Caffeine-Induced Ca(2+) Oscillations in Type I Horizontal Cells of the Carp Retina and the Contribution of the Store-Operated Ca(2+) Entry Pathway

The mechanisms of release, depletion, and refilling of endoplasmic reticulum (ER) Ca(2+) were investigated in type I horizontal cells of the carp retina using a fluo-3-based Ca(2+) imaging technique. Exogenous application of caffeine, a ryanodine receptor agonist, induced oscillatory intracellular free Ca(2+) concentration ([Ca(2+)](i)) responses in a duration- and concentration-dependent manner. In Ca(2+)-free Ringer’s solution, [Ca(2+)](i) transients could also be induced by a brief caffeine application, whereas subsequent caffeine application induced no [Ca(2+)](i) increase, which implied that extracellular Ca(2+) was required for ER refilling, confirming the necessity of a Ca(2+) influx pathway for ER refilling. Depletion of ER Ca(2+) by thapsigargin triggered a Ca(2+) influx which could be blocked by the store-operated channel inhibitor 2-APB, which proved the existence of the store-operated Ca(2+) entry pathway. Taken together, these results suggested that after being depleted by caffeine, the ER was replenished by Ca(2+) influx via store-operated channels. These results reveal the fine modulation of ER Ca(2+) signaling, and the activation of the store-operated Ca(2+) entry pathway guarantees the replenishment of the ER so that the cell can be ready for response to the subsequent stimulus.