Plasmalemmal Na+/Ca2+ exchanger modulates Ca2+-dependent exocytotic release of glutamate from rat cortical astrocytes

Astroglial excitability operates through increases in Ca²⁺_cyt (cytosolic Ca²⁺), which can lead to glutamatergic gliotransmission. In parallel fluctuations in astrocytic Na⁺_cyt (cytosolic Na⁺) control metabolic neuronal-glial signalling, most notably through stimulation of lactate production, which on release from astrocytes can be taken up and utilized by nearby neurons, a process referred to as lactate shuttle. Both gliotransmission and lactate shuttle play a role in modulation of synaptic transmission and plasticity. Consequently, we studied the role of the PMCA (plasma membrane Ca²⁺-ATPase), NCX (plasma membrane Na⁺/Ca²⁺ exchanger) and NKA (Na⁺/K⁺-ATPase) in complex and coordinated regulation of Ca²⁺_cyt and Na⁺_cyt in astrocytes at rest and upon mechanical stimulation. Our data support the notion that NKA and PMCA are the major Na⁺ and Ca²⁺ extruders in resting astrocytes. Surprisingly, the blockade of NKA or PMCA appeared less important during times of Ca²⁺ and Na⁺ cytosolic loads caused by mechanical stimulation. Unexpectedly, NCX in reverse mode appeared as a major contributor to overall Ca²⁺ and Na⁺ homoeostasis in astrocytes both at rest and when these glial cells were mechanically stimulated. In addition, NCX facilitated mechanically induced Ca²⁺-dependent exocytotic release of glutamate from astrocytes. These findings help better understanding of astrocyte-neuron bidirectional signalling at the tripartite synapse and/or microvasculature. We propose that NCX operating in reverse mode could be involved in fast and spatially localized Ca²⁺-dependent gliotransmission, that would operate in parallel to a slower and more widely distributed gliotransmission pathway that requires metabotropically controlled Ca²⁺ release from the ER (endoplasmic reticulum).