Voltage dependence and activation kinetics of pharmacologically defined components of the high-threshold calcium current in rat neocortical neurons.

1. As a first step toward identification of the functional significance of the spatial distribution of calcium channels we examined the high voltage-activated calcium current (HVA current) in acutely isolated pyramidal neurons from rat sensorimotor cortex using whole-cell voltage clamp. The goals of this study were (1) to determine whether the pharmacologically separable components of the HVA current differed in voltage dependence or activation kinetics and (2) to develop an empirical model that could predict the HVA current evoked by action potentials or other physiological responses. 2. Cells with short dendrites were chosen for study. Input resistance averaged 3.5 +/- 0.4 (SE) G omega. Specific membrane resistance averaged 51.9 +/- 6.8 K omega-cm2 on the basis of estimated membrane surface area. 3. Using 2 mM calcium in the extracellular solution, we evoked the HVA current by depolarizations positive to -45 mV from a holding potential of -60 mV, a potential where the low-threshold calcium current is fully inactivated. Maximum HVA current amplitude (484.9 +/- 42.3 pA) occurred near -15 mV. The evoked current was completely and reversibly blocked by 200 microM cadmium. 4. Tail current amplitude at a fixed potential increased as a sigmoidal function of prepulse potential. A plot of normalized tail current amplitude, taken as the fraction of HVA channels open at each prepulse potential, was best described by a Boltzmann function (maximum slope: e-fold per 11.3 mV; half activation: -24.6 mV) raised to the power of 2. This relation was not altered by extracellular application of 5 microM nifedipine or 10 microM omega-conotoxin, each of which reduced a separate component of the HVA current uniformly at all potentials. We conclude that the pharmacologically separable components of the HVA current do not differ significantly in voltage dependence. 5. The time course of current onset during a step depolarization was best described by second-order activation kinetics. Activation time constants ranged from a maximum of 1.2 ms at -40 mV to 0.3 ms at +25 mV. Neither activation nor tail current time constants were altered by extracellular application of 5 microM nifedipine or 10 microM omega-conotoxin. After application of 1 microM Bay K 8644 tail current decay was best described by a fast time constant similar to control values and a slow time constant. We conclude that the pharmacologically separable components of the HVA current in these neurons do not differ significantly in kinetics.(ABSTRACT TRUNCATED AT 400 WORDS)