ISSN:
1471-4159
Quelle:
Blackwell Publishing Journal Backfiles 1879-2005
Thema:
Medizin
Notizen:
Abstract: Although dopamine agonists can recognize two states of the D2 dopamine receptor in the anterior pituitary (Dhigh2 and Dlow2), we examined whether the dopamine antagonists such as [3H]spiperone could recognize these two sites with different affinities. Using up to 30 concentrations of [3H]spiperone, however, we could only detect a single population of binding sites (porcine anterior pituitary homogenates) with a dissociation constant (KD) of 130 pM. When specific [3H]spiperone binding was defined by a low concentration of (+)-butaclamol (100 nM), the apparent density was low. When defined by a high concentration of (+)-butaclamol (10 μM), nonspecific sites became detectable, thus revealing two apparent populations of sites for [3H]spiperone, only one of which was specific for dopamine. Sodium chloride reduced the KD of the single population of specific D2 sites to 64 pM. Guanine nucleotide by itself had no effect on the KD, but enhanced the density by 25%. Since the density-enhancement could be eliminated by extensive washing of membranes, and could be restored by preincubation with dopamine, the nucleotide-induced elevation of D2 density appeared to be a result of the release of tightly bound endogenous dopamine. Thus, monovalent cations and guanine nucleotides appear to have separate regulatory effects on the anterior pituitary D2 receptor that modulate antagonist-receptor interactions. Several maneuvers were used to test whether [3H]spiperone could differentiate between the two agonist-detected subpopulations of sites. Twentyfold different concentrations of [3H]spiperone (47 pM and 1000 pM) were found to label identical proportions of receptors in the Dhigh2 and Dlow2 states as detected by the agonist 6,7-dihydroxyaminotetralin (ADTN), suggesting that spiperone labelled equal proportions of Dhigh2 and Dlow2 sites without differential affinity for them. In addition, competition of spiperone for Dhigh2 sites selectively labelled by the agonist [3H]n-propylnora pomorphine (NPA) had a virtually identical KD for spiperone as did the total D2 receptor population as determined by direct binding studies (75 pM versus 64 pM). [3H]Spiperone also bound to a uniform population of Dlow2 sites induced by preincubation with guanine nucleotide with identical affinity as to the total D2 population. Thus, these data do not support a “reciprocal model’ for the D2 receptor (i.e., antagonist having low affinity for Dhigh2 and high affinity for Dlow2 in a manner reciprocal to agonists). It is therefore concluded from these studies that spiperone recognizes the Dhigh2 and Dlow2 states of the receptor with equal affinity and no evidence is provided in support for reciprocal modulation of antagonist/agonist affinities.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1111/j.1471-4159.1985.tb08740.x
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