Nortriptyline, a tricyclic antidepressant, inhibits voltage-dependent K+ channels in coronary arterial smooth muscle cells

Abstract

We demonstrated the effect of nortriptyline, a tricyclic antidepressant drug and serotonin reuptake inhibitor, on voltage-dependent K⁺ (Kv) channels in freshly isolated rabbit coronary arterial smooth muscle cells using a whole-cell patch clamp technique. Nortriptyline inhibited Kv currents in a concentration-dependent manner, with an apparent IC₅₀ value of 2.86±0.52 µM and a Hill coefficient of 0.77±0.1. Although application of nortriptyline did not change the activation curve, nortriptyline shifted the inactivation current toward a more negative potential. Application of train pulses (1 or 2 Hz) did not change the nortriptyline-induced Kv channel inhibition, suggesting that the effects of nortiprtyline were not use-dependent. Preincubation with the Kv1.5 and Kv2.1/2.2 inhibitors, DPO-1 and guangxitoxin did not affect nortriptyline inhibition of Kv channels. From these results, we concluded that nortriptyline inhibited Kv channels in a concentration-dependent and state-independent manner independently of serotonin reuptake.

Keywords: Coronary artery; Nortriptyline; Voltage-dependent K+ channel.

Fig. 1. Effects of nortriptyline on voltage-dependent K⁺ (Kv) channel currents.

Representative Kv currents were elicited by 600-ms depolarizing step pulses from −60 to +60 mV in steps of 10-mV at a holding potential of −60 mV in the control condition (A) and in the presence of 10 µM nortriptyline (B). (C) The current-voltage (I~V) relationship at steady-state Kv current under control conditions (○) and in the presence of 10 µM nortriptyline (●). n=7. ^*p<0.05. (D) Voltage-dependence of Kv current inhibition by nortriptyline. Normalized inhibition shown as relative current from panel in (C) are plotted as a function of membrane potential.

Fig. 2. Concentration-dependent inhibition of Kv channels by nortriptyline.

(A) Superimposed currents were elicited by 600-ms depolarizing pulse of +60 from a holding potential of −60 mV in the presence of 0, 0.1, 0.3, 1, 3, 10, 30, and 100 mM nortriptyline. (B) Concentration-dependent curve for nortriptyline-induced inhibition of the Kv current measured at steady-state. n=8.

Fig. 3. Influence of nortriptyline on steady-state activation and steady-state inactivation curves.

(A) Activation currents in the absence (○) and presence of 10 µM nortriptyline (●). Tail currents were elicited by short depolarizing pulses (20~50 ms) from −80 to +60 mV in steps of 10 mV at a holding potential of −80 mV. Subsequent pulses returned to −40 mV. The current was normalized from the peak value of the tail current. n=10. (B) Inactivation currents in the absence (○) and presence of nortriptyline (●). The currents were elicited by a two-pulse protocol with a test step to +40 mV after 7-s conditioning pre-pulses between −80 and −30 mV. The elicited currents were normalized to the current of the peak test pulse. n=9. ^*p<0.05.

Fig. 4. The effects of pre-treatment with Kv1.5 and/or Kv2.1/2.2 inhibitors on the nortriptyline-induced inhibition of Kv current.

Current traces were obtained by one-step depolarizing pulses to +60 mV at a holding potential of −60 mV. (A) Superimposed currents under control, in the presence of DPO-1, and in the presence of DPO-1+nortriptyline. (B) Summary of the results shown in panel (A). n=5. ^*p<0.05. (C) Superimposed currents under control, in the presence of guangxitoxin, and in the presence of guangxitoxin+nortriptyline. (D) Summary of the results shown in panel (C). n=5. ^*p<0.05. (E) Superimposed currents under control, in the presence of DPO-1+guangxitoxin, and in the presence of DPO-1+guangxitoxin+nortriptyline. (F) Summary of the results shown in panel (E). n=6. ^*p<0.05.

Fig. 5. Frequency-dependent inhibition of Kv currents by nortriptyline.

Twenty depolarizing pulses from −60 mV to +60 mV in duration of 150-ms were applied in the absence (○) and presence of nortriptyline (●), at frequencies of 1 Hz (A) and 2 Hz (B). Each peak current was normalized by the peak current at first pulse and plotted against the pulse number. n=5.