  Using a self-consistent delta-hole model the pion propagation in isospin
asymmetric nuclear matter is studied. In neutron-rich matter, corresponding to
heavy nuclei, a significant difference in positive and negative pion light-cone
distributions is obtained leading to a nuclear enhancement of up antiquark
distribution compared to the down antiquark one. This means that the sea-quark
asymmetry in the free nucleon cannot be extracted model independently from an
experiment on a neutron-rich nucleus.
