Journals of Gerontology Series A: Biological Sciences and Medical Sciences, Vol 53, Issue 4 B268-B273, Copyright © 1998 by The Gerontological Society of America
Decrease in apparent alpha1-adrenoceptor-g protein coupling during maturation in rat aorta
H Gurdal, E Friedman, MD Johnson and HO Onaran
Department of Pharmacology, MCP Hahnemann School of Medicine, Philadelphia, PA.
We have primarily previously shown that the adrenoceptor agonist
norepinephrine (NE) is more potent in eliciting contraction in aortas from
1-month-old Fischer 344 rats than it is in older animals. In the present
study, we examined alpha1-adrenoceptor-guanine nucleotide regulatory
binding protein (G protein) coupling in aortic membranes in order to
investigate the mechanism for the age-dependent reduced responsiveness of
aorta to NE. We used the guanosine 5' (by-imido)triphosphate
(Gpp[NH]p)-induced shift in agonist binding affinity as a measure of the
efficiency of alpha1-adronoceptor-G protein coupling. The binding of NE
was assessed by measuring the displacement of
2-[B-(4-hydroxy-3-[125I]iodephenyl)ethylaminomethyl] tetralone (125]-HEAT)
by NE in aortic membranes. In 1-, 6-, and 24-month-old rat aortas, two
apparent binding sites were deteced in the competition isotherms for NE.
This heterogeneous binding pattern was independent of Gpp (NH) at all ages,
and is likely to be due to a heterogeneous receptor population (alpha1a,
alpha 1b, and alpha 1d subtypes). In 1-month-old rats, the high affinity
binding of NE to alpha1-adrenoceptors was sensitive to Gpp(NH)p, indicating
a significant interaction between the receptor and G protein. This
Gpp(NH)p-sensitive high affinity binding was not observed in aortas from 6-
or 24-month-old animals. Despite the lack of Gpp(NH)p-sensitive high
affinity binding of agonist in 6- or 24-month-old aortas, NE was still able
to induce maximal contraction in these aortas, albeit, with a relatively
low potency. A partial reduction in alpha1-adrenoceptor-G protein coupling
between 1 and 6 months of age can explain the observed decrease in agonist
potency and the loss of Gpp(NH)p-sensitive high affinity binding of NE.
This phenomenon can be explained as a reduction of allosteric coupling
between the bindings of ligand and G protein to the receptor, that has been
formulated in the ternary complex model. Computer stimulation using the
simple ternary complex model shows that manipulating the reciprocal
coupling factor alone can lead to a loss of Gpp(NH)p-sensitive high
affinity agonist binding, along with a reduction in agonist potency for
contraction without altering the maximal response. Thus, a change in the
relative expression of different alpha1-adrenoceptor subtypes, which we
have previously observed in the aorta, and which possess diverse allosteric
coupling, may be speculated to be the mechanism for the apparent reduction
of alpha1-adrenoceptor-G protein coupling during maturation.
