Decrease in apparent alpha1-adrenoceptor-G protein coupling during maturation in rat aorta

We have 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'(betagamma-imido)triphosphate (Gpp[NH]p)-induced shift in agonist binding affinity as a measure of the efficiency of alpha11-adrenoceptor-G protein coupling. The binding of NE was assessed by measuring the displacement of 2-[beta-(4-hydroxy-3-[125I]iodophenyl)ethylaminomethyl] tetralone ([125]-HEAT) by NE in aortic membranes. In 1-, 6-, and 24-month-old rat aortas, two apparent binding sites were detected in the competition isotherms for NE. This heterogeneous binding pattern was independent of Gpp(NH)p at all ages, and is likely to be due to a heterogeneous receptor population (alpha(1a), alpha(1b), and alpha(1d) subtypes). In 1-month-old rats, the high affinity binding of NE to alpha,-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 ago- nist 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 simulation 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 alpha,-adrenoceptor subtypes, which we have previously observed in the aorta, and which possess diverse intrinsic allosteric couplings, may be speculated to be the mechanism for the apparent reduction of alpha,-adrenoceptor-G protein coupling during maturation.