Effect of arterial blood pressure reduction on renal hemodynamics in the developing lamb

The relationship between pressure and flow in the kidney has been examined in 2-9 and 31-48 day old lambs. Renal blood flow (RBF), determined by the microsphere technique, and glomerular filtration rate (GFR) were first studied under control conditions. The abdominal aorta was then constricted above the renal arteries until the pressure ranged between 60 and 70 mmHg, 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 mmHg) than in the older lambs (107 mmHg). During aortic constriction total RBF and GFR were reduced. In both age groups GFR was reduced out of proportion to RBF. The sodium excretion fell around 60% in both age groups. The fall in perfusion pressure resulted in a more pronounced blood flow reduction to the outer than to the inner cortical glomerular capillaries. This pressure-induced blood flow redistribution was found in both age groups. The consequences of the pronounced effect of reducing the perfusion pressure to 60-65 mmHg for the young lambs with their basally low arterial blood pressure are discussed.