Summary
Intracellular potassium activity (a K i ) was measured in control conditions in mid-cortical rabbit proximal convoluted tubule using two methods: (i) by determination of the K+ equilibrium potential (E K) using Ba2+-induced variations in the basolateral membrane potential (V BL) during transepithelial current injections and (ii) with double-barrel K-selective microelectrodes. Using the first method, the meanV BL was −48.5±3.2 mV (n=16) and the meanE K was −78.4±4.1 mV corresponding to aa K i of 68.7mm. With K-selective microelectrodes,V BL was −36.6±1.1 mV (n=19),E K was −64.0±1.1 mV anda K i averaged 40.6±1.7mm. While these lastE K andV BL values are significantly lower than the corresponding values obtained with the first method (P<0.001 andP<0.01, respectively), the electrochemical driving force for K transport across the basolateral membrane (μ K =V BL −E K) is not significantly different for both techniques (30.1±3.3 mV for the first technique and 27.6±1.8 mV for ion-selective electrodes). This suggests an adequate functioning of the selective barrel but an underestimation ofV BL by the reference barrel of the double-barrel microelectrode. Such double-barrel microelectrodes were used to measure temporal changes ina K i andμ K in different experimental conditions where Na reabsorption rate (J Na) was reduced.a K i was shown to increase by 12.2±2.7 (n=5) and 14.1±4.4mm (n=5), respectively, whenJ Na was reduced by omitting in the luminal perfusate: (i) 5.5mm glucose and 6mm alanine and (ii) glucose, alanine, other Na-cotransported solutes and 110mm Na. In terms of the electrochemical driving force for K exit across the basolateral membrane,μ K, a decrease of 5.4±2.0 mV (P<0.05,n=5) was measured when glucose and alanine were omitted in the luminal perfusate whileμ K remained unchanged whenJ Na was more severely reduced (mean change =−1.7±2.1 mV, NS,n=5). In the latter case, this means that the electrochemical driving force for K efflux across the basolateral membrane has not changed while both the active influx through the Na−K pump and the passive efflux in steady state are certainly reduced. If the main pathway for K transport is through the basolateral K conductance, this implies that this conductance must have decreased in the same proportion as that of the reduction in the Na−K pump activity.
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Laprade, R., Lapointe, JY., Breton, S. et al. Intracellular potassium activity in mammalian proximal tubule: Effect of perturbations in transepithelial sodium transport. J. Membrain Biol. 121, 249–259 (1991). https://doi.org/10.1007/BF01951558
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DOI: https://doi.org/10.1007/BF01951558