IDENTIFICATION OF THE PROTEIN THAT REGULATES THE SENSORY LALENCY IN TASTE TRANSDUCTION MECHANISM

用全细胞电压钳技术分析了糖精诱发牛蛙味细胞味觉反应的味觉信号转导机制。在44个视杆细胞中的10个（23%）中，在-50 mV的膜电位下，浴用30 mM糖精诱导了-34 12 pA（平均值±SEM，n=10）的内向电流。糖精门控电流的浓度-反应关系与味觉神经反应的浓度-反应关系一致。在低Cl^-条件下（E =-56 mV），糖精诱导的电流随电导的增加而增加<Cl>。这表明糖精激活了阳离子电导。糖精诱导电流的反向电位为-17 2 mV（n=10）。0.5 mM GDPβS的细胞内透析完全阻断了糖精诱导的反应，表明G蛋白参与了转导。肝素（1 mg/ml）透析也几乎完全抑制该反应，而1 mM 8-Br-cAMP对该反应无明显影响。胞内50μM的1，4，5-InsP_3也能引起5个（38%）视杆细胞的内向电流，而胞内的多杀性巴氏杆菌毒素（5μg/ml，Gα q偶联的PLC激活剂）则不能引起任何反应。结果表明，糖精主要通过介导IP_3的产生而激活蛙味觉细胞的阳离子电导。

The taste transduction mechanism of the response elicited by saccharin was analysed in isolated bullfrog taste cells under whole-cell voltage-clamp. Bath application of 30 mM saccharin induced an inward current of -34±12 pA (mean±SEM, n=10) at a membrane potential of -50 mV in 10 (23%) of 44 rod cells. The concentration-response relationship for the saccharin-gated current was consistent with that of the gustatory neural response. The saccharin-induced current was accompanied with a conductance increase under internal low Cl^- condition (E_<Cl>=-56 mV). This suggests that saccharin activated a cation conductance. The reversal potential of the saccharin-induced current was -17±2 mV (n=10). Intracellular dialysis of 0.5 mM GDPβS completely blocked the saccharin-induced response, suggesting the involvement of a G protein in the transduction. The dialysis of heparin (1mg/ml) also inhibited the response almost completely, but the 1 mM 8-Br-cAMP did not affect the response significantly. Intracellular 50μM 1,4,5-InsP_3 also induced the inward current in five (38%) of 13 rod cells, but intracellular Pasteurella multocida toxin (5μg/ml, Gαq-coupled PLC activator) did not elicit any response in the cells. The results suggest that saccharin mainly activates a cation conductance in frog taste cells through the mediation of IP_3 production.