The Na+‐Ca2+ exchange system in rat glial cells in culture: Activation by external monovalent cations

Abstract

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.