Na/Ca exchangers in collecting cells of rat kidney

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Abstract
Single pieces of fura-2-loaded cortical collecting tubule (CCT) isolated either from normal or adrenalectomized (ADX) rats were superfused in vitro, and the cytosolic calcium concentration ([Ca2+]i) was calculated from fluorescence recordings. The effects of altering the sodium gradient across cell membranes were investigated. Switching external sodium from 164 mM to 27 mM (low [Na+]o) had little effect on [Ca2+]i in normal tubules (106±9 versus 101±9 nM, n=15) whereas it resulted in a large peak of [Ca2+]i in CCT from ADX-rats (270±32 versus 135±11 nM, n=21). Since CCT from ADX rats are known to have a reduced Na-pump activity, the effect of ouabain treatment on CCT from normal rats was also tested. When CCT from normal rats were exposed to 1 mM of ouabain in the presence of 164 mM of [Na+]o, [Ca2+]i increased only moderately (123±15 versus 111±11 nM, n=13); when the low [Na+]o solution was applied to these ouabain-treated tubules, a large and transient increase in [Ca2+]i was obtained (287±38 versus 123±15 nM, n=13). This response was absent with [Ca2+]o=0. The data suggest the presence of 3 Na+/1 Ca2+ exchangers in cell membranes of rat CCT. The calcium flux equation derived by Läuger for the exchanger indicates a non-linear relationship between net calcium flux and driving force which could account for the difference observed here between the poor effect of applying either low [Na+]o or ouabain alone and the large peak of [Ca2+]i induced by combining these two conditions.