Set role dynamically from proc

Hello!

May you know the way how to set role dynamically.

DO
$$
DECLARE act_dbowner varchar(100);
BEGIN

SELECT u.usename into act_dbowner FROM pg_database d
JOIN pg_user u ON (d.datdba = u.usesysid)
WHERE d.datname = (SELECT current_database());
raise notice 'DB owner: %', act_dbowner;

*set role to act_dbowner; -- THIS LINE*
END
$$;

-------------

ERROR: role "act_dbowner" does not exist
CONTEXT: SQL statement "set role to act_dbowner"
PL/pgSQL function inline_code_block line 10 at SQL statement

I try to use $act_dbowner, but it have no effect.

Thank you for your help!

Best wishes
dd