Dopamine receptors in the 7315a transplantable rat anterior
pituitary tumor were characterized using radioligand binding assays with [3H]
spiroperidol ([3H]SPD) and assays of
adenylate cyclase activity. Scatchard analysis of the binding of [3H]SPD yielded linear plots and a Kd value of 73 pM. Studies of the inhibition of the binding of [3H]SPD were performed with a series of competing
ligands, including the antagonists
domperidone, (+)-
butaclamol and
sulpiride and the
agonists dopamine,
bromocriptine and
N-propylnorapomorphine. Inhibition curves for the antagonists gave Hill coefficients of approximately 1, consistent with the presence of only a single class of binding sites with a high affinity for [3H]SPD. In contrast, the Hill coefficient for
dopamine was significantly less than 1. When assays were carried out in the presence of 300 microM
GTP, the inhibition curve for
dopamine was shifted to the right and the Hill coefficient increased to approximately 1. An effect of
GTP on the affinity of a receptor for agonists is consistent with the existence of at least two agonist affinity states. Inhibition of the binding of [3H]SPD by the partial agonist
bromocriptine was not affected when assays were carried out in the presence of
GTP. The uniform low affinity of the selective
serotonin antagonist ketanserin for these sites indicated that the radioligand was not labeling
serotonin-2 receptors in this tissue. A good correlation was observed between the Ki values for competing
ligands measured in the
tumor and in homogenates of rat striatal tissue.
Dopamine was shown to inhibit
forskolin-stimulated
adenylate cyclase activity.(ABSTRACT TRUNCATED AT 250 WORDS)