Figure 3.

The inward current evoked by decreasing Ca²⁺ is carried by Na⁺ and K⁺ but not by Cl⁻. (A) Example traces showing the responses of cultured hippocampal neurons to a decrease in Ca²⁺. Ca²⁺ was reduced from 2.0 to 0 mM in four different extracellular solutions: (i) No Na⁺ or K⁺, replacement with 150 mM NMDG; (ii) in the presence of 140 NaCl and 5.4 mM KCl; (iii) in 20 mM NaCl; and (iv) in 20 mM KCl. In the absence of monovalent cations, step reductions of Ca²⁺ evoked little or no inward current, demonstrating that the response was not due to some kind of nonspecific breakdown of the membrane. However, when the usual extracellular solution (140 NaCl, 5.4 KCl) was used, an inward current with an amplitude of about 500 pA was recorded from the same cell. A smaller current was recorded when the extracellular solution contained only 20 mM Na⁺ or only K⁺. The holding potential was −60 mV. (B) A step reduction in the concentration of Ca²⁺ was used to evoke responses in the presence of a solution containing 150 mM NaCl and 5.4 mM KCl or a second solution with half the concentration of Na⁺ (75 mM) and K⁺ (2.7 mM). In each case the holding potential of the cell was varied between −60 and +20 mV. (C) Such data (n = 3) was then used to construct I-V curves. When the concentration of monovalent ions was reduced by one-half, the reversal potential shifted toward more hyperpolarized values (see text). (D) Substitution of all of the extracellular Cl⁻ with gluconate had little influence on the responses to lowered Ca²⁺ and did not alter the I-V curve (n = 3) for this response (E).