Hypoxia increases intracellular calcium in glutamate-activated horizontal cells of goldfish retina via mitochondrial K channels and intracellular stores.

Central neurons of the common goldfish (Carassius auratus) are exceptional in their capacity to survive Ca-induced excitotoxicity and cell death during hypoxia. Horizontal cells (HCs) are inhibitory interneurons of the retina that are tonically depolarized by the neurotransmitter, glutamate, yet preserve intracellular Ca homeostasis. In HCs isolated from goldfish, and in the absence of glutamatergic input, intracellular Ca concentration ([Ca]) is protected from prolonged exposure to hypoxia by mitochondrial ATP-dependent K (mK) channel activity. In the present study, we investigated the effects of hypoxia upon [Ca] in isolated HCs during tonic activation by glutamate to better predict the effects of hypoxia in the active retina. Dynamic changes in [Ca] were measured using the ratiometric Ca indicator, Fura-2. Application of 100 μM glutamate during hypoxia (P = 25 mmHg) produced a 1.3-fold greater rise in [Ca] compared to the same glutamate stimulus during normoxia. The hypoxia-dependent increase in [Ca] was abolished by application of 5-hydroxydecanoic acid, which renders mK channels inactive. Extracellular Ca did not contribute to the elevated [Ca] observed during hypoxia, as the effect persisted in Ca-free solution and during application of verapamil, an L-type Ca channel blocker. By contrast, inhibition of the mitochondrial Ca uniporter or ryanodine receptors (with ruthenium red or ryanodine, respectively) abolished the hypoxia-dependent rise in [Ca]. This study reports an mK-dependent rise in [Ca] during hypoxia in HCs activated by glutamate, and suggests roles for the mitochondria and intracellular Ca stores in regulating this mechanism.