Dopamine (DA) and DA agonists bind with high affinity to anterior pituitary receptors which mediate the inhibition of PRL release. Spiperone (SPIP), a DA antagonist, has also been successfully used to characterize pituitary DA receptors with a dissociation constant (Kd) of less than 1 nM. We studied the binding of SPIP to GH3D6 cells which secrete only PRL and GH. This clone was derived from a radiation-induced tumor of the rat anterior pituitary. Equilibrium binding of [3H]SPIP to living GH3 cells showed no high affinity receptors, but a low affinity (Kd = 0.83 microM) and saturable (0.06 fmol/cell) population of sites was observed. In addition, saturable binding with a similar affinity (Kd = 0.57 microM) was noted in broken GH3 cells. The interaction was completely reversible and temperature dependent. The concentration of various ligands required to compete for half of the [3H]SPIP binding to whole cells were: chlorpromazine, 0.17 microM; haloperidol, 0.68 microM; pimozide, 0.77 microM; d-butaclamol, 1.16 microM; 1-butaclamol, 1.30 microM; SPIP, 1.49 microM; bromergocryptine, 4.98 microM; apomorphine, 13.9 microM; and DA, 100 microM. The absence of a high affinity site in GH3 cells is consistent with the decreased effectiveness of various agonists and antagonists on PRL secretion. It is possible that the low affinity interactions observed in GH3 cells are normally present in the anterior pituitary and brain and do not simply represent an alteration of receptor affinity.
Animals Binding, Competitive