IN-VIVO MEASUREMENT OF ANP OVERALL TURNOVER AND IDENTIFICATION OF ITSMAIN METABOLIC PATHWAYS UNDER STEADY-STATE CONDITIONS IN HUMANS

Using a tracer method, we evaluated, in vivo, the main turnover parame ters and the main metabolic pathways of ANP in 10 normal subjects. HPL C was used to purify the labeled hormone and the principal labeled met abolites present in venous plasma samples collected at determined time s after tracer injection. The main ANP kinetic parameters were derived from the disappearance curves of [I-125] ANP, which were satisfactori ly fitted by a biexponential function in all subjects. Newly produced ANP initially distributes in a large, plasma equivalent space (10.9+/- 3.6 Vm(2) body surface); the hormone rapidly leaves this space due to both degradation and to distribution in peripheral spaces. The mean re sidence time in the body (19.4+/-19.8 min) and the plasma equivalent t otal distribution volume (28.2+/-11.5 Vm(2)) indicate that ANP is also widely distributed outside the initial space in humans (circulating A NP is no more than 1/15 of the body pool). Metabolic clearance rate va lues were distributed across a wide range (from 740 ml/min/m(2) to 258 1 ml/min/m(2), mean 1849 ml/min/m(2)), and were shown to strongly corr elate (R=0.962) with the daily urinary excretion of sodium. A complete separation of labeled ANP from its labeled metabolites was achieved b y the HPLC technique; at least 3 different peaks due to labeled metabo lites in vivo produced from the injected [I-125]ANP(1-28) were found. The first chromatographic peak eluted showed an identical elution time to monoiodotyrosine. At least two other peaks due to in vivo generate d labeled metabolites were well identified in the chromatograms: one p eak (coeluting with labeled COOH-terminal tripeptide, H-Phe-Arg-Tyr-OH ) was eluted ahead and one (coeluting with labeled peptide fragments A NP(7-28), ANP(13-28), and ANP(18-28)) behind the elution peak of the l abeled ANP. The peak of labeled tyrosine appearing in the plasma range d between 3 and 5 min after tracer injection; the other two peaks of r adioiodinated metabolites showed their highest activity in the first s ample (1.5 min), suggesting an earlier occurrence of their peaks. Thes e labeled metabolites seem to be intermediate peptides, between the in tact circulating form of the hormone and the final labeled metabolite (tyrosine), which is the last amino acid of the peptide hormone, produ ced in vivo after injection of the tracer. In conclusion, our kinetic data indicate that: 1) newly produced ANP is rapidly distributed and d egraded; 2) the body pool of the hormone can be considered a combinati on of two exchanging spaces, circulating ANP representing no more than 1/15 of the body pool; 3) MCR of ANP is closely related to sodium int ake; 4) labeled tyrosine is the main endogenous metabolite of the horm one in humans; 5) both receptor-mediated and enzymatic degradation pla y an important role in the turnover of ANP in humans.