Fig. 5. Effects of withdrawal of extracellular Ca²⁺ on the rate of decline of STOCs and on the response to InsP₃. Depolarisation to -10 mV from a holding potential of -70 mV (iii) induced STOCs (i) and raised [Ca²⁺]_i (ii). Removal of extracellular Ca²⁺ for the duration indicated by the bar abolished STOCs and lowered [Ca²⁺]_i to pre-depolarisation levels. The times on the bars indicate the period in Ca²⁺-free/1mM EGTA solution prior to the release of InsP₃ (ii). A 4 minute period separated the traces as indicated by the gap. At -10 mV the responses to InsP₃ (, iii) were elicited 0.5, 1, 2, 4 and 8 minutes after Ca²⁺ withdrawal and compared with control responses to InsP₃ obtained before removal of extracellular Ca²⁺. Ca²⁺ withdrawal reduced the amplitude of both STOCs and the InsP₃ responses, the rate of decline of the former exceeded that of the latter (iv). These results suggest that the Ca²⁺ store content required to support STOCs is greater that that to maintain InsP₃ responses.