Caffeine-sensitive Ca2+ stores in carp retinal bipolar cells

High K+- or caffeine-induced Ca2+ signal was studied in freshly dissociated carp retinal ON-type bipolar cells using a confocal laser-scanning microscope. In response to 35 mM K+ exposure, a rise in [Ca2+]i appeared in both the terminal and soma, but was absent after removal of external Ca2+ or in the presence of 100 microM nifedipine. It is indicated that, for high K+-induced increase of [Ca2+]i, Ca2+ influx through voltage-gated L-type Ca2+ channels is essential and Ca2+ entry through reversed Na+/Ca2+ exchange may be negligible. Interestingly, caffeine-induced elevation of [Ca2+]i was restricted to the soma, and could be abolished by 50 microM ryanodine, suggesting that caffeine-sensitive Ca2+ stores gated by ryanodine receptors were present in the soma but not in the terminal of bipolar cells. After treatment with 50 microM ryanodine for 20 min, the peak of the Ca2+ transients evoked by 35 mM K+ in the soma decreased to 48.2+/-5.7% of the control. The results suggest that depolarization-evoked Ca2+ influx can cause Ca2+ release from caffeine-sensitive Ca2+ stores, and in turn amplify Ca2+ signal in the soma of retinal bipolar cells.