Directional antennas can significantly improve the spatial reuse of a mobile ad hoc network (MANET), leading to higher network throughput. This gain comes with a substantial energy saving that results from beamforming the transmitter and/or receiver antennas in the appropriate directions. However, several medium access problems resurface when directional antennas are integrated into existing MAC protocols. We propose a power-controlled MAC protocol for directional antennas that ameliorates these problems. Our protocol allows for dynamic adjustment of the transmission power for both data and clear-to-send (CTS) packets to optimize energy consumption. It provides a mechanism for permitting interference-limited concurrent transmissions and choosing the appropriate tradeoff between throughput and energy consumption. The protocol enables nodes to implement load control in a distributed manner, whereby the total interference in the neighborhood of a receiver is upper-bounded. Simulation results demonstrate that the combined gain from concurrent transmissions using directional antennas and power control results in up to 89% saving in energy compared to a previously proposed protocol and to the CSMA/CA scheme used in the IEEE 802.11 standard. At the same time, network throughput is improved by 79% and 185% over these protocols, respectively.