Receptor regulation of osmolyte homeostasis in neural cells.

The capacity of cells to correct their volume in response to hyposmotic stress via the efflux of inorganic and organic osmolytes is well documented. However, the ability of cell-surface receptors, in particular G-protein-coupled receptors (GPCRs), to regulate this homeostatic mechanism has received much less attention. Mechanisms that underlie the regulation of cell volume are of particular importance to cells in the central nervous system because of the physical restrictions of the skull and the adverse impact that even small increases in cell volume can have on their function.

Muscarinic receptor stimulation of D-aspartate uptake into human SH-SY5Y neuroblastoma cells is attenuated by hypoosmolarity.

In addition to its function as an excitatory neurotransmitter, glutamate plays a major role as an osmolyte within the central nervous system (CNS). Accordingly, mechanisms that regulate glutamate release and uptake are of physiological importance not only during conditions in which cell volume remains constant but also when cells are subjected to hypoosmotic stress. In the present study, the ability of muscarinic cholinergic receptors (mAChRs) to regulate the uptake of glutamate (monitored as D-aspartate) into human SH-SY5Y neuroblastoma cells under isotonic or hypotonic conditions has been examined.

Muscarinic receptor regulation of osmosensitive taurine transport in human SH-SY5Y neuroblastoma cells.

The ability of G protein-coupled receptors to regulate osmosensitive uptake of the organic osmolyte, taurine, into human SH-SY5Y neuroblastoma cells has been examined. When monitored under isotonic conditions and in the presence of physiologically relevant taurine concentrations (1-100 microM), taurine influx was mediated exclusively by a Na(+)-dependent, high-affinity (K(m) = 2.5 microM) saturable transport mechanism (V(max) = 0.

Volume-dependent osmolyte efflux from neural tissues: regulation by G-protein-coupled receptors.

The CNS is particularly vulnerable to reductions in plasma osmolarity, such as occur during hyponatremia, the most commonly encountered electrolyte disorder in clinical practice. In response to a lowered plasma osmolarity, neural cells initially swell but then are able to restore their original volume through the release of osmolytes, both inorganic and organic, and the exit of osmotically obligated water. Given the importance of the maintenance of cell volume within the CNS, mechanisms underlying the release of osmolytes assume major significance.

Activation of muscarinic cholinergic receptors on human SH-SY5Y neuroblastoma cells enhances both the influx and efflux of K+ under conditions of hypo-osmolarity.

The ability of receptor activation to regulate osmosensitive K+ fluxes (monitored as 86Rb+) in SH-SY5Y neuroblastoma has been examined. Incubation of SH-SY5Y cells in buffers rendered increasingly hypotonic by a reduction in NaCl concentration resulted in an enhanced basal efflux of Rb+ (threshold of release, 200 mOsM) but had no effect on Rb(+) influx. Addition of the muscarinic cholinergic agonist, oxotremorine-M (Oxo-M), potently enhanced Rb+ efflux (EC50 = 0.

Prostanoid receptors regulate the volume-sensitive efflux of osmolytes from murine fibroblasts via a cyclic AMP-dependent mechanism.

The ability of prostanoid receptors to regulate the volume-dependent efflux of the organic osmolyte taurine from murine fibroblasts (L cells) via a cAMP-dependent mechanism has been examined. Incubation of L cells under hypoosmotic conditions resulted in a time-dependent efflux of taurine, the threshold of release occurring at 250 mOsM. Addition of prostaglandin E(1) (PGE(1)) potently (EC(50) = 2.

Regulation of volume-sensitive osmolyte efflux from human SH-SY5Y neuroblastoma cells following activation of lysophospholipid receptors.

The ability of the lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) to promote the release of the organic osmolyte taurine in response to hypoosmotic stress has been examined. Incubation of SH-SY5Y neuroblastoma cells under hypoosmotic conditions (230 mOsM) resulted in a time-dependent release of taurine that was markedly enhanced (3-7-fold) by the addition of micromolar concentrations of either S1P or LPA. At optimal concentrations, the effects of S1P and LPA on taurine efflux were additive and mediated via distinct receptors.

Potentiation of the osmosensitive release of taurine and D-aspartate from SH-SY5Y neuroblastoma cells after activation of M3 muscarinic cholinergic receptors.

The ability of muscarinic cholinergic receptors (mAChRs) to regulate the volume-sensitive efflux of two organic osmolytes, namely, taurine and d-aspartate, from human SH-SY5Y neuroblastoma cells has been examined. Incubation of the cells with hypoosmolar buffers resulted in an efflux of both osmolytes, with the threshold for release occurring at approximately 225 mOsM for taurine and d-aspartate. Inclusion of oxotremorine-M (Oxo-M), a muscarinic agonist, resulted in a marked enhancement of the volume-dependent efflux of both osmolytes and increased the threshold osmolarity for taurine and d-aspartate release to 340 (isotonic) and 320 mOsM, respectively.

Activation of muscarinic cholinergic receptors enhances the volume-sensitive efflux of myo-inositol from SH-SY5Y neuroblastoma cells.

A mechanism used by cells to regulate their volume under hypo-osmotic conditions is the release of organic osmolytes, one of which is myo-inositol. The possibility that activation of phospholipase-C-linked receptors can regulate this process has been examined for SH-SY5Y neuroblastoma cells. Incubation of cells with hypo-osmolar buffers (160-250 mOsm) led to a biphasic release of inositol which persisted for up to 4 h and could be inhibited by inclusion of anion channel blockers - results which indicate the involvement of a volume-sensitive organic anion channel.