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

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Abstract
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.