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

在全细胞电压箝位下，分析了糖精诱导牛蛙味觉细胞的味觉转导机制。在44个杆细胞中的10个（23%）中，30 mM糖精溶液在-50 mV的膜电位下诱导向内电流为-34±12 pA（平均±SEM， n=10）。糖化电流的浓度-反应关系与味觉神经反应的浓度-反应关系一致。在内部低Cl^-条件下（E_<Cl>=-56 mV），糖精感应电流伴随着电导增加。这表明糖精激活了阳离子电导。糖精诱导电流的逆转电位为-17±2 mV （n=10）。细胞内透析0.5 mM GDPβS完全阻断了糖精诱导的反应，提示G蛋白参与了转导。透析肝素（1mg/ml）也几乎完全抑制了反应，但1mm 8-Br-cAMP对反应没有明显影响。细胞内50μM 1,4,5- insp_3也诱导了13个杆状细胞中的5个（38%）向内电流，但细胞内多杀性巴氏杆菌毒素（5μg/ml， g αq偶联PLC激活剂）未引起细胞反应。结果表明，糖精主要通过介导IP_3的产生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.