ISSN:
1432-1912
Keywords:
Divalent cation site
;
α2-Adrenoceptors
;
Magnesium
;
Clonidine
;
Prazosin
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary α2-Adrenoceptors in rat brain membranes, identified with [3H] clonidine, have divalent cation sites. Nickel, manganese, magnesium, barium, and strontium are among the active cations which potentiate clonidine binding. The effects of magnesium were investigated in detail: 1. The potentiation of binding by magnesium is caused by an increase in the association constant (Ka) and by increased availability of sites to the agonist. Both, availability and affinity increase, are dependent on temperature. Ka increased with increasing temperature when free magnesium is present. However, whether the divalent cation is present or not, clonidine binding to α2-adrenoceptors as well as the binding of the antagonist prazosin to α2-adrenoceptors are mainly entropy driven. 2. The effects of magnesium on α2-adrenoceptors can be reversed rapidly (t 1/2〈1 min, 30°C) by chelation; the conversion of α2-adrenoceptors into the divalent cation-liganded state is also rapid (t 1/2〈1 min, 30°C). GTP, presumably via a guanyl nucleotide protein, is a potent inhibitor of agonist binding in the presence of magnesium. 3. The magnesium-induced affinity increase of clonidine to α2-adrenoceptors is shared by other agonists but not by the antagonists phentolamine and yohimbine. 4. The α1-adrenoceptors antagonist prazosin looses 30-fold in affinity for the α2-adrenoceptors when the divalent cation requirement if fulfilled with magnesium. 5. Brain membrane α2-adrenoceptors can be converted into an almost completely divalent cationdependent state by a brief and mild thermal denaturation. We are able to discriminate four conformations of α2-adrenoceptors in rat brain membranes: α0 is a species of presumably very low affinity for clonidine (Ka〈0.01 nM−1) which cannot be detected by our current methodology, αL 1 has a Ka around 0.12 nM−1 (at 30°C) and is predominant species in native membranes in the absence of free divalent cations, αL2 is intermediate between α0 and αL 1 in its affinity and is found after a brief heat treatment of membranes in EDTA containing buffers, αH is the high affinity conformation of α2-adrenoceptors. It can be formed in vitro by fulfilling the divalent cation requirement with magnesium. The divalent cation converts α02, αL and α2 z into αH. Since the effects of magnesium are agonist-specific, they may relate to signal transfer. It is proposed that the different conformations, identified by their affinity for agonists, represent free receptors and different complexes of receptors with effector and/or coupling proteins.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00504525
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