Natriuretic peptides and the acclimation of aglomerular toadfish to hypo-osmotic media

Since the aglomerular toadfish (Opsanus tau) experiences a natriuresis foll owing transfer to 10% seawater, we examined the role of natriuretic peptide s in the acclimation of toadfish to hypo-osmotic media. Gel filtration chro matography of acid extracts of toadfish heart and kidney identified a broad peak of atrial natriuretic peptide-like immunoreactivity in both tissues, with maximal immunoreactivity in fractions coeluting with human alpha-atria l natriuretic peptide. Using a homologous bioassay to measure changes in ao rtic ring tension, both vasorelaxing and, surprisingly, vasoconstricting bi oactivities were identified in gel fractions of heart extract. No significa nt Vasorelaxing activity was identified in kidney extract or fractions. Ins tead, a potent vasoconstricting activity was observed, with maximal activit y in gel fractions with an estimated MW greater than 1000 Da. Levels of atr ial natriuretic peptide immunoreactivity in plasma from the caudal vein wer e very low in seawater toadfish and were unchanged 12 h after transfer of t oadfish to 10% seawater. We conclude, that natriuretic peptides are present in the heart and kidney of toadfish. However, atrial natriuretic peptideli ke peptides of cardiac origin circulating to the kidney via the caudal vein do not appear responsible for the natriuresis that ensues upon the transfe r of toadfish to 10% seawater. In the absence of glomeruli, this tubular na triuresis may be regulated by natriuretic peptides present in the kidney.