Na+ channels as sites of action of the cardioactive agent DPI 201-106 with agonist and antagonist enantiomers.

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Abstract
This paper shows the interaction of the cardiotonic agent 4-[3-(4-diphenylmethyl-1-piperazinyl)-2-hydroxypropoxy]-1H-indole-2-carbonitrile (DPI 201-106) and its optic enantiomers R-DPI (205-249) and S-DPI (205-430) with the Na+ channel of a variety of excitabe cells. Voltage-clamp experiments show that DPI 201-106 acts on neuroblastoma cells and rat cardiac cells. S-DPI (205-403) increases the peak Na+ current, slows down the kinetics of Na+ channel inactivation, and is cardiotonic on heart cells. Conversely, R-DPI (205-429) reduces the peak Na+ current and blocks Na+ channel activity and cardiac contractions. Binding experiments using radioactivity labeled toxins indicate that DPI 201-106 and its enantiomers do not interact with sites already identified for tetrodo toxin or sea anemone and scorpiontoxins. DPI 201-106 and its enantiomers inhibit binding of a 3H-labeled batrachotoxin derivative, [3H]batrachotoxinin A 20-.alpha.-benzoate, to brain membranes. The dissociation constant of the complex formed between the Na+ channel and both R-DPI and S-DPI is Kd .simeq. 100 nM. 22Na+ uptake experiments using different cell types have shown that R and S enantiomers of DPI 201-106 are active on the different Na+ channel subtypes with similar IC50 values. These results are discussed in relation with the cardiotonic properties of DPI 201-106 that are not accompanied by cardiotoxic effects.