A decrease in renal medullary tonicity stimulates anion transport in the henle's loop of the rat kidney.. Capasso, Giovambattista, Caterina Saviano, Francesca Ciani, Florian Lang, Ferdinando Russo, Natale G. De Santo. Chair of Nephrology, Second University of Napoli (Italy); Department of Biological Structures and Function, University Federico II of Napoli (Italy); Institute of Physiology, University of Tubingen (Germany).
APStracts 5:0015F, 1998.
To investigate the effect of reduction in renal medulla osmolality on loop of Henle net bicarbonate reabsorption, clearance and microperfusion experiments were performed on Sprague-Dawley rats. The decrease of renal medulla osmolality was induced by intravenous infusion of either a large dose of mannitol (mannitol protocol) or an hypotonic solution (hypotonic protocol) delivered at a rate to match the sodium and bicarbonate load of the control period. During the mannitol protocol, clearance data demonstrated a rise in GFR, RPF, urine pH and fractional bicarbonate excretion. On the contrary, microperfusion experiments, performed in the absence of mannitol in the tubular perfusate, revealed a significant increase both in the absolute and fractional loop of Henle bicarbonate transport. During the hypotonic protocol there was a decrease in GFR, associated with an increase in fractional excretion of bicarbonate. In the microperfusion experiments, hypotonic saline, similar to mannitol, stimulated absolute and fractional loop of Henle bicarbonate transport. Net reabsorption of chloride, measured under the same experimental conditions, was also found to be activated. Therefore, the i.v. infusion of hypotonic solution affected the loop of Henle trans-epithelial net reabsorption of both bicarbonate and chloride. We hypothesize that the increase in the transport rate of these two anions, along the same segment and in similar experimental conditions, may be mediated, at least in part, by decreased medullary tonicity that is one factor common both to hypertonic mannitol and hypotonic saline infusion. 

Received 25 April 1997; accepted in final form 5 January 1998.
APS Manuscript Number F139-7.
Article publication pending Am. J. Physiol. (Renal Physiology).
ISSN 1080-4757 Copyright 1998 The American Physiological Society.
Published in APStracts on 28 January 1998