title: Ca²+ buffering at a drosophila larval synaptic terminal.
abstract: A quantitative analysis of Ca²+ dynamics requires knowledge of the Ca²+-binding ratio (κ(S) ); this has not been measured at Drosophila synaptic terminals or any invertebrate synaptic terminal. We measured κ(S) at a Ib motor terminal in Drosophila larvae comparing single-AP Ca²+ transients in synaptic terminals that contained varying concentrations of the Ca²+ indicator, Oregon Green 488 BAPTA-1 (OGB-1). Using a linear single-compartment model, κ(S) was calculated based upon the effect of [OGB-1] on the time constant (τ(decay) ) for the decay of intracellular free Ca²+ concentration ([Ca²+](i)). This gave a κ(S) of 77 indicating that nearly 99% of entering Ca²+ is immediately bound by endogenous fast Ca²+ buffers. Extrapolation to zero [OGB-1] gave a τ(decay) of 46 ms and a Ca²+-removal rate constant of 1641 s⁻¹ for single APs. We calculated that a single AP produced an increase in [Ca²+](i) of 196 nM and an increase in the total intracellular [Ca²+](free + bound) of 15.3 μM for measurements made in 1.0 mM external Ca²+. The increase in [Ca²+](i) for AP trains was 185 nM/ 10 Hz; this gave a Ca²+ extrusion rate constant of 827 s⁻¹, which likely reflects the activity of the plasma membrane Ca²+ ATPase. Experiments were performed to examine the effect of altering external Ca²+ or Mg²+ on Ca²+ influx at these terminals.
authors.affiliation: Department of Biological Sciences, University at Albany, Suny, Albany, New York 12222, USA.
authors.forename: Tao
authors.initials: T
authors.lastname: He
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Lnenicka
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D004331:Drosophila melanogaster / Q000502:physiology*; D007814:Larva; D017729:Presynaptic Terminals / Q000502:physiology*
chemical_list: D002021:Buffers; D002118:Calcium