This project addresses the cellular signaling cascade in endocrine and neuroendocrine cells, and the interactions between plasma membrane electrical events and receptor-mediated intracellular signaling and secretion. Current emphasis is on the characterization of cell type-specific basal pituitary hormone secretion and its underlying mechanism. Experiments were performed in pituitary somatotrophs, lactotrophs, and gonadotrophs. Although these cells exhibited spontaneous and extracellular calcium-dependent firing of action potentials (APs), voltage-gated calcium influx triggered secretion only in lactotrophs and somatotrophs. The secretory vesicles in these cell-types also had a similar sensitivity to voltage-gated calcium influx. However, the pattern of AP-driven calcium influx differed among cell types studied. Spontaneous activity in gonadotrophs was characterized by high amplitude, sharp APs that had a limited capacity to promote calcium influx, whereas lactotrophs and somatotrophs fired plateau-bursting APs, which generated high-amplitude calcium signals. Furthermore, a shift in the pattern of firing from sharp spikes to plateau-like spikes in gonadotrophs triggered LH secretion. These cells express similar groups of ionic channels, but there were marked differences in the expression levels of some of the individual channels. Specifically, lactotrophs and somatotrophs exhibited low expression levels of sodium channels and high expression levels of the large-conductance, calcium-activated potassium (BK) channel compared to those observed in gonadotrophs. In contrast, functional expression of the transient potassium channel was much higher in lactotrophs and gonadotrophs than in somatotrophs. Furthermore, in somatotrophs and lactotrophs, BK channel activation prolonged membrane depolarization, leading to the generation of plateau-bursting activity and facilitated calcium entry. Such a paradoxical role of BK channels was determined by their rapid activation by domain calcium, which truncated the AP amplitude and thereby limited the participation of delayed rectifying potassium channels during membrane repolarization. Conversely, gonadotrophs expressed relatively few BK channels and fired single APs with a low capacity to promote calcium entry, whereas elevation in BK current expression in a gonadotroph model system led to the generation of plateau-bursting activity and high amplitude calcium transients. These results indicate that the cell-type specific AP-secretion coupling in pituitary cells is determined by the capacity of their plasma membrane oscillator to generate threshold calcium signals.
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