摘要：

Lipophilic cations such as safranine and methyltriphenylphosphonium (MTPP +) are commonly employed to obtain measures of the membrane potential (Δψ) exhibited by energized biological membrane systems. These probes reflect the presence of Δψ (inside negative) by accumulating in the interior of the membrane bound system to achieve transmembrane distributions dictated by the Nernst equation. In this work, we characterize the ability of model membrane large unilamellar vesicle systems to accumulate safranine and other biologically active lipophilic cations in response to a K + diffusion potential (interior negative) across the large unilamellar vesicle membrane. We show that safranine, MTPP +, chlorpromazine and vinblastine can be rapidly accumulated to achieve interior lipophilic cation concentrations which may be more than two orders of magnitude higher than exterior concentrations. In the case of safranine, for example, incubation of 2 mM safranine with large unilamellar vesicle systems exhibiting a Δψ of 100 mV or more can lead to interior safranine concentrations in excess of 120 mM. This accumulation appears to proceed as an antiport K +-safranine exchange process, and the optical 'safranine response' observed can be attributed to precipitation of the dye inside the vesicle as the interior concentrations of safranine exceeds its solubility (96 mM). These observations are discussed in terms of the utility of probes such as safranine and MTPP + for determinations of Δψ as well as their implications for the equilibrium transbilayer distributions of biologically active lipophilic cations in vivo.

展开