Distribution of mechanoreceptor channels in theParamecium surface membrane

Summary

Different sites of theParamecium surface were mechanically stimulated with a fine-tipped glass stylus. Ciliated and deciliated specimens showed a decrease in depolarizing mechanosensitivity and a subsequent increase in hyperpolarizing mechanosensitivity, when the site of stimulation was shifted from the anterior to the posterior end of the cell. Maximal depolarizing sensitivity was observed slightly posterior to the front end of the cell. Specimens without cilia failed to produce action potentials, but no differences in mechanosensitivity were detected. Stimuli given to dorsal surfaces produced larger hyperpolarizations, or smaller depolarizations, than stimuli applied to ventral surfaces at the same latitude. In voltage-clamped cells posterior stimulation elicited outward receptor currents with 5 ms half-time of decay. Inward receptor currents following anterior stimulation decayed with half-times between 20 and 70 ms. Reversal potentials of the receptor currents became more positive, when more anterior surfaces were stimulated. Measurements of the reversal potentials in solutions with varied K and Ca concentrations showed that the posterior receptor currents are exclusively carried by K, while the anterior receptor currents are the sum of K and Ca currents. Ca and K mechanoreceptor currents cancel out in the midposterior region of the cell. Quantitative evaluations of the data suggest that the receptor potentials inParamecium occur due to activated Ca and K mechanoreceptor channels distributing over the somatic cell surface in the manner of overlapping gradients.