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@silcom.com.br>