PRL cells respond rapidly to changes in extracellular osmolality (mOsmolal, indicated in the bar above the time axis). After hyposmotic stimulation, an increase in cell volume precedes a rise in [Ca²⁺]_i, which in turn precedes PRL release. Cell volume and PRL release were measured from the same preparation of cells [54]. The trace for [Ca²⁺]_i was recorded from cells treated identically and aligned to match the temporal scale of the other two parameters. Intracellular Ca²⁺ concentration was monitored by fluorescence imaging with the Ca²⁺-sensitive dye, fura-2, whose excitation wavelengths are 340 and 380 nm, for fura-2 bound to Ca²⁺ and free fura-2, respectively [54].