  We calculate spin relaxation rates in lateral quantum dot systems due to
electron exchange between dots and leads. Using rate equations, we develop a
theoretical description of the experimentally observed electric current in the
spin blockade regime of double quantum dots. Single expression fits the entire
current profile and describes the structure of both the conduction peaks and of
the suppressed (`valley') region. Extrinsic rates calculated here have to be
taken into account for accurate extraction of intrinsic relaxation rates due to
the spin-orbit and hyperfine spin scattering mechanisms from spin blockade
measurements.
