ROLE OF NA+/CA2+ EXCHANGE IN TRANSCELLULAR CA2+ TRANSPORT ACROSS PRIMARY CULTURES OF RABBIT KIDNEY COLLECTING SYSTEM

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-OMEGA-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)+). Isosmotic 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 Ca-- 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-mu-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.