Interaction between ?- and ?-adrenoceptors in rat pineal adenosine cyclic 3?,5?-monophosphate phosphodiesterase activation

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Abstract
The adrenergic regulation of the low-Km pineal cAMP phosphodiesterase (PDE) activity was studied in adult female rats. PDE activity showed a transient enhancement (up to 42%) during the process of degeneration of pineal sympathetic nerve terminals that followed superior cervical ganglionectomy (SCGx), thus confirming the neural modulation of the enzyme. Treatment with isoproterenol (0.3–5.0 mg/Kg) increased significantly PDE activity within 2 hours. Phenylephrine induced a significant increase of pineal PDE only at a 10 mg/Kg dose, while at a lower dose (1 mg/Kg) it potenciated the stimulatory effect of isoproterenol. Treatment of pineal organ cultures with 100 ΜM propranolol inhibited norepinephrine (NE)-induced PDE activity while 100 ΜM phentolamine had no significant effect. Propranolol at doses unable to alter thein vitro NE-induced stimulation of pineal PDE activity (1 ΜM), antagonized such NE effect when used in combination with 1 uM phentolamine. At equimolecular concentrations (1 ΜM) the mixed α-Β-adrenergic agonist NE was more effective than the Β-adrenergic agonist isoproterenol to increase pineal PDEin vitro. These results

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