Nuclear estradiol binding in rat adipocytes. Regional variations and regulatory influences of hormones

The nuclear estrogen receptor was characterised in isolated rat adipocytes. The binding reaction with [³H]estradiol was performed with intact isolated rat adipocytes and the radioactivity associated with the nucleus was subsequently determined after cell lysis. The nuclear uptake of [³H]estrogen in rat adipocytes was temperature dependent and steroid specific. The steady-state binding was achieved after 30 min at 37°C wnd was constant for several hours. Estradiol was found to bind to a homogeneous class of nuclear receptors in epididymal adipocytes with an apparent K_d of 3.1 ± 0.76 nM and a B_max of 7.98±1.11 fmol/10⁶ cells corresponding to about 4800 receptors per nucleus. The estradiol binding exhibited regional variations in isolated adipocytes. In lean rats the highest receptor number was found in epididymal adipocytes, whereas there was a significantly lower number of nuclear binding sites in perirenal and subcutaneous adipocytes (P < 0.05), unlike in older and more obese rats where the nuclear estradiol binding was greatest in adipocytes from the perirenal fat depot. Incubations with isoproterenol (10 μM) and dibutyryl-cAMP (2.5 mM) both reduced estradiol binding by 56% (P < 0.005), while insulin (1 nM) enhanced the estradiol binding by 37% (P < 0.01). In conclusion, a specific and high affinity nuclear estradiol receptor was demonstrated in rat adipocytes and regional differences in nuclear estradiol binding were detected. Furthermore, it was demonstrated that nuclear estradiol binding could be modulated by other agents known to affect adipocyte metabolism.