Understanding the behavior of matter under extreme conditions of density and temperature, such as those found in neutron stars or in heavy-ion collisions, requires a good knowledge on the modification of the properties of hadrons in the nuclear medium. In particular, the in-medium effects on antikaons have received much attention over the last years, especially after the speculation of the possible existence of an antikaon condensed phase in neutron stars and the analysis of the experimental data coming from the programs developed at SIS-GSI, SPS-CERN and RHIC-BNL.The purpose of this thesis is to present a self-consistent calculation of the antikaon properties in dense and hot matter in order to explore the typical conditions found in heavy-ion collisions at GSI, studying the possible implications that the inclusion of the in-medium effects at finite temperature on the antikaon optical potential would have on the K-/ K+ ratio.Solving the Bethe-Goldstone equation for G-matrix, we obtain the K- optical potential from the K- N effective interaction in nuclear matter at T=0 taking, as a bare meson-baryon interaction, the meson-exchange potential of the Julich group. The momentum dependence and the effect of higher partial waves of the K- N effective interaction, beyond the L=0 component, are studied using this microscopic and self-consistent calculation. Afterwards, the model is extended by incorporating finite temperature effects in order to adapt our calculations to the experimental conditions in heavy-ion collisions. In the rank of densities studied, the finite temperature K- optical potential shows a smooth behavior if we compare it to the T=0 outcome. Finally, our model is applied to the study of the ratio between K- and K+ produced at GSI with T around 70 MeV in the framework of thermal models. Different approaches for the K- self-energy are taken into account so as to analyze the effects on the determination of this ratio.