Complex sentence: Ouabain is the familiar name of G-strophanthin, a poisonous cardiac glycoside that blocks the sodium-potassium ATPase in higher (toxical?) concentrations which are attainable in vitro or with high intravenous dosage.
Split into simpler sentences: Ouabain is the familiar name of G-strophanthin, a poisonous cardiac glycoside. Ouabain blocks the sodium-potassium ATPase in higher concentrations or with intravenous dosage.