Effects of inhibitors of Na +,K +-ATPase on the membrane potentials and neurotransmitter efflux in rat brain slices

The potassium potential, E K, of rat brain slices was estimated by determining the uptake of 86Rb +. The E Rb was the same for slices prepared from five rostral brain regions, the average value being 66.4 mV. The E Rb values in the presence of 20 μM ouabain were only slightly lower than the resting values; increasing concentrations of ouabain above 20 μM resulted in a graded depolarization in all five brain regions. High concentrations (1 mM) of two other inhibitors of Na +,K +-ATPase, dihydro-ouabain and strophanthidin, produced no more depolarization than did 20 μM ouabain. Competitive binding studies indicated that the differential effects were due to the relative binding to brain slices. Erythrosin B, an inhibitor of Na +,K +-ATPase, had no measurable effect on E Rb. Intermediate concentrations of the Na +/H + ionophore monensin slightly hyperpolarized striatal slices, whereas the same monensin concentrations plus 20 μM ouabain, 1 mM strophanthidin or 70 μM erythrosin B resulted in marked depolarization. Measurement of the membrane potential via uptake of methyltriphenylphosphonium cation indicated that E Rb was indeed a valid estimation of the membrane potential. E K was measured directly by monitoring 42K + uptake in striatal slices and was found to be essentially identical to E Rb. Uptake of 22Na + was consistent with the values for E Rb or E K. Several conditions that resulted in little or no measurable depolarization of striatal slices did induce efflux of exogenously loaded GABA and dopamine; these conditions included 20 μM ouabain, 1 mM dihydro-ouabain or strophanthidin, and 70 μM erythrosin B. Neurotransmitter efflux in the absence of general cell depolarization was not accompanied by altered rates of respiration or decreased ATP levels.