Interaction between alpha- and beta-adrenoceptors in rat pineal adenosine cyclic 3',5'-monophosphate...

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 potentiated the stimulatory effect of isoproterenol. Treatment of pineal organ cultures with 100 microM propranolol inhibited norepinephrine (NE)-induced PDE activity while 100 microM phentolamine had no significant effect. Propranolol at doses unable to alter the in vitro NE-induced stimulation of pineal PDE activity (1 microM), antagonized such NE effect when used in combination with 1 microM phentolamine. At equimolecular concentrations (1 microM) the mixed alpha-beta-adrenergic agonist NE was more effective than the beta-adrenergic agonist isoproterenol to increase pineal PDE in vitro. These results suggest an alpha-beta-adrenergic interaction in the sympathetic modulation of low-Km PDE activity of rat pineal gland.