Validation of the use of the lipophilic thiocyanate anion for the determination of membrane potential in Ehrlich ascites tumor cells

Summary The utility of the lipophilic anion thiocyanate (SCN⁺) as a probe for the indirect estimation of the cell membrane potential (V _m ) in Ehrlich ascites tumor cells has been evaluated by comparison to direct electrophysiological measurements. SCN accumulation is consisten with first-order uptake into a single kinetically-identifiable cellular compartement, achieving steadystate distribution in 20–30 min at 22°C. The steady-state distribution ratio (SCN^–] _c /SCN^–] _e ) in physiological saline is 0.44±0.02. Treatment of the cells with proparanolol (0.13 mM), an activator of Ca²⁺ dependent K⁺ channels, reduces the steady-state distribution ratio to 0.19±0.02. Conversely, treatmetn with BACl₂ (10 mM), an antagonist of the pathway, increases the SCN^– distribution ratio to 0.62±0.01. The equilibrium potentials (V _SCN ) calculated under these conditions are virtually identical to direct electrophysiological measurements of theV _m made under the same conditions. The effect of varing extracellular K⁺](K⁺] _e ) in the presence of constant Na⁺] _e =100 mM has also been tested. In control cells, elevation of K⁺] _e from 6 to 60 mM reducesV _SCN from –20.6±1.0 to –13.2±1.2 mV. Again, microelectrode measurements give excellent quantitative agreement. Propranolol increases the sensitivity of the cells to varying K⁺] _e , so that a 10-fold elevation reducesV _SCN by approximately 31 mV. BaCl₂ greatly reduces this reponse: a 10-fold elevation in K⁺] _e yielding only a 4-mV rediction inV _SCN . It is concluded that the membrane potential of Ehrlich cells can be estimated accurately from SCN^– distribution measurements.