Electrophysiological Properties of the Microstome and Macrostome Morph of the Polymorphic Ciliate Tetrahymena vorax

Polymorphic ciliates, like Tetrahymena vorax, optimize food utilization by altering between different body shapes and behaviours. Microstome T. vorax feeds on bacteria, organic particles, and solutes, whereas the larger macrostome cells are predators consuming other ciliates. We have used current clamp and discontinuous single electrode voltage clamp to compare electrophysiological properties of these morphs. The resting membrane potential was approximately −30 mV in both morphs. The input resistance and capacitance of microstomes were approximately 350 MΩ and 105 pF, whereas the corresponding values for the macrostomes were 210 MΩ and 230 pF, reflecting the larger cell size. Depolarizing current injections elicited regenerative Ca2+ spikes with a maximum rate of rise of 7.5 Vs−1 in microstome and 4.7 Vs−1 in macrostome cells. Depolarizing voltage steps from a holding potential of −40 mV induced an inward Ca2+ -current (Ica) peaking at −10 mV, reaching approximately the same value in microstome (−1.4 nA) and macrostome cells (−1.2 nA). Because the number of ciliary rows is the same in microstome and macrostome cells, the similar size of ICa in these morphs supports the notion that the voltage-gated Ca2+ channels in ciliates are located in the ciliary membrane. In both morphs, hyperpolarizing voltage steps revealed inward membrane rectification that persisted in Na+-free solution and was only partially inhibited by extracellular Cs+. The inward rectification was completely blocked by replacing Ca2+ with Co2+ or Ba2+ in the recording solution, and is probably due to Ca2+ -activated inward K+ current secondary to Ca2+ influx through channels activated by hyperpolarization.