2.1. Animals
All experiments were approved by the appropriate Committee on Animal Care and Use. Male C57BL/6J mice (7–8 weeks old) were purchased from SPF Biotechnology Co., Ltd (Beijing, China). βarr2−/− mice were kindly gifted by Professor Ma Lan of Fudan University. This strain was generated by the laboratory of R. J. Lefkowitz (Duke University Medical Center, Durham, NC, USA) and backcrossed onto a C57BL/6J background. The animals used in the present study were wild-type, βarr2−/−, or βarr2+/− littermates resulting from heterozygous breeding, and the descendants of mice separately bred as homozygous wild-type, βarr2−/−, or βarr2+/− mice after the initial backcrossing. βarr2−/− mice were backcrossed onto the C57BL/6J background every six generations. All mice were housed in groups of 5–6 mice per cage and maintained on a 12-h/12-h light/dark cycle with food and water available ad libitum. All animals were adapted for at least 2 days before experimentation.