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

Action potentials and voltage-dependent membrane currents have been investigated in the fresh-water hypotrich ciliate Stylonychia mytilus, using two intracellular micro-electrodes. The inward current-voltage (Iin-V) relationship has two maxima, the first around -45 mV, and the second 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 inward currents exhibit a differential sensitivity to both external CO and Cd; the inward current activated at potentials greater than or equal to -40 mV is more sensitive to these divalent cations than the inward current activated at around -45 mV. This suggests the presence of two different types of Ca inward currents. Both types of inward currents are present when Ca is replaced by Ba (or Sr). The small inward current recorded between -48 and -40 mV relaxes similarly in Ca and in Ba solutions. The larger inward current, recorded at -30 or -20 mV, relaxes rapidly in Ca solution but only slowly and incompletely in Ba solution. A two-pulse protocol revealed that for both types of inward currents inactivation may depend partially upon the influx and/or intracellular accumulation of the charge-carrying divalent cation. There appears to be a significant difference in the degree of inactivation of the two types of inward currents, however, when Ba is the charge carrier. When the cell spontaneously released, or was induced to release its membranellar band (row of compound cilia), the second, "all-or-none' component of the action potential, and the maximum of the Iin-V relationship at -45 mV disappeared. The first, graded peak of the action potential and the larger maximum of the Iin-V relationship remained essentially unaltered. The smaller Ca current and the action potential shoulder also disappeared when the anterior half of the cell (with most of the membranellar band) was severed, but not when the posterior half was cut off. When recording from a membranellar band vesicle both types of inward currents were present. The results suggest that the two components of the action potential may correspond to the two 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.(ABSTRACT TRUNCATED AT 400 WORDS)