Endocrine Abstracts

Aims: A critical function of astrocytes is to recycle glutamate to neurons as glutamine to sustain glutamatergic neurotransmission. In the hypothalamus, this is required for effective counterregulatory hormone release in response to hypoglycaemia. However, after recurrent hypoglycaemia in vivo, this is attenuated. The aim of this study was to characterise how rat primary hypothalamic and cortical astrocytes adapt to recurrent low glucose (RLG) with repeated exposure to glutamate. We hypothesised that the additional metabolic challenge of glutamate recycling would alter cellular metabolism and the astrocytes would metabolise it as an alternative fuel source.

Methods: Rat primary cortical (CRTAS) and hypothalamic (HTAS) astrocytes were exposed to 0, 1, or 4 bouts of low (0.1 mM) glucose for three hours over four days with and without glutamate (100 μM). Mitochondrial and glycolytic flux were measured using the Seahorse Bioanalyser platform as well as extracellular and intracellular glutamate and glutamine levels.

Results: Like human primary astrocytes, RLG increased basal mitochondrial oxygen consumption rate (OCR) in HTAS and CRTAS. However, concurrent glutamate treatment attenuated the adaptation in HTAS but exacerbated it in CRTAS. This coincided with an increased dependency for glutamine metabolism in HTAS, and enhanced depletion of glutamate in the extracellular medium. Intracellular glutamate concentrations were unchanged.

Conclusions: Together these data show that HTAS increase their dependency for and use of glutamate as a fuel source after RLG with concurrent glutamate treatment. This adaptation we believe helps maintain intracellular energy supply during decreased glucose availability. If this is the case in vivo, then glutamate, which should be recycled to neurons, is metabolised by the ‘selfish’ astrocyte and may impair hypoglycaemia-induced glutamatergic signalling in the hypothalamus and attenuate counterregulatory hormone release.