An API is a synapse configuration and they are invoked in API Manager through transports. The default transport used is NIO but you can switch to API Manager uses HTTP-NIO transport by default to invoke APIs. You can switch to a different transport such as PassThrough. For information on each of these transports, see ESB Transports.
To change the default transport, go to <APIM_HOME>/repository/conf/axis2 folder and rename axis2.xml_PT file to axis2.xml. Similarly, you can switch back to NHTTP by simply renaming axis2.xml_NHTTP file to axis2.xml.