ABSTRACT
Recent evidence suggests that atriopeptin (AP) may play a role in osmoregulation in fishes (Evans, 1990) via hemodynamic and ionic transport effects analogous to those described for mammalian volume regulation (e.g. Genest and Cantin, 1988). Specifically, heterologous mammalian AP has now been shown to produce natriuresis (Duff and Olson, 1986), even in an aglomerular teleost (Lee and Malvin, 1987), inhibit intestinal salt uptake (O’Grady et al. 1985) and stimulate gill sodium extrusion (Scheide and Zadunaisky, 1988) in teleosts, all responses that favor osmoregulation in dehydrating sea water, rather than volume-loading fresh water. The proposition that AP may be primarily involved in salt extrusion rather than volume regulation is supported by recent studies showing that plasma immunoreactive atriopeptin (APir) levels are higher when euryhaline teleost fishes are acclimated to high salinities (Westenfelder et al. 1988; Evans et al. 1989). The recent finding that mammalian AP produces vasodilation in the ventral aorta and branchial vasculature of the marine toadfish (Evans et al. 1989) suggests that AP may also play a role in controlling gill hemodynamics. However, an increase in gill perfusion would presumably exacerbate the osmoregulatory problems in either the marine or freshwater environment, since the gill is the site of passive ionic and water movements in teleost fishes (Evans, 1979).
