Mode of Action
There is no direct evidence of a particular mode of action of ophanin blocking depolarization-induced contractions of the smooth muscles. However, based on the hypothesis of Yamazaki and colleagues in regards to ablomin, another snake venom toxin from the CRISP family that also blocks depolarization-induced smooth muscle contraction, we can postulate a similar mechanism might be in place for ophanin.
Since ablomin only blocks contraction induced by depolarization, but not by caffeine, the effect of ablomin is likely to be caused by inhibition of voltage-gated ion channels. An activation of smooth muscle cells through caffeine activates ryanodine receptors of the sarcoplasmic reticulum, whereas an activation through high levels of extracellular potassium depolarizes the membrane (due to the change of the reversal potential for potassium towards more positive values) and would then activate voltage-gated calcium-ion channels leading to high levels of intracellular calcium ions. The intracellular calcium ion concentration correlates well with contraction force in the rat-tail artery. Thus, contraction following extracellular application of high-potassium solution depends on the influx of the extracellular calcium ions through voltage-gated calcium channels. Therefore ablomin (and by extension ophanin) most likely targets voltage-gated calcium channels on smooth muscle.
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