Na+-Ca2+Exchanger Controls the Gain of the Ca2+Amplifier in the Dendrites of Amacrine Cells

Abstract

We have previously shown that disabling forward-mode Na+-Ca2+exchange in amacrine cells greatly prolongs the depolarization-induced release of transmitter. To investigate the mechanism for this, we imaged [Ca2+]iin segments of dendrites during depolarization. Removal of [Na+]oproduced no immediate effect on resting [Ca2+]ibut did prolong [Ca2+]itransients induced by brief depolarization in both voltage-clamped and unclamped cells. In some cells, depolarization gave rise to stable patterns of higher and lower [Ca2+] over micrometer-length scales that collapsed once [Na+]owas restored. Prolongation of [Ca2+]itransients by removal of [Na+]ois not due to reverse mode operation of Na+-Ca2+exchange but is instead a consequence of Ca2+release from endoplasmic reticulum (ER) stores over which Na+-Ca2+exchange normally exercises control. Even in normal [Na+]o, hotspots for [Ca2+] could be seen following depolarization, that are attributable to local Ca2+-induced Ca2+release. Hotspots were seen to be labile, probably reflecting the state of local stores or their Ca2+release channels. When ER stores were emptied of Ca2+by thapsigargin, [Ca2+] transients in dendrites were greatly reduced and unaffected by the removal of [Na+]oimplying that even when Na+-Ca2+exchange is working normally, the majority of the [Ca2+]iincrease by depolarization is due to internal release rather than influx across the plasma membrane. Na+-Ca2+exchange has an important role in controlling [Ca2+] dynamics in amacrine cell dendrites chiefly by moderating the positive feedback of the Ca2+amplifier.