Intra- and Intercellular Ca²⁺ Signaling in Paraneurons and Other Secretory Cells

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Paraneurons are endocrine and sensory cells which share structural, functional, and metabolic features with neurons. They produce identical with or related to neurotransmitters or neurohormones, which are synthesized and secreted by regulated secretion. They are receptoconductile-secretory in function, which is shared by specific proteins distributed at proper regions of cell membrane. A substantial advance has been made in the molecular machinery underlying protein sorting and transport within the endoplasmic reticulum and Golgi apparatus, and the mechanism of targeted membrane fusion by constitutive secretion. Various patterns of [Ca²⁺]_c dynamics play cardinal signaling roles in stimulus-secretion coupling in individual secretory cells. Long-lived recurrent Ca²⁺ spikes or oscillation may maintain prolonged secretory responses, ATP synthesis in mitochondria, cell growth, difterentiation, and division. In the neurons and the paraneurons of neuroectodermal origin, action potentials propagate along a conductile region to the secretory region of each cell and hardly be transmitted to the adjacent cells. In the paraneurons of gut endodermal origin, intracellular signalling including Ca²⁺ spikes can be propagated to the adjacent cells, and in turn may maintain coordination of individual cells forming a cell society.