CA2-CA2+ EXCHANGER IN RETINAL HORIZONTAL CELLS DEPOLARIZED BY L-GLUTAMATE( REGULATION BY THE NA+)

This study is concerned with regulation of the intracellular Ca2+ conc entration ([Ca2+](i)) of horizontal cells isolated from cyprinid fish retinae, with the main emphasis on the role of the Na+-Ca2+ exchanger. An inward current was blocked by Co2+ (4 mM) during prolonged (>1 h) depolarization by L-glutamate (100 mu M) in the whole-cell voltage-cla mp configuration, suggesting the persistent activation of voltage-gate d Ca2+ channels. This Co2+-sensitive current was absent when extracell ular Na+ was replaced by Li+ to suppress Na+-Ca2+ exchange. Measuremen t of [Ca2+](i) using the Fura-2 ratiometric method gave the following results. (1) L-Glutamate (100 mu M) caused [Ca2+](i) to increase from the resting level of 75.4 +/- 36.8 nM (mean +/- S.D., n = 11) to the m aximum level (2.2 +/- 1.4 mu M, n = 11) within 15 s and then to decrea se to a steady level of 0.59 +/- 0.23 mu M (n = 11). (2) Nifedipine (1 00 mu M) lowered the L-glutamate-induced steady [Ca2+](i) level, which was still higher than the resting level. (3) L-Glutamate caused [Ca2](i) to increase even after blockading the voltage-gated Ca2+ channels by nifedipine or by clamping the membrane voltage at -55 mV. (4) Na+- free superfusate elevated the L-glutamate-induced steady [Ca2+](i) lev el. (5) The time course of the [Ca2+], decrease from the L-glutamate-i nduced steady level to the resting level was prolonged in the Na+-free superfusate. These results suggest that the Na+-Ca2+ exchanger extrud es intracellular Ca2+ to maintain a low [Ca2+](i) level by counteracti ng the continuous Ca2+ influx through the voltage-gated Ca2+ channels and glutamate-gated channels when horizontal cells in situ are tonical ly depolarized by L-glutamate released from the photoreceptors. The Na +-Ca2+ exchange current isolated by a voltage-clamp experiment depends exponentially on the membrane potential. (C) 1998 Elsevier Science Ir eland Ltd. All rights reserved.