Agonist-directed Trafficking of Porcine alpha(2A)-adrenergic Receptor Signaling in Chinese Hamster Ovary Cells: L-Isoproterenol Selectively Activates G(s)

J Pharmacol Exp Ther. 2000 Aug;294(2):539-47.

Abstract

In this study, we investigated the hypothesis of agonist-directed trafficking of receptor signaling for the alpha(2A)-adrenergic receptor (alpha(2A)-AR). alpha(2A)-ARs couple to both G(s) and G(i) to stimulate or inhibit adenylyl cyclase activity. Chinese hamster ovary-K1 cell lines expressing the porcine alpha(2A)-AR at high (alpha(2A)-H) and low (alpha(2A)-L) levels were used to estimate the relative efficacies (R.e.s) of a series of agonists for the G(s) and G(i) pathways. G(s)-mediated responses were measured after pertussis toxin treatment to inactivate G(i) in alpha(2A)-H, whereas G(i) responses were measured in alpha(2A)-L, where G(s) responses were absent. The full agonist UK-14,304 showed a large receptor reserve for G(i) responses in alpha(2A)-H but little receptor reserve for G(s) responses in alpha(2A)-H or for G(i) responses in alpha(2A)-L. With the exception of l-isoproterenol (ISO), all agonists showed similar R.e.s at the alpha(2A)-AR for G(s) and G(i) responses, with rank orders of R.e.s as follows: l-epinephrine = l-norepinephrine = UK-14,304 > p-aminoclonidine > or = BHT-920 > or = BHT-933 > clonidine = p-iodoclonidine > or = xylazine > or = guanabenz. Interestingly, ISO had the highest efficacy at the alpha(2A)-AR for activating G(s) versus G(i) (9-fold higher); however, it had low potency for both. By several criteria, the ISO response was mediated by the alpha(2A)-AR, supporting the hypothesis of agonist-directed trafficking of receptor signaling or agonist-specific G protein selectivity. In contrast, the apparent G(i) pathway selectivity of oxymetazoline appears to be mediated by an endogenous serotonergic receptor. It is intriguing that a classic beta-AR agonist that activates G(s) through beta(2)-ARs also appears to produce a G(s)-selective conformation of the G(i)-coupled alpha(2A)-AR.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adrenergic alpha-2 Receptor Agonists*
  • Adrenergic alpha-Agonists / pharmacology*
  • Adrenergic alpha-Antagonists / pharmacology
  • Adrenergic beta-Agonists / pharmacology*
  • Animals
  • Binding Sites
  • Brimonidine Tartrate
  • CHO Cells
  • Cricetinae
  • Drug Interactions
  • GTP-Binding Protein alpha Subunits, Gi-Go / agonists
  • GTP-Binding Protein alpha Subunits, Gi-Go / physiology
  • GTP-Binding Protein alpha Subunits, Gs / agonists
  • GTP-Binding Protein alpha Subunits, Gs / physiology*
  • Isoproterenol / pharmacology*
  • Kinetics
  • Oxymetazoline / antagonists & inhibitors
  • Oxymetazoline / pharmacology
  • Pindolol / analogs & derivatives
  • Pindolol / pharmacology
  • Protein Binding
  • Quinoxalines / pharmacology
  • Receptors, Adrenergic, alpha-2 / metabolism
  • Receptors, Adrenergic, alpha-2 / physiology
  • Serotonin Antagonists / pharmacology
  • Signal Transduction / drug effects
  • Signal Transduction / physiology*
  • Swine
  • Yohimbine / pharmacology

Substances

  • Adrenergic alpha-2 Receptor Agonists
  • Adrenergic alpha-Agonists
  • Adrenergic alpha-Antagonists
  • Adrenergic beta-Agonists
  • Quinoxalines
  • Receptors, Adrenergic, alpha-2
  • Serotonin Antagonists
  • Yohimbine
  • cyanopindolol
  • Brimonidine Tartrate
  • Oxymetazoline
  • Pindolol
  • GTP-Binding Protein alpha Subunits, Gi-Go
  • GTP-Binding Protein alpha Subunits, Gs
  • Isoproterenol