Research article

The presence of β₂-adrenoceptors sensitizes α_2A-adrenoceptors to desensitization after chronic epinephrine treatment

Tasneem Bawa-Khalfe^1,4 , Ghazi F Altememi⁴ , Chitra D Mandyam^2,4 , Lindsay A Schwarz⁴ , Douglas C Eikenburg⁴ and Kelly M Standifer^3,4

¹Tasneem Bawa-Khalfe, Research Center for Cardiovascular Diseases, Brown Foundation Institute of Molecular Medicine for the Prevention of Human Diseases, The University of Texas-Houston Health Science Center, Houston, TX, USA

²Chitra D. Mandyam, Committee on the Neurobiology of Addictive Disorders, The Scripps Research Institute, La Jolla, CA, USA

³Kelly M. Standifer, Ph.D., Dept. Pharmaceutical Sciences, 1110 N. Stonewall, Suite 329, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA

⁴Department of Pharmacological and Pharmaceutical Sciences, University of Houston, Houston, TX, USA

author email corresponding author email

BMC Pharmacology 2007, 7:16doi:10.1186/1471-2210-7-16

Published:	20 December 2007

Abstract

Background

In addition to the regulation of blood pressure, α₂- and β-adrenoceptor (AR) subtypes play an important role in the modulation of noradrenergic neurotransmission in the human CNS and PNS. Several studies suggest that the α₂-AR responsiveness in cells and tissues after chronic epinephrine (EPI) or norepinephrine (NE) exposure may vary, depending on the β-AR activity present there. Recently, we reported that in BE(2)-C human neuroblastoma cells (endogenously expressing α_2A- and β₂-AR), chronic EPI treatment (300 nM) produced a dramatic β-adrenoceptor-dependent desensitization of the α_2A-AR response. The aim of this study is to determine if stable addition of a β₂-AR to a second neuroblastoma cell line (SH-SY5Y), that normally expresses only α_2A-ARs that are not sensitive to 300 nM EPI exposure, would suddenly render α_2A-ARs in that cell line sensitive to treatment with the same EPI concentration.

Methods

These studies employed RT-PCR, receptor binding and inhibition of cAMP accumulation to confirm α₂-AR subtype expression. Stable clones of SH-SY5Y cells transfected to stably express functional β₂-ARs (SHβ₂AR4) were selected to compare sensitivity of α₂-AR to EPI in the presence or absence of β₂-ARs.

Results

A series of molecular, biochemical and pharmacological studies indicated that the difference between the cell lines could not be attributed to α₂-AR heterogeneity. We now report that after transfection of functional β₂-AR into SH-SY5Y cells (SHβ₂AR4), chronic treatment with modest levels of EPI desensitizes the α_2A-AR. This effect results from a β₂-AR dependent down-regulation of native α_2A-ARs by EPI accompanied by enhanced translocation of GRK2 and GRK3 to the membrane (required for GRK-mediated phosphorylation of agonist-occupied receptors).

Conclusion

This study further supports the hypothesis that the presence of the β-AR renders the α_2A-AR more susceptible to desensitization with physiological levels of EPI.