  We present a simple derivation of an expression for the superfluid density $
n_s \propto 1/\lambda^2 $ in superconductors with the tight binding energy
dispersion. The derived expression is discussed in detail because of its
distinction from the known expressions for ordinary superconductors with
parabolic energy dispersion. We apply this expression for the experimental data
analysis of the isotope effect in London penetration depth parameter $ \lambda
$ in the BiSrCuO and YBaCuO family compounds near optimal doping, taking into
account the orthorhombic distortion of crystal structure, and estimate the
isotopic change of hopping parameters from the experimental data. We point out
that $1/\lambda^2$ temperature behaviour is very sensitive to the ratio $
2\Delta_m(T=0)/ k_B T_c $ and estimate this quantity for a number of compounds.
