COMPARISON BETWEEN THERMORECEPTOR AND MECHANORECEPTOR CURRENTS IN PARAMECIUM CAUDATUM

1. A voltage-clamped Paramecium produced an inward membrane current upon thermal or mechanical stimulation of its anterior region, whereas it produced an outward membrane current upon similar stimulation of its posterior region. 2. Anterior thermo- and mechanoreceptor currents decreased when the membrane potential was shifted in a positive direction, showing sign reversal at a positive membrane potential, whereas posterior thermo- and mechanoreceptor currents decreased when the membrane potential was shifted in a negative direction, showing sign reversal at a membrane potential more negative than the resting potential. 3. The reversal potential for both anterior receptor currents shifted in a positive direction when external [Ca2+] was increased, whereas those for both posterior receptor currents shifted in a positive direction when external [K+] was increased. 4. External [Mg2+], [Mn2+], [Na+], [Rb+] and [TEA+] had similar effects on the thermo- and mechanoreceptor currents. 5. Thermoreceptor currents decreased whereas mechanoreceptor currents increased as the ambient temperature was raised. 6. When a mechanical stimulus was applied to the membrane where a thermoreceptor current was being produced, an algebraic summation of these receptor currents occurred. 7. It is concluded that thermoreceptor currents are dependent on ion channels different from those responsible for the mechanoreceptor currents, although the ionic pores for the channels are similar to each other in various respects. 8. A possibility that a thermoreceptor mechanism exclusively shares a Ca2+ pore in the anterior membrane, or a K+ pore in the posterior membrane, with a mechanoreceptor mechanism is discussed.