The Activity of the Na+/Ca2+ Exchanger Largely Modulates the Ca2+i Signal Induced by Hypo-Osmotic Stress in Rat Cerebellar Astrocytes. The Effect of Osmolarity on Exchange Activity

Abstract

Abstract:We recently demonstrated that rat cerebellar Type-1 astrocytes express a very active Na + /Ca 2+ exchanger highly colocalized with ryanodine receptors (RyRs), which in turn play a key role in glutamate-induced Ca 2+ signaling through a calciuminduced calcium release (CICR) mechanism. In this work we have explored whether the Na + /Ca 2+ exchanger has any role in the Ca 2+ i signal induced by hypo-osmotic stress in these cells, using microspectrofluorometric measurements with Fura-2, pharmacological tools, and confocal microscopy image analysis. We present evidence for the first time that the increase in [Ca 2+ ] i in rat cerebellar Type-1 astrocytes, resulting from moderate hypotonic shock, is mediated by Ca 2+ release from ryanodine-operated Ca ] i by a calcium-induced calcium release mechanism (CICR); and (iv) [Ca 2+ ] i increase induced by L -glutamate is the consequence of the activation of the reverse Na + / Ca 2+ exchange, as a result of Na + entry through the electrogenic glutamate transporter [13,14].

Astrocyte ERK phosphorylation precedes K+-induced swelling but follows hypotonicity-induced swelling

“…The most important information obtained about swelling induced by hypotonicity under the presently used conditions (22 min of exposure to 155 mOsM saline medium) is that in contrast to K + ‐mediated swelling it was not inhibited by GM6001, AG1478 or bumetanide. This is in spite of the observation that [Ca 2+ ] i is increased during exposure to a hypotonic medium but that the increase, except for an initial brief peak 64 amounts to little more than 100 nM, 65,66 which may not suffice to induce transactivation. However, ERK 1/2 phosphorylation was increased, which is a common phenomenon during cell swelling, 67,68 including that of astrocytes exposed to hypotonic medium 69,70 .…”

Glial cells in (patho)physiology

“…Another pathway for Ca 2+ entry across the plasma membrane into the astrocytic cytosol is offered by NCXs operating in the reverse mode (Kirischuk et al. 1997; Rojas et al. 2008).…”

Astroglial Excitability and Gliotransmission: An Appraisal of Ca2+ as a Signalling Route

“…When operating in the reverse mode, the NCX can deliver Ca 2+ to the cytosol (Kirischuk et al, 1997; Rojas et al, 2008). Hence, mild depolarization of astrocytes cultured from adult rats caused NCXs to operate in the reverse mode to generate cytosolic Ca 2+ increases leading to glutamate release that were sensitive to 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiurea (KB-R7943) (Paluzzi et al, 2007) (Figure 1).…”

“…An alternative pathway for trans -plasmalemmal Ca 2+ movements is controlled by NCXs (Na + /Ca 2+ exchangers), abundantly expressed in astrocytes (Goldman et al, 1994; Takuma et al, 1994; Matsuda et al, 1996; Kirischuk et al, 1997). When operating in the reverse mode, the NCX can deliver Ca 2+ to the cytosol (Kirischuk et al, 1997; Rojas et al, 2008). Hence, mild depolarization of astrocytes cultured from adult rats caused NCXs to operate in the reverse mode to generate cytosolic Ca 2+ increases leading to glutamate release that were sensitive to 2-[2-[4-(4-nitrobenzyloxy)phenyl] ethyl]isothiurea (KB-R7943) (Paluzzi et al, 2007) (Figure 1).…”