Effect of Arterial Blood Pressure Reduction on Renal Hemodynamics in the Developing Lamb

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Abstract
The relationship between blood pressure and blood flow [BF] in the kidney was examined in 2-9 and 31-48 day old lambs. Renal [R]BF, determined by the microsphere technique, and glomerular filtration rate (GFR) were 1st studied under control conditions. The abdominal aorta was then constricted above the renal arteries until the pressure ranged between 60 and 70 mm Hg, i.e., just below the normal autoregulatory range, and the hemodynamic recordings were repeated. During control conditions the arterial pressure was lower in the younger (93 mm Hg) than in the older lambs (107 mm Hg). During aortic constriction total RBF and GFR were reduced. In both age groups GFR was reduced out of proportion to RBF. Na excretion fell around 60% in both age groups. The fall in perfusion pressure resulted in a more pronounced BF reduction to the outer than to the inner cortical glomerular capillaries. This pressure-induced BF redistribution was found in both age groups. The consequences of the pronounced effect of reducing the perfusion pressure to 60-65 mm Hg for the young lambs with their basally low arterial blood pressure are discussed.