Triphenylmethylphosphonium (TPMP⁺) as a Probe for Peritubular Membrane Potential in the Kidney Slice

The present work was undertaken to assess the validity of the use of TPMP⁺, a lipophilic cation, as a basolateral membrane-potential probe in the rabbit kidney slice. Cortical slices incubated in media containing ³H-TPMP⁺ reached an equilibrium slice/medium concentration ratio (S/M) for TPMP⁺ after 90 min. The equilibrium S/M TPMP⁺ was ∼20 at TPMP⁺ concentrations between 30 and 50 μM. Concentrations of TPMP⁺ above 50 μM resulted in changes in tissue electrolytes and O₂ consumption. Anaerobic incubation of slices in the presence of iodoacetamide decreased the S/M TPMP to ∼3. When medium K⁺ was progressively increased, the S/M TPMP⁺ decreased continuously. There was a linear correlation between cell water/medium concentration ratios (C/M) for TPMP⁺ and K⁺ (r = 0.629, P < 0.01). However, the C/M TPMP⁺ was 29.0 even at C/M K⁺ of 1.0, strongly suggesting the presence of a tissue binding component for TPMP⁺. When corrections are made for the above binding, the average C/M TPMP⁺ in slices incubated in the normal medium was ∼13, equivalent to a Nernst potential of −64 mV. This value compared favorably to −54.3 mV obtained by microelectrodes in slices treated in the same fashion. It is concluded that the C/M TPMP⁺ corrected for binding may be used as a measure of the basolateral membrane potential in the steady state.