Akademik Veri Yönetim Sistemi
MOLECULAR PHARMACOLOGY, cilt.47, sa.4, ss.772-778, 1995 (SCI-Expanded)
beta-Adrenoceptor (beta AR) responsiveness, receptor density, receptor-G protein coupling, and the possible role of membrane fluidity in receptor-G protein coupling were investigated in the rat aorta with age. The beta AR agonist isoproterenol (ISO) produced relaxation of KCl-induced aortic contractions by 97%, 21%, and 0% in aortae from 1-, 6-, and 24-month-old Fischer 344 rats, respectively. Forskolin completely relaxed the contractions at all ages. beta AR density was determined in aortic membranes by saturation binding of I-125-cyanopindolol (I-125-CYP). beta AR density was 76, 52, and 47 fmol/mg of protein in 1-, 6-, and 24-month-old rats, respectively. To investigate beta AR coupling to G proteins, displacement by ISO of I-125-CYP binding was determined in aortic membranes in the presence and absence of the GTP analog guanosine-5'-(beta-gamma-imido)triphosphate [Gpp(NH)p] (0.1 mM). The effect of Gpp(NH)p on the ISO displacement curve for I-125-CYP binding was greatest in 1-month-old rats and decreased markedly with age. In 1-month-old aorta, in the absence of Gpp(NH)p the ISO displacement curve was biphasic and two affinity constants were determined (K-H = 0.061 mu M and K-L = 2.4 mu M). In the presence of Gpp(NH)p the ISO displacement curve was monophasic (K-d = 0.72 mu M). In 6-month-old aorta, whereas an effect of Gpp(NH)p on the ISO displacement curve could still be observed [in the absence of Gpp(NH)p, K-H = 0.2 mu M and K-L = 3.5 mu M; in the presence of Gpp(NH)p, K-d = 0.83 mu M], the affinity constant for high affinity agonist binding and the percentage of receptors with high affinity for agonist were decreased significantly. In 24-month-old aorta there was no effect of Gpp(NH)p on the ISO displacement curve and a single affinity constant was detected [0.7 mu M and 0.8 mu M in the presence and absence of Gpp(NH)p, respectively]. The presence of two affinity constants for ISO in 1- and 6-month-old aorta in the absence of Gpp(NH)p and single affinity constants in the presence of Gpp(NH)p presumably represent the G protein-coupled and uncoupled states of the beta ARs, which are not observed in 24-month-old aorta. The ability of the beta AR to form the high affinity nucleotide-sensitive complex with the agonist was restored by treatment of the membranes with cis-vaccenic acid, which increases the fluidity of the membrane. The agonist-independent basal interactions of beta ARs with G proteins were examined using an immunoprecipitation approach. Aortic membranes were solubilized and then immunoprecipitated using specific antisera directed against the individual G protein alpha subunits (G(alpha s), G(alpha l), G(alpha o), and G(alpha q)). beta ARs in the immunoprecipitate were then determined by measurement of the specific binding of I-125-CYP. In solubilized preparations from 1-month-old aorta, beta AR binding was immunoprecipitated by G(alpha s)-specific antiserum but not by G(alpha l)-, or G(alpha o)-, or G(alpha q)-specific antiserum. G(alpha s)-specific antiserum did not immunoprecipitate beta AR binding in solubilized preparations from 6- or 24-month-old aorta. These results indicate that beta ARs are coupled to G(alpha s) in aortae from 1-month-old rats but that coupling is decreased or absent under basal and agonist-stimulated conditions in aortae from 6- and 24-month-old rats.