Phosphodiesterase isozymes: molecular targets for novel antiasthma agents

Phosphodiesterase inhibitors suppress alpha2-adrenoceptor-mediated 5-hydroxytryptamine release from tracheae of newborn rabbits

The outflow of 5-hydroxytryptamine (5-HT) from isolated tracheae of newborn rabbits was determined by high pressure liquid chromatography with electrochemical detection. This 5-HT outflow reflects release from neuroendocrine epithelial cells of the airway mucosa, as previously shown. Phenylephrine, via alpha2B-adrenoceptors, caused a transient increase in 5-HT outflow, maximally by about 250%, an effect mediated by liberation of intracellular Ca2+, as previously shown. The non-selective phosphodiesterase inhibitor 2-isobutyl-1-methylxanthine (IBMX) concentration-dependently inhibited phenylephrine-induced 5-HT release (completely at 100 microM, IC50: 1.3 microM). Likewise, benzafentrine (inhibitor of phosphodiesterase 3 and 4) and siguazodan (inhibitor of phosphodiesterase 3) also almost completely inhibited phenylephrine-induced 5-HT release with IC50 values of 1.7 and 4.2 microM, respectively. Rolipram (inhibitor of phosphodiesterase 4), in a concentration of 10 microM, which exceeds more than 10-fold the reported IC50 for phosphodiesterase 4, did not significantly affect phenylephrine-induced 5-HT release. 5-HT release induced by depolarizing concentrations of K+ (45 mM), which largely depends on extracellular Ca2+, was not affected by IBMX. In conclusion, phosphodiesterases, with characteristics of phosphodiesterase 3, appear to play an important role in the control of cyclic nucleotide mediated inhibition of 5-HT release from neuroendocrine epithelial cells.

Modulation of carbachol-induced inositol phosphate formation in bovine tracheal smooth muscle by cyclic AMP phosphodiesterase inhibitors

An investigation was made of a range of agents capable of elevating tissue cyclic AMP levels, or acting as a stable analogue of cyclic AMP, upon carbachol induced inositol phosphate responses in bovine tracheal smooth muscle slices. Whereas the beta 2 adrenoceptor agonist salbutamol (1 microM) and the membrane permeable analogue of cyclic AMP, 8-bromo-cyclic AMP (1 mM) were without effect upon total [3H]inositol phosphate formation induced by carbachol, 3-iso-butyl-1-methylaxanthine (IBMX) (EC50 140 microM), the high Km, cyclic AMP selective phosphodiesterase inhibitor rolipram (EC50 41 microM) and theophylline (EC50 76 microM) all inhibited the inositol phosphate response to low (1 microM) concentrations of carbachol. IBMX (IC50 13 microM), rolipram (IC50 4.6 microM) and theophylline (IC50 180 microM) all relaxed bovine tracheal muscle strips precontracted with methacholine (1 microM). The adenylate cyclase activator forskolin (1 microM), produced a much smaller (10% inhibition) effect upon inositol phosphate formation induced by carbachol. Carbachol (1 microM-1 mM) did not inhibit forskolin induced [3H]cyclic AMP formation. An inhibitor of the cyclic GMP preferring phosphodiesterase isozyme, M&B 22948 (1-100 microM), was without effect upon either carbachol induced inositol phosphate formation or trachealis tone. It is concluded that IBMX, rolipram and theophylline inhibit carbachol stimulated inositol phosphate formation, possibly through a cyclic AMP independent mechanism.