Alpha 2-adrenoceptor in HT29 human colon adenocarcinoma cell-line: study of [3H](-)-adrenaline binding.
The HT29 human adenocarcinoma cell-line was used to investigate the binding of [3H](-)-adrenaline at alpha 2-adrenoceptors. All aspects of the study indicated that alpha 2-adrenoceptors were specifically labeled. [3H](-)-adrenaline bound with high affinity (KD = 2.28 +/- 0.41 nM) to a single population of non-interacting sites. The rank order of adrenoceptor antagonists (yohimbine greater than phentolamine much greater than prazosin) and agonists (UK-14,304 greater than clonidine greater than (-)-adrenaline greater than (-)-noradrenaline greater than (+)-adrenaline greater than (+)-noradrenaline greater than amidephrine) to compete with [3H](-)-adrenaline binding showed that the labeled sites were alpha 2-selective and stereospecific. Comparison of the binding parameters of [3H](-)-adrenaline with those of [3H]clonidine (partial-agonist) and [3H]yohimbine (antagonist) indicated that [3H](-)-adrenaline and [3H]clonidine labeled a similar number of sites (156 +/- 13 versus 175 +/- 21 fmol/mg protein) and that [3H]yohimbine (Bmax = 246 +/- 22 fmol/mg protein) labeled more sites than the 3H-agonists. Data on the inhibition of [3H]yohimbine binding by (-)-adrenaline was better fitted to a two-site model and revealed (1) that the KiL/KiH ratio was higher for (-)-adrenaline than for clonidine (2) that both agonists recognized the same percentage of high-affinity receptors. The results from a kinetic study of [3H](-)-adrenaline binding were apparently inconsistent with the equilibrium data. Both the association and dissociation were bi-exponential, suggesting a relative heterogeneity of the labeled sites. The tritiated physiological full-agonist was moreover able to induce tight-binding. The practical consequences of this property are discussed.
Sénard
J M
JM
INSERM U-317, Institut de Physiologie, Université Paul Sabatier, Toulouse, France.
Mauriège
P
P
Daviaud
D
D
Paris
H
H
D000230:Adenocarcinoma / Q000378:metabolism*; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D003000:Clonidine / Q000378:metabolism; D003110:Colonic Neoplasms / Q000378:metabolism*; D004837:Epinephrine / Q000378:metabolism*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006801:Humans; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D015016:Yohimbine / Q000378:metabolism
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D006165:Guanylyl Imidodiphosphate; D003000:Clonidine; D004837:Epinephrine