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Isoproterenol, Forskolin, and cAMP-Induced Nitric Oxide Production in Pig Ciliary Processes

From the Laboratory of Ocular Pharmacology and Physiology, University Eye Clinic, Basel, Switzerland.

PURPOSE. To investigate whether isoproterenol and forskolin, two adenylylcyclase activators, or 8-bromo-cAMP, an adenosine 3',5'-cyclic monophosphate (cAMP) analog, increase nitric oxide (NO) production in isolated porcine ciliary processes.

METHODS. Nitrite (an NO metabolite) was measured (Griess reaction) before and 2 hours after exposure to 0.1 to 100 µM isoproterenol (a ß-adrenoreceptor agonist), 0.01 to 100 µM forskolin, or 0.1 to 1000 µM 8-bromo-cAMP. Some experiments were conducted in the presence of 0.5 mM N^G-nitro-L-arginine methyl ester (L-NAME; a nitric oxide synthase [NOS] inhibitor), 10 µM propranolol (a ß-adrenoreceptor antagonist), or 1 µM KT 5720 (a cAMP-dependent protein kinase inhibitor). cAMP production was also measured (by immunoassay).

RESULTS. Nitrite production was increased by isoproterenol (maximum, 10 µM: 164%; P < 0.001), forskolin (maximum, 10 µM: 254%; P < 0.001), and 8-bromo-cAMP (maximum, 100 µM: 184%; P < 0.001), an effect prevented by L-NAME (P < 0.05–0.001). Propranolol inhibited only isoproterenol-induced (10 µM) nitrite production (P < 0.05), whereas KT 5720 (P < 0.05) inhibited isoproterenol- (10 µM) and 8-bromo-cAMP–induced (10 µM) nitrite production. Furthermore, cAMP production evoked by isoproterenol (10 µM, P < 0.05) but not by forskolin (10 µM, P < 0.001) was inhibited by propranolol (P < 0.05).

CONCLUSIONS. In isolated porcine ciliary processes, drugs activating adenylylcyclase or mimicking cAMP increase the production of NO by a mechanism that appears to involve both a cAMP-dependent protein kinase and NOS.

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