Evidence for two voltage‐dependent calcium currents in the membrane of the ciliate Stylonychia.

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Abstract
Action potentials and voltage-dependent membrane currents were investigated in the freshwater hypotrich ciliate S. mytilus, using 2 intracellular microelectrodes. The inward current-voltage (Iin-V) relationship has 2 maxima, the 1st around -45 mV, and the 2nd around -17 mV (resting and holding membrane potential being -50 mV). The shape of the Iin-V relationship is virtually unaltered in the presence of the K-channel blockers tetraethylammonium, 4-aminopyridine or internal Cs. The 2 components of the action potential may correspond to the 2 types of Ca currents. These Ca currents are separable by their localization in the membrane. The smaller Ca current appears to be restricted to the membranellar band. The presence of the 2 voltage-dependent Ca currents in the membrane of Stylonychia may be functionally related to the difference between the beating behavior of the membranellar band cilia and that of the rest of the compound cilia.