Role of deep nephrons and the terminal collecting duct in a mannitol-induced diuresis.

Recollection micropuncture in Munich-Wistar rats was used to study the effects of intravenous hypertonic mannitol infusions on fluid reabsorption by surface nephrons, prior to the bend of Henle's loop of deep nephrons, and along the papillary collecting duct. During mannitol diuresis, single nephron glomerular filtration rate rose significantly in surface nephrons but fell in deep nephrons. Although mannitol increased the delivery of sodium and water to the end of the proximal tubule and to the first portion of the distal tubule of surface nephrons, water and sodium were reabsorbed between these two sites. In deep nephrons, water reabsorption prior to the bend of the loop of Henle was significantly decreased. Absolute sodium delivery to this site was reduced despite a marked decrease in fractional sodium reabsorption prior to the bend. Papillary osmolality was decreased. Renal plasma flow and inner medullary plasma flow (IMPF) increased proportionally. The reduced water extraction prior to the bend of deep nephrons and the decrease in papillary osmolality could have been partly due to a concomitant increase in IMPF and a decrease in sodium delivery to the medulla. The reabsorption of delivered sodium and water by the papillary collecting duct was reduced to a greater extent than could be expected from the increase in sodium delivery.