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MD Snavely, MG Ziegler and PA Insel
We have developed a method for the assessment of agonist-induced down- regulation of receptors in vivo in terms of rates of receptor appearance and disappearance. This method involves computer-assisted analysis of the kinetics of receptor loss during agonist infusion and of the recovery of receptor number upon the removal of the agonist. These kinetics are analyzed in terms of a steady state model that allows estimation of the rate constants for receptor appearance, kappa ap, and receptor disappearance, kappa dp. Several tests establish that the model can fit experimentally derived data very well. In testing this model, we examined the in vivo down-regulation and recovery of rat renal cortical membrane beta 1- and beta 2-adrenergic receptors in response to infusion of the agonist isoproterenol from subcutaneously implanted osmotic minipumps. During recovery from down-regulation, the beta 1-receptors have a t 1/2 of 45 hr and a kappa ap of 1.6%/hr, and the beta 2-adrenergic receptors a t 1/2 of 18 hr and a kappa ap of 3.9%/hr. During down-regulation, the t 1/2 for both receptors is 12 hr, while kappa ap for beta 1-receptors and beta 2-receptors are 3 and 2.3%/hr, respectively. To the extent that the kinetics of recovery from down-regulation reflect "basal" receptor metabolism, the data indicate that enhanced receptor clearance of both receptor subtypes from the plasma membrane contribute to down-regulation, but changes in rates of receptor appearance may occur as well. The use of this computer modeling technique for defining kinetics of changes in receptor number from one steady state level to another should provide a generally useful means to assess hormone and neurotransmitter receptor metabolism in vivo.
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