NA-CA2+ EXCHANGER-LIKE IMMUNOREACTIVITY AND REGULATION OF INTRACELLULAR CA2+ LEVELS IN FISH RETINAL GANGLION-CELLS()

1. We have used two experimental approaches to examine regulation of i ntracellular calcium ion levels in fish retinal ganglion cells. In the first set of experiments, we ratio-imaged fura-2 emission intensity t o estimate the concentration of free intracellular calcium ions ([Ca2](i)) in isolated goldfish retinal ganglion cells depolarized by incre ases in extracellular levels of potassium ions ([K+](o)), in the prese nce and absence of extracellular sodium ions (Na+). Stepwise increases in [K+](o) from 5 mM to as high as 60 mM produced stepwise increases in [Ca2+](i). These increases were sustained in the absence of externa l Na+, but transient and smaller in the presence of external Na+. The decline of [Ca2+](i) in high-K+, Na+-containing saline could be revers ed by application of the ionophore monensin, or by replacement of exte rnal Na+ with either N-methyl-D-glucamine or lithium. In Na+-containin g saline, [Ca2+](i) fell to control levels after [K+](o) was restored to control levels. 2. In the second set of experiments, we assessed Na +-Ca2+ exchanger-like immunoreactivity in goldfish retinal ganglion ce lls with the use of a polyclonal antiserum directed against Na+-Ca2+,K + exchanger purified from bovine rod outer segments. This antiserum sp ecifically stained the somata, neurites, and growth cones of isolated ganglion cells, the outer segments of rod photoreceptors, and (on West ern blots prepared from mechanically isolated rods) protein displaying an apparent molecular mass of 210 kDa. 3. These results provide measu rements ofchanges in [Ca2+](i) of retinal ganglion cells depolarized i n Na+-containing saline, and the distribution and apparent molecular w eight of Na+-Ca2+ exchanger-like immunoreactivity in teleost retina. T hese results are the first demonstration that retinal ganglion cells p ossess mechanisms for buffering intracellular Ca2+ consistent with Na-Ca2+ exchange.