Circulatory model for in vivo kinetic studies of atrial natriuretic peptide (ANP) in humans

A new model, which takes into account the circulation of blood as well as different tracee concentrations in venous and arterial blood, has been developed and applied for the study of ANP, a hormone involved in sodium-water balance in physiological and pathophysiological conditions in humans. The body is considered as 2 blocks in series with recirculation. Heart and lung represent one block in which ANP is endogenously formed, distributed and partially metabolized, and the rest of the body represents the second one, where the hormone is only distributed and metabolized. Transmission and extraction can be calculated, under the assumption of linearity and stationarity, for whole body as well as for each of the blocks separately after labelled ANP administration into right atrium and later sampling in aorta and in pulmonary artery. The authors were able to measure different clearance values of the hormone for different sampling sites, as determined from the ratio dose/area under tracer concentration curve, in 2 patients. The model enables an estimation of fractional hormone extraction in each of the blocks, thus representing a significant advantage in respect to an integral model which yields only whole body clearance