Characterization of tight binding of isoprenaline and terbutaline to beta-adrenoceptors in lung membranes.

Preincubation of guinea-pig lung membranes with the beta-agonist isoprenaline or with the partial beta-agonist terbutaline, followed by repeated washing, causes a 35% decrease in the number of beta-adrenoceptors (R). This decrease corresponds to the tight binding of the agonist (H) to R as a result of the formation of a complex between H.R. and the stimulatory guanine nucleotide regulatory component (Ns). Tight binding of the agonist is dependent on the presence of magnesium ions and is reversed by GTP. However, the stability of the H.R.Ns-complex is limited even in the absence of GTP. Under these conditions, the complex is more stable when induced by isoprenaline than by terbutaline. When the alkylating reagent N-ethylmaleimide is included in the agonist preincubation step, tight binding of isoprenaline and terbutaline is increased to about 55% of the receptor population and is no longer dependent on the presence of magnesium ions. This value is in good agreement with the percentage of agonist high affinity sites, determined by isoprenaline/[3H]-dihydroalprenolol competition binding studies.