title: Activation of muscarinic cholinergic receptors on human SH-SY5Y neuroblastoma cells enhances both the influx and efflux of K+ under conditions of hypo-osmolarity.
abstract: 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 = 0.45 microM) and increased the threshold of release to 280 mOsM. Oxo-M elicited a similarly potent, but osmolarity-independent, enhancement of Rb+ influx (EC50 = 1.35 microM). However, when incubated under hypotonic conditions in which osmolarity was varied by the addition of sucrose to a fixed concentration of NaCl, basal- and Oxo-M-stimulated Rb+ influx and efflux were demonstrated to be dependent upon osmolarity. Basal- and Oxo-M-stimulated Rb+ influx (but not Rb+ efflux) were inhibited by inclusion of ouabain or furosemide. Both Rb+ influx and efflux were inhibited by removal of intracellular Ca2+ and inhibition of protein kinase C activity. In addition to Oxo-M, agonists acting at other cell surface receptors previously implicated in organic osmolyte release enhanced both Rb+ efflux and influx under hypotonic conditions. Oxo-M had no effect on cellular K+ concentration in SH-SY5Y cells under physiologically relevant reductions in osmolarity (0-15%) unless K+ influx was blocked. Thus, although receptor activation enhances the osmosensitive efflux of K+, it also stimulates K+ influx, and the latter permits retention of K+ by the cells.
authors.affiliation: Molecular and Behavioral Neuroscience Institute, University of Michigan, Ann Arbor, Michigan, USA.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Foster
authors.forename: Anne M
authors.initials: AM
authors.lastname: Heacock
authors.forename: Richard F
authors.initials: RF
authors.lastname: Keep
authors.forename: Stephen K
authors.initials: SK
authors.lastname: Fisher
mesh_terms: D002118:Calcium / Q000378:metabolism; D045744:Cell Line, Tumor; D006801:Humans; D018721:Muscarinic Agonists / Q000494:pharmacology*; D009994:Osmolar Concentration; D010095:Oxotremorine / Q000031:analogs & derivatives* / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism; D011976:Receptors, Muscarinic / Q000378:metabolism*; D012414:Rubidium Radioisotopes; D028021:Sodium-Potassium-Chloride Symporters / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D014867:Water / Q000378:metabolism
chemical_list: D018721:Muscarinic Agonists; D011976:Receptors, Muscarinic; D012414:Rubidium Radioisotopes; D028021:Sodium-Potassium-Chloride Symporters; D014867:Water; D010095:Oxotremorine; C042743:oxotremorine M; D011493:Protein Kinase C; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium; D002118:Calcium