Role of low Km cyclic AMP phosphodiesterase inhibition in tracheal relaxation and bronchodilation in the guinea pig.

This study evaluated the relationship between inhibition of the rolipram-sensitive and the CI-930-sensitive low Km cyclic AMP-specific phosphodiesterase (PDE) isozymes (PDE IIIRO and PDE IIIc, respectively) and bronchomotor tone in the guinea pig. Rolipram and CI-930 exhibited biphasic concentration-response relationships for relaxation of carbachol-, histamine- and leukotriene D4-contracted trachea. However, each agent produced a monophasic (sigmoidal) concentration-response curve when tested in the presence of a fixed concentration (3 microM) of the other. The same relationships were observed for inhibition of tracheal peak III PDE isolated via diethylaminoethyl-cellulose chromatography. Whereas CI-930 was approximately equipotent inhibiting PDE IIIc and relaxing rolipram-pretreated trachea, rolipram was substantially more potent (EC50 = 0.02 microM) in relaxing CI-930-pretreated trachea than in inhibiting CI-930-pretreated PDE III (PDE IIIRO, IC50 = 2.6 microM). Among a series of PDE inhibitors, there was a highly significant correlation (r = 0.89, P less than .01) between PDE IIIc inhibition (i.e., PDE III in the presence of rolipram) and rolipram-pretreated tracheal relaxation, but not between PDE IIIRO inhibition and CI-930-pretreated tracheal relaxation (r = 0.23). Nine of the PDE inhibitors used in this study have been reported to displace rolipram from a high-affinity binding site in rat brain. A highly significant correlation between relaxation of CI-930-pretreated trachea and displacement of rolipram binding by these agents was observed (r = 0.97, P less than .0001).(ABSTRACT TRUNCATED AT 250 WORDS)