The activity of the Na+/Ca2+ exchanger largely modulates the Ca2+i signal induced by hypo-osmotic stress in rat cerebellar astrocytes. The effect of osmolarity on exchange activity.
We recently demonstrated that rat cerebellar Type-1 astrocytes express a very active Na(+)/Ca(2+) exchanger highly colocalized with ryanodine receptors (RyRs), which in turn play a key role in glutamate-induced Ca(2+) signaling through a calcium-induced calcium release (CICR) mechanism. In this work we have explored whether the Na(+)/Ca(2+) exchanger has any role in the Ca(2+)(i) signal induced by hypo-osmotic stress in these cells, using microspectrofluorometric measurements with Fura-2, pharmacological tools, and confocal microscopy image analysis. We present evidence for the first time that the increase in [Ca(2+)](i) in rat cerebellar Type-1 astrocytes, resulting from moderate hypotonic shock, is mediated by Ca(2+) release from ryanodine-operated Ca(2+)(i) stores, and that the magnitude of the intracellular Ca(2+) signal induced by hypotonicity in the short term (up to 240 s) is small and controlled by the activity of the Na(+)/Ca(2+) exchanger operating in its extrusion mode. With longer times in the hypotonic medium, intracellular Ca(2+) store depletion leads to Ca(2+) entry through store-operated Ca(2+) channels. We found it interesting that the activity of the Na(+)/Ca(2+) exchanger measured during this reverse mode operation (Ca(2+) entry in exchange for internal Na(+)) was found to be greatly increased in hypotonic solutions and decreased in hypertonic ones. The buffering of the [Ca(2+)](i) rise induced by hypo-osmotic stress may prevent excessive increases in [Ca(2+)](i), which otherwise might impair the normal function of this glial cell.
Rojas
Héctor
H
Laboratorio de Fisiología Celular, Centro de Biofísica y Bioquímica, Instituto Venezolano de Investigaciones Científicas, Apartado 21827, Caracas 1020A.
Ramos
Magaly
M
Benaim
Gustavo
G
Caputo
Carlo
C
DiPolo
Reinaldo
R
D000818:Animals; D001253:Astrocytes / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling; D048429:Cell Size; D002478:Cells, Cultured; D002531:Cerebellum / Q000166:cytology / Q000378:metabolism*; D007038:Hypotonic Solutions; D018613:Microscopy, Confocal; D015591:Microspectrophotometry; D009997:Osmotic Pressure; D051381:Rats; D017207:Rats, Sprague-Dawley; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013997:Time Factors
D007038:Hypotonic Solutions; D019837:Ryanodine Receptor Calcium Release Channel; D019831:Sodium-Calcium Exchanger; D012964:Sodium; D002118:Calcium