  We investigate the question whether the unusual doping dependence of the
isotope exponent observed in underdoped high-Tc superconductors might be
related to another unusual phenomenon observed in these systems: the pseudogap
phenomenon. Within different approximations we study the influence of a
phenomenological pseudogap on the isotope exponent and find that it generally
strongly increases the isotope exponent, in qualitative agreement with
experiments on underdoped high-Tc compounds. This result is stable against
strong-coupling self-energy corrections and also holds for recently proposed
spinfluctuation exchange models, if a weak additional electron-phonon coupling
is considered.
