Figure 1.

Tripartite Ca²⁺_cyt handling in Ca²⁺- dependent vesicular glutamate release from astrocytes. A drawing of sources of Ca²⁺ for [Ca²⁺]_cyt increase. [Ca²⁺]_cyt can be affected by the endoplasmic reticulum (ER), store-operated Ca²⁺ (SOC) entry and mitochondria. Ca²⁺_cyt accumulation could be caused by the entry of Ca²⁺ from the internal stores of the ER that possess IP₃ and ryanodine receptors. The pre-incubation of astrocytes with ryanodine (Ry) can block Ca²⁺ release from ryanodine/caffeine-sensitve ER stores, while diphenylboric acid 2-aminoethyl ester (2-APB), an IP₃R antagonist, reduces Ca²⁺ release from an IP₃-senistive stores. Thapsigargin (Thaps) and cyclopiazonic acid (CPA), blockers of store specific Ca²⁺-ATPase, prevent filling of these stores by Ca²⁺ and, thus, deplete stores of this ion. Ultimately, the (re)filling of ER Ca²⁺ stores requires Ca²⁺ entry from the extracellular space (ECS) through SOC channels, in particular those containing canonical type 1 transient receptor potential (TRPC1) protein. A specific TRPC1 antibody blocks TRPC1 containing SOC channels and reduces Ca²⁺ entry to the cytosol. A negative membrane potential (about −160 mV) exists across the inner mitochondrial membrane. Mitochondrial Ca²⁺ uptake is mediated by the uniporter as the electropotential gradient drives Ca²⁺ into the matrix. Ru360 blocks Ca²⁺ influx through the uniporter. Free Ca²⁺ exits the mitochondrial matrix through the Na⁺/Ca²⁺ exchanger, and this process can be blocked by CGP37157. Ca²⁺ accumulation in mitochondria is increased by cyclosporin A (CsA) via preventing formation of the mitochondrial permeability transition pore, which releases Ca²⁺ and other components of the mitochondrial matrix at high Ca²⁺ loads. Increase of Ca²⁺_cyt is sufficient and necessary to cause vesicular fusions and release of glutamate, which requires action by proteins of the ternary SNARE complex and their associated proteins (not shown). Additionally, vesicles possess the vacuolar type H⁺-ATPase which creates the proton gradient used by vesicular glutamate transporters to fill these organelles with glutamate (Glu). Drawing is not to scale.