title: Effects of low extracellular sodium on cytosolic ionized calcium. Na+-Ca2+ exchange as a major calcium influx pathway in kidney cells.
abstract: The effects of extracellular Na+ (Na+o) on cytosolic ionized calcium (Ca2+i) and on calcium and sodium fluxes were measured in monkey kidney cells (LLC-MK2). Ca2+i was measured with aequorin and the ion fluxes with 45Ca and 22Na. Na+-free media rapidly increased Ca2+i from 60 to a maximum of about 700 nM in 2-3 min. After the peak, Ca2+i declined and reached a plateau of about twice the resting Ca2+i. The peak Ca2+i was inversely proportional to Na+o and directly proportional to the extracellular calcium concentration (Ca2+o). On the other hand, a pH of 6.8 reduced and Ca2+o substitution with Sr2+ completely blocked the Ca2+i response to low Na+o. A Na+-free medium stimulated calcium efflux from the cells 4-5-fold, a response which was abolished in the absence of extracellular Ca2+. Na+-free media also stimulated calcium influx and sodium efflux. The cell calcium content, however, was not increased. These results indicate that removal of extracellular Na+ increases Ca2+i by stimulating calcium influx and not by inhibiting calcium efflux; the increased calcium influx takes place on the Na+-Ca2+ antiporter operating in the reverse mode in exchange for sodium efflux. The increased calcium efflux occurs as a consequence of the rise in Ca2+i and presumably takes place on the (Ca2+-Mg2+) ATPase-dependent calcium pump.
authors.forename: K W
authors.initials: KW
authors.lastname: Snowdowne
authors.forename: A B
authors.initials: AB
authors.lastname: Borle
mesh_terms: D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002460:Cell Line; D002794:Choline / Q000378:metabolism; D003470:Culture Media; D003600:Cytosol / Q000378:metabolism; D000882:Haplorhini; D007668:Kidney / Q000166:cytology* / Q000378:metabolism; D008094:Lithium / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*; D013324:Strontium / Q000378:metabolism
chemical_list: D003470:Culture Media; D000644:Quaternary Ammonium Compounds; D008094:Lithium; D012964:Sodium; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; C027917:tetramethylammonium; D002794:Choline; D002118:Calcium; D013324:Strontium