liver imidazoline receptors. Although the need of an endogenous ligand for imidazoline sites is obvious, it has yet to be determined whether CDS is the endogenous ligand for imidazoline receptors in general, and for liver cells in particular, and whether K+ channels are involved in the mechanism of action of these sites.
AB - 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°C within 3.5 h and at 30°C within 30 min, respectively. Rat lung membranes had more imidazoline sites (Bmax = 578 ± 30 fmol/mg protein, Kd = 14 ± 1.4 nM) than α2-adrenoceptors (Bmax = 175.0 ± 20.0 fmol/mg protein,Kd = 4.8 ± 2.0 nM). We also screened other tissues for imidazoline 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 α2-adrenoceptors, although it is not a subtype of the adrenoceptor, since it bound neither the endogenous agonists norepinephrine and epinephrine, nor the selective α2-antagonists yohimbine and phentolamine. All guanidine type α2-adrenoceptor drugs (e.g. guanabenz, 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 α2-adrenoceptors. Ionic changes were considered as a possible mechanism underlying the α2-adrenoceptor effects in various cells. Opening of K+ channels by α2-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 α2-adrenoceptors in brain membranes and human platelets competed with idazoxan at rat liver imidazoline receptors. Although the need of an endogenous ligand for imidazoline sites is obvious, it has yet to be determined whether CDS is the endogenous ligand for imidazoline receptors in general, and for liver cells in particular, and whether K+ channels are involved in the mechanism of action of these sites.
KW - Clonidine displacing substance (CDS)
KW - Imidazoline receptors
KW - α-Adrenoceptors
UR - http://www.scopus.com/inward/record.url?scp=0025044010&partnerID=8YFLogxK
U2 - 10.1016/0014-2999(90)94127-J
DO - 10.1016/0014-2999(90)94127-J
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C2 - 1981751
AN - SCOPUS:0025044010
SN - 0014-2999
VL - 190
SP - 203
EP - 215
JO - European Journal of Pharmacology
JF - European Journal of Pharmacology
IS - 1-2
ER -