Effects of thyroidectomy, insulin, and phospholipids on cyclic AMP phosphodiesterase in rat adipocyte plasma...

The effect of thyroid status on the cyclic nucleotide phosphodiesterase in adipocyte plasma membranes has been studied. In euthyroid rat fat cells, about 7% of the total cAMP phosphodiesterase was found in the plasma membrane. Thyroidectomy doubled both the enzyme's total activity in this fraction, and its specific activity (60 versus 37 pmol/min/mg) compared to euthyroid rat plasma membrane. In membranes from thyroidectomized rats, phosphodiesterase hydrolyzed cAMP with a single Km of 2 microM, whereas in euthyroid rat membranes, Lineweaver Burk plots were non-linear, with apparent Kms of 0.5 and 5 microM. This phosphodiesterase activity was insensitive to exogenous guanine nucleotides and calcium. In vivo injection of triiodothyronine restored phosphodiesterase activity in plasma membranes from thyroidectomized rats to the values obtained for euthyroid rats. Centrifugation on a 10 to 45% sucrose density gradient of the plasma membrane fractions gave two main peaks of phosphodiesterase activity which hydrolyzed the cAMP in adipocyte plasma membranes from both euthyroid and thyroidectomized rats. The distribution profiles for these activities were very similar in the two plasma membrane preparations. The peaks of phosphodiesterase and 5'nucleotidase activity coincided. Thyroidectomy raised the phosphodiesterase activity of these two peaks, particularly of the first. The cAMP phosphodiesterase activities in both hypothyroid and control plasma membrane preparations were also sensitive to insulin and were activated by phospholipase A2 and three anionic phospholipids. Thyroid hormones therefore regulate the degradation of cAMP in plasma membranes by a mechanism which seems different from the one involved in the action of insulin, and is independent of the membrane phospholipid composition.