Low extracellular Ca2+ enhances synaptic transmitter release from salamander photoreceptors (PHs) by promoting Ca2+ influx through PH membrane

Purpose: We have previously reported that reducing extracellular Ca2+ could relieve the block of PHs to horizontal cells (HCS) transmission induced by Co2+ Zn2+ or Ni2+ in the turtle retina, and that this effect could be accounted for by a paradoxical increased Ca2+ influx through PH membrane via a surface charge effect. To verify this hypothesis, and to ascertain whether a similar mechanism could explain the apparent Ca2+ independence of synaptic transmission in amphibian retina (Schwartz, l986), we carried out experiments on salamanders. Methods: Isolated eyecups were prepared from Ambystoma salamanders and intracellular responses recorded by thin microelectrodes; PHs were dissociated from salamander retina using an enzymatic method, and whole-cell patch-clamped. Results: As in turtle, lowering Ca2 + in the pcrfusing media to 250-10 microM resulted in a complete recovery of HC light responses after the block induced by Co2+ (0.2-1 mM, Ni2+ (0.1-0.5 mM) or Zn2+ (0.1-0.2 mM), still in me presence of the exogenous divalents. In isolated PHs, lowering external Ca2+ to 0.3-0-5 mM. in the presence of Co2+, Ni2+ or Zn2+ (5—50 microM), resulted in a reduction of lhe maximum Ca2+ current elicited at more depolarized potentials, but led to an increase of Ca2+ current measured at potentials close to the resting (dark) potential, due to a shift in Ca2+ current activation curve. Conclusions: On the basis of these and other observations we suggest that, as in turtle, low Ca2+ media may enhance synaptic transmission from salamander PHs by promoting Ca2+2 influx into PHs as a consequence of shift of the Ca2+ current activation curve. The apparent Ca2+-independence of synaptic transmission at this level can thus be explained within the framework of the classical Cal2+-hypothesis.