The permeability of endplate channels to monovalent and divalent metal cations.

Abstract

The relative permeability of endplate channels to monovalent and divalent metal ions was determined from reversal potentials. Tl is the most permeant ion with a permeability ratio relative to Na+ of 2.5. The selectivity among alkali metals is weak with a sequence, Cs+ > Rb+ > K+ > Na+ > Li+, and permeability ratios of 1.4, 1.3., 1.1, 1.0 and 0.9. The selectivity among divalent ions is also weak, with a sequence for alkaline earths of Mg2+ > Ca2+ > Ba2+ > Sr2+. The transition metal ions Mn2+, Co2+, Ni2+, Zn2+ and Cd2+ are also permeant. Permeability ratios for divalent ions decreased as the concentration of divalent ion was increased. With 20 mM XCl2 and 85.5 mM glucosamine.cntdot.HCl in the external solution, the apparent permeability ratios for the alkaline earth cations (X2+) are 0.18-0.25. Alkali metal ions see the endplate channel as a water-filled, neutral pore without high-field-strength sites inside. Their permeability sequence is the same as their aqueous mobility sequence. Divalent ions have a permeability sequence almost opposite from their mobility sequence and must experience some interaction with groups in the channel. The concentrations of monovalent and divalent ions are increased near the channel mouth by a weak negative surface potential.