The Relationship between Single-Channel and Whole-Cell Conductance in the T-type Ca2+ Channel CaV3.1

In T-type Ca2+ channels, macroscopic IBa is usually smaller than ICa, but at high Ca2+ and Ba2+, single-channel conductance (γ) is equal. We investigated γ as a function of divalent concentration and compared it to macroscopic currents using CaV3.1 channels studied under similar experimental conditions (TEAo and Ki). Single-channel current-voltage relationships were nonlinear in a way similar to macroscopic open-channel I/Vs, so divalent γ was underestimated at depolarized voltages. To estimate divalent γ, concentration dependence, iDiv, was measured at voltages <−50 mV. Data were well described by Langmuir isotherms with γmax(Ca2+) of 9.5 ± 0.4 pS and γmax(Ba2+) of 10.3 ± 0.5 pS. Apparent KM 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 γ, macroscopic GmaxCa was larger than GmaxBa at 5 mM (GmaxCa2+/Ba:2+1.43 ± 0.14) and similar at 60 mM (GmaxCa2+/Ba:2+1.10 ± 0.02). However, over the range of activation, ICa was larger than IBa under both conditions. This was a consequence of the fact that Vrev was more negative for IBa than for ICa, so that the driving force determining IBa was smaller than that determining ICa over the range of potentials in standard current-voltage relationships.