A, trains of ten 500 ms voltage-clamp depolarisations from -70 to 0 mV were applied at a frequency of 1 Hz (Vm) using the standard whole-cell configuration in single rat α-cells. The trains were applied 4 min (a) or and 7 min (b) after establishment of the whole-cell configuration. The pipette contained 3 mm Mg-ATP and 0.1 mm cAMP. The cells were exposed to 10 μm forskolin for ≥ 5 min before establishment of the standard whole-cell configuration to maximise the size of the RRP. B, as in A but ATP was replaced by the non-hydrolysable analogue AMP-PCP. C, as in A but somatostatin (Sst, 400 nm) was present for 1 min as indicated schematically and the train applied only once. D, as in C but ATP was replaced by AMP-PCP. The traces are typical for a total of 5-7 cells for each experimental condition. In C and D, the trains were applied 4 min after the establishment of the whole-cell configuration.