The regulation factor of intracellular Ca^<2+> level in taste transduction

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The bullfrogs (Rana catesbeiana) were used for the experiments. Taste cells were isolated from the tongue of decapitated and pithed animals. Voltage clamp recording was performed in the whole-cell configuration. The membrane potentials were held to -50 mV.The current-voltage (I-V) relationships were obtained by a voltage ramp (167 mV/s) from -100 to 100 mV.When the pipette was filled with a K^+-internal solution containing 1 mM EGTA which kept Ca^<2+> level of 17 nM at resting state, taste cells displayd sustained inward current at -50 mV in response to a Ca^<2+>-ionophore 3 muM ionomycin. Simultaneous removals of external Na^+ and Ca^<2+> eliminated the ionomycin-induced current completely, suggesting that the rise of intracellular Ca^<2+> elicits the opening of nonselective cation channels. Intracellular dialysis with 1 mM Ca^<2+> or 50 muM inositol 1,4,5-trisphosphate (IP_3) in K^+-internal solution also induced an similar inward current at -50 mV.With internal 104 mM Cl^-, GTP_<gamma>S (0.5 mM) dialysis induced fast transient and slow sustained currents at -50 mV in about 40% of rod cells examined. When internal Cl^- was reduced from 104 to 10 mM,slow component disappeared, but fast one was still observed, suggesting that fast current is cationic. Addition of 10 muMU73211 (inhibitor of phospholipase C) to GTP_<gamma>S in the pipette decreased the GTP_<gamma>S-induced current to 23% of the control. Tne results suggest that the IP_3-Ca^<2+> cascade coupled to G protein elicits cation current in frog taste transduction.