Dynamically access to field on a RECORD variable

Supose I have this function

CREATE OR REPLACE my_func(TEXT) RETURNS text AS '
DECLARE
var_name ALIAS FOR $1;
rec RECORD;
BEGIN
SELECT * INTO rec FROM my_table WHERE my_key = 1;
-- Here is my problem
RETURN rec.var_name;
END;
' LANGUAGE plpgsql;

SELECT my_func('my_field');

I want the return row in the function executes as:

RETURN rec.my_field;

Is it possible?

Thank you.

--
Ricardo Vaz Mannrich <rvm_l1(at)silcom(dot)com(dot)br>