Molecular mechanisms of ion channels and second messengers in gustatory transduction of taste cells.

1.当分析NaCl刺激引起的菌状乳头味细胞的内向电流时，发现小鼠的味细胞根据顶端感受膜中存在和不存在或几乎不存在阿米洛利可阻断的上皮Na^+通道（ENaC）而分为两种类型。当阿米洛利阻断NaCl刺激引起的大内向电流时，ENaC的亚基可能由α、β和γ组成。然而，当内向电流没有完全或几乎完全被阿米洛利阻断时，很可能味觉细胞的顶端膜缺乏ENaC的整个亚基或β和γ。阿米洛利完全不能阻断小鼠由酸刺激诱导的内向电流。因此，小鼠味觉细胞的酸反应不是通过阿米洛利可阻断的ENaC诱发的。我们最近的发现表明，有两种途径产生酸反应。一个是质子与顶膜中的离子受体结合和阳离子通过阳离子通道流入的级联反应。另一个途径是通过蛋白激酶激活G蛋白和效应分子、产生第二信使和关闭基底外侧膜中的C1^-通道的级联反应。沙鼠味觉细胞对两种甜味剂产生甜味反应的途径有两种。糖刺激的一个途径是通过蛋白激酶激活G蛋白和腺苷酸环化酶、产生cAMP和关闭K^+通道的级联反应。非糖甜味剂的另一个途径是G蛋白和PLC的激活、1 P3的产生和Ca^2^+从细胞内Ca^2^+库中的释放的级联反应。

1. When inward currents in the fungiform papilla taste cells induced by NaCi stimuli were analysed, it was found that the taste cells in the mouse were divided into two types according to the presence and absence or almost absence of amiloride-blockable epithelial Na^+ channels (ENaC) in the apical receptive membrane. When large inward currents elicited by NaCi stimuli were blocked by amiloride, the subunits of ENaC may be composed of alpha, beta and gamma. However,when the inward currents were not blocked completely or almost completely by amiloride, it is likely that the apical membrane of a taste cell lacks the whole subunits or beta and gamma of ENaC.2. The inward current in the mouse induced by an acid stimulus was not blocked at all by amiloride. Therefore, the acid response of mouse taste cells is not evoked via the amiloride-blockable ENaC.Our recent finding suggests that there are two pathways for generation of acid responses. One is a cascade of binding of proton to ionotoropic receptor in the apical membrane and inflow of cations through the cation channels. The other pathway is a cascade of activation of G-protein and effector molecule, production of a second messenger and closure of C1^- channels in the basolateral membrane via a protein kinase.3. Two pathways are concerned with generation of sweet responses in gerbil taste cells for two types of sweeteners. One pathway for sugar stimuli is a cascade of activation of G-protein and adenylate cyclase, production of cAMP and closure of K^+ channels via a protein kinase. The other pathway for non-sugar sweeteners is a cascade of activation of G-protein and PLC, production of 1P3 and release of Ca^2^+ from the intracellular Ca^2^+ store.