Fig. 7. BAPTA in the membrane-permeable (AM) form prevents quantal Ca²⁺ release. Depolarization (–70 to +10 mV, 3 seconds) (C) activated I_Ca (D) and increased [Ca²⁺]_c (A). At –70 mV (C), CCh (50 µM; B) produced a small, and CCh (500 µM) a substantial, [Ca²⁺]_c increase (A). Approximately 7 minutes after the introduction of BAPTA AM (25 µM; A, unfilled bar) to the bathing solution, the depolarization-evoked [Ca²⁺]_c rise was significantly reduced owing to increased cytoplasmic buffering, as revealed by the reduced [Ca²⁺]_c rise for a similar Ca²⁺ influx (A,D), as was the CCh-evoked [Ca²⁺]_c rise (A,B). Scaling up the [Ca²⁺]_c transients obtained in BAPTA so that the depolarization-evoked transients in the presence and absence of the chelator are of comparable size allowed a compensation for the increased buffer capacity of the cell to be made (E). Application of the same scaling factor to the CCh-evoked [Ca²⁺]_c increases allowed comparison of the CCh-evoked [Ca²⁺]_c transients in the presence of the chelator (G-I). When the cytoplasmic Ca²⁺ buffer capacity had been increased (with BAPTA), the lower concentration of CCh was affected to a smaller extent than the higher CCh concentration (inset shown on an expanded scale; note the colour coding). The change in noise in G (red trace) during CCh (500 µM) occurred because of a decrease in data sampling rate (from 10 Hz to 1 Hz).