  We study cuprates within Dynamical Cluster Approximation and find the
pseudogap displays an isotope effect of the same sign as observed
experimentally. Notwithstanding the non-phononic origin of the pseudogap the
interplay between electronic repulsion and retarded phonon-mediated attraction
gives rise to an isotope dependence of the antinodal spectra. Due to the strong
momentum differentiation, such interplay is highly non-trivial and leads to the
simultaneous presence of heavier quasiparticles along the nodal direction. We
predict an isotope effect in electron-doped materials.
