Modulation of calcium channels by taurine acting via a metabotropic-like glycine receptor

Abstract

Taurine is one of the most abundant free amino acids in the central nervous system, where it displays several functions. However, its molecular targets remain unknown. It is well known that taurine can activate GABA-A and strychnine-sensitive glycine receptors, which increases a chloride conductance. In this study, we describe that acute application of taurine induces a dose-dependent inhibition of voltage-dependent calcium channels in chromaffin cells from bovine adrenal medullae. This taurine effect was not explained by the activation of either GABA-A, GABA-B or strychnine-sensitive glycine receptors. Interestingly, glycine mimicked the modulatory action exerted by taurine on calcium channels, although the acute application of glycine did not elicit any ionic current in these cells. Additionally, the modulation of calcium channels exerted by both taurine and glycine was prevented by the intracellular dialysis of GDP-β-S. Thus, the modulation of voltage-dependent calcium channels by taurine seems to be mediated by a metabotropic-like glycinergic receptor coupled to G-protein activation in a membrane delimited pathway.

Fig. 1

GΑBA-induced response in chromaffin cell. a Graph plots current–response relationship obtained by application of 100 μM of GABA at various membrane potentials using an intracellular solution of 130 mM Cl⁻. Each point represents the mean of individual measurements at each potential in 13 cells. Note the linearity of the I _GABA–voltage relationship (values for R ² equal to 0.975). Data are means ± S.E.M. b Original traces recorded at −80 mV holding potential when applying GABA (left panel), GABA plus taurine evoked current (middle) or taurine alone (right). Note that taurine was not able to induce chloride current

Fig. 2

Blockade of whole-cell I _Ca by taurine. a Time course of I _Ca generated by test pulses to 20 mV. Taurine (TAU, 30 and 100 μM) was applied when indicated by the top horizontal bars. b I–V curve plots the current peak obtained by stimulating with test pulses in 10 mV increment steps, in the absence (control) and in the presence of taurine (100 μM). c Original I _Ca traces recorded at the condition indicated. d Concentration–response curve for the inhibition of I _Ca by taurine (4–13 cells). e Original records of I _Ca elicited by 250 ms depolarising pulses (HP −80 mV) in 10 mM Ca²⁺. Taurine (100 μM) was superfused during 2 min. Note the enhancement of current inactivation exerted by taurine. f Averaged results on the fraction of peak current and late current blocked by taurine at 50 and 250 ms stimulation pulses (n = 19). Data are means ± S.E. from at least three different cultures. * P < 0.05, Student’s t test

Fig. 3

GABA-B receptors are not involved in the blockade of calcium current exerted by taurine. Time courses of I _Ca recorded in 10 mM Ca²⁺. a Taurine (30–100 μM) and baclofen (30 μM) were superfused during the time period indicated by the top horizontal bars. Inset: original traces recorded at 20 mV test potential at the indicated experimental conditions. b Averaged results on the fraction of current blocked by taurine and baclofen (n = 13). c Taurine (TAU, 30–100 μM), CGP 55845 (2 μM), or both (TAU 100 μM plus CGP) were superfused during the time period indicated by the top horizontal bars. Note that the inhibition of GABA-B receptors did not modify I _Ca indicating that there is not a tonic activation of these receptors. Insets: original traces at the indicated experimental conditions (left). Effects exerted by baclofen (30 μM) applied alone or in combination with the CGP 55845 (2 μM) were evaluated (right). d Averaged results on the fraction of current blocked by taurine, CGP55845 or both (n = 8). Data are means ± S.E

Fig. 4

Glycine elicited a G-protein-mediated blockade of whole-cell calcium currents. a Time course for I _Ca. Glycine (100 μM) and taurine (100 μM) were applied when indicated by the top horizontal bars. Insets represent original traces corresponding at the indicated experimental conditions (left panel) and time course of simultaneous application of taurine and glycine (right panel). b I–V curve plots current peaks evoked by test pulses in 10-mV steps, applied in the absence (control, filled squares) and in the presence of taurine (100 μM, filled circles) or glycine (100 μM, open squares). c Traces recorded after a regular test pulse or preceded by a pre-pulse (filled circle) in control conditions (left) and during 100 μM glycine application (right). d Time course for I _Ca (10 mM Ca²⁺) during the intracellular application of GDP-β-S (500 μM). 50-ms pulses (20 mV) applied at 10-s intervals (HP −80 mV). Glycine (100 μM) and taurine (100 μM) were applied during the time indicated by the top horizontal bars. Insets represent current traces obtained at the beginning of recording and after the progressive G-protein inactivation by GDP-β-S as indicated. The first I _Ca was obtained soon after having established the whole-cell recording conditions. Note that the strong inhibition observed at the beginning of recording was almost fully removed after 200 s of cell dialysis