Selective Uptake and Vacuolar Storage of Primary Cardiac Glycosides by Suspension-cultured Digitalis lanata Cells

Abstract

Summary Cardenolide uptake by Digitalis cells and protoplasts was examined. The uptake of secondarycardiac glycosides (digitoxin, digoxin, β-methyldigitoxin, β-methyldigoxin) was not affected by ATPase inhibitors, uncoupling agents, or electron transport inhibitors and was thus inferred to result from simple diffusion of the relatively hydrophobic glycosides across the plasma-lemma. The uptake of primary glycosides (purpurea-glycoside A, lanatoside A, deacetyl-lanatoside C, lanatoside C), on the other hand, was inhibited reversibly by uncoupling agents and electron transport inhibitors. It was not affected by ATPase inhibitors. Cell cultures from plant species which do not synthesize cardenolides were unable to accumulate primary cardiac glycosides. Wash-out studies with Digitalis cells and protoplasts showed that secondary glycosides were easily released into the culture medium, while the primary glycosides were retained by the cells. To investigate the intracellular distribution of cardiac glycosides, vacuoles were prepared from cardenolide-containing Digitalis cells and analysed for their cardenolide content using an HPLC method. The primary glycosides were located almost exclusively in the vacuoles. A model for cardenolide uptake, transformation, and transport across the tonoplast is proposed.