Membrane transport of antiviral drugs in plants: an electrophysiological study in grapevine explants infected by Grapevine leafroll associated virus 1

Abstract

Antiviral drugs have demonstrated significant therapeutic potential against phytoviruses, such as inosine monophosphate dehydrogenase inhibitors like mycophenolic acid or thiopurines. However, drug delivery across cellular barriers is a challenging task that calls for investigation. In this study, membrane transport of three antiviral drugs such as mycophenolic acid (MPA), 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) was investigated in order to determine the mechanism involved in transmembrane flux. Electrophysiological tests were carried out on leaves of in vitro grapevine explants naturally infected by Grapevine leafroll associated virus 1 as well as virus-free explants treated with antiviral drugs. Results indicate that membrane electrical responses of the tested antiviral dugs are supported by the metabolism, and virus infection did not cause differences in membrane potential tests. The extent of depolarization was slight for MPA and 6-TG and greater for 6-MP. Complete repolarization always occurred and, in many cases, the electrical potential was greater than the starting level (overshoot). Treatments at 5 °C or in presence of CCCP, an uncoupler of oxidative phosphorylation, led to inhibition of drug effects of the membrane potential, suggesting a metabolic dependence of the transport. However, treatments carried out under sodium orthovanadate, a specific inhibitor of H+-ATPase, prove, although only partially, the activity of a proton pump. The complex of electrophysiological tests highlights that drugs such as MPA, 6-MP and 6-TG, when applied to infected or healthy grapevine leaf segments, lead to clear, biphasic, electrical membrane response suggesting an operation of H+ co-transport through systems that link substrate translocation across the membrane to the free energy available in a proton electrochemical potential difference.