Open channel block of Kv1.5 currents by citalopram

Abstract

Aim: To examine whether selective serotonin reuptake inhibitor citalopram interacts with Kv1.5, one of the cardiovascular-specific Kv channel isoforms.

Methods: The interaction between citalopram and Kv1.5 expressed in Chinese hamster ovary cells was studied using the whole-cell patch-clamp technique.

Results: Citalopram reduced Kv1.5 whole-cell currents in a reversible concentration-dependent manner, with an IC(50) value and a Hill coefficient of 2.8+/-1.1 micromol/L and 0.8+/-0.3, respectively. Citalopram-induced inhibition of Kv1.5 is associated with time-dependent development of block without modifying the kinetics of current activation. The inhibition increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. In the voltage range positive to 0 mV, inhibition displayed an additional voltage dependence, consistent with an electrical distance delta of 0.19. Citalopram slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of citalopram, were superimposed. Inhibition of Kv1.5 by citalopram was use-dependent.

Conclusion: The present results suggest that citalopram acts on Kv1.5 currents as an open-channel blocker, and much caution about arrhythmogenic risk is required when using citalopram in the treatment with depressed patients.

Figure 1

Concentration dependence of citalopram-induced inhibition of Kv1.5 currents. (A) Superimposed current traces were produced by applying 250-ms depolarizing pulses from a holding potential of −80 to +50 mV followed by a 250-ms repolarizing pulse to −40 mV every 10 s in the absence and presence of 0.3, 1, 3, 10, and 30 μmol/L citalopram, as indicated. The dotted line represents zero current. (B) Concentration-dependent curve of inhibition by citalopram. Current amplitudes of Kv1.5 measured at the end of the depolarizing pulses were used as an index of steady-state inhibition, and percentage inhibition was plotted against various concentrations of citalopram. The solid line is fitted to the data points by the Hill equation. (C) Time course for inhibition in the presence of 3 μmol/L citalopram. The current amplitudes were measured at the end of a 250-ms depolarizing pulses from a holding potential of −80 to +40 mV every 10 s in the presence of 3 μmol/L citalopram and normalized to the first current amplitude and the normalized data were plotted as a function of time. Data are expressed as mean±SEM.

Figure 2

Voltage dependence of citalopram-induced inhibition of Kv1.5 currents. The Kv1.5 currents were produced by applying 250-ms pulses between −60 and +50 mV in 10-mV increments followed by a 250-ms repolarizing pulse to −40 mV every 10 s, from a holding potential of −80 mV under control conditions (A), and after the addition of 3 μmol/L citalopram (B). The dotted lines in (A) and (B) represent zero current. (C) Resultant I-V relationships taken at the end of the test pulses in the absence (open circle) and presence (closed circle) of 3 μmol/L citalopram. (D) Percentage current inhibition (closed square) from data in (C) was plotted against the membrane potential. For potentials positive to 0 mV, the data of percentage current inhibition was recalculated by using ln{(I_control–I_citalopram)/I_citalopram} (closed triangle) and replotted against membrane potential. The voltage dependence was linear fitted with equation 5 (see Materials and methods), shown by the solid line with the indicated values for the equivalent electrical distance (δ=0.19±0.02, n=4). The dotted line represents the activation curve of Kv1.5 under control conditions, which was obtained from a deactivating tail current amplitude at −40 mV after 250-ms depolarizing pulses to potentials between −60 to +50 mV in steps of 10 mV from a holding potentials of −80 mV and thereafter normalization using equation 2 (see Materials and methods). Data are expressed as mean±SEM.

Figure 3

Concentration-dependent kinetics of Kv1.5 inhibition by citalopram. (A) Superimposed Kv1.5 current traces were elicited by applying 250-ms depolarizing pulses from a holding potential of −80 to +50 mV every 10 s in the presence of citalopram (3, 10, and 30 μmol/L). The drug-induced time constants were obtained by a single exponential fitting to the decaying traces of Kv1.5 currents. The dotted line represents zero current. (B) Summary data obtained form (A). The time constants (τ) were plotted versus various concentrations of citalopram (n=4; ^bP<0.05 vs control data). Data are expressed as mean±SEM.

Figure 4

Effects of citalopram on deactivation kinetics of Kv1.5. (A) Tail currents were induced at the repolarizing pulses of −40 mV after a 250-ms depolarizing pulse of +50 mV from a holding potential of −80 mV in the absence and presence of 3 μmol/L citalopram. The dotted lines represent a zero current. Tail crossover phenomenon (indicated by the arrow) observed by superimposing the two tail currents. The solid lines over the current traces represent the single exponential fitting to the data. (B) Deactivation time constants (τ) obtained from (A). n=4, ^bP<0.05 vs control data. Data are expressed as mean±SEM.

Figure 5

Effects of repetitive depolarization on citalopram-induced inhibition of Kv1.5 currents. (A) Original current traces under control conditions and in the presence of 3 μmol/L citalopram obtained from applying 20 repetitive 125-ms depolarizing pulses of +50 mV from a holding potential of −80 mV at 1 or 2 Hz. The dotted lines represent a zero current. (B) Plot of normalized current at two different frequencies, 1 and 2 Hz under control conditions (open circle and open triangle, n=4) and in the presence of 3 μmol/L citalopram (closed circle and closed triangle, n=4) as a function of the number of pulses. The peak amplitudes of the current at every pulse were normalized to the peak amplitudes of current obtained at the first pulse. Data are expressed as mean±SEM.