The ultrastructural localization of membrane ATPase in rat thin limbs of the loop of Henle.

The cytochemical distribution of nonspecific membrane ATPase activity in the epithelial membranes of the thin limbs of the loops of Henle of rat nephrons was studied at the ultrastructural level. Membrane ATPase activity was localized in the luminal, lateral, and (to a lesser extent) basal membranes of only the outer medullary segment of the thin descending limbs of long nephrons (Type II epithelium). The reaction product was lacking in the thin limb of short nephrons (Type I epithelium) as well as in the inner medullary descending (Type III epithelium) and ascending (Type IV epithelium) segments of the thin limbs of long nephrons. These data reinforce the concept of thin limb heterogeneity and may indicate a specialized role for the outer medullary segment of thin descending limbs of long nephrons in the concentrating mechanism.