  We derive the expressions of the vector and axial current from a chiral
Lagrangian restricted to nucleons and pions. They display mixing terms between
the axial and vector currents. We study the modifications in the nuclear medium
of the coupling constants of the axial current, namely the pion decay constant
and the nucleonic axial one due to the requirements of chiral symmetry. We
express the renormalizations in terms of the local scalar pion density. The
latter also governs the quark condensate evolu- tion and we discuss the link
between this evolution and the renormaliza- tions. In the case of the nucleon
axial coupling constant this renormali- zation corresponds to a new type of
exchange currents, with two exchanged pions. We give an estimate for the
resulting quenching. Although moderate it helps explaining the quenching
experimentally observed.
