The differences between the inhibitory activity of CRISPs may be explained through sequence comparisons that suggest a site that may be critical for inhibition of channel activity. Phe189 and Glu186 are the most likely functional residues: strong blockers of smooth muscle contraction (ablomin, triflin, and latisemin) all have Phe189, and all blockers of smooth muscle contraction except ophanin have Glu186. The significance of this lack of the probable functional residues in ophanin has not yet been addressed. However, it is likely that the picture is more complex and other residues contribute to the inhibitory activity of CRISPs on smooth muscle contraction and some data supports this. For example, pseudecin, while also having Phe189, does not affect depolarization-induced contraction. 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. The of the venom in mice is ~1.2 to 3.5 mg/kg via intravenous injection. The LD50 of ophanin is not yet known.