Ca 2+ buffering at a drosophila larval synaptic terminal

A quantitative analysis of Ca 2+ dynamics requires knowledge of the Ca 2+ ‐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 Ca 2+ transients in synaptic terminals that contained varying concentrations of the Ca 2+ 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 Ca 2+ concentration ([Ca 2+ ] i ). This gave a κ S of 77 indicating that nearly 99% of entering Ca 2+ is immediately bound by endogenous fast Ca 2+ buffers. Extrapolation to zero [OGB‐1] gave a τ decay of 46 ms and a Ca 2+ ‐removal rate constant of 1641 s −1 for single APs. We calculated that a single AP produced an increase in [Ca 2+ ] i of 196 nM and an increase in the total intracellular [Ca 2+ ](free + bound) of 15.3 μM for measurements made in 1.0 mM external Ca 2+ . The increase in [Ca 2+ ] i for AP trains was 185 nM/ 10 Hz; this gave a Ca 2+ extrusion rate constant of 827 s −1 , which likely reflects the activity of the plasma membrane Ca 2+ ATPase. Experiments were performed to examine the effect of altering external Ca 2+ or Mg 2+ on Ca 2+ influx at these terminals. Synapse, 2011. © 2011 Wiley‐Liss, Inc.