Response of Nitella internodal cell to chemical stimulation. A model for olfactory receptor system

Electrical response to excitable internodal cell of Nitella was studied by applying various kinds of odorants to the cell. Changes in membrane potential and resistance during responses induced by odorants were measured intracellularly under a variety of ionic environments in the media. Results were: 1) Some odorants (coumarin, isoamylacetate, methylacetate, 1-octanol, 1-butanol, 1-propanol) produced an all-or-nothing type action potential when the concentration of odorant exceeded a certain threshold. The action potential was followed by a gradual depolarization of the potential whose amplitude depended on the odorant concentration, C. Other odorants (heptanoic acid, beta-ionon) induced gradual depolarization of the membrane potential without evoking an action potential. 2) Membrane resistance Rm changed in various ways during depolarization: some odorants led to a temporal or gradual decrease in Rm, and others caused an increase in Rm when the membrane potential was depolarized by the application of odorants. 3) Magnitude of response to odorants OR was found to be represented by the following equation: OR =(alpha + beta square root I) log (C/Cth) for C greater than or equal to Cth where alpha and beta are constants for a given odorant, I the ionic strength in the medium, and Cth the threshold concentration of the odorant. 4) Plots of olfactory threshold of human and of internodal cell of Nitella gave a straight line having slope unity. 5) Local application of odorants on the internodal cell induced impulses which transmitted from the part treated by odorants to the other portion. Physico-chemical and physiological implications of the results obtained were discussed.