Fig. 4.

Simultaneous records of intracellular calcium and membrane potential from a rat pancreatic β cell. The cell was loaded with 1 μM fura-2/AM at 37°C for 30 min. The cell was excited at 350 nm and 380 nm and the emitted light was collected at 510 nm using an lonoptix imaging system. Intracellular [Ca²⁺] was calculated from

$${\left[{\mathrm{Ca}}^{2+}\right]}_i=K_d\beta (R-R_{\min })∕(R_{\max }-R)$$

where K_d = 224 nM, the dissociation constant of fura-2 for Ca²⁺; β = ratio of free/bound fura-2 fluorescences at 380 nm; R = ratio of 350/380 nm fluorescences measured; R_min = ratio of 350/380 fluorescences in Ca²⁺-free solution; and R_max = ratio of 350/380 fluorescences in saturating [Ca²⁺]. (Top) Intracellular [Ca²⁺]. Oscillations of [Ca²⁺] can be seen with the cell bathed in saline containing 7.5 mM glucose, with no other form of stimulation. (Bottom) The record of membrane potential obtained with the perforated patch technique. Initially the cell was under current clamp with a resting potential near −60 mV. At the point marked by the arrowhead, the cell was switched to voltage clamp and held at −70 mV. The oscillations of intracellular [Ca²⁺] can be seen to occur without membrane depolarization and continue with the cell held at −70 mV. After two elevations of intracellular [Ca²⁺], a depolarizing voltage step was applied to activate voltage dependent Ca²⁺ channels and the cell responded with a large increase of intracellular [Ca²⁺].