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

Abstract

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 taurine influx was mediated exclusively by a Na+‐dependent, high‐affinity (Km=2.5 μM) saturable transport mechanism (Vmax=0.087 nmol/mg protein/min). Reductions in osmolarity of >20% (under conditions of constant NaCl) resulted in an inhibition of taurine influx (>30%) that could be attributed to a reduction in Vmax, whereas the Km for uptake remained unchanged. Inclusion of the muscarinic cholinergic agonist, oxotremorine‐M (Oxo‐M), also resulted in an attenuation of taurine influx (EC50~0.7 μM). Although Oxo‐M‐mediated inhibition of taurine uptake could be observed under isotonic conditions (~25‐30%), the magnitude of inhibition was significantly enhanced by hypotonicity (~55‐60%), a result that also reflected a reduction in the Vmax, but not the Km, for taurine transport. Oxo‐M‐mediated inhibition of taurine uptake was dependent upon the availability of extracellular Ca2+ but was independent of protein kinase C activity. These results suggest that taurine uptake is reduced upon muscarinic receptor activation in a volume‐dependent fashion, thus facilitating taurine depletion from cells upon exposure to hypotonicity. Supported by NS23831 (SKF) GM007767 (DJF) and DK64959 (RB).