You can find more details by reading through the "Getting Started with ANT Development in Embedded Systems" thread and particularly the "AN13 Power States" application note. These documents may be found here .
For the UART interface, using the SLEEP line appropriately, would let you reopen channels without having to go through the entire configuration. The Sleep pin in UART tells ANT it can turn off the serial port when it is unneeded thus saving power.
In SPI state transitions are generally automatic except for AP2's "Deep Sleep" which is also available on the UART.
However, AP2 does offer a "Deep Sleep" state designed for long durations between activity as it causes a full reset of the AP2, but in this mode AP2 will only draw ~0.5uA. That state is entered through a command specific to the AP2.