There is a popular (e.g. link 1, link 2) theory, that is, if you train big muscle groups (e.g. legs, chest), it will increase the overall growth of muscles all over the body; the trained muscle will release signals (what are they?) into the blood stream and in turn, triggers the release of muscle growth/repair hormones (insulin-like growth factor?) which have effects on any (skeleton) muscle cells.
Is this story true? Will the muscle growth/repair hormones also enhance the smooth muscle or cardiac muscle?