Inner medullary hemodynamics in chronic salt-depleted dogs

The effects of chronic salt depletion on medullary hemodynamics remain unknown. In the present study, sodium excretion, renal hemodynamics including papillary plasma flow, measured by the albumin-accumulation technique, and papillary tissue solute content were determined during hydropenia in 13 anesthetized sodium-replete and 10 sodium-depleted dogs. Salt depletion induced a significant rise in plasma renin activity and aldosterone without potassium depletion. Mean arterial pressure, GFR, and renal blood flow were similar in sodium-depleted and sodium-replete dogs. Despite a similar distribution of cortical blood flow (measured by the microsphere method) in the two groups, papillary plasma flow was markedly reduced in sodium-depleted dogs (8.8 +/- 1.7 vs. 22.8 +/- 1.9 ml X min-1 X 100 g-1 in sodium-replete dogs), associated with a significant decrease in renal sodium excretion. Furthermore, papillary osmolality and sodium concentration were significantly greater in sodium-depleted dogs. Ultrastructure examination revealed smooth muscle cells surrounding the efferent arterioles and pericytes with contractile potential encircling descending vasa recta. These results suggest that included in the complex hemodynamic adjustment to chronic sodium depletion is a significant reduction in inner medullary blood flow that may be important in maintaining enhanced papillary solute concentration. In addition, the anatomy of the medullary vasculature is compatible with regional regulation of medullary blood flow.