THE EFFECTS OF PHORBOL 12,13-DIACETATE ON RESPONSES OF GUINEA-PIG ISOLATED TRACHEA TO METHYLXANTHINES, ISOPRENALINE AND RYANODINE

Using guinea-pig isolated trachea we have studied how phorbol 12,13-di acetate (PDA) modulates mechanical responses of the tissue to methylxa nthines, isoprenaline and ryanodine. 2 Caffeine (10 mu M-5 mM), theoph ylline (10 mu M-5 mM) and isoprenaline (1 nM - 1 mu M), each inhibited the spontaneous tone of the trachea. Pretreatment with PDA (0.1-10 mu M) converted relaxant responses to high concentrations of the methylx anthines into contractions. PDA produced no equivalent effect against isoprenaline. Pretreatment with verapamil (1 or 10 mu M), nifedipine ( 0.1 mu M) Or incubation with Ca2+-free, EGTA (0.1 mM)-containing physi ological salt solution (PSS) suppressed the contraction produced by ca ffeine or theophylline in PDA (5 mu M)-treated tissues. 3 The ability of PDA (5 mu M) to convert caffeine-induced relaxation into caffeine-i nduced contraction was retained in tissues pretreated with a combinati on of atropine (1 mu M) and mepyramine (1 mu M) and in tissues denuded of the airway epithelium. 4 Caffeine (10 mu M - 5 mM), theophylline ( 10 mu M - 5 mM) and isoprenaline (1 nM - 1 mu M), each relaxed trachea contracted with histamine (0.1 mM). The relaxation induced by caffein e, theophylline and isoprenaline was markedly reduced in the presence of PDA (5 mu M) and the responses to high concentrations of caffeine a nd theophylline, but not those to isoprenaline, were reversed to contr actions. Verapamil (10 mu M) prevented the effects of PDA against caff eine- or theophylline-induced relaxation. 5 PDA (1 mu M) enhanced the tracheal spasm produced by caffeine (10 mM) and theophylline (10 mM) i n indomethacin (2.8 mu M)-treated trachea maintained at 20 degrees C. This enhancement was reduced in the presence of verapamil (10 mu M). 6 Tested in trachea bathed by K+-rich (40 mM), Ca2+-free PSS, CaCl2 (0. 1-20 mM) caused concentration-dependent spasm. PDA (1-5 mu M) did not significantly modify the shape or position of the log concentration-ef fect curve for CaCl2. In contrast, verapamil (1 and 10 mu M) antagoniz ed CaCl2. 7 Tested in trachea bathed by indomethacin (2.8 mu M)-contai ning PSS, ryanodine (1-100 mu M) caused concentration-dependent spasm. PDA (5 mu M) potentiated ryanodine. Verapamil (10 mu M) inhibited rya nodine in inducing spasm and suppressed the ability of PDA to potentia te ryanodine. 8 It is concluded that, in guinea-pig isolated trachea, PDA augments the spasmogenic activity of the methylxanthines and ryano dine. This effect of PDA does not result from PDA-induced suppression of spontaneous tone, from increased cellular entry of Ca2+ through L-t ype channels or from sensitization of the intracellular contractile ma chinery to activator Ca2+. The evidence suggests, instead, that PDA fa cilitates methylxanthine- or ryanodine-induced release of Ca2+ from th e intracellular store.