Beta-adrenoceptor-G alpha S coupling decreases with age in rat aorta.
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 125I-cyanopindolol (125I-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 125I-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 125I-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 (KH - 0.061 microM and KL = 2.4 microM). In the presence of Gpp(NH)p the ISO displacement curve was monophasic (Kd - 0.72 microM). 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, KH = 0.2 microM and KL = 3.5 microM; in the presence of Gpp(NH)p, Kd - 0.83 microM], 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 microM and 0.8 microM 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.(ABSTRACT TRUNCATED AT 400 WORDS)
Gurdal
H
H
Department of Pharmacology, Medical College of Pennsylvania, Philadelphia 19129, USA.
Friedman
E
E
Johnson
M D
MD
D000375:Aging / Q000502:physiology*; D000818:Animals; D001011:Aorta / Q000502:physiology; D019204:GTP-Binding Proteins / Q000502:physiology*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D066298:In Vitro Techniques; D020120:Iodocyanopindolol; D008297:Male; D009829:Oleic Acids / Q000494:pharmacology; D010869:Pindolol / Q000031:analogs & derivatives / Q000378:metabolism; D011233:Precipitin Tests; D051381:Rats; D011916:Rats, Inbred F344; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D014664:Vasodilation
D009829:Oleic Acids; D011943:Receptors, Adrenergic, beta; D006165:Guanylyl Imidodiphosphate; C065593:cis-vaccenic acid; D020120:Iodocyanopindolol; D010869:Pindolol; D019204:GTP-Binding Proteins