Biological actions of atrial natriuretic factor in flatfish

Flounder adapted to seawater were chronically cannulated and received a single i.v. injection of either saline (control) or 10 µg/kg b.w. of human ANF. Compared to controls, ANF significantly reduced (p<0.001) mean arterial blood pressure; full recovery was evident after 4 hours. Blood samples taken at intervals after saline or ANF injection showed that ANF caused a marked increase of 33.7 µg/100 ml in plasma cortisol concentration (p<0.001) 5 hours post injection. The rate of recovery of²²Na in seawater after a single i.v. injection of 14×10⁶ cpm/kg²²NaCl was significantly increased (p<0.01) following ANF injection compared to controls suggesting that ANF stimulates Na⁺ efflux. This observation was confirmed in plaice and dab. The steroidogenic action of ANF and its ability to promote Na⁺ efflux are discussed in relation to its potential osmoregulatory role in teleost fish.