Dynamic changes in spontaneous intracellular free calcium oscillations and their relationship to prolactin gene expression in single, primary mammotropes

Cytosolic calcium plays a critical role in the control of a number of genes, including that of the pituitary hormone PRL. Cells that secrete this hormone, termed mammotropes, display spontaneous oscillations of intracellular free calcium ([Ca2+](i)) that are positively correlated to PRL release. However, the precise contribution of calcium signaling to the expression of any gene including PRL has remained obscure owing to the requirement for and lack of a strategy for monitoring both of these dynamic variables (gene expression and [Ca2+](i) oscillations) in the same living cell. In the present study, we overcame this technical limitation by making real-time measurements of PRL gene expression in transfected, primary rat mammotropes previously subjected to [Ca2+](i) determinations by digital imaging fluorescence microscopy of fura-2. Our results showed that the majority of mammotropes (75%) exhibited distinct oscillatory behaviors that could be subgrouped on the basis of frequency/amplitude of [Ca2+](i) changes, whereas the remainder (25%) were quiescent (nonoscillatory). Interestingly, most mammotropes displayed spontaneous transitions between oscillatory and quiescent states over the course of several hours. As a consequence of this oscillatory plasticity, there was not a positive correlation between [Ca2+](i) dynamics and gene expression at any point in time, as would be predicted by studies with entire populations of cells. Instead, the relationship was distinctly inverse, suggesting that dynamic changes in PRL gene expression may be regulated by temporally dissociated transitions between quiescent and oscillatory states.