  We analyze the data for the pressure and boron isotope effect on the
temperature dependence of the magnetization near $T_{c}$. Invoking the
universal scaling relation for the magnetization at fixed magnetic field it is
shown that the relative shift of $T_{c}$, induced by pressure or boron isotope
exchange, mirrors essentially that of the anisotropy. This uncovers a novel
generic property of anisotropic type II superconductors, inexistent in the
isotropic case. For MgB$_{2}$ it implies that the renormalization of the Fermi
surface topology due to pressure or isotope exchange is dominated by a
mechanism controlling the anisotropy.
