The relationship between single-channel and whole-cell conductance in the T-type Ca2+ channel CaV3.1

In T-type Ca2+ channels, macroscopic I-Ba is usually smaller than I-Ca, but at high Ca2+ and Ba2+, single-channel conductance (gamma) is equal. We investigated gamma as a function of divalent concentration and compared it to macroscopic currents using Ca(V)3.1 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 gamma was underestimated at depolarized voltages. To estimate divalent gamma, concentration dependence, i(Div), was measured at voltages, < -50 mV. Data were well described by Langmuir isotherms with gamma(max)(Ca2+) of 9.5 +/- 0.4 pS and gamma(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 gamma, macroscopic G(maxCa) was larger than G(maxBa) at 5 mM (G(max)Ca(2+)/Ba:(2+) 1.43 +/- 0.14) and similar at 60 mM(G(max)Ca(2+)/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.