Atrial natriuretic factor in the freshwater turtle Pseudemys scripta: a partial characterization

The presence of natriuretic and vasorelaxant materials in the atria and ventricles of a Chelonian reptile, the freshwater turtle Pseudemys scripta, was verified and the active substance partially characterized. Crude atrial and ventricular extracts were acutely natriuretic and diuretic when administered to anesthetized rats increasing sodium excretion 23.5 +/- 7.9 and 5.11 +/- 18 microM Na/10 min/mg extract, respectively. Although atrial extracts were relatively more natriuretic than ventricular extracts, the total natriuretic content of ventricular extracts was approximately twofold greater. Moreover, partially purified atrial extracts were found to relax precontracted isolated rat aortic ring segments and exhibited immunoreactivity to mammalian ANP antisera. Also, the natriuretic and immunoreactive substance was purified further by size exclusion chromatography and a molecular weight of 3-5 kDa was determined. Using the bovine adrenal glomerulosa ANP receptor, this partially purified natriuretic substance displayed an apparent binding affinity similar to that of mammalian ANP. Finally, in order to demonstrate that this reptile is physiologically responsive to ANP, synthetic rat ANP (10 micrograms/kg) was shown to decrease arterial pressure in conscious turtles from a control value of 37.2 +/- 5.2 to 30.3 +/- 2.7 mm Hg. These data demonstrate that unlike other non-Chelonian reptiles, both the atria and the ventricles of this Chelonian reptile synthesize and store substantial levels of biologically active ANP-like materials, and further, that ANP is a highly conserved and primitive cardiac hormone.