The turnover of beta 1- and beta 2-adrenergic receptors was measured after both
isoproterenol-induced down-regulation and irreversible blockade of receptors. Changes in the density of receptors were quantified using the radioligands 125I-iodopindolol and 125I-iodocyanopindolol. Treatment of intact L6 myoblasts or C6
glioma cells with
N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (
EEDQ) inactivated
beta-adrenergic receptors on membranes prepared from these cells. At a concentration of 100 microM
EEDQ, more than 90% of beta 1- and beta 2-adrenergic receptors were inactivated within 2 hr of treatment. Recovery of
beta-adrenergic receptors on intact cells after inactivation by
EEDQ required more than 24 hr and was prevented by
cycloheximide, an inhibitor of
protein synthesis. The kinetics of recovery of the density of receptors were analyzed in terms of a model that allows estimation of the rate constants for receptor appearance in and disappearance from the membrane, assuming that the rate of appearance of receptors is constant and the rate of disappearance of receptors is proportional to the number of receptors. Beta 2-Adrenergic receptors on L6 myoblasts were incorporated into the membrane at a rate of 28 fmol/mg of
protein/hr and had a half-life of 12.6 hr. On C6
glioma cells, Beta 1- and beta 2-adrenergic receptors appeared at rates of 13.3 and 6.6 fmol/mg of
protein/hr, respectively, with half-lives of 9.4 and 6.4 hr. Recovery of receptors on C6 cells after
isoproterenol-induced down-regulation was inhibited by
cycloheximide. The rate of recovery of beta 1- and beta 2-adrenergic receptors was reduced
after treatment with
isoproterenol for 8 hr when compared to recovery
after treatment with
EEDQ. The major effect of treatment with
isoproterenol was a persistent decrease in the rate of appearance of beta 1- and beta 2-adrenergic receptors (rate of synthesis and insertion into the membrane
after treatment with
isoproterenol = 4.0 fmol/mg of
protein/hr). Since treatment with
isoproterenol did not alter the rate of cell division or total
protein synthesis, the
isoproterenol-induced alteration was probably a specific effect on the rate of synthesis of
beta-adrenergic receptors.