Vol. 301, Issue 2, 599-604, May 2002

Inhibition of cAMP Accumulation by -Receptor Activation in Isolated Iris-Ciliary Bodies: Role of Phosphodiesterase and Protein Kinase C

Juanita Dortch-Carnes and David E. Potter

Department of Pharmacology/Toxicology, Morehouse School of Medicine, Atlanta, Georgia

The present study was designed to examine the roles of protein kinase C (PKC) and phosphodiesterase (PDE) in modulating the action of  receptor stimulation on cAMP accumulation in isolated iris-ciliary bodies (ICBs) of New Zealand White rabbits. The  receptor agonist, (±)-1-(3,4-dichlorophenyl)acetyl-2-(1-pyrrolidinyl)methylpiperidine (BRL-52537) (BRL), and the PKC activator, phorbol 12,13-dibutyrate (PDBu), both caused a concentration-dependent inhibition of forskolin-stimulated cAMP production. The inhibitory effect of BRL on cAMP levels was significantly reduced in the presence of the selective  receptor antagonist, norbinaltorphimine (10⁶ M), but the effect of PDBu was not, thus supporting the involvement of -opioid receptors in the response to BRL. In the presence of 3-isobutyl-1-methylxanthine or rolipram (10⁵ M), the inhibitory effect of BRL or PDBu (10⁶ M) on cyclic AMP accumulation was abolished. In the presence of the selective PKC antagonist, chelerythrine (10⁶ M), the inhibitory effect of PDBu or BRL (10⁶ M) was significantly reduced. Direct measurement of PDE activity demonstrated the ability of BRL and PDBu (10⁶ M) to augment the activity of these enzymes. Preincubation of ICBs with rolipram (10⁵ M) or chelerythrine (10⁶ M) caused significant reversal of both BRL- and PDBu-induced increases in PDE activity. These results indicate that stimulation of PKC and PDE4 activity is part of the complex mechanism whereby -opioid receptor agonists reduce levels of cAMP in the rabbit ICB. This mechanism of action could contribute to the ability of -opioid agonists to suppress aqueous flow rate and to lower intraocular pressure.