Changes in renal hemodynamics and sodium excretion during saline infusion in lambs

The renal response to a progressive isotonic extracellular volume (ECV) expansion was studied in 13 lambs of two age groups (5-28 days and 48-57 days). Changes in renal hemodynamics induced by the ECV expansion were followed. Intrarenal blood flow was determined by the microsphere method. For determination of the glomerular filtration rate (GFR) standard clearance techniques were used. Recordings were made during control conditions and when normal saline had been infused in amounts up to 4.5% of the body weight. During the infusion there was an increase in sodium excretion both in absolute values and in relationship to GFR. The increase was, however, much less pronounced in the younger lambs. The GFR did not change significantly during saline infusion. The cortical blood flow increased only in the older lambs. As a consequence the quotient between GFR and cortical blood flow decreased in the older lambs. The possibility of a causal relationship between the fall in filtration fraction so obtained and the more pronounced natriuretic response in the older lambs is discussed. The inner to outer cortical blood flow ratio increased more in the younger lambs during saline infusion. The functional significance of an age related blood flow redistribution is, however, not clear.