isomorphism extension theorem

Here's the problem:

Let K be an algebraically closed field. Show any isomorphism $\displaystyle \sigma$ of K onto a subfield of K such that K is algebraic over $\displaystyle \sigma[K] $ is an automorphism of K, that is show $\displaystyle \sigma[K]=K$.

I know $\displaystyle \sigma^{-1}:\sigma[K] \rightarrow K $ can be extended to an isomorphism $\displaystyle \mu:K\rightarrow K'$ where K'<=K. And since K<=K'<=K we know $\displaystyle \sigma^{-1}$ can only be extended to an automorphism of K. But does this help me? I don't see how to make the connection with $\displaystyle \sigma[K] $.

Any advice would be great! :-)