Na/Ca exchange in the basolateral membrane of the A6 cell monolayer: role in Cai homeostasis.
The presence of a Na/Ca exchanger in A6 cells was investigated by measuring intracellular calcium (Cai) fluctuations and the 45Ca fluxes through the basolateral membranes (blm) of the cell monolayer. Removal of Na+ from the medium produced a transient increase in Cai followed by a regulatory phase returning Cai to control levels in 3-4 min, this phase being greatly accelerated (< 60 s) by NaCl addition (apparent Km of approximately 5 mM Na+). The Cai increase was only found with the Na(+)-free medium on the basolateral side of the cell monolayer. A twofold increase in the 45Ca influx was observed under these conditions. In Ca(2+)- depleted cells, the initial Cai increase after Ca2+ addition to the medium was greater when the putative Na/Ca exchanger was not functioning (i.e. in a Na(+)-free medium). 45Ca effluxes through the blm of the monolayer were greatly and transiently increased by a Na(+)-free medium on the serosal side and blocked by orthovanadate (1 mM). The Cai increased induced by a hypo-osmotic shock was greater in cells bathed in a Na(+)-medium, conditions expected to block the activity of the Na/Ca exchanger. These findings support the hypothesis that a Na/Ca exchanger is present on the blm of A6 cells and affirm its role in Cai homeostasis in steady-state conditions and following osmotic shock. In addition, a Ca2+ pump also located on the blm and Ca2+ stores sensitive to inositol 1,4,5-trisphosphate were found to be implicated in Cai homeostasis.
Brochiero
E
E
Department of Cellular and Molecular Bilogy CEA/URA 1855 (CNRS), Villefranche/Mer, France.
Raschi
C
C
Ehrenfeld
J
J
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002352:Carrier Proteins / Q000187:drug effects / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D002478:Cells, Cultured; D006706:Homeostasis / Q000187:drug effects; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D017136:Ion Transport / Q000187:drug effects; D007668:Kidney / Q000187:drug effects / Q000502:physiology; D009997:Osmotic Pressure; D012964:Sodium / Q000378:metabolism* / Q000493:pharmacokinetics; D019831:Sodium-Calcium Exchanger; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D014982:Xenopus laevis
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D013729:Terpenes; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D012964:Sodium; D000252:Calcium-Transporting ATPases; D002118:Calcium