Superposition of the I-V relations obtained from voltage ramps (
Figure 1C) and theoretical I-V curves calculated using the Goldman-Hodgkin-Katz current equation (red curves) with a permeability of TRPV1 for Na
+ ions that is 20-fold larger than that for NMDG
+. Theoretical I-V relations were calculated with the following equation:
where
I(V) is the current as a function of voltage (V),
N is the number of channels,
Po,minis the minimal open probability at V << 0,
Po,max is the maximal open probability at V >> 0,
z is the gating charge of the channel,
V1/2 is the voltage of half-maximal channel activation,
F is Faraday’s constant,
R is the gas constant,
T is the temperature,
PX1 is the permeability of the intracellular cation (i.e. Na
+),
zX1 and
zX2 are the charges of the intracellular and extracellular cations, respectively,
[X1]iand
[X2]o are the molar concentrations of the intracellular (Na
+) and extracellular (Na
+ or NMDG
+) cations, respectively, and
f is the permeability ratio for cations 1 and 2 (
PX2/PX1). At saturating capsaicin, the parameters used were:
Po,min = 0.05;
Po,max = 0.9;
z = 0.31 e
0;
V1/2 = 71 mV and
f = 1 for 130 Na
o or 0.05 for 130 NMDG
o. For 130 NMDG
o the parameters were:
Po,min = 0;
Po,max = 0.30; z = 0.72 e
0; V
1/2 = 99 mV and
f = 0.05. A permeability for Na
+ of 2.04721 x 10–19 m/s was used.