Activation of muscarinic cholinergic receptors (mAChRs) in SH‐SY5Y neuroblastoma cells potentiates both the influx and efflux of potassium following hypotonic stress

Incubation of SH‐SY5Y neuroblastoma cells in buffers rendered increasingly hypotonic by a reduction in NaCl concentration resulted in an enhanced basal efflux of K + (threshold of release, 200 mOsM), but had no effect on K + influx. Addition of oxotremorine‐M (Oxo‐M) potently enhanced K + efflux (EC 50 = 0.45 μM) increasing the threshold of release to 280 mOsM, and elicited a similarly potent, volume‐independent, enhancement of K + influx (EC 50 = 1.35μM). When incubated under hypotonic conditions in which osmolarity was varied by the addition of sucrose to a fixed concentration of NaCl, both basal and Oxo‐M‐stimulated K + influx and efflux were dependent upon osmolarity. Basal and Oxo‐M‐stimulated K + influx (but not K + efflux) were inhibited by inclusion of ouabain or furosemide. In addition to the mAChR, activation of other GPCRs previously implicated in osmolyte release also enhanced both K + efflux and influx under hypotonic conditions. Oxo‐M had no effect on cellular K + content under physiologically relevant reductions in osmolarity (0–15%) unless K + influx was blocked by ouabain and furosemide. The results indicate that, although GPCR activation enhances K + efflux under hypotonic conditions, it also stimulates K + influx, and that this results in retention of K + by the cells. Supported by NS23831 (SKF) and GM007767 (DJF).