Fig 1.
Simulated electrical activity in the GLUTag cell line.
A: Simulation of electrical activity triggered by SGLT1-substrate uptake, corresponding to αMG application, with default parameters and extracellular SGLT1-substrate concentration, GSGLT1, changing from 0.5 mM to 1.5 mM and 10 mM as indicated. B: SGLT1-associated current (left axis) and average calcium current (red, right axis) corresponding to the simulation in panel A. C: Simulation of electrical activity triggered by K(ATP)-channel closure with default parameters and K(ATP)-channel conductance, gK(ATP), changed from from 30 pS/pF to 23 pS/pF as indicated by the bar. D: Voltage peak amplitude as a function of GSGLT1 and gK(ATP). E: Spiking frequency as a function of GSGLT1 and gK(ATP). In panels D and E, the black arrows indicate the parameter changes in panels A, B and C. The red arrow indicates the effect of an increase in intracellular glucose concentration due to GLUT2 transport.
Fig 2.
Model characterization of SGLT1-associated currents during voltage clamp as a function of SGLT1-substrate concentration, GSGLT1, and voltage pulse, Vpulse and parameters as for GLUTag model.
The voltage-clamp protocol consisted in applying 1 s depolarization at t = 0.5 s from a holding potential of -70 mV. A: Simulated SGLT1-associated current in response to different voltage pulses (Vpulse = −50, −30, −10, 10, 30 mV) and constant GSGLT1 = 5 mM. B: Simulated SGLT1-associated current in response to a voltage pulse (Vpulse = −10 mV) and different GSGLT1 = 0.1, 1, 5, 10, 20 mM. C: Simulated peak SGLT1-associated current in response to voltage pulses as a function of GSGLT1 and Vpulse. D: Simulated steady-state SGLT1-associated current in response to voltage pulses as a function of GSGLT1 and Vpulse.
Fig 3.
Effect of stimulation with glucose at different concentrations (indicated by grey bars) on GLUTag electrical activity.
The stimulation with 1.5 mM glucose was simulated by changing extracellular SGLT1-substrate concentration, GSGLT1, from 0.1 mM to 1.5 mM, while gK(ATP) remained unchanged from its default value. Subsequent 20 mM glucose application was simulated by changing GSGLT1 from 1.5 mM to 20 mM, and gK(ATP) from 30 pS/pF to 25 pS/pF. A: Simulation of electrical activity triggered by different glucose concentrations. B: SGLT1-associated current corresponding to the simulation in panel A. C: K(ATP)-current corresponding to the simulation in panel A. D: Calcium current (blue) and its average (red) corresponding to the simulation in panel A.
Fig 4.
Simulated electrical activity in primary L-cells.
A: simulation of electrical activity triggered by SGLT1-substrate uptake, corresponding to αMG application, in primary L-cells with default parameters and extracellular SGLT1-substrate, GSGLT1, changing from 0.1 mM to 1 mM and 20 mM as indicated. B: SGLT1-associated current (left axis) and average calcium current (red, right axis) corresponding to the simulation in panel A. C: Simulation of electrical activity triggered by the K(ATP)-channel blocker tolbutamide. Default parameters and K(ATP)-channel conductance, gK(ATP), changing from from 3 pS/pF to 0 pS/pF. Grey bars indicate application of the substances. D: Voltage peak as a function of GSGLT1 and gK(ATP). E: Spiking frequency as a function of GSGLT1 and gK(ATP). In panels D and E, the black arrows indicate the parameter changes in panels A, B and C. The red arrow indicates the effect of an increase in intracellular glucose concentration due to GLUT2 transport.
Fig 5.
Membrane currents during a single action potential.
A: Zoom on an action potential in GLUTag cells from Fig 1 with GSGLT1 = 1.5 mM. B: Membrane currents during the action potential in panel A. The currents have for clarity been divided according to their amplitude. Upper panel: ISGLT1 (blue), IK(ATP) (black), IKA (cyan), IK,hyper (green). Lower panel: INa (black), ICaHV A (red), IKv (blue). C: Zoom on an action potential in primary L-cells from Fig 4 with GSGLT1 = 1 mM. D: Membrane currents during the action potential in panel C. Upper panel: ISGLT1 (blue), IK(ATP) (black), ICaT (red). Lower panel: INa (black), ICaHV A (red), IKv (blue).
Fig 6.
Simulation of application of Na+-channel blocker TTX.
A: Simulation of glucose-induced electrical activity (GSGLT1 = 10 mM and gK(ATP) = 0.015 nS/pF) and subsequent application of TTX in GLUTag cells with default parameters. Red line represents mean voltage during electrical activity. B: Calcium current corresponding to the simulation in panel A. Red line represents mean Ca2+ current during electrical activity. C: Simulation of current-induced electrical activity (Iapp = 0.1 pA/pF, GSGLT1 = 1 mM and gK(ATP) = 0.0035 nS/pF), and subsequent application of TTX in primary L-cells. The average Ca2+ current is indicated by the red lines (right axis) D: As in panel C, except gCaT = 0.11 nS/pF. E: Simulation of current-evoked action potential in primary L-cells in control case (black line) and in presence of TTX (grey line). Black bar indicates the current application Iapp = 5 pA/pF. Grey bars indicate glucose and TTX application as indicated.
Fig 7.
Simulation of application of Ca+-channel blocker.
A: Simulation of application of the L-type Ca2+-channel blocker, nifedipine, in GLUTag cells, with default parameter except GSGLT1 = 10 mM, gK(ATP) = 0.015 nS/pF. The average Ca2+ current is indicated in red (right axis). B: Simulation of application of partial block of HVA Ca2+-channels in primary L-cells, with default parameters except GSGLT1 = 10 mM. The average Ca2+ currents are in both panels indicated in red (right axes). Grey bars indicate Ca2+-channel blocker application.
Table 1.
Default parameters of the different ion channels.
Table 2.
Default parameters of the SGLT1 model.
Fig 8.
Comparison of Na+ current (INa) activation function (A), inactivation function (B) and I-V relationship (C) between GLUTag (solid line) and primary L-cells (dashed line).
Fig 9.
A: Comparison of HVA Ca2+ current (ICaHVA) activation function between GLUTag (solid line) and primary L-cells (dotted line), superimposed T-type Ca2+ current activation function (dot-dashed line). B: Comparison of HVA Ca2+ current (ICaHVA) inactivation function between GLUTag (solid line) and primary L-cells (dotted line), superimposed T-type Ca2+ current inactivation function (dot-dashed line). C: Comparison of HVA Ca2+ current (ICaHV A) -V relationship between GLUTag (solid line) and primary L-cells (dotted line), superimposed T-type Ca2+ current activation function (dot-dashed line) and the total Ca2+ current (dashed line) in primary L-cells.
Fig 10.
Comparison of delayed-rectifier K+ current (IKv) activation function (A), time constant (B) and I-V relationship (C) between GLUTag (solid line) and primary L-cells (dashed line).
Fig 11.
Six-state model of the sodium/glucose co-transporter SGLT1.