Cardiac role of frog ANF: negative inotropism and binding sites in Rana esculenta

To elucidate the role of atrial natriuretic peptides (NPs) in the amphibian heart, the myotropic effects and the cardiac distribution of frog atrial natriuretic factor (fANF) have been studied in Rana esculenta. Spontaneously, beating in vitro isolated working heart preparations were treated with increased concentrations (10(-11)- 10(-8) M) of fANF-(1-24). The peptide at 10(-9) and 10(-8) M significantly reduced heart rate (HR) and, on the electrically paced preparations, decreased cardiac output (CO), stroke volume (SV) and work. Such negative inotropism was abolished by pretreatment with the pertussis toxin or by blocking the particulate guanylate cyclase (GC) with anantin while it was independent both from the functional impairment of the endocardium-endothelium by Triton X-100 and the inhibition of the soluble guanylate cyclase by 1 H-(1,2,4,) oxadiazolo-(4,3-a) quinoxalin-1-one (ODQ). By autoradiography, two classes of high and low affinity NPs binding sites were detected in the ventricular endocardium and myocardium and in the bulbus arteriosus. The analysis of displacement binding data using the radioligand [I-125]-rat atrial natriuretic peptide [I-125-rANP-(1-28)], its cold counterpart and the fANF-(1-24) showed that in the ventricular myocardium, the low affinity NPs sites bound both the heterologous and the homologous ligands at a concentration close to that responsible for the negative inotropism and chronotropism.