Localization of the epithelial Ca2+ channel in rabbit kidney and intestine

Abstract

The epithelial Ca2+ channel (ECaC), which is exclusively expressed in 1,25-dihydroxyvitamin D3-responsive tissues, i.e., kidney, intestine, and placenta, is postulated to constitute the initial step in the process of transcellular Ca2+ transport. To strengthen this postulated function, the present study compares the segmental and cellular distribution of ECaC and the other Ca2+ transport proteins known to be involved in transcellular Ca2+ transport. In rabbit kidney, ECaC mRNA and protein expression were primarily present in the connecting tubule. Immunopositive staining for the ECaC protein was exclusively found at the apical domain of this tubular segment. Importantly, ECaC completely colocalized with calbindin-D(28K), Na+-Ca2+ exchanger (NCX), and plasma membrane Ca2+-ATPase (PMCA). A minority of cells along the distal tubule lacked immunopositive staining-for ECaC and the other Ca2+ transporting proteins. These negative cells were identified as intercalated cells. In intestine, ECaC was present in a thin layer along the apical membrane of the duodenal villus tip, whereas the crypt and goblet cells were negative. Again, a complete colocalization was observed between ECaC, calbindin-D(9K), and PMCA. In contrast to the kidney, NCX could not be detected in duodenum. The present finding that ECaC completely colocalizes with the Ca2+ transport proteins in the connecting tubule and duodenum, together with its apical localization, further substantiates the postulated function of ECaC as the gatekeeper of active Ca2+ (re)absorption.