Potentiating effect of phenylephrine on isoproterenol activation of thyroxine type II deiodinase in the pineal gland of adult rats.

In the present study we show, for the first time, that phenylephrine (PHE), an alpha-adrenergic receptor agonist, potentiates the effect of isoproterenol (ISO), a beta-adrenergic agonist, in activating pineal type II5'-deiodinase (5'-D) activity. The potentiating effect of PHE was observed only at doses of ISO which induce submaximal activation of the enzyme. However, at doses which lead to maximal activation of the enzyme, PHE was ineffective. The results suggest that not only beta-, but also alpha-adrenergic receptors, are involved in the sympathetic noradrenergic regulation of pineal 5'-D activity in the adult rat.