Membrane permeabilities determining resting,action and mechanoreceptor potentials inStentor coeruleus

Summary In response to mechanical stimuli the protozoan,Stentor coeruleus, contracts in an all-or-none fashion and simultaneously reverses the direction of its ciliary beat. These behaviors have previously been shown to be correlated with the presence of a mechanoreceptor potential and all-or-none action potential (Wood 1970, 1973a). In the studies reported below the ionic bases of the resting, receptor and action potentials ofStentor were determined by use of intracellular microelectrodes penetrating animals chilled to 8.5–10 °C. The resting potential is most dependent on the extracellular concentration of KCl but some dependence on CaCl₂ concentration was also observed. If allowance is made for the large increases in membrane conductance observed in solutions containing 2–8 mM KCl it is found that the resting potential data are well described by a modified form of the Goldman equation whereP_Ca/P_K = 0.068 andP_Cl/P_K = 0.072. The intracellular ionic activities (K_i⁺ = 13.1 mM, Cl_i^–= 9.9 mM, Ca_i⁺ = 0 mM) which provide the best fit of this equation to the resting potential data are in close agreement with the intracellular concentration values measured by flame microspectrophotometry (K_i=12.4 mM, Cl_i = 9.4 mM) except in the case of Ca_i where most of the intracellular concentration is presumed to be bound. 65 to 75 mV action potentials are produced by suprathreshold depolarizations but contractions were not generally seen in these chilled animals, only ciliary reversals. The action potential peak varies with CaCl₂ concentration with a slope of 12.6 mV/10 fold change but varies only slightly with KCl or Cl^– concentration. These peak potentials are well described by assuming that theP_Ca/P_k = 7.9 andP_Cl/P_K=1.0 at the time of the action potential peak. Depolarizing receptor potentials and brief inward receptor currents were observed for all forms of punctate and gross bodily mechanical stimulation employed. No evidence was found for any form of hyperpolarizing mechanoreceptor potentials as observed in some other ciliates. The reversal potential of the mechanoreceptor current varied with CaCl₂ concentration in a manner similar to that of the action potential peak. As in the case of the action potential both theP_Ca/P_k andp_cl/p_k ratios appear to increase as a result of mechanical stimulation to 9.3–15 and 1.2–1.95 respectively. Mechanoreceptor currents are voltage dependent being increased when the membrane is depolarized above resting potential and decreased when the membrane is hyperpolarized. In general the electrophysiological characteristics ofStentor appear similar to those ofParamecium andStylonychia, but its resting membrane appears more selectively permeable to K⁺, it produces only depolarizing receptor potentials when mechanically stimulated and the initial action potential elicited by depolarizing current pulses can be all-or-none even in culture medium.