postgres=# create procedure p (in out a int) as 'begin a := 42; end;' language plpgsql;
CREATE PROCEDURE

postgres=# call p(null);
 a  
----
 42
(1 row)

postgres=# \c demo
demo=# create extension dblink;
CREATE EXTENSION

demo=# do $g$begin perform dblink_exec('dbname=postgres', 'do $$declare l int; begin CALL p(l); end;$$'); end;$g$;
DO

demo=# do $$ declare r record; begin select * into r from dblink('dbname=postgres', 'CALL p(NULL)') AS t(x int); end;$$;
DO