TITLE sodium calcium exchange
: taken from Courtemanche et al Am J Physiol 1998 275:H301
NEURON {
SUFFIX ncx
THREADSAFE
USEION ca READ cao, cai WRITE ica
USEION na READ nao, nai
RANGE ica, itot, imax, rate
GLOBAL kna, kca, gamma, cont_factor, ksat, naf, is_fixed
NONSPECIFIC_CURRENT i_na
NONSPECIFIC_CURRENT icont : remove electrogenic effect
}
UNITS {
(mA) = (milliamp)
(mV) = (millivolt)
F = (faraday) (coulombs)
R = (kmole) (joule/degC)
}
PARAMETER {
imax = 3.2 (mA/cm2)
kna = 87.5 (mM)
kca = 1.38 (mM)
gamma = .35 : voltage dependence factor
cont_factor = 1 (1)
ksat = 0.1 (1)
naf = 15.0 (mM)
is_fixed = 0 (1)
}
ASSIGNED {
celsius (degC)
v (mV)
ica (mA/cm2)
i_na (mA/cm2)
itot (mA/cm2)
icont (mA/cm2)
rate (mA/cm2)
cao (mM)
cai (mM)
nao (mM)
nai (mM)
}
BREAKPOINT {
:LOCAL rate
if (is_fixed==0) {
rate = pumprate(v,nai,nao,cai,cao)
}
if (is_fixed==1) {
rate = pumprate(v,naf,nao,cai,cao)
}
i_na = 3*rate
ica = 2*rate
itot=i_na+ica
icont=itot*cont_factor
}
FUNCTION pumprate(v,nai,nao,cai,cao) {
LOCAL q10, Kqa, KB, k
k = R*(celsius + 273.14)/(F*1e3)
q10 = 3^((celsius  37)/10 (degC))
Kqa = exp(gamma*v/k)
KB = exp( (gamma  1)*v/k)
pumprate = q10*imax*(Kqa*nai*nai*nai*caoKB*nao*nao*nao*cai)/((kna*kna*kna + nao*nao*nao)*(kca + cao)*(1 + ksat*KB))
}
