313 THE α-ADRENERGIC RECEPTORS IN BROWN ADIPOCYTES OF DEVELOPING RATS

Abstract

Specific binding of α-adrenergic ligands [3H] olonidine and [3H]RX 781097 was observed with plasma membrane fragments isolated from interscapular brown adipose tissue of 7-day-old rats. Scatchard and Woolf analyses of the data and results on specific displacement of [3H] norepinephrine by low concentrations of epinephrine and yohimhine were consistent with the existence in brown adipocytes of a homogeneous class of the binding sites. Competition experiments on both α-agonists and α-antagonists binding were consistent with pharmacological definition of the α2-adrenoreceptor subtype. Chemical denervation by 6-hydroxy-dopamine resulted in a more pronounced distinction of the two affinity states of the receptors. A two-slope Scatchard plot showed mean apparent KD of 29 and 250 nM for the α2H and α2L, respectively. The maximum number of sites was somewhat higher (0.58 pmol/mg protein) than in control animals. Pre-treatment of infant rats with yohimbine (7 days of 10 mg/kg b.w. daily) has resulted in increased specific binding of [3H] yohimbine. Comparison studies using the β-receptor ligand [3H] dihydroal-prenolol seem to indicate “hybrid” characteristics. Competition binding studies may be interpreted in terms that both the α-and β-adrenoreceptors in brown adipocytes of infant rats are “hybrid” species which may in fact reside within integrated protein molecules. (Supported by Canadian Medical Research Council Grant MA-7217).