Currents carried by monovalent cations through calcium channels in mouse neoplastic B lymphocytes

Membrane currents through the Ca2+ channel were studied in a hybridoma cell line (MAb-7B) constructed by fusion of S194 myeloma cells and splenic B lymphocytes from the mouse. The whole-cell variation of the patch-electrode voltage-clamp technique was used. When [Ca2+]o = 2.5 mM, [Na+]o = 150 mM and [Na+]i = 155 mM, the current reversed from inward to outward at 20.9 +/- 2.4 mV (mean +/- S.D., n = 62). Both inward and outward currents showed voltage-dependent inactivation with the same membrane potential dependence of steady-state inactivation. The decay time constant of the current decreased from about 27 ms at -44 mV to a saturation value of 16 ms at about -20 mV, and remained at this value even when the current became outward. From the above results both the inward and outward currents were considered to flow through Ca2+ channels. The inward current showed no change when the external Na+ was replaced with Cs+ or tetraethylammonium and increased when [Ca2+]o was increased. Also, the reversal potential became more positive with increasing [Ca2+]o with a slope of 29 mV/decade change of [Ca2+]o. Effects of different divalent cations examined at 10 mM concentration showed the reversal potential to become more positive in the order of Mn2+, Sr2+ approximately equal to Ba2+ and Ca2+ whereas the relative maximum amplitudes of peak inward current were 1.0 for Ca2+, 1.24 for Sr2+, 0.99 for Ba2+ and 0.07 for Mn2+. When [Ca2+]o or [Mg2+]o was reduced by chelators, monovalent cations became capable of carrying inward current through the Ca2+ channel. These monovalent currents share common kinetic properties with the Ca2+ current, as judged from the steady-state inactivation and the decay time constant of the current. The monovalent cation current was blocked by divalent cations in a voltage-dependent manner. The half-blocking concentrations of Ca2+ and Mg2+ at -45 mV were 2.0 X 10(-6) M and 3.0 X 10(-5) M respectively. The same voltage-dependent binding mechanism can explain the outward current carried by monovalent cations at large positive potentials at normal Ca2+ concentrations. The suppression of the monovalent currents by Ca2+ and Mg2+ showed different voltage dependences. The suppression by Ca2+ increased and then decreased as the membrane potential was made negative, whereas the suppression by Mg2+ increased monotonically. This difference can be explained by considering the fact the Ca2+ is permeant and Mg2+ is impermeant through the Ca2+ channel.