The renal concentrating mechanism: micropuncture studies of the renal medulla.

Micropuncture of the rat renal papilla has disclosed an outward transepithelial gradient for NaCl at the bend of Henle's thin loop and an electrical potential difference, lumen positive, in the ascending thin limb. Substantial water extraction and urea secretion occur somewhere proximal to the bend, but direct evidence for transepithelial NaCl movement across the rat descending thin limb is lacking. In the hamster, water is extracted and urea secreted, but no NaCl gradient has been found, and in Psammomys there is indirect evidence for transepithelial entry of NaCl into the descending limb. Fluid is diluted in the ascending thin limb by reabsorption of NaCl. The lack of unequivocal evidence for active NaCl reabsorption has stimulated a search for alternative mechanisms of osmotic work in the inner medulla. The collecting duct plays a crucial role by its differential reabsorption of water (primarily in the cortex) and urea (exclusively in the inner medulla) but has not yet been shown to supply useful energy to the concentrating mechanism by active sodium reabsorption. Exposure of the papillary tip by ureteral excision impairs urinary osmolality. Ureteral peristalsis normally causes intermittent flow of fluid in the collecting duct, but abolition of intermittent flow by paralysis of the ureter does not decrease urinary osmolality. Superperfusion of the exposed papilla by a urea solution prevents the decline in osmolality but the amount of urea used greatly exceeds that available from the urine. Nevertheless, it is the intactness of the ureter that is somehow essential to maximum urinary concentration, perhaps by preventing loss of solute from the papilla rather than by supplying energy.