Immunoreactive atriopeptin in plasma of fishes: its potential role in gill hemodynamics

With the use of antibodies raised against human atriopeptin (AP), immunoreactive AP (APir) was quantified in the plasma of five species of marine fishes, including members of the Agnatha, Chondrichthyes, and Osteichthyes. Concentrations of APir in fish plasma are in the same range as those described for mammalian species, indicating that AP was present in the earliest vertebrates and has retained at least partial structural similarity during the course of vertebrate evolution. Acclimation of two species to very dilute seawater was associated with a significant reduction in plasma APir, suggesting that salt loading, rather than volume expansion, may be the primary stimulus for AP release in fishes. Heterologous rat atriopeptin (AP101-126) vasodilated preconstricted, perfused gills and unstimulated isolated vascular rings from the ventral aorta of the marine teleost Opsanus beta with an apparent half maximum effective concentration (EC50) of 3-4 x 10(-9) M, similar to sensitivities to AP described for mammalian vascular smooth muscle. Acclimation of toadfish to approximately 5% seawater (hyposmotic to plasma) did not alter the sensitivity of the perfused gills but reduced the apparent EC50 of rat AP on aortic rings to 3 x 10(-10) M. Extracts from O. beta atrium, ventricle, and brain also produced dilation of aortic rings, with ventricular extracts producing the greatest effect per milligram extracted tissue, suggesting that the ventricle may be a major site source of atriopeptin in fishes. An atriopeptin-induced increase in blood flow to the fish gill would theoretically have detrimental osmotic consequences, but may stimulate salt transport, again suggesting that a putative atriopeptin may be involved in salt, rather than osmotic, balance in fishes.