LIMIT/OFFSET doesn't work on PL/PGSQL

I try to limit the amount of rows retrieved (on a message base), I need
to do some kind of "pages" with X messages each one... so I create these
function than retrieve messages from user (ID_user_in), the amount of
messages by page (IN_cant_pag) and the offset (to skip messages from
prev pages).

I don't know why the last query doesn't work inside function... I run
the query from psql and every goes ok, but when I use the function, I
can't see anything (the function doesn't retrieve anything). I put fixed
value on LIMIT and the function execute correctly.

The problem is than I need to use a variable to LIMIT.

Any ideas why doesn't work?

Thanks a lot!

CREATE OR REPLACE FUNCTION messages_rec(
,NUMERIC,NUMERIC,NUMERIC,NUMERIC,refcursor ) RETURNS refcursor AS '
DECLARE
ID_instalation_in alias for $1;
ID_user_in alias for $2;
OFF_set_in alias for $3;
IN_cant_pag alias for $4;
OUT_cursor alias for $5;

OPEN OUT_cursor FOR
SELECT m.id_mensaje
,m.fecha
,m.id_mensaje_operador
,m.contenido
,m.tema
,m.id_tipo_mensaje
,m.id_usuario
,m.id_instalacion
,u.username
,m.leido
FROM
men_mensajes m
,core_usuarios u
WHERE m.id_usuario_destinatario = ID_user_in
AND m.id_instalacion_destinatario = ID_instalation_in
AND upper(m.id_tipo_mensaje) in (''OT'',''UT'')
AND upper(m.estado_origen) in (''OK'',''BO'')
AND upper(m.estado_destino) = ''OK''
AND m.id_usuario = u.id_usuario
AND u.id_instalacion = m.id_instalacion
ORDER BY leido,fecha desc
LIMIT IN_cant_pag
OFFSET OFF_set_in
;

RETURN OUT_cursor;

--
Fernando O. Papa
DBA