Ca2+ responses to ATP via purinoceptors in the early embryonic chick retina.

1. The action of adenosine triphosphate on cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in the retinal cell of early embryonic chicks with fura-2 fluorescence measurements. The fluorescence was measured from the whole neural retina dissected from chick embryos at embryonic day three (E3). 2. Bath application of ATP (> or = 30 microM; EC50, 128 microM) raised [Ca2+]i by the release of Ca2+ from intracellular Ca2+ stores, since the Ca2+ response to ATP occurred even in a Ca(2+)-free medium. 3. The Ca2+ response to ATP was mediated by P2U purinoceptors. An agonist for P2U purinoceptors, uridine triphosphate (UTP), evoked Ca2+ rises more potently (> or = 3 microM; EC50, 24 microM) than ATP. Agonists for P2X purinoceptors, alpha, beta-methylene ATP and beta, gamma-methylene ATP, or an agonist for P2Y purinoceptors, 2-methylthio ATP (500 microM each), caused no Ca2+ response. Suramin (100 microM) and Reactive Blue 2 (50 microM) almost completely blocked the Ca2+, responses to 500 microM ATP and 200 microM UTP. 4. The developmental profile of the Ca2+ response to ATP was studied from E3 to E13. The Ca2+ response to ATP was largest at E3, drastically declined towards E8 and decreased further until E11-13. 5. These results suggest that the Ca2+ mobilization by ATP via P2U purinoceptors is characteristic of early embryonic retinal cells.