INCREASED ALPHA-1-ADRENERGIC RECEPTOR DENSITY IN BROWN ADIPOSE-TISSUE OF COLD-ACCLIMATED RATS AND HAMSTERS

Binding sites for [3H]prazosin were characterized in crude membrane fractions from rat brown adipose tissue. Based on agonist (norepinephrine= phenylephrine .mchgt. isoprenaline) and antagonist (prazosin .mchgt. yohimbine > propranolol) potencies to compete with [3H]prazosin, the binding sites were identified as .alpha.1-receptors, not previously described in rat brown adipose tissue. As the [3H]prazosin binding sites could be observed in isolated brown fat cell preparations, they were probably postsynaptic. The effect of cold acclimation was studied in crude membrane fractions from control and cold-acclimated (4.degree. C) rats and hamsters. Cold acclimation did not change the affinity of the receptor for agonists and antagonists, but there was a significant increase in the number of .alpha.1-receptors (per mg protein), both in rat (100% increase) and hamster (40% increase) brown fat. Evidently, activation of brown adipose tissue is associated with an increase in the relative density of .alpha.1-receptors (i.e., in the .alpha.1/.beta. ratio) and an increased significance of .alpha.1-adrenergic pathways for the function of the tissue.