Estradiol and guanine nucleotide modulation of dopamine receptor agonist and antagonist binding sites in 7315a pituitary tumors

Abstract

The agonist high- and low-affinity states of the dopamine (DA) receptor were investigated with apomorphine competition for [ 3H]spiperone binding to DA receptors in 7315a tumors grown in intact female rats, while the antagonist site was investigated with saturation of [ 3H]spiperone binding. Such as for the intact pituitary, the antagonist binding site density in 7315a tumors was not affected by NaCl and/or Gpp(NH)p, and its binding affinity was increased in the presence of NaCl. The DA receptor in 7315a tumors existed in high- and low-affinity agonist states, and the two apomorphine sites had similar affinities in tumoral and intact tissue. However, the proportion of the high affinity state was slightly lower in the 7315a tumor compared to intact tissue. Tumor (7315a) growth in ovariectomized rats was slower than in intact animal; chronic 17β-estradiol treatment inhibited growth of these tumors. Prolactin (PRL) concentration and density of DA receptors were higher in tumors grown in ovariectomized than in intact female rats, whereas both decreased after 23 days of 17β-estradiol treatment. Estradiol treatment decreased the affinity of the high- and the low-apomorphine binding sites, whereas their proportions remained unchanged. Thus, changes of DA receptors and 7315a tumor growth seem to be related; however, their relationship is complex.