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 microM GABA, and a maximal response was elicited by 500 microM 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 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.
Haugh-Scheidt
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Department of Ophthalmology and Visual Sciences, UIC Eye Center, University of Illinois College of Medicine, Chicago 60612, USA.
Malchow
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Ripps
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D000818:Animals; D001418:Baclofen / Q000031:analogs & derivatives / Q000494:pharmacology; D001640:Bicuculline / Q000494:pharmacology; D001692:Biological Transport / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D003620:Dantrolene / Q000494:pharmacology; D004594:Electrophysiology; D018756:GABA Antagonists / Q000494:pharmacology; D008564:Membrane Potentials / Q000502:physiology; D009125:Muscle Relaxants, Central / Q000494:pharmacology; D009543:Nifedipine / Q000494:pharmacology; D010852:Picrotoxin / Q000494:pharmacology; D012160:Retina / Q000166:cytology* / Q000378:metabolism; D012860:Skates, Fish; D005680:gamma-Aminobutyric Acid / Q000378:metabolism* / Q000494:pharmacology
D001418:Baclofen; D001640:Bicuculline; D002118:Calcium; D002121:Calcium Channel Blockers; D003620:Dantrolene; D018756:GABA Antagonists; D009125:Muscle Relaxants, Central; D009543:Nifedipine; D010852:Picrotoxin; D005680:gamma-Aminobutyric Acid; C052329:phaclofen