Influence of prolonged fasting on thyroid hormone modulation of lipolysis in isolated epididymal adipocytes of Wistar rats.

Male Wistar rats, 3 weeks old, were thyroidectomized surgically, kept for 1 month at 25 degrees C and then fasted for 3 days, with or without daily intraperitoneal injection of 3,5,3'-triiodo-L-thyronine (4.6 nmol T3/100 g body weight). Age-matched fed euthyroid rats were used as controls. All the experiments were carried out using isolated epididymal adipocytes. Basal lipolysis was higher during fasting in euthyroid or T3-treated adipocytes than in hypothyroid adipocytes. Adipocytes of fed hypothyroid rats were quite unresponsive to theophylline alone or combined with adrenaline or isoproterenol, whereas lipolysis was stimulated by these drugs in euthyroid or T3-treated adipocytes. Such a stimulated lipolysis was increased partially by fasting in hypothyroid adipocytes and was restored to a euthyroid level in T3-treated adipocytes. Lipolysis was more stimulated by adenosine deaminase in fasted euthyroid adipocytes than in fed ones. Hypothyroid and T3-treated adipocytes were unresponsive to adenosine deaminase except in fasted T3-treated rats. In these adipocytes, lipolysis was activated by the combination of adenosine deaminase plus theophylline. Finally, lipolysis was inhibited strongly in hypothyroidism while it was activated weakly by fasting. Lipolysis was inhibited slightly in fasted hypothyroid rats and thyroid hormone restored lipolysis. The findings are discussed in terms of the dual regulation of lipolysis by fasting and thyroid hormones.