Spontaneous cAMP-dependent [Ca²⁺]_i transients in voltage-clamped δ-cells. Spontaneous thapsigargin-sensitive [Ca²⁺]_i spikes evoked by cAMP. (A–D) [Ca²⁺]_i recorded in δ-cells held at −70 mV and infused with cAMP (concentrations as indicated) using the standard whole-cell technique. Three consecutive depolarizations from −70 mV to 0 mV with increasing durations were applied: 10 ms (black dotted lines), 20 ms (blue dotted lines), and 50 ms (red dotted lines). Depolarizations were applied with >20-s intervals. (E) As in D but after preincubation with 10 µM thapsigargin for >1 h. Responses are normalized to the [Ca²⁺]_i increase evoked by a 50-ms depolarization to 0 mV. Recordings are representative of six (A), five (B), three (C), five (D), and four (E) independent experiments from two to five animals. Arrows above traces indicate the spontaneous [Ca²⁺]_i spikes.