GABA TRANSPORT AND CALCIUM DYNAMICS IN HORIZONTAL CELLS FROM THE SKATE RETINA

1. Changes in intracellular calcium concentration [Ca2+](i) in response to extracellularly applied gamma-aminobutyric acid (GABA) were studied in isolated horizontal cells from the all-rod skate retina. 2. Calcium measurements were made using fura-2 AM, both with and without whole-cell voltage clamp. Superfusion with GABA, in the absence of voltage clamp, resulted in an increase in [Ca2+](i); the threshold for detection was approximately 50 mu M GABA, and a maximal response was elicited by 500 mu M GABA. 3. The rise in [Ca2+](i) was not mimicked by baclofen nor was it blocked by phaclofen, picrotoxin or bicuculline. However, the GABA-induced [Ca2+](i) increase was completely abolished when extracellular sodium was replaced with N-methyl-D-glucamine. 4. With the horizontal cell voltage clamped at -70 mV, GABA evoked a large inward current, but there was no concomitant change in [Ca2+](i). Nifedipine, which blocks L-type voltage-gated Ca2+ channels, suppressed the GABA-induced increase in [Ca2+](i). These findings suggest that the calcium response was initiated by GABA activation of sodium-dependent electrogenic transport, and that the resultant depolarization led to the opening of voltage-gated gated Ca2+ channels, and a rise in [Ca2+](i). 5. The GABA-induced influx of calcium appears not to have been the sole source of the calcium increase. The GABA-induced rise in [Ca2+](i) was reduced by dantrolene, indicating that internal Ca2+ stores contributed to the GABA-mediated Ca2+ response. 6. These observations demonstrate that activation of the GABA transporter induces changes in [Ca2+](i) which may have important implications for the functional properties of horizontal cells.