Regulation of water homeostasis

Astrocytes regulate water exchange between blood and brain and within the brain compartments through numerous aquaporins (water channels), the latter being particularly concentrated in the endfeet enwrapping blood vessels. On a systemic level, water homeostasis in the brain is controlled by several neuropeptides, produced and released by neurosecretory cells; these peptides are vasopressin, atrial natriuretic peptide (ANP or atriopeptin), angiotensinogen and angiotensin.

Vasopressin increases water content in the brain through the increase in the water permeability of astrocytes. This effect is mediated through astroglial vasopressin Vx receptors that control intracellular Ca2+ release. The effects of vasopressin are antagonized by ANP which itself is produced by astrocytes. ANP is accumulated into vesicles resembling secretory granules, and is released through Ca2+-dependent exocytosis. Water homeostasis is also controlled by the rennin-angiotensin system, which is present in the brain. This system converts angiotensinogen into angiotensin II, which acts as a potent hormone regulating fluid homeostasis and blood pressure. Astrocytes are the main source of brain angiotensinogen, which is present in astroglial cells in all brain regions. How and where angiotensinogen is converted into angiotensin II remains unknown; although many astrocytes express functional angiotensin II receptors of the ATX type. Activation of these receptors causes intracellular Ca2+ release and secretion of prostacyclin from a subpopulation of astrocytes in the cerebellum and medulla.

Astrocytes are also able to sense changes in extracellular osmolarity. When the osmolarity of the extracellular milieu is decreased (hypo-osmotic stress), astrocytes rapidly swell; this swelling is followed by a so-called regulatory volume decrease (RVD), which corrects the initial increase in cell volume. RVD is a complex process, which involves extrusion of intracellular osmotically active substances, including K+ and Cl-, as well as some organic molecules, such as organic amines. Hypo-osmotic shock also triggers efflux of neurotransmitters glutamate, glycine, taurine and GABA. The precise mechanisms of RVD are not yet fully understood; a particular role may be played by volume(swell)-activated K+ and Cl- channels; the latter may also be permeable to glutamate and taurine. Release of taurine is especially important in the osmosensitive regions of the brain. In conditions of hypo-osmotic stress astrocytes in the supra-optic nucleus and in circumventric-ular organs release taurine, which activates glycine receptors of osmosensitive neurones.

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