Intracellular potassium activity in mammalian proximal tubule: Effect of perturbations in transepithelial sodium transport |
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Authors: | Raynald Laprade Jean-Yves Lapointe Sylvie Breton Marcelle Duplain Jean Cardinal |
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Institution: | (1) Groupe de recherche en transport membranaire, Université de Montréal, H3C 3J7 Montreal, Quebec, Canada;(2) Maisonneuve-Rosemont Research Center and Division of Nephrology, H1T 2M4 Montreal, Quebec, Canada |
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Abstract: | 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 (
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
and
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,
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 while
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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Keywords: | potassium Ba2+ proximal tubule electrophysiology ion-selective microelectrode membrane cross-talk |
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