Depletion of the Renal Medullary Osmotic Gradient Following Hemorrhagic Hypotension in Hydropenic Rabbits

The influence of blood pressure on the renal medullary osmotic pressure gradient was studied in hemorrhagic hypotension of hydropenic rabbits. Slice analysis was made for urea, sodium and potassium concentrations in the kidney slices. The tissue fluid osmolality was estimated as the sum of the urea concentration and 2×electrolyte concentrations. A drop of systemic blood pressure, induced by bleeding, resulted in decrements of urea and sodium concentrations with resultant reduced osmotic pressure gradient in the medulla, but did not significantly affect solute and osmotic concentrations in the cortex. A loss of urea from the medulla was more rapid and greater than that of sodium. The reduction of the medullary osmotic pressure gradient became greater as mean blood pressure decreased to 50-60mm.Hg, but was reduced with progressively decreased blood pressure below 50mm.Hg. Renal arterial occlusion for 1 hour gave rise to a slight decrease in the medullary osmotic pressure gradient, chiefly due to loss of urea from the medulla. The finding indicates that the depletion of the medullary osmotic pressure gradient in hemorrhagic hypotension is dependent upon the blood pressure level. The data were also discussed in relation to the renal hemodynamic alterations in hemorrhagic hypotension.