Sodium and guanine nucleotide regulation of dopamine receptor agonist and antagonist binding sites in MtTW15 pituitary tumors

The modulation of the dopamine receptor in MtTW15 tumors was investigated. The antagonist dopaminergic binding site in MtTW15 tumors labelled with [3H]spiperone remains unchanged at 25 °C in the presence or absence of sodium or guanine nucleotides (Gpp(NH)p); by contrast at 37 °C sodium increases the affinity while Gpp(NH)p decreases it slightly. The dopamine receptor in this tumor, such as the intact adenohypophysis, exists in a high and low affinity state for dopamine agonists. These agonist affinity states evaluated with apomorphine competition for [3H]spiperone binding show similar affinities as those of intact tissue but have a lower proportion of the high affinity state. At 25 °C, a partial conversion of the high into the low affinity state is obtained in the presence of both sodium and Gpp(NH)p, while at 37 °C a complete conversion is observed. These data show differences in the modulation of antagonist and agonist dopaminergic binding sites in MtTW15 pituitary tumors compared with the intact pituitary.