Rhythmic bursts of calcium transients in acute anterior pituitary slices

Endocrine cells isolated from the anterior pituitary fire intracellular Ca2+ ([Ca2+](i)) transients due to voltage-gated Ca2+ entry. However, the patterns of [Ca2+](i) transients within the glandular parenchyma of the anterior pituitary are unknown. Here we describe, using real-time confocal laser microscopy, several spontaneous patterns of calcium signaling in acute pituitary slices prepared from male as well as cycling and lactating female rats. Forty percent of the cells demonstrated a spontaneous bursting mode, consisting of an active period of [Ca2+](i) transients firing at a constant frequency, followed by a rest period during which cells were either silent or randomly active. The remaining recordings from endocrine cells either demonstrated random [Ca2+](i) transients or were silent. These rhythmic bursts of [Ca2+](i) transients, which required extracellular calcium, were detected in lactotrophs, somatotrophs, and corticotrophs within the acute slices. Of significance was the finding that the bursting mode could be adjusted by hypothalamic factors. In slices prepared from lactating rats, TRH recruited more bursting cells and finely adjusted the average duty cycle of [Ca2+](i) bursts such that cells fired patterned bursts for approximately 70% of the recording period. Eighty-six percent of these cells were lactotrophs. Thus, the rhythmic [Ca2+](i) bursts and their tuning by secretagogues may provide timing information that could encode for one or more cellular functions (e.g. exocytosis and/or gene expression) critical for the release of hormones by endocrine cells in the intact gland.