ISSN:
1432-1912
Keywords:
Adenosine receptors
;
Rat fat cells
;
Adenylate cyclase
;
Lipolysis
;
Radioligand binding
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary A new adenosine analogue, (−)-iodo-N6-p-hydroxyphenylisopropyladenosine [(−)-IHPIA], has been developed for radioligand binding studies of Ri adenosine receptors. In addition, the effects of (−)IHPIA on adenosine-mediated responses of rat fat cells have been characterized. (−)IHPIA is slightly less potent at Ri adenosine receptors than (−)N6-phenylisopropyladenosine [(−)PIA] as assessed by adenylate cyclase and lipolysis studies. (−)IHPIA inhibited basal adenylate cyclase activity with an IC50 of 60 nmol/l compared to an IC50 of 16.3 nmol/l for (−)PIA. (−)PIA and (−)IHPIA inhibited adenosine deaminase-stimulated lipolysis of intact rat fat cells with an IC50 of 0.55 and 3.6 nmol/l. The potency of (−)N6-p-hydroxyphenylisopropyladenosine [(−)HPIA] was intermediate. (−)HPIA has been labelled with carrier-free Na[125I] to very high specific activity (2,175 Ci/mmol) and used as agonist radioligand in binding studies of Ri adenosine receptors. The binding of (−)[125I]HPIA was saturable, reversible and stereospecific. Saturation analysis revealed two affinity states with dissociation constants (K D) of 0.7 and 7.6 nmol/l and maximal number of binding sites (B max) of 0.94 and 0.95 pmol/mg protein. The rate constant of association, k 1, was 3.7×108 l×mol−1×min−1. Binding was slowly reversible with a t1/2 of 88 min. In competition experiments specific binding was most potently inhibited by (−)PIA, N6-cyclohexyladenosine (CHA), (−)HPIA and (−)IHPIA, followed by 5′-N-ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine. 1,3-Diethyl-8-phenylxanthine (DPX) and 8-phenyltheophylline were the most potent adenosine antagonists with K i-values of 67 and 83 nmol/l, whereas the methylxanthines 3-isobutyl-1-methylxanthine, theophylline and caffeine had K i-values between 1 and 21 μmol/l. Binding is highly stereospecific, as indicated by an approximately 20-fold higher K i-value of the (+)isomer of PIA in comparison to the (−)isomer. The pharmacological profile of (−)[125I]HPIA binding sites is consistent with an interaction at R i adenosine receptors. (−)[125I]HPIA appears to be a suitable agonist for radioligand binding studies at R i adenosine receptors.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00505324
Permalink
