  We analyze the evolution with increasing isospin asymmetry of the proton
single-particle states 2s1/2 and 1d3/2 in Ca isotopes, using non-relativistic
and relativistic mean field approaches. Both models give similar trends and it
is shown that this evolution is sensitive to the neutron shell structure, the
two states becoming more or less close depending on the neutron orbitals which
are filled. In the regions where the states get closer some parametrizations
predict an inversion between them. This inversion occurs near $^{48}$Ca as well
as very far from stability where the two states systematically cross each other
if the drip line predicted in the model is located far enough. We study in
detail the modification of the two single-particle energies by using the
equivalent potential in the Schroedinger-like Skyrme-Hartree-Fock equations.
The role played by central, kinetic and spin-orbit contributions is discussed.
We finally show that the effect of a tensor component in the effective
interaction considerably favors the inversion of the two proton states in
$^{48}$Ca.
