Binding of [3H]dihydroergocryptine to an alpha-adrenergic site in the stalk median eminence of the steer.
Dihydroergocryptine (DHE), a potent dopamine agonist and alpha-adrenergic antagonist, has been used as a radioligand to characterize both dopamine and alpha-adrenergic receptors. In the present study, the binding of [3H]DHE to particulate fractions of the steer stalk median eminence was characterized using a filtration assay. Specific binding was defined by the presence of 10 microM phentolamine or by an iterative nonlinear hyperbolic curve-fitting program. Scatchard analysis of equilibrium isotherms of specific binding defined a single high affinity (Kd = 1.78 +/- 0.22 nM), saturable (maximum binding, 481 +/- 39 fmol/mg protein), stereoselective binding site. The Kd, calculated from the ratio of the rate constants k2 and k1, was 2.8 +/- 0.14 nM. The rank order of potency of agonists to compete for [3H]DHE binding (l-epinephrine greater than l-norepinephrine greater than dopamine greater than l-isoproterenol) was consistent with interactions at an alpha-adrenergic site. The rank order of potency of alpha-antagonists (phentolamine greater than yohimbine greater than prazosin) suggested that this was an alpha 2-adrenergic receptor. The affinity of dopamine agonists for the [3H]DHE-binding site was 10-fold lower relative to their potency at known dopamine receptors, while the affinity of dopaminergic antagonists was 100-fold lower. Furthermore, Scatchard analysis of specific [3H]DHE binding in the presence of a concentration of spiperone which should saturate dopamine receptors, only decreased the number of binding sites by 9%. These data demonstrate the presence of large numbers of alpha-adrenergic receptors in the stalk median eminence of the steer. Only a small number of dopaminergic binding sites for [3H]DHE appeared to be present.
D000818:Animals; D001667:Binding, Competitive; D002417:Cattle; D004088:Dihydroergotoxine / Q000378:metabolism*; D007030:Hypothalamo-Hypophyseal System / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008473:Median Eminence / Q000378:metabolism*; D011941:Receptors, Adrenergic / Q000378:metabolism*; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014316:Tritium
D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D014316:Tritium; D004088:Dihydroergotoxine