Effects of cartap on isolated mouse phrenic nerve diaphragm and its related mechanism

Cartap, a nereistoxin analogue pesticide, is reported to have no irritation to eyes in rabbits. However, we have demonstrated recently that cartap cou ld actually cause acute death in rabbits via ocular exposure. Our prelimina ry study with isolated mouse phrenic nerve diaphragms has shown that instea d of neuromuscular blockade, cartap caused muscular contracture. The object ive of the study was to examine the effect of cartap on the neuromuscular j unction in more detail and to investigate its possible underlying mechanism with isolated mouse phrenic nerve diaphragms and sarcoplasmic reticulum (S R) vesicles. Cartap or nereistoxin at various concentrations was added in t he organ bath with isolated mouse phrenic nerve diaphragm and both nerve- a nd muscle-evoked twitches were recorded, Instead of blocking the neuromuscu lar transmission as nereistoxin did, cartap caused contracture in stimulate d or quiescent isolated mouse phrenic nerve diaphragm, Both the cartap-indu ced muscular contracture force and the time interval to initiate the contra cture were dose-dependent. The contracture induced by cartap was not affect ed by the pretreatment of the diaphragm with the acetylcholine receptor blo cker alpha-bungarotoxin; the Na+ channel blocker tetrodotoxin; or various C a2+ channel blockers, NiCl2, verapamil, and nifedipine, On the contrary, th e contracture was significantly inhibited when the diaphragm was pretreated with ryanodine or EGTA containing Ca2+-free Krebs solution or in combinati on. This suggested that both internal and extracellular Ca2+ might particip ate in cartap-induced skeletal muscle contracture. Moreover, cartap inhibit ed the [H-3]-ryanodine binding to the Ca2+ release channel of SR in a dose- dependent manner. Additionally, cartap could induce a significant reduction in Ca2+-ATPase activity of SR vesicles at a relatively high dose, The resu lts suggested that cartap might cause the influx of extracellular Ca2+ and the release of internal Ca2+, with subsequent induction of muscular contrac ture in the isolated mouse phrenic nerve diaphragm, Based on these findings , we propose that the acute death of rabbits following ocular exposure to c artap might have resulted from respiratory failure secondary to diaphragm c ontracture.