Role of Na+/Ca2+ exchange in transcellular Ca2+ transport across primary cultures of rabbit kidney collecting system |
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Authors: | R J M Bindels P L M Ramakers J A Dempster A Hartog C H van Os |
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Institution: | (1) Department of Physiology, University of Nijmegen, Trigon Building, P.O. Box 9101, NL-6500 HB Nijmegen, The Netherlands |
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Abstract: | Cells from connecting tubule and cortical collecting duct of rabbit kidney were isolated by immunodissection with mAb R2G9 and cultured on permeable filters. Confluent monolayers developed an amiloride-sensitive transepithelial potential difference of –50±1 mV (lumen negative) and a transepithelial resistance of 507±18 cm2. Transepithelial Ca2+ transport increased dose-dependently with apical Ca2+] and, in solutions containing 1 mM Ca2+, the active transcellular Ca2+ transport rate was 92±2 nmol h–1 cm–2. Transcellular Ca2+ transport was dependent on basolateral Na+ (Na
b
+
). Isoosmotic substitution of Na
b
+
for N-methylglucamine resulted in a concentration-dependent decrease in Ca2+ absorption, with maximal inhibition of 67±5%. A Hill plot of the Na+-dependence yielded a coefficient of 1.9±0.4, indicating more than one Na+ site on a Na+-dependent Ca2+ transport system. In addition, the absence of Ca
b
2+
resulted in a significant increase in Ca2+ transport both in the presence and absence of Na
b
+
. Added basolaterally, ouabain (0.1 mM) inhibited Ca2+ transport to the same extent as did Na+-free solutions, while bepridil (0.1 mM), an inhibitor of Na+/Ca2+ exchange, reduced Ca2+ transport by 32±6%. Methoxyverapamil, felodipine, flunarizine and diltiazem (10 M) were without effect. Depolarisation of the basolateral membrane, by raising K+]b to 60 mM, significantly decreased transcellular Ca2+ transport, which is indicative of electrogenic Na+/Ca2+ exchange. In conclusion, active Ca2+ transport in the collecting system of rabbit kidney is largely driven by basolateral Na+/Ca2+ exchange. However, a residual Ca2+ absorption of about 30% was always observed, suggesting that other Ca2+ transport mechanisms, presumably a Ca2+-ATPase, participate as well. |
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Keywords: | Na+/Ca2+ exchange Rabbit kidney Connecting tubule Cortical collecting duct Ca2+ reabsorption Bepridil Ca2+ entry blockers Ca2+ transport |
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