Atrial natriuretic factor (ANF), a 28-amino-acid peptide hormone produced in the heart, circulates in both freshwater and seawater rainbow trout. In mammals, two other peptide hormones, proANF 1-30 and proANF 31-67, derived from the same 126-amino-acid prohormone as ANF (amino acids 99&shyp;126), circulate and have natriuretic and diuretic properties. It has never been determined whether these peptides circulate in fish. The present investigation was designed to determine (1) whether proANF 1-30 and/or proANF 31-67 circulate in perfused hearts from freshwater- and seawater-acclimated rainbow trout (Oncorhynchus mykiss) in situ, and (2) if they do, to determine whether increasing the filling pressure of the heart causes their release in trout as it does in mammals. High-performance gel-permeation chromatography of fish plasma revealed that both proANF 1-30 and 31-67 circulate in freshwater- and seawater-acclimated trout plasma at threefold higher concentrations than does ANF. The basal rates of release of ANF and proANF 1-30 and 31-67 were similar in both freshwater and seawater trout, with the rate of release of proANF 1-30 being 10 times higher and that of proANF 31-67 20 times higher than that of ANF. When the filling pressure was increased to the peak of the Starling curve (max), the rate of release of ANF and proANFs 1-30 and 31-67 increased fivefold for each peptide in the freshwater trout, while in seawater trout the rates of release increased six- to ninefold. We conclude that proANF 1-30 and 31-67, as well as ANF, circulate in both freshwater-and seawater-acclimated trout and do so at concentrations higher than that of ANF. Increasing the filling pressure to the trout heart was found to cause a similar increase in the release rates for each of these peptides, but the maximal increase was higher in the seawater-acclimated trout, apparently because they showed a larger increase in cardiac output.

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