  Within a relativistic single particle model, we calculate the Coulomb sum
rule of inclusive electron scattering from $^{40}$Ca and $^{208}$Pb in
quasielastic region. Theoretical longitudinal and transverse structure
functions are extracted for three momentum transfers from 300 to 500 MeV/c and
compared with the experimental data measured at Bates and Saclay. We find that
there is no drastic suppression of the longitudinal structure function and that
the Coulomb sum rule depends on nucleus in our theoretical model.
