Reduction of calcium currents by Lambert-Eaton syndrome sera: motoneurons are preferentially affected, and L-type currents are spared

Abstract

Previous work has demonstrated that Lambert-Eaton syndrome (LES) antibodies reduce calcium currents in nonneuronal cells and neurons and reduce the amplitude of extracellularly recorded currents at mouse motor nerve terminals. We compared effects of LES sera on whole-cell currents of cultured nerve and muscle. LES sera more strongly reduced calcium currents in motoneurons than in sensory neurons. Motoneuronal potassium currents were unaffected. The sera minimally affected calcium currents in skeletal and cardiac muscle. In motoneurons, both low voltage-activated (LVA) and high voltage-activated (HVA) components of calcium current were decreased, demonstrating that the sera targeted more than one calcium channel type. The HVA current remaining in LES-treated motoneurons was little affected by micromolar omega-conotoxin MVIIC but was reduced > 70% by micromolar nimodipine. This pharmacological profile contrasts with untreated cells and suggest that LES sera primarily spare L-type currents in motoneurons.

Fig. 1.

Voltage-activated calcium currents in a motoneuron treated with serum from a control individual (left) and a motoneuron treated with serum from LES Patient II (right). Representative calcium currents were elicited by 300 msec depolarizing pulses to test potentials between −40 and +30 mV at 10 mV intervals. LVA and HVA calcium currents are present in both control and treated cells. Note the prominent transient and sustained components of HVA current in the control motoneuron and that the HVA current in the LES serum-treated cell is predominantly sustained.

Fig. 2.

Peak calcium current density plotted as a function of test potential in control (circles) and LES serum-treated (squares) motoneurons. Average current density was smaller at each test potential for cells treated with serum from each of the four LES patients than for cells treated with control serum. The reduction in current was least substantial for cells treated with serum from Patient I (n = 11) and Patient III (n = 8) and greatest for cells treated with serum from Patient II (n = 11) and Patient IV (n = 10).

Fig. 3.

Maximal LVA (G_maxL) and HVA (G_maxH) calcium conductances for treated and control motoneurons. Calcium conductances were calculated by least-squares fitting of the experimental data with the equation: $I=\frac{G_{\text{maxL}}\ast (V-V_R)}{1+exp[(V-V_{b1})/k_1]}+\frac{G_{\text{maxH}}\ast (V-V_R)}{1+exp[(V-V_{b2})/k_2]},$in which V is the test potential,V_R is the calcium current reversal potential, G_maxL andG_maxH are the maximal LVA and HVA conductances, respectively, V_b1 andV_b2 are potentials for half-maximal activation, and k₁ andk₂ are related to the steepness of the voltage dependence of activation. Reductions in bothG_maxL and G_maxHwere statistically significant (p < 0.05) for motoneurons treated with serum from any of the four patients. Following are average values ± SEM for V_R,V_b1, k₁,V_b2, and k₂, respectively: control 66.1 (8.2), −27.3 (2.6), 10.5 (1.5), 13.8 (2.9), and 5.5 (0.7); Patient I 60.0 (9.2), −27.5 (2.4), 10.2 (1.3), 13.1 (2.9), and 3.9 (0.6); Patient II 84.0 (11.3), −33.2 (6.0), 8.7 (2.1), 7.8 (5.5), and 5.2 (3.0); Patient III 70.3 (8.1), −34.2 (5.0), 9.7 (2.4), 8.3 (1.8), and 7.6 (2.7); Patient IV 66.1 (5.7), −25.1 (4.8), 6.5 (1.8), 6.2 (2.4), and 5.8 (1.6).

Fig. 4.

Normalized, peak potassium current density as a function of test potential in motoneurons treated with control serum (circles; n = 13), serum from Patient III (squares; n = 8), or serum from Patient IV (triangles; n = 8). At high potentials, average potassium currents were slightly smaller than control for motoneurons treated with serum from Patient III and slightly greater for motoneurons treated with serum from Patient IV. However, these changes were not statistically significant. The inset shows a representative family of control potassium currents elicited by test potentials of −30 to 60 mV at 10 mV intervals.

Fig. 5.

Comparison of calcium currents in cardiac myocytes treated with control or LES sera. A, Normalized, peak current–voltage relationships from control (n = 20) cardiomyocytes and cardiomyocytes treated with serum from Patients II (n = 12), III (n = 14), or IV (n = 14). The LVA current (−20 mV) was not altered significantly by serum from any of the three patients; the only statistically significant (p < 0.05) decrease in HVA current (+20 mV) was for cells treated with serum from Patient IV. B, Average maximal LVA (G_maxL) and HVA (G_maxH) calcium conductances for control and LES serum-treated cardiac myocytes. Differences in conductance were statistically significant only for cells treated with serum from Patient IV. C, Representative calcium currents evoked by test potentials ranging from −20 to +30 mV at 10 mV intervals.

Fig. 6.

Effects of LES serum on calcium currents in skeletal myotubes. A, Normalized, peak current density versus voltage relationship for myotubes treated with control serum (circles; n = 12) or serum (squares) from Patient III (n = 14) or IV (n = 14). Changes in LVA current were not significant. The decrease in HVA current was significant for Patient IV but not for Patient III. For Patient IV, G_maxH(determined as described in Fig. 3) was 0.21 ± 0.06 nS/pF compared with 0.30 ± 0.08 nS/pF in control. B, Representative current traces from a control (left) and Patient IV serum-treated (right) skeletal myotube at test potentials of −30, −20, −10, 20, and 30 mV. Serum from Patient IV had no obvious effect on voltage dependence or kinetics of calcium current.

Fig. 7.

Representative calcium currents elicited at test potentials of −30, −20, −10, 10, 20, and 30 mV from motoneurons incubated in control serum and either 10 μmnimodipine (A) or 5 μm ω-CTx MVIIC (B). C, Averaged, peak current density versus test potential in control and test serum-treated motoneurons that had been incubated 1.5 hr in medium with 10 μmnimodipine (squares), 5 μm ω-CTx MVIIC (triangles), or without antagonist (circles; data replotted from Fig. 2). In control serum-treated motoneurons, LVA current was decreased significantly (65%) by nimodipine but not by ω-CTx MVIIC.

Fig. 8.

Effects of acute perfusion of calcium channel blockers on control serum- and Patient II serum-treated motoneurons. Calcium currents were evoked at +10 mV in control serum-treated motoneurons (A) or Patient II serum-treated motoneurons (B) 1 min before and 5 min after acute perfusion with 10 μm nimodipine (left) or 5 μm ω-CTx MVIIC (right).C, Normalized peak calcium current at +10 mV is plotted with respect to time for motoneurons treated with control serum (left) or serum from Patient II (right) and exposed to either nimodipine (squares) or ω-CTx MVIIC (triangles). The circles plot peak current as a function of time in control serum-treated motoneurons not exposed to either antagonist (n = 7).

Fig. 9.

Summary of effects of LES sera on LVA and HVA calcium conductances in DRG neurons, motoneurons, and cardiac myocytes. The vertical axis plots the average calcium conductance remaining in treated cells as a percentage of control.