How can I integrate an equation that contains a time varying variable

I have a simple 1st oder differential equation I am solving. The function is as below along with my plot.m file.

To test it I intially used a constant value for T so I could plot it against an exact solution.

Now I want to input a matrix of T which varies with time i.e

t T

0.0 300.0

0.005 350.0

0.01 500.o

Can anyone suggest what I need to do to the T variable in the

K = 1e006 * exp (-5e003/T)

line.

Thanks in advance

.m

%%%%%%%%%%%%%%%%%%%%%%%%%%

function df = letsgo(t,f)
% Input constant temperature for Integration. 
% T is variable. 
T = 2000
K = 1e006 * exp (-5e003/T) 
df = zeros(1,1);
df = -K * f;

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

My plot file is plot.m

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    [t,f] = ode45('letsgo',[0 0.01], 1.0);
    plot(t,f);
    xlabel('time');
    ylabel('f(t)');
    title('Plot');

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%