FURTHER ANALYSIS OF THE MECHANISMS UNDERLYING THE TRACHEAL RELAXANT ACTION OF SCA40

1 SCA40 (1nM-10 mu M), isoprenaline (1-300 nM) and levcromakalim (100 nM-10 mu M) each produced concentration-dependent suppression of the s pontaneous tone of guinea-pig isolated trachea. Propranolol (1 mu M) m arkedly (approximately 150 fold) antagonized isoprenaline but did not antagonize SCA40. The tracheal relaxant action of SCA40 was unaffected by suramin (100 mu M) or 8-(p)-sulphophenyltheophylline (8-SPT; 140 m u M). 2 An isosmolar, K+-rich (80 mM) Krebs solution increased trachea l tone, antagonized SCA40 (approximately 60 fold), antagonized isopren aline (approximately 20 fold) and very profoundly depressed the log co ncentration-effect curve for levcromakalim. Nifedipine (1 mu M) did no t itself modify the relaxant actions of SCA40, isoprenaline or levcrom akalim. However, nifedipine prevented the rise in tissue tone and the antagonism of SCA40 and isoprenaline induced by the K+-rich medium. In contrast, nifedipine did not prevent the equivalent antagonism of lev cromakalim. 3 Charybdotoxin (100 nM) increased tracheal tone, antagoni zed SCA40 (approximately 4 fold) and antagonized isoprenaline (approxi mately 3 fold). Nifedipine (1 mu M) prevented the rise in tissue tone and the antagonism of SCA40 and isoprenaline induced by charybdotoxin. 4 Quinine (30 mu M) caused little or no change in tissue tone and did not modify the relaxant action of isoprenaline. However, quinine anta gonized SCA40 (approximately 2 fold). Nifedipine (1 mu M) prevented th e antagonism of SCA40 induced by quinine. 5 Tested on spontaneously-be ating guinea-pig isolated atria SCA40 (1 nM-10 mu M) increased the rat e of beating in a concentration-dependent manner. Over the concentrati on-range 1 mu M-10 mu M, SCA40 also caused an increase in the force of atrial contraction. 6 Intracellular electrophysiological recording fr om guinea-pig isolated trachealis showed that the relaxant effects of SCA40 (1 mu M) were often accompanied by the suppression of spontaneou s electrical slow waves but no change in resting membrane potential. W hen the concentration of SCA40 was raised to 10 mu M, its relaxant act ivity was accompanied both by slow wave suppression and by plasmalemma l hyperpolarization. 7 SCA40 (10 nM-100 mu M) more potently inhibited the activity of cyclic AMP phosphodiesterase (PDE) than that of cyclic GMP PDE derived from homogenates of guinea-pig trachealis. Theophylli ne (1 mu M-10 mM) also inhibited these enzymes but was less potent tha n SCA40 in each case and did not exhibit selectivity for inhibition of cyclic AMP hydrolysis. 8 Tested against the activity of the isoenzyme s of cyclic nucleotide PDE derived from human blood cells and lung tis sue, SCA40 proved highly potent against the type III isoenzyme. It was markedly less potent against the type IV and type V isoenzymes and ev en less potent against the isoenzymes types I and II. 9 It is conclude d that the tracheal relaxant action of SCA40 (1 nM-1 mu M) does not in volve the activation of beta-adrenoceptors or P-1 or P-2 purinoceptors . Furthermore, this action is unlikely to depend upon the opening of B KCa, channels with consequent cellular hyperpolarization and voltage-d ependent inhibition of Ca2+ influx. The tracheal relaxant action of SC A40 (up to 1 mu M) is more likely to depend upon its selective inhibit ion of the type III isoenzyme of cyclic nucleotide PDE. At concentrati ons above 1 mu M, SCA40 exerts more general inhibition of the isoenzym es of cyclic nucleotide PDE and may then promote the opening of BKCa c hannels.