Abstract

Cold-induced vasoconstriction in cutaneous blood vessels is mediated by increased constrictor activity of vascular α₂-adrenoceptors (α₂-ARs). In mouse cutaneous arteries, α₂-AR constriction at 37°C is mediated by α_2A-ARs, whereas after cold exposure (28°C), α_2C-ARs are no longer silent and mediate the remarkable cold-induced augmentation of α₂-AR responsiveness. The goals of the present study were to develop a cell model of cutaneous thermoregulation and to determine the mechanisms underlying the thermosensitivity of α_2C-ARs. Human embryonic kidney 293 cells were transiently transfected with the mouse α_2A- or α_2C-AR. In cells expressing α_2A-ARs, UK-14,304 (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine), an α₂-AR agonist, inhibited (10 pM) and stimulated (1–10 nM) the accumulation of cAMP evoked by forskolin. Similar responses were obtained at 37°C and 28°C. In contrast, in cells expressing α_2C-ARs, UK-14,304 did not affect forskolin-stimulated cAMP accumulation at 37°C but did cause a concentration-dependent inhibitory effect at 28°C. Subcellular fractionation revealed that at 37°C α_2C-ARs were localized predominantly to Golgi compartments, whereas α_2A-ARs localized predominantly to the plasma membrane. After cooling (28°C), α_2C-ARs relocated from Golgi compartments to the plasma membrane, whereas the α_2A-AR remained at the plasma membrane. Immunofluorescence microscopy confirmed that, at 37°C, α_2A-ARs were localized to the cell surface, whereas α_2C-ARs colocalized with a trans-Golgi marker. Cooling did not affect localization of α_2A-ARs, but shifted α_2C-ARs to the cell surface. Moderate cooling, therefore, caused a selective redistribution of α_2C-ARs from the Golgi compartments to the cell surface, allowing the rescue of the α_2C-adrenergic functional response. This mechanism may explain the role of α₂-ARs in thermoregulation of the cutaneous circulation.