differential Equation Problem

I have been trying to implement the below differential equation but I can't
get it to work:

[code]
lpmk10[u_] := 1/(1 + Exp[(u + 12.3)/-11.8]);
lptaumk1[u_] := 14.4 - 12.8/(1 + Exp[(u + 28.3)/-19.2]);

solution =
NDSolve[{lpmk1'[t] == (lpmk10[lpv[t]] - lpmk1[t])*(1/lptaumk1[lpv[t]]),
lpmk1[0] == lpmk10[-80]*(1/lptaumk1[-80])
lpv'[t] ==
-1*(+1*125*(lpmk1[t]^4)*(lpv[t] + 80)),
lpv[0] == -80},
{lpv, lpmk1}, {t, 0, 1500}, Method -> Automatic];
ParametricPlot[Evaluate[{t, lpv[t]} /. solution], {t, 0, 1500},
PlotPoints -> 1500, PlotRange -> All];

[/code]



I cannot see what is wrong as it is a relativly simple set of equations.


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