Lower airway smooth muscle contraction induced by Ptychodiscus brevis (red tide) toxin

Ptychodiscus brevis toxin (PBTX) is produced by the organism Ptychodiscus brevis. This toxin causes a phenomenon that has come to be known as Florida red tide. It also stimulates neuronal sodium channels, resulting in activation of the cholinergic and adrenergic nerve endings of the autonomic nervous system in upper airway smooth muscle and rat vas deferens, respectively, as previously reported. It is the cholinergic stimulating action that has been implicated as a possible "triggering" event in bronchial asthma. This article concerns the investigation of whether PBTX may also affect lower airways and by what mechanism any contractile response to PBTX in lower airways may be induced. PBTX was found to elicit contractions in isolated canine lower airway smooth muscle. The threshold concentration was 0.15 microgram/ml, the peak response occurred at 6.0 micrograms/ml, and the concentration causing half-maximal response of the group was 0.57 microgram/ml. Pharmacologic analysis demonstrated that atropine (10(-6) mol/L) blocked the response to both PBTX and acetylcholine, tetrodotoxin (10(-7) mol/L) blocked the response to PBTX but not to acetylcholine, and verapamil (10(-5) mol/L) blocked the response to PBTX and reduced the response to acetylcholine. Four consecutive contractile responses to PBTX (3 micrograms/ml) produced rapid tachyphylaxis. The fourth contraction was 60% less than the initial response. A fifth response to PBTX after exposure to indomethacin (2.8 X 10(-6) mol/L) was increased and resulted in a contraction that was only 25% less than the initial response. The exogenous addition of prostaglandins (PG), PGE1 and PGE2, to indomethacin-treated lower airway strips selectively suppressed the contractile response to PBTX. Other PGs tested (PGA2, PGB2, PGD2, PGF2 alpha and PGI2) did not affect the PBTX response. These results indicate that PBTX produces spasm in lower airway smooth muscle and that it does this by stimulation of sodium channels in the cholinergic nerve fibers. The results also demonstrate a rapid reduction in the contractile response to PBTX. The results also demonstrate that the reduction is mediated by PGs of the E series.