IDENTIFICATION, CHARACTERIZATION AND FUNCTIONAL ROLE OF PHOSPHODIESTERASE ISOZYMES IN HUMAN AIRWAY SMOOTH MUSCLE

Abstract

Purpose of the Study. Theophylline, a widely used bronchodilator, has been traditionally thought to exert its effect by inhibiting phosphodiesterase (PDE), which increases cAMP levels. It has become apparent that there are actually five different PDE isozymes, each encoded by a different gene or gene family. It may be possible to selectively target the lung PDE isozyme using specific inhibitors for the pulmonary smooth muscle type of PDE. The specific type of airway smooth muscle PDE isozyme, the effect of specific PDE inhibitors, individually or additively, and regional differences in PDE enzyme types was the focus of this study. Methods. Human trachea was obtained from brain-dead subjects, processed within 24 hours by separating the muscle from surrounding tissues, and homogenized. Human lung was similarly obtained and processed, with the ultimate isolation of individual bronchial smooth muscle from several generations of bronchial diameters. PDE isozyme separation was performed by using a combination of DEAE-Sepharose and calmodulin-Sepharose affinity chromatography. Results. Trachael smooth muscle contained all five PDE isozymes. Using specific inhibitors of each PDE isozyme, inhibitors of PDE type III were >PDE Type IV which were >PDE type III, respectively, as bronchial smooth muscle relaxants. This bias toward Type III and IV PDE was found in large and small bronchial preparations. Inhibitors of PDE III and IV acted synergistically, and PDE III inhibitors assisted beta-agonist-induced relaxation. Reviewer's Comments. If the only action of theophylline is its' effect on PDE, drugs that do a better job of blocking specific PDE isozyme(s) in bronchial smooth muscle should be developed.