Brown
adipose tissue (BAT) has emerged as a potential target to combat
obesity and diabetes, but novel strategies to activate BAT are needed.
Adenosine and A_2A receptor (A2AR) agonism activate BAT in
rodents, and endogenous adenosine is released locally in BAT as a
by-product of noradrenaline, but physiological data from humans is
lacking. The purpose of this pilot study was to investigate the effects
of exogenous adenosine on human BAT perfusion, and to determine the
density of A2ARs in human BAT in vivo for the first time, using PET/CT
imaging.

Healthy, lean men (n = 10)
participated in PET/CT imaging with two radioligands. Perfusion of BAT,
white adipose tissue (WAT) and muscle was quantified with [¹⁵O]H₂O
at baseline, during cold exposure and during intravenous administration
of adenosine. A2AR density of the tissues was quantified with [¹¹C]TMSX at baseline and during cold exposure.

Adenosine
increased the perfusion of BAT even more than cold exposure (baseline
8.3 ± 4.5, cold 19.6 ± 9.3, adenosine 28.6 ± 7.9 ml/100 g/min, p < 0.01). Distribution volume of [¹¹C]TMSX in BAT was significantly lower during cold exposure compared to baseline. In cold, low [¹¹C]TMSX binding coincided with high concentrations of noradrenaline.

Adenosine
administration caused a maximal perfusion effect in human
supraclavicular BAT, indicating increased oxidative metabolism. Cold
exposure increased noradrenaline concentrations and decreased the
density of A2AR available for radioligand binding in BAT, suggesting
augmented release of endogenous adenosine. Our results show that
adenosine and A2AR are relevant for activation of human BAT, and A2AR
provides a future target for enhancing BAT metabolism.