title: Calcium transport across rat inner medullary collecting duct perfused in vitro.
abstract: Ca2+ transport by the inner medullary collecting duct (IMCD) of normal rats was studied using the "in vitro" microperfusion technique. Net (Jnet), lumen-to-bath (Jl----b), and bath-to-lumen (Jb----l) fluxes of Ca2+ were measured in the absence of net water absorption using 45Ca as a tracer. In the absence of an electrochemical gradient, an important net absorption of Ca2+ (11.1 +/- 1.6 pmol.cm-2.s-1), similar to the difference between the Jl----b and Jb----l, was observed by direct determination at low (5-6 nl/min) and high (12-17 nl/min) perfusion rates. The Jl----b of Ca2+ was reduced by the addition to the bath fluid of ouabain (10(-3) M) and verapamil (10(-4) M), by the presence of amiloride (10(-5) and 10(-3) M) and verapamil (10(-4) M) in the luminal fluid, or by perfusion with a Na+-free solution. Neither the presence of verapamil (10(-4) M) and ouabain (10(-3) M) in the bath nor the withdrawal of Na+ from bath and perfusion solution was able to modify the Jb----l of Ca2+. Incrementing Ca2+ bath concentration increased proportionally the Jb----l of Ca2+. Therefore Ca2+ outflux is in part dependent on Na+-K+-adenosinetriphosphatase luminal membrane Na+ transport and in part inhibited by verapamil. However, Ca2+ influx is independent of Na+ transport, is not blocked by verapamil, but is increased by Ca2+ transtubular gradient, indicating the presence of a passive diffusion mechanism.
authors.affiliation: Laboratório de Pesquisa Básica da Faculdade de Medicina da Universidade de São Paulo, Brazil.
authors.forename: A J
authors.initials: AJ
authors.lastname: Magaldi
authors.forename: A A
authors.initials: AA
authors.lastname: van Baak
authors.forename: A S
authors.initials: AS
authors.lastname: Rocha
mesh_terms: D000042:Absorption; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D066298:In Vitro Techniques; D007679:Kidney Medulla; D007684:Kidney Tubules / Q000378:metabolism*; D007685:Kidney Tubules, Collecting / Q000378:metabolism*; D008297:Male; D010477:Perfusion; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism; D014700:Verapamil / Q000494:pharmacology
chemical_list: D000584:Amiloride; D012964:Sodium; D014700:Verapamil; D002118:Calcium