ARGININE VASOPRESSIN MOBILIZES INTRACELLULAR CALCIUM VIA V1-RECEPTOR ACTIVATION IN ASTROCYTES (PITUICYTES) CULTURED FROM ADULT-RAT NEURAL LOBES

An extremely close association exists between the membranes of the neurosecretory endings and the resident astrocytes' (pituicytes) of the neurohypophysis. Indeed, synaptoid contacts involving neurosecretory vesicle-containing axons contacting pituicytes have been observed, suggesting pituicytes as targets of the products released from neurosecretory axons. We have investigated the effects of various neural lobe peptides on pituicytes in primary culture from adult neurohypophyses. Using Fura-2 loaded cells and dynamic ratio imaging, we have determined that arginine vasopressin (AVP) or V1- but not V2-receptor agonists, mobilise pituicyte intracellular Ca2+([Ca2+]i) in the absence of extracellular Ca2+. AVP was consistently effective, at concentrations of 10 nM or higher in elevating [Ca2+]i by 200-1000 nM. These responses could be blocked by V1-antagonists and were shown to be associated with accumulation of phosphoinositides. Oxytocin was also found to mobilise [Ca2+]i but was effective only at higher concentrations than for AVP. Oxytocin-evoked [Ca2+]i elevations were also blocked by V1-antagonists. Raising [K+]0 was ineffective in changing [Ca2+]i suggesting that these cells lack voltage-gated Ca2+ channels. We conclude that pituicytes possess V1-receptors, activation of which mobilises [Ca2+]i possibly functioning to initiate a Ca2+-activated K+ conductance which could contribute to further depolarisation of secretory terminals and facilitate exocytosis.