Sodium and guanine nucleotide regulation of dopamine receptor agonist and antagonist binding sites in MtTW15 pituitary tumors |
| |
Authors: | T Di Paolo D Lévesque |
| |
Institution: | School of Pharmacy, Laval University, Québec, Canada. |
| |
Abstract: | 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 degrees C in the presence or absence of sodium or guanine nucleotides (Gpp(NH)p); by contrast at 37 degrees 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 degrees 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 degrees 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. |
| |
Keywords: | |
|
|