Abstract
In T-type Ca(2+) channels, macroscopic I(Ba) is usually smaller than I(Ca), but at high Ca(2+) and Ba(2+), 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)(Ca(2+)) of 9.5 +/- 0.4 pS and gamma(max)(Ba(2+)) of 10.3 +/- 0.5 pS. Apparent K(M) was lower for Ca(2+) (2.3 +/- 0.7 mM) than for Ba(2+) (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 Ca(2+). 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.