The Na(+)-Ca2+ exchange system in rat glial cells in culture: activation by external monovalent cations.
Cultured rat glial cells display a Na(+)-Ca2+ exchange system located at the plasma membrane levels. This was evidenced by the Na+ (i)-dependency of a Na+ (o)-inhibitable influx of Ca2+, or reversal exchange mode. This antiporter has an external site where monovalent cations (K+, Li+, and Na+ were investigated) stimulate the exchange by a chemical action. The monovalent cation is not transported during the exchange cycle. The mechanism of that stimulation agrees with an increase in the apparent affinity of the carrier for Ca2+(o) without effect on the maximal translocation rate. Two models can equally well account for the data: i) the formation of ECa(o) is essential for the binding of the monovalent cation, or ii) the activating cation can bind even when the carrier is free of Ca2+(o). The cations K+ and Li+ produced only stimulation, although that of K+ seem to require actions other than the chemical effect. The response to Na+ was biphasic; this can be fully explained considering that at low concentrations, Na+(o) binds preferentially to the activating monovalent site while at high concentrations it displaces Ca2+ from its external transporting site. Pure type I astrocytes displayed the same Na(+)-Ca2+ exchange mechanism.
Holgado
A
A
División de Biofisica, Instituto de Investigación Médica Mercedes y Martin Ferreyra, Córdoba, Argentina.
Beaugé
L
L
D000818:Animals; D001253:Astrocytes / Q000378:metabolism; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002478:Cells, Cultured / Q000378:metabolism; D017136:Ion Transport / Q000502:physiology; D008094:Lithium / Q000378:metabolism / Q000494:pharmacology*; D009457:Neuroglia / Q000166:cytology* / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism / Q000494:pharmacology*; D051381:Rats; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D013997:Time Factors
D008094:Lithium; D012964:Sodium; D011188:Potassium; D002118:Calcium