Mechanism of A2 adenosine receptor activation. I. Blockade of A2 adenosine receptors by photoaffinity labeling.
It has previously been shown that covalent incorporation of the photoreactive adenosine derivative (R)-2-azido-N6-p-hydroxy-phenylisopropyladenosine [(R)-AHPIA] into the A1 adenosine receptor of intact fat cells leads to a persistent activation of this receptor, resulting in a reduction of cellular cAMP levels [Mol. Pharmacol. 30:403-409 (1986)]. In contrast, covalent incorporation of (R)-AHPIA into human platelet membranes, which contain only stimulatory A2 adenosine receptors, reduces adenylate cyclase stimulation via these receptors. This effect of (R)-AHPIA is specific for the A2 receptor and can be prevented by the adenosine receptor antagonist theophylline. Binding studies indicate that up to 90% of A2 receptors can be blocked by photoincorporation of (R)-AHPIA. However, the remaining 10-20% of A2 receptors are sufficient to mediate an adenylate cyclase stimulation of up to 50% of the control value. Similarly, the activation via these 10-20% of receptors occurs with a half-life that is only 2 times longer than that in control membranes. This indicates the presence of a receptor reserve, with respect to both the extent and the rate of adenylate cyclase stimulation. These observations require a modification of the models of receptor-adenylate cyclase coupling, which is described in the accompanying paper [Mol. Pharmacol. 39:524-530 (1991)].
Lohse
M J
MJ
Pharmakologisches Institut, Universität Heidelberg.
Klotz
K N
KN
Schwabe
U
U
D000241:Adenosine / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D019830:Adenosine-5'-(N-ethylcarboxamide); D000262:Adenylyl Cyclases / Q000378:metabolism / Q000502:physiology; D000527:Alprostadil / Q000494:pharmacology; D001386:Azides / Q000378:metabolism / Q000494:pharmacology*; D001792:Blood Platelets / Q000378:metabolism / Q000648:ultrastructure; D006801:Humans; D007700:Kinetics; D008566:Membranes / Q000378:metabolism; D010660:Phenylisopropyladenosine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D058914:Purinergic Antagonists; D011869:Radioligand Assay; D011983:Receptors, Purinergic / Q000502:physiology; D014316:Tritium; D014665:Vasodilator Agents / Q000378:metabolism; D014970:Xanthines / Q000378:metabolism
D001386:Azides; D058914:Purinergic Antagonists; D011983:Receptors, Purinergic; D014665:Vasodilator Agents; D014970:Xanthines; D014316:Tritium; D010660:Phenylisopropyladenosine; D019830:Adenosine-5'-(N-ethylcarboxamide); C051869:8-(4-((2-aminoethyl)aminocarbonylmethyloxy)phenyl)-1,3-dipropylxanthine; C049136:2-azido-N(6)-4-hydroxyphenylisopropyladenosine; D000262:Adenylyl Cyclases; D000527:Alprostadil; D000241:Adenosine