DECREASE IN APPARENT ALPHA(1)-ADRENOCEPTOR G-PROTEIN COUPLING DURING MATURATION IN RAT AORTA

We heave previously shown that the adrenoceptor agonist norepinephrine (IVE) is more potent in eliciting contraction ill aortas from 1-month -old Fischer 344 rats than it is in older animals. in the present stud y, we examined alpha(1)-adrenoceptor-guanine nucleotide regulatory bin ding protein (G protein) coupling 81 aortic membranes in order to inve stigate the mechanism for the age-dependent reduced responsiveness of aorta to NE. We used the guanosine S'(beta gamma-imido)tritphosphate ( Gpp[NH]p)-induced shift in agonist binding affinity as a measure of th e efficiency of alpha(1)-adrenoceptor-G protein coupling. The binding of NE was assessed by measuring the displacement of -(4-hydroxy-3-[(12 5)l]iodophenyl)ethylaminomethyl] tetralone ([(125)]-HEAT) by NE in aor tic membranes. In 1-, 6-, and 24-month-old rat aortas, two apparent bi nding sites were detected ill the competition isotherms for NE. This h eterogeneous 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 mts, the high affinity binding of NE to alpha(1)-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 observ ed ill aortas from 6- or 24-month-old animals. Despite the lack of Gpp (NR)p-sensitive high affinity binding of agonist ill 6- or 24-month-ol d aortas, NE was still able to induce maximal contraction ill these ao rtas, albeit, with a relatively low potency. A partial reduction in al pha(1)-anrenoceptor-G protein coupling between I and 6 profiles of age call explain the observed decrease ill agonist potency and the loss o f Gpp(NH)p-sensitive high affinity binding of NE. This phenomenon can be explained as a reduction of allosteric coupling between the binding s of ligand and G protein to the receptor, that has been formulated in the ternary complex model. Computer simulation using the simple terna ry complex model shows that manipulating the reciprocal coupling facto r alone cart lead to a loss of Gpp(NH)p-sensitive high affinity agonis t binding, along with a reduction ill agonist potency for contraction without altering the maximal response. This, a change ira the relative expression of different alpha(1)-adrenoceptor subtypes, which,ve have previously observed ill the aorta, and which possess diverse intrinsi c allosteric couplings, may be speculated to be the mechanism for the apparent reduction of alpha(1)-adrenoceptor -G protein coupling during maturation.