Paracellular calcium transport across renal and intestinal epithelia

Calcium ([Ca.sup.2+]) is a key constituent in a myriad of physiological processes from intracellular signalling to the miner alization of bone. As a consequence, [Ca.sup.2+] is maintained within narrow limits when circulating in plasma. This is accomplished via regulated interplay between intestinal absorption, renal tubular reabsorption, and exchange with bone. Many studies have focused on the highly regulated active transcellular transport pathways for [Ca.sup.2+] from the duodenum of the intestine and the distal nephron of the kidney. However, comparatively little work has examined the molecular constituents creating the paracellular shunt across intestinal and renal epithelium, the transport pathway responsible for the majority of transepithelial [Ca.sup.2+] flux. More specifically, passive paracellular [Ca.sup.2+] absorption occurs across the majority of the intestine in addition to the renal proximal tubule and thick ascending limb of Henle's loop. Importantly, recent studies demonstrated that [Ca.sup.2+] transport through the paracellular shunt is significantly regulated. Therefore, we have summarized the evidence for different modes of paracellular [Ca.sup.2+] flux across renal and intestinal epithelia and highlighted recent molecular insights into both the mechanism of secondarily active paracellular [Ca.sup.2+] movement and the identity of claudins that permit the passage of [Ca.sup.2+] through the tight junction of these epithelia. Key words: kidney, NHE3, claudins, solvent drag. Le calcium ([Ca.sup.2+]) est un important constituant d'une myriade de processus physiologiques allant de la signalisation intracellulaire a la mineralisation de l'os. Consequemment, le [Ca.sup.2+] est maintenu a l'interieur de limites etroites lorsqu'il circule dans le plasma. Cela s'accomplit grace a l'influence reciproque regulee entre l'absorption intestinale, la reabsorption renale et l'echange avec l'os. Plusieurs etudes se sont concentrees sur la voie hautement regulee du transport actif transellulaire du [Ca.sup.2+] du duodenum et du nephron distal. Cependant, en comparaison, peu d'etudes ont examine les constituants moleculaires qui creent une derivation para-cellulaire a travers l'epithelium intestinal et renal, la voie de transport responsable de la majorite du flux de [Ca.sup.2+] trans-epithelial. Plus specifiquement, l'absorption para-cellulaire passive de [Ca.sup.2+] survient le long de la plus grande partie de l'intestin en plus du tubule proximal du rein et de la branche ascendante large de l'anse de Henle. Fait important, des etudes recentes demontrent que le transport de [Ca.sup.2+] a travers la derivation para-cellulaire est sujet a une importante regulation. Les auteurs ont donc fait la synthese des indices qui suggerent qu'il existe differents modes de flux para-cellulaires de [Ca.sup.2+] a travers l'epithelium du rein et de l'intestin, et souligne les recentes percees sur les plans du mecanisme moleculaire du mouvement para-cellulaire de [Ca.sup.2+] secondairement actif et de l'identite des claudines qui permettent le passage du [Ca.sup.2+] a travers les jonctions serrees de ces epitheliums. [Traduit par la Redaction] Mots-cles: rein, NHE3, claudines, deplacement de solvant.
Introduction Calcium ([Ca.sup.2+]) participates in a diverse array of physiological processes, including neural transmission, muscle contraction, blood coagulation, intracellular signal transduction as a second messenger, and the mineralization of bone [...]