SPECIFIC CYCLIC-NUCLEOTIDE PHOSPHODIESTERASE INHIBITORS DIFFERENTLY MODULATE CONTRACTILE KINETICS IN AIRWAY SMOOTH-MUSCLE

The involvement of various phosphodiesterases (PDEs) in controlling th e time-dependent mechanical properties of guinea pig trachealis smooth muscles was determined by using different classes of PDE inhibitors a s pharmacological tools. These drugs produced low amplitude and long-l asting dose-dependent relaxations on the resting tone with the followi ng EC(50) values: rolipram, 3 nM; indolidan, 0.11 mu M; and zaprinast, 0.5 nM and 1 mu M. These PDE inhibitors were 50% less active than 1 m u M norepinephrine. The effects of the drugs were also tested on carba chol-induced contractions and norepinephrine-evoked relaxations. Zapri nast, but not rolipram nor indolidan, decreased the rate of rise of co ntraction, thus prolonging the time to reach the plateau by 75% withou t modifying the magnitude of the responses. Zaprinast and rolipram sig nificantly increased the total length of the norepinephrine effect by 25 and 35%, respectively. Similar results were obtained in a dose-depe ndent manner on isoproterenol-induced relaxations. In contrast, a high er concentration of indolidan was required to affect the amplitude, du ration, and time to peak of isoproterenol- or norepinephrine-induced r elaxations. These results indicate that PDE IV (rolipram sensitive) an d PDE I, and less likely PDE V (both zaprinast sensitive), are involve d in the control of guinea pig airway contractile kinetics, whereas PD E III (indolidan sensitive) is essentially involved in the modulation of the resting tone. Four cytosolic isozymes were identified in bovine airway smooth muscles (ASMs): PDE I (calmodulin-dependent PDE), PDE I I (cGMP-stimulated PDE), PDE IV (cAMP-specific and rolipram-sensitive PDE), and PDE V (cGMP-specific and zaprinast-sensitive PDE). Character ization of PDE isoforms present in the microsomal fraction by HPLC sho wed the presence of PDE IV, PDE V, and to a lesser extent PDE III. How ever, PDE III was not detected in ASM cytosol. Using newly synthesized radioligands, binding studies confirmed the low level of expression o f PDE III and the presence of PDE IV. We conclude that PDE I controls the rate of contraction, whereas PDE V and PDE IV prolong the time of relaxation induced by NE. PDE V would control the ASM responsiveness b y regulating the intracellular cGMP concentration, which in turn would both activate PKG and stimulate PDE II (cGS-PDE). Since the various i sozymes of PDE are differently involved in the kinetic control of the mechanical events in ASM, they represent physiologically relevant and important pharmacological targets.