Permeation of divalent cations through the Ca²⁺ channel of rabbit portal vein myocytes

The divalent selectivity of the Ca²⁺ channel in the rabbit portal vein myocyte was examined by the whole cell clamp method. A concentration- dependent selectivity of divalent ion permeation was found such that when Ca²⁺ was replaced by Ba²⁺ or Sr²⁺, the order of maximum current was Ca²⁺ = Ba²⁺ > Sr²⁺ at 2 mM and Ba²⁺ > Sr²⁺ ≥ Ca²⁺ at 5-10 mM. The possibility of block of the Ca²⁺ channel by micromolar concentrations of 'contaminant' Ca²⁺ as a determinant of change in the order of selectivity of divalents was examined. Ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (500 μM) significantly increased maximum Ba²⁺ current (I(Ba)) or I(Sr) in solution containing 5 mM Ba²⁺ or Sr²⁺. Furthermore, at 5 mM extracellular Ba²⁺ concentration, addition of 10, 20, 50, and 100 μM Ca²⁺ caused a 6, 14, 22, and 33% decrease in I(Ba), respectively. These results suggest that the portal vein Ca²⁺ channel has three orders of magnitude higher selectivity for Ca²⁺ over Ba²⁺ and Sr²⁺ such that micromolar Ca²⁺ may block permeation of other divalents through the channel.