Extremes of urine osmolality: lack of effect on red blood cell survival

Rats were allowed a third of normal water intake for 20 days, and food consumption decreased. The reticulocyte count indicated a suppression of erythropoiesis. Urine osmolality increased from 2,000 mosmol/kg to 3,390 mosmol/kg. Random hemolysis and senescence of a cohort of red blood cell (RBC) previously labeled with [2-14C]glycine was monitored via the production of 14CO. Neither hemolysis nor senescence was affected. Following water restriction, the polydipsic rats generated a hypotonic urine. Urine osmolality decreased to 1,300 mosmol/kg for at least 6 days; a reticulocytosis occurred, but RBC survival was unaffected. These results contradict those previously reported, which suggest that RBC survival is influenced by the osmotic stress imposed on the RBC by extremes of urine tonicity. This discrepancy, we conclude, is due to differences in the methods employed for measuring RBC survival. The random-labeling technique employed previously assumes a steady state between RBC production and destruction. The cohort-labeling technique used here measures hemolysis and senescence independent of changes in RBC production, which is known to be depressed by fasting.