In experiments employing confluent monolayers, nanomolar ouabain has been previously shown to increase the expression of claudin-1, -2 and -4 in MDCK II cells [20], claudin-1 and -11 in Sertoli cells [21] and claudin-1 in ADPKD cells [22]. This evidence concerns the gene CLDN1 and autosomal dominant polycystic kidney disease.