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

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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.