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Qun-Fang Wan, Ruth Heidelberger; Regulation Of Presynaptic Calcium In A Rod Bipolar Cell. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1160.
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© ARVO (1962-2015); The Authors (2016-present)
Presynaptic Ca2+ regulates many aspects of synaptic physiology, including neurotransmitter release. We investigated the roles of the plasma membrane calcium ATPase (PMCA), and the sodium/calcium exchanger (NCX) in presynaptic calcium handling in the synaptic terminals of a mammalian rod bipolar cell.
Isolated mouse rod bipolar cells were held under voltage-clamp control via a whole-cell patch pipette positioned on the cell soma. The intracellular calcium concentration in a synaptic terminal or soma was measured with calcium-sensitive fluorescent indicator dyes.
Under control conditions, the resting calcium concentration in synaptic terminals was ~ 50nM (55 ± 7nM, n=23). Treatment with Na3VO4 to block PMCA or removal of Na+ from external solution to block NCX resulted in an approximate doubling of resting calcium (Na3VO4: 131 ± 20nM, n=14; 0 external Na+: 107 ± 12nM, n=33). Thapsigargin, an inhibitor of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) had no effect on resting calcium. Following a 1s depolarization, recovery of intraterminal Ca2+ was prolonged in the presence of Na3VO4 (2.8 ± 0.4s, n=5) relative to controls (1.6 ± 0.2s, n=15). Calcium recovery was not significantly prolonged by 0 external Na+ (2 ± 0.2s, n=25) or thapsigargin (1.8 ± 0.4s, n=5). When dialyzed with a Ca2+-buffered internal solution with a free Ca2+ concentration of 50 µM, the calcium concentration in synaptic terminals remained close to resting levels (77 ± 16nM; n=14). Inhibition of PMCA or NCX resulted in a modest elevation of intraterminal Ca2+ to ~ 1 µM (n=12), but in the presence of mitochondrial inhibitors, the intraterminal calcium concentration rose to tens of µM (n=4).
PMCA and NCX are instrumental in setting the resting calcium concentration in synaptic terminals of rod bipolar cells. Restoration of intraterminal calcium levels following depolarization-evoked calcium entry is regulated by PMCA, but not NCX. Under conditions of internal Ca2+ dialysis, a dominant role is played by mitochondria, although PMCA and NCX also contribute.
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