  We study the kaon nuclear optical potential including the effect of the
\Theta^+ pentaquark in a self-consistent approach, starting from an extension
of the Juelich meson-exchange potential as bare kaon-nucleon interaction.
Significant differences between a fully self-consistent calculation and the
low-density T\rho approximation are observed. Whereas te influence of the KN ->
\Theta^+ absorption process is found to be negligible, due to the small
\Theta^+ KN coupling, the two-nucleon mechanism (KNN -> \Theta^+ N), estimated
from the two-meson coupling of the pentaquark, provides the necessary
additional kaon absorption to reconcile with data the systematically low
theoretical K^+ nucleus reaction cross sections.
