Spontaneous calcium oscillatory patterns in mammotropes display non-random dynamics

We previously showed that primary rat mammotropes exhibited four distinct patterns of 'spontaneous' free intracellular calcium ([Ca2+]i) oscillatory behavior: a quiescent state A and three oscillatory states B,C&D, which differed in frequency/amplitude characteristics. When [Ca2+]i was monitored in 10 min windows separated by several hours, these phenotypes were frequently found to interconvert, raising the question about whether these transitions were random or ordered events. We reasoned that if such activity were random, then neither episode duration nor transitional probabilities should differ among phenotypes. We tested this logic in the current study by making long-term, continuous measurements of [Ca2+]i in mammotropes microinjected with Fura-2-dextran and identified by their ability to express a prolactin promoter-driven reporter plasmid. We found that transitions occurred in ~25% of cells (n = 36 from 9 independent experiments) once every 1-5 h and demarcated phenotype episodes of different duration (A, 1.04 +/- 0.2 h; B, 1.64 +/- 0.3 h; C, 2.45 +/- 0.62 h; D, 0.90 +/- 0.2 h, mean +/- SEM). Moreover, some transitions occurred more frequently than others and linked specific phenotypes into a common pattern: C to B to A. Our results demonstrate that the seemingly spontaneous nature of [Ca2+]i phenotype transitions are, in fact, ordered and support the view that they comprise a structured 'code' like that proposed to underlie calcium-dependent regulation of exocytosis and gene expression.