Specific binding sites for atrial natriuretic peptide in the freshwater turtle, Amyda japonica

Specific binding sites for atrial natriuretic peptide (ANP) were investigated by in vitro autoradiographic techniques in various tissues of the freshwater turtle, Amyda japonica. A high density of binding sites for 200 pM of 125I-labelled rANP(1-28) was located in the glomeruli of the kidney and the cortical portion of the adrenal gland. A moderate density of binding sites was seen in the arachnoid matter and choroid plexus of the third and lateral ventricles of the brain and the epididymis. A low density of binding sites was revealed in lamina propria of the mucosa of stomach and intestine, the seminiferous tubules of testes, and the epithelial layer of oviduct. In the presence of excess unlabelled rANP(1-28) (1 microM), binding to these structures were completely displaced. Therefore, specific ANP receptors exist in the kidney, adrenal gland, stomach, intestine, oviduct, epididymis, seminiferous tubules and brain. The ANP system may be involved in physiological regulatory function in the freshwater turtle, Amyda japonica.