The renin-angiotensin system in nonmammalian vertebrates

The purpose of this review is to appraise the results of the comparative approach to an endocrine system, the renin-angiotensin system (RAS). Recognizing that animal life is a product of history, comparative endocrinology tries to account for the structure and function of endocrine systems as products of evolution. Its characteristic method is the study of homologies, those resemblances among species resulting from derivation from a common ancestry on which natural selection has operated to produce adaptations. Why the comparative approach to the RAS? In addition to their intrinsic interest, comparative studies are of contextual importance to the understanding of mammalian endocrinology. Insights gained from the study of primitive organisms often are valid even at the level of complexity represented by the most recent vertebrates because animals considered primitive on phylogenetic scales nevertheless possess many of the same components as organisms at higher levels. Furthermore, the comparative approach favors serendipitous discoveries: some physiological relationships are most obvious in primitive animals and yet are fundamental to the lives of all vertebrates. Another advantage is heuristic: some animals, or parts of animals, are more favorable for particular experimental paradigms than are others.