NO EVIDENCE OF A COUNTERCURRENT MULTIPLIER IN THE INTESTINAL VILLUS OF THE DOG

The hypothesis that a countercurrent multiplier within the intestinal villus increases osmolality in the distal villus tip was tested. Total sodium, potassium, and water content of intestinal tissue samples was measured. The results showed no significant difference in the total Na plus K concentration (millimoles per kilogram H2O ± SEM) between the villi tips (185.1 ± 7.7), whole villi (179.8 ± 5.4), or intestine minus villi (177.2 ± 1.7). In contrast, in the kidney (where the existence of a countercurrent multiplier has been demonstrated), the renal medulla had a total Na plus K concentration of 284 ± 17.1 that was significantly more than the renal cortical concentration of 163 ± 3.1. Villous tissue osmolality should be in osmotic equilibrium with intestinal luminal fluid. Sampling of intestinal luminal fluid revealed a total Na plus K concentration of 145-165 mmol/kg H2O, a figure compatible with normal luminal osmolality of 280-320 mosmol/kg H2O. These results deny the existence of a countercurrent multiplier in the intestinal villus of the dog. © 1990.