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The Relationship between Single-Channel and Whole-Cell Conductance in the T-type Ca²⁺ Channel Ca_V3.1

Committee of Neurobiology, University of Chicago, Chicago, Illinois

Correspondence: Address reprint requests to Dorothy A. Hanck, PhD, Committee of Neurobiology, University of Chicago, 5841 S. Maryland Ave., MC 6094, Chicago, IL 60637. Tel.: 773-702-1758; Fax: 773-702-6789; E-mail: dhanck{at}uchicago.edu.

In T-type Ca²⁺ channels, macroscopic I_Ba is usually smaller than I_Ca, but at high Ca²⁺ and Ba²⁺, single-channel conductance () is equal. We investigated as a function of divalent concentration and compared it to macroscopic currents using Ca_V3.1 channels studied under similar experimental conditions (TEA_o and K_i). Single-channel current-voltage relationships were nonlinear in a way similar to macroscopic open-channel I/Vs, so divalent was underestimated at depolarized voltages. To estimate divalent , concentration dependence, i_Div, was measured at voltages <–50 mV. Data were well described by Langmuir isotherms with _max(Ca²⁺) of 9.5 ± 0.4 pS and _max(Ba²⁺) of 10.3 ± 0.5 pS. Apparent K_M was lower for Ca²⁺ (2.3 ± 0.7 mM) than for Ba²⁺ (7.9 ± 1.3 mM). A subconductance state with an amplitude 70% that of the main state was observed, the relative occupancy of which increased with increasing Ca²⁺. As predicted by , macroscopic G_maxCa was larger than G_maxBa at 5 mM (G_maxCa²⁺/Ba:²⁺1.43 ± 0.14) and similar at 60 mM (G_maxCa²⁺/Ba:²⁺1.10 ± 0.02). However, over the range of activation, I_Ca was larger than I_Ba under both conditions. This was a consequence of the fact that V_rev was more negative for I_Ba than for I_Ca, so that the driving force determining I_Ba was smaller than that determining I_Ca over the range of potentials in standard current-voltage relationships.