SINGLE-PASS GILL EXTRACTION AND TISSUE DISTRIBUTION OF ATRIAL-NATRIURETIC-PEPTIDE IN TROUT

A new method is described in the present experiments to quantify atrial natriuretic peptide (I-125-ANP) extraction from plasma in a single transit through the gill vasculature of an unanesthetized trout. Tissue distribution of I-125 relative to an inert extracellular space marker, Co-58-EDTA, was also measured 1 h after injection of I-125-ANP. Single-pass extraction of I-125-ANP by the gill was 60% in control fish; it fell to 18% in fish previously injected with saline and became negative (relative to Co-58-EDTA) after treatment with the clearance (C-type) receptor inhibitor SC 46542. Approximately 90% of a I-125-ANP bolus injected into control trout is removed from the circulation within 2 min. I-125 concentration in gill tissue was nearly seven times greater than that predicted by the Co-58-EDTA space. I-125 was also concentrated in brain (2x), gallbladder, gastrointestinal tract, and eye (all 1x). SC 46542 decreased gill I-125 binding from seven times to one times the Co-58-EDTA space and increased I-125 accumulation in heart, kidney, fat, and skeletal muscle. Extraction of I-125-ANP by the isolated perfused gill was 53%, similar to that observed in vivo. These results show that the gill is a major site for ANP removal from trout plasma and that C-type receptors are predominant in this process.