Spontaneous coupling of the beta-adrenergic receptor to Ns in mammalian cardiac membranes.

The beta-adrenergic receptor in membranes from cat, rat, and guinea pig hearts appears to undergo spontaneous coupling to the stimulatory nucleotide-regulatory protein, Ns. This was demonstrated by incubating cardiac membranes from acutely reserpinized animals with the alkylating reagent N-ethylmaleimide (NEM), which is known to "freeze" the beta-receptor Ns-complex. The concentration of norepinephrine in these membrane preparations was less than 0.1 nM. Cat heart membranes were preincubated for 10 min at 30 degrees with NEM (10(-7)-10(-3) M) and subsequently incubated with (-)-[125I]iodopindolol (IPIN) (18 min, 30 degrees). NEM caused a concentration-dependent decrease in specific IPIN binding with a maximal reduction of about 20% at 0.1 mM. This decrease occurred even in the absence of MgCl2 (0.5 mM EDTA added as well). A comparable reduction was also observed in myocardial membranes from rat and guinea pig. This fall reflects a decrease in the number of beta-adrenergic receptor sites, as demonstrated by saturation binding experiments with IPIN. The equilibrium dissociation constant of the radioligand for the remaining receptors was not affected. When increasing concentrations of GTP were included in the preincubation mixture, it resulted in a dose-dependent protection of NEM-induced decrease in IPIN binding. The protection was complete at 0.1 mM GTP. In addition, GTP reversed the NEM effect when added to the incubation mixture 10 min after NEM. The apparent reduction in cardiac beta-adrenergic receptor number by NEM (in the absence of beta-receptor agonist) is compatible with a model in which part of the receptor population is able to undergo spontaneous coupling to Ns.