Ionic currents in the human serotonin transporter reveal inconsistencies in the alternating access hypothesis

Abstract

We have investigated the conduction states of human serotonin transporter (hSERT) using the voltage clamp, cut-open frog oocyte method under different internal and external ionic conditions. Our data indicate discrepancies in the alternating access model of cotransport, which cannot consistently explain substrate transport and electrophysiological data. We are able simultaneously to isolate distinct external and internal binding sites for substrate, which exert different effects upon currents conducted by hSERT, in contradiction to the alternating access model. External binding sites of coupled Na ions are likewise simultaneously accessible from the internal and external face. Although Na and Cl are putatively cotransported, they have opposite effects on the internal face of the transporter. Finally, the internal K ion does not compete with internal 5-hydroxytryptamine for empty transporters. These data can be explained more readily in the language of ion channels, rather than carrier models distinguished by alternating access mechanisms: in a channel model of coupled transport, the currents represent different states of the same permeation path through hSERT and coupling occurs in a common pore.

FIGURE 1

Raw current traces from a typical oocyte expressing hSERT. In each panel, the upper (dark) trace shows the current before adding the test reagent or exchanging solution, and the gray line shows the response. The lowermost trace shows the voltage protocol. The difference between the steady-state current (dark) and test current (gray) defines the 5HT (I_5HT), Li (I_Li), and leak (I_Leak) currents. The transitory current, visible at the beginning of each response to a voltage step, is a combination of presteady state and capacitance transient currents. (A) Schematic of the cut-open oocyte preparation. The top and bottom chambers are amenable to perfusion and set voltage; the guard chamber isolates the top and bottom chambers. Exposed membrane represents ∼10% of the total oocyte membrane. (B) Current evoked by application of 15 μM external 5HT. (C) Current evoked by complete exchange of external Na with Li. (D) Current revealed by application of 2 μM external desipramine.

FIGURE 2

Effect of 5HT_in on hSERT currents. Plotting the steady-state current (I) as a function of the test voltage (V) defines the I(V) characteristic for hSERT (see Fig. 1 for details). (A) I(V) showing dose-dependent inhibition of I_5HT by 5HT_in. I_5HT is elicited with [5HT]_out = 15 μM. (B) I(V) showing inhibition of I_Li by 5HT_in. I_Li is activated by total replacement of Na_out with Li. 5HT_in doses are represented by same symbols as in A. (C) I(V) showing lack of effect of 5HT_in on I_Leak. I_Leak elicited by DS_out = 2 μM. 5HT_in doses are represented by same symbols as in A. (D) Average inhibition of I_5HT (squares) (5HT_out = 15 μM) and I_Li (circles) at −80 mV. Solid lines are fits to the logistic equation I = 1/(1 + [5HT]_in/IC₅₀); with IC₅₀ = 99 ± 26 μM, the fitting algorithm generates the uncertainty and the IC₅₀ applies to both curves normalized to 1.0 at 0 5HT_in.

FIGURE 3

Effect of Na_in on hSERT currents. Steady-state current (Fig. 1) as a function of the test voltage defines I(V) plots. (A) I(V) showing dose-dependent inhibition of I_5HT ([5HT]_out = 15 μM) by Na_in. (B) I_Li I(V) with varied [Na]_in. Symbols are the same as in A. [Na]_in has no effect upon I_Li. (C) The reversal potential of I_Leak changes with [Na]_in.

FIGURE 4

Combined effect of elevated [Na]_in and [5HT]_in on hSERT currents. Steady-state current (Fig. 1) as a function of the test voltage defines I(V) plots. (A) I(V) of I_5HT ([5HT]_out = 15 μM), [5HT]_in as indicated on figure. (B) I(V) of I_Li, with changing [5HT]_in. (C) I_Leak is insensitive to [5HT]_in. Symbols in B and C have the same meaning as in A.

FIGURE 5

Effect of [K]_in on hSERT currents. Steady-state current (Fig. 1) as a function of the test voltage defines I(V) plots. (A) I(V) of I_5HT ([5HT]_out = 15 μM), [K]_in as indicated on figure. (B) I(V) of I_Li, with changing [K]_in. (C) I_Leak is insensitive to [K]_in. Symbols in B and C have the same meaning as in A.

FIGURE 6

Effect of [5HT]_in on hSERT currents with [K]_in = 0. Steady-state current (Fig. 1) as a function of the test voltage defines I(V) plots. (A) I(V) of I_5HT ([5HT]_out = 15 μM), [5HT]_in as indicated on figure. (B) I(V) of I_Li, with changing [5HT]_in. (C) I_Leak is insensitive to [5HT]_in. Symbols in B and C have the same meaning as in A.

FIGURE 7

Effect of [Cl]_in on hSERT currents. Steady-state current (Fig. 1) as a function of the test voltage defines I(V) plots. (A) I(V) of I_5HT ([5HT]_out = 15 μM), [Cl]_in as indicated on figure. (B) I(V) of I_Li, with changing [Cl]_in. (C) I_Leak is weakly sensitive to [Cl]_in. Symbols in B and C have the same meaning as in A.

FIGURE 8

The carrier model (distinguished by the alternating access mechanism) used for comparison with voltage-clamp data. (A) Alternating access cotransport scheme incorporating ordered binding in which Na (N) binds before 5HT (S). The state TS_out is assumed to conduct I_Leak, whereas TNS_out conducts I_5HT. For simplicity, in this diagram the stoichiometric ratio Na:5HT is assumed to be 1:1. (B) The alternating access model including nonobligatory K countertransport. Internal K is presumed to stimulate transport when the rate of transition TK₁→TK₀ exceeds the rate T₁→T₀.