Identification and characteristics of a Na+/Ca2+ exchanger in cultured human mesangial cells.
Excitable cells express Na+/Ca2+ exchange activity among other mechanisms modulating rapid fluctuations of cytosolic free Ca2+ ([Ca2+]i). We studied functions and regulation of a Na+/Ca2+ exchanger in cultured human glomerular mesangial cells. Fura-2-loaded confluent monolayers reacted to removal of ambient Na+ with an immediate, transient elevation of [Ca2+]i, assessed by single/dual wavelength fluorometry. Peak [Ca2+]i was inversely correlated with the extracellular Na+ concentration. Ca2+ influx was the sole mechanism implicated, as the [Ca2+]i rise was prevented by EGTA. The process was inhibited by 1 mM amiloride, but not by blockers of voltage-operated Ca2+ channels. Re-addition of Na+ resulted in a rapid decrease of [Ca2+]i, indicating bimodal operation of the exchanger. Na(+)-loading the cells with monensin and ouabain enhanced Ca2+ uptake. Prior stimulation of [Ca2+]i with the thromboxane A2 mimetic, U-46619, or angiotensin II also increased Ca2+ uptake upon subsequent Na+ removal, suggesting induction of the exchanger by vasoconstrictors. Moreover, the magnitude of agonist-induced [Ca2+]i transients was amplified by Na+ removal, indicating that the exchanger modulates the effects of vasoconstrictors. These results demonstrate that an inducible Na+/Ca2+ antiporter is operative in resting and stimulated human mesangial cells, further confirming their smooth muscle origin and potential regulatory role on glomerular hemodynamics.
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Cattedra di Nefrologia Medica, University of Rome La Sapienza, Italy.
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Faraggiana
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D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry / Q000302:isolation & purification*; D002478:Cells, Cultured; D005920:Glomerular Mesangium / Q000737:chemistry* / Q000502:physiology; D006801:Humans; D066298:In Vitro Techniques; D019831:Sodium-Calcium Exchanger
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D002118:Calcium